Abstract
This chapter presents extensive and updated knowledge from scientific and technical reports on the management of agriculture pests using detergents and soaps (D + S), with emphasis on their utility in integrated pest management (IPM) schemes. It includes a review on their environmental, ecological, and toxicological impacts, and their possibilities to become important tools for pest control, especially for those D + S having minimum risk, considering both current and newer products. The present knowledge of their modes of action on arthropods is addressed, revealing the need to better identify the mechanisms to optimize their use against crop pests. Their disadvantages are also analyzed, mainly the lack of residual effect and the potential toxicity to plants. Some ways these problems have been overcome are presented. A comparison of the direct costs of the use of conventional pesticides versus D + S, achieving statistically similar levels of control, is discussed, and scenarios where detergents are competitive (representing lower costs) are presented. There is also a review of the type of compounds reported in the specific literature, which leads to highlight the opportunities to develop agriculture detergents and soaps suited to local agriculture needs. New findings on D + S as co-adjuvants for conventional and biological pesticides, and their potential utilization as safe postharvest treatments against pest, are also presented. Finally, the authorization for soaps and detergents is also discussed, highlighting the need for a joint effort (state agencies, producers, researchers, etc.), in order to increase the offer and the use of detergents and soaps, partially replacing conventional pesticides, to take advantage of their potential as sustainable pest management tools, particularly for IPM programs, but also for organic and conventional productive schemes.
Keywords
- detergents
- IPM
- soaps
- surfactants
- sustainability
- toxicity
1. Introduction
1.1. Chemical control in integrated pest management (IPM) programs and detergents and soaps
Integrated pest management is a strategy developed to control agricultural pests and, at the same time solve problems derived from the extensive and intensive implementation of chemical control in conventional agriculture, where broad spectrum, specific action site, and persistent pesticides are used. Compounds with this profile have been called “conventional pesticides” and are responsible for causing resistance in pest populations, destruction of beneficial arthropods, and presence of pesticide residues in foods, soils, water, and air [1]. In order to obtain an economic, environmental, and ecologically sustainable food production, IPM encompasses several components, including cultural, biological, and chemical control [2]. Therefore, the use of pesticides is not excluded from IPM programs, for instance, when there is no other available tools to avoid economic damage [3], synergy occurs between chemical and biological control [4], or a diverse pest complex affects the crop [5]. Under those circumstances, the products used should target several sites and mechanisms (multisite), be shortly persistent in the environment and crops (non-residual), and have both a narrow spectrum (selective) and low toxicity to mammals. Many compounds having these attributes have been called “alternative pesticides”, including oils, pheromones, botanicals, entomopathogens, and soaps and detergents, among the most frequently used [6, 7]. For definition purposes, agriculture detergents and soaps, from now on “D + S”, correspond to surfactants from either natural or synthetic origin, formulated specifically for pest control or other uses in crops. Within these options, D + S have additional particularities, being relatively inexpensive, easy to produce and apply, versatile (controlling juvenile and adults), allowed as postharvest treatment, etc. [8, 9].
1.1.1. Resistance management
Resistance is a consequence of the elimination of susceptible genotypes and selection, over time, of the tolerant part of the population by the frequent and wide use of pesticides with specific sites of action that lose afterwards their capability to control pests [1]. The alternating use of conventional products with different action sites has been one way to face resistance, but a more holistic approach is necessary to provide a sustainable solution [10]. That is why IPM was developed during the second half of the twentieth century, attempting to either avoid or reverse resistance by replacing chemical control by other strategies, and/or by using several different chemicals with multiple modes of action, as D + S that, therefore, should become useful tools for IPM [8, 11].
1.1.2. Environmental, ecological, and toxicological issues
Environmental contamination, diversity threatening, and toxic effects on mammals and other animal species are well known and severe impacts from the use of conventional pesticides. Environmental toxicity by soaps, on the other hand, is considered very low [12], but detergents in wastewater (sometimes in large concentrations) are considered important pollutants when they reach rivers and streams, where they form foam layers and affect the aquatic fauna. However, the greater biodegradability of current surfactants has significantly reduced those problems [13]. Besides, sprays in farms should not massively reach water courses, therefore minimizing the potential impact in surface and groundwater. Based on studies of wastewater used for irrigation [14], some surfactants alter physical, chemical, and biological properties of some types of soils [15]. However, linear alkylbenzene sulfonates (LAS, widely used in detergents) are considered not to be a threat to terrestrial ecosystems on a long-term basis because of biodegradation [16], although nonylphenol has been questioned [17]. Thus, their impact depends largely on the type of surfactant chemistry, providing room for testing, selecting, and using those less hazardous products.
In general, D + S have low acute toxicity [18], particularly non-ionic or anionic detergents, which are, by far, less dangerous than conventional insecticides [19]. For instance, the soap Safer has an oral LD50 of 16.500 ppm (= median lethal dose, i.e., the amount of active substance per body weight required to kill half of an exposed population), which is by far less dangerous than conventional insecticides, including botanicals [12]. The risk should be even lower considering both the necessary dilution and the small chance of ingestion. Conventional pesticides on the foliage are an important risk for applicators by dermal exposure, making necessary reentry intervals after their application, which are not needed when D + S are used. Detergents can cause dermal [20] or eye irritation, but in general this type of exposure represents a very low risk to agriculture workers wearing the basic personal protective equipment, although some respiratory disorders have been reported to detergent exposure, mainly on asthma sufferers [21, 22]. There are some concerns regarding specific housecleaning products (e.g., those containing alkyl phenols), which have been related to breast cancer [23], although under normal exposure in the field the risks are reduced, since no systemic toxicity is expected for most D + S and several components of their formulations [18, 19, 24], but this issue needs a case-by-case analysis. Another important issue is the persistence of conventional pesticide residues in/on the marketable part of the crop that makes necessary to establish regulations of MRLs (maximum residue limits) for foods. Thus, PHIs (preharvest intervals) are established to comply with the law, whereas most D + S are not subjected to this type of restrictions. In fact, some D + S are applied right before harvest [9] and others are authorized for postharvest treatments [25], being easily washed off from the epidermis of fruits and vegetables by rinsing before consumption, having minimum risk and being therefore exempt of MRLs [26].
Regarding the impact on beneficial fauna in crops, D + S have been considered more selective than conventional insecticides, being compatible with biological control due to their low adverse impact on not sprayed insect and mites and the lack of residual activity [4, 27]. The only threat occurs by the direct spray or when the solution persists on the foliage, usually for short periods, killing predators and parasitoids. Therefore, the release of beneficial arthropods after a spray, once deposits are dry, allows them to survive. Available EIQ (environmental impact quotient that considers environmental and ecological threats) values for soaps indicate their low impact (e.g., 19.45 for M-Pede), close to most botanicals or IGRs (insecticide growth regulators), and smaller than those of horticulture oils [28]. However, no data on detergents were available. Therefore, research to identify efficient (current or new), but also nontoxic and ecologically safe D + S for pest control is required.
1.1.3. Legal and economic issues
Conventional pesticides are subject to a complex and expensive registration process where, after agronomic and toxicological reviews, they might obtain legal authorization to be used on crops. On the other hand, D + S are not necessarily subjected to registration, since some products are not labeled as pesticides, but as tree cleaners. However, it is important to transparent the real purpose of its use in agriculture [11]. Even when explicitly recommended to control pests, D + S should be easier to register after considering their risk assessment due to their low acute and chronic toxicity and, in some cases, their status as food additives or edible surfactants [29]. Considering the growing demand for residue-free foods, the eventual replacement of conventional pesticides for D + S will make those foods preferred by customers, increasing their value and making their trade easier. Therefore, all the actors involved should deeply assess D + S uses for pest control.
1.2. Modes of action of detergents and soaps as pesticides
The modes of action for D + S against pests have not been well understood yet [30, 31]. In fact, D + S are not considered on the IRAC (Insecticide Resistance Action Committee) lists that classify the pesticides mode of action for those with known specific target sites [32]. This is because D + S are not known to act at specific target sites, but at multiple sites [11]. Despite that, wax removal, arthropod dislodging, and drowning have been mentioned as lethal mechanism in D + S.
1.2.1. Wax removal
The arthropod epicuticle is mainly made of lipids. The outermost part is a wax layer constituted mostly by hydrocarbons, serving mainly for waterproofing to avoid dehydration [33]. This is a serious threat for small insects and mites, particularly those sessile and exposed individuals. It has been proposed that when arthropods are sprayed with detergent, lipids are removed from the epicuticle, losing its waterproof ability, which in turn causes important water losses and, finally, the death of treated pests [34]. In fact, a significant reduction in both residual epicuticular lipids and body weight (assumed to occur mainly due to water losses) on the obscure mealybug
Santibáñez [35] proposed that mealybug mortality by exposure to detergents might be caused by several mechanisms, including the initial wax removal that might lead to further damage of the integument, but this was not demonstrated. Many reports of pest management with D + S reveal that individuals present a degreased and dehydrated aspect after exposure, suggesting that water losses might be involved in mortality. For instance, the cotton aphid
Treatments1 | Detergent (mL a.i.2/100 mL) | Water loss3 (mg) | Residual waxes4 (mg/mL) |
---|---|---|---|
LC90 | 8.17 | 1.85 a5 | 14.95 b5 |
LC50 | 4.45 | 1.48 b | 6.85 b |
LC10 | 0.74 | 0.89 c | 54.76 a |
Control | 0.00 | 0.47 c | 55.06 a |
Detergents1 (%, v/v) | % dislodgment2 (D) | % mortality (M) | CD = 100×D/[D + M]) |
---|---|---|---|
1.00% | 22.2 | 59.4 | 27.2 |
0.50% | 21.7 | 32.6 | 40.0 |
0.25% | 14.1 | 17.6% | 44.5 |
Control3 | 3.2 | 18.8% | 14.5 |
1.2.2. Arthropod dislodgement
Detergents and soaps contain surfactants, that is, compounds that reduce the surface tension of solutions, enhancing their capability to wet and wash arthropods off. Thus, sprays can dislodge motile forms of phytophagous pests, as nymphs and adults of mites, thrips, etc. (particularly when the solution runoffs on the leaves). Even not necessarily all removed individuals die, and dislodgement causes significant reductions of populations infesting the foliage. Dislodgement has been highlighted as an anti-herbivore trait [36] that reduces their phytophagous performance on the plant. In a laboratory study, up to 22% dislodgment of the citrus red mite
Not many reports have demonstrated dislodgment when soaps and detergents are used for pest control, although surfactants have been mentioned as useful tools to wash out arthropods plant substrates (including plant organs) for cleaning produce or pest sampling purposes [39]. For instance, ca. 28% of the western flower thrips,
1.2.3. Drowning
Arthropod respiratory system is formed by a net of conducts (traqueae) that allow direct gas exchange with tissues. It is connected to the exterior by spiracles that regulate opening by muscles [33]. The surfactant properties of detergents and soaps allow the solutions to enter the spiracles [41, 42]. The solutions fill the traqueae, causing drowning and death. No reports have been found describing this mechanism for pest control, but several papers have mentioned drowning as a mortality factor after surfactant sprays on insects and mites [43, 44]. In larger insects, this seems to be a lethal mechanism after exposure to D + S [43].
1.2.4. Other mechanisms
Interference with cellular metabolism [41], repellency [30], breakdown of cell membranes [42], abnormal juvenile development [12], caustic activity, uncoupling oxidative phosphorylation, and/or even nervous system disruption [45] have been also indicated as possible modes of action of D + S, but further details have not been found. Interestingly, in nature, surfactants have been highlighted as a mechanism of defense developed by some insects against their predators by producing oral secretions containing surfactants that, for instance, stop ants attacking beet armyworm,
1.3. Detergents and soaps used for pest control in agriculture
1.3.1. Formulations
Table 3 presents the characteristics and origin of 16 detergents and soaps used for pest control, or as co-adjuvants, reported in here. Many are liquids that perform better as insecticides and miticides [47], and a few are bars or powders. All were mixed in water to be applied, but bars needed, additionally, chipping and boiling before dilution. Several of the main world producers of cleaning products are represented in the list. About 44% of the products listed in Table 3 correspond to either dishwashing, housecleaning, or personal cleaning products tested or used as alternatives to conventional pesticides. Thus, most products were not registered for pest control or agriculture use, but the results from research led later, in some cases, to the development of agriculture detergents (e.g., TS 2035 or SU 120 in Chile). Some D + S are produced locally, by relatively small producers, with raw materials easy to obtain, making suppliers and growers, particularly in developing countries, more independent from foreign surfactant producers. Information on D + S formulae was not always readily available and their components were not completely described, indicating only generically the type of compound (no chemical names given) or giving the range of the total surfactant content, but not precise figures. In fact, in many scientific publications reporting on the topic, there are no details on the specific inert ingredients or the surfactants (considered the active ingredients), or their respective proportions [47, 48].
Commercial names and formulations2 | Companies3 and countries | Surfactants (a.i.) and %4 in c.p. | Declared uses5 and references |
---|---|---|---|
Acco Highway Plant Spray Soap, L | Acme Chemical Company, PA, USA | Coconut oil soap6 (38.5) | ASo, Moore et al. [63] |
Break, L | BASF, Chile | Trisiloxane7 (75) | Co, Sazo et al. [54] |
Disolkyn, L | Bramell Ltda., Chile | Sodium disoctyl sulfosuccinate6 (70) |
Su, Sazo et al. [66] |
Ivory Clear detergent, L |
Proctor and Gamble, OH, USA |
Acids salts of coconut oil and tallow6 |
HCD, Sclar et al. [69] |
Key soap, B | Unilever, Ghana | Not provided | PCSo, Asiedu et al. [48] |
LK dishwashing, P | Biotec S.A., Chile | Not provided | DiD, Arias et al. [47] |
M-Pede, L | Mycogen Corp., CA, USA | Potassium salts of fatty acids6 (49) |
ASo, Butler et al. [30] |
Nobla, P | Johnson and Diversey, Chile |
Sodium alkyl benzene- sulfonate6 (5-15) |
HCD, Curkovic et al. [57] |
Palmolive, L | Colgate-Palmolive S.A., Chile |
Total fatty acids6 (71) | PCSo, Arias et al. [47] |
Quix, L | Lever S.A., Chile | Sodium benzene- sulfonate6 (15-30%) |
HCD, Curkovic et al. [34] |
Safer, L | Agro-Chem, CA, USA | Potassium salts of fatty acids6 (50) |
ASo, Osborne and Petit [65] |
SU 120, L | Johnson and Diversey, Chile | Sulfonates (14.9); lauryleter sulpfnate6 (17.8) |
AD, Ripa et al. [55] |
Sunlight Dishwashing Detergent, L | Unilever, Ghana | LAS6 (10-20) + sodium lauryl ether sulfate6 (5-10) | DiD, Asiedu et al. [48] |
Tecsa fruta, L | Protecsa, Chile | Xylene sulfonate6 + nonylphenol7 (1.5-2) |
AD, Curkovic et al. [38] |
Triton X, L | Sigma, MO, USA | Octyl-phenol hydrophobe series Polyethylene glycol ether7 |
ASu, Warnock and Loughner [40] |
TS 2035, L | Pace Intl., Chile | 15-17% sodium dodecyl sulfate6, 4-6 ethoxilated alcohol%7 |
AD, Curkovic et al. [9] |
1.3.2. Surfactants
The first synthesized surfactants were soaps, molecules with a relatively long hydrocarbon hydrophobic chain in one extreme, capable of binding lipids, and a hydrophilic carboxylic group in the other extreme bonded to either sodium or potassium [49]. Soaps are relatively easy to produce from natural raw materials (animal fat or vegetable oils). They were used in pest control as far back as the eighteenth century [50]. However, soaps did not perform efficiently in hard water (where they precipitate) or at low temperatures. Therefore, and also considering the shortage of raw materials in Europe after World War I, detergents were developed in the 1930s, overcoming the limitations of soaps [20], mainly by substituting the carboxylic end by a sodium sulfate or sulfonate, or other hydrophilic group. The main uses of both types of compounds worldwide are housecleaning (laundry and dishwashing), personal care (body washers, shampoos), but also in agriculture, food processing, etc. Today, the main raw materials used to produce surfactants are petroleum-based materials and plant oils (mainly from soybean and palm). The latter has an increasing production due to, among other factors, its low cost and toxicity, and natural origin. In fact, from the point of view of their use in agriculture, detergents, unlike soaps, cannot be used in organic farms because they are synthetic, nonnatural products. The recent changes in surfactant markets (including the need for safer, environmentally friendly, and economical products) have stimulated the production of new compounds. For instance, food and pharmaceutical processing surfactants or edible surfactants are available, providing alternatives that need to be tested as pesticides, besides older compounds [29, 51]. Surfactants in D + S reported herein are described in Table 3. In solution, surfactants tend to adsorb to the surface or interphase of materials, reducing hydrogen bridges between water molecules, thus improving their wetting capabilities. Besides, in contact with water, surfactants form micelles or small spheres, usually having the hydrophobic end inside, binding lipids, and the hydrophilic end outside. In this way, lipids are removed (degreasing effect) from the substrate and get diluted (solubilized). The electric charge of the hydrophilic end in solution can be neutral (non-ionic surfactants), negative (anionic, the most common among the D + S reported herein), positive (cationic), or both (negative and positive) [49]. Ionic surfactants can modify the pH of the solution. For instance, anionic surfactants tend to slightly acidify the pH, but they perform better at basic pH; therefore, the detergent formulae include some buffer agents. In fact, it was found that agriculture detergents (including all co-adjuvants) tend to alkalinize the solution in distilled water (pH: 7.8–8.9, depending on the concentration) [35], but only when above 1% (v/v) was prepared, maintaining the pH neutral otherwise [52]. In many cases, the surfactants vary between D + S formulations (in their chemistry and/or proportions), affecting their insecticide/miticide performance [38, 53]. Therefore, the activity of D + S needs some standardizing procedure in order to compare their activities as pesticides, for instance, comparing the proportion of surfactants (see below the case of some mealybugs), although differences can also be due to the particular type of surfactant, so this issue needs further research. Besides the house or personnel cleaning products, and some agriculture detergents, other sources for pest control are the co-adjuvants commercialized for specific functions, for example, wetting agents when mixed with pesticides or fertilizers in agriculture. Some of them have been individually or in mixtures tested as insecticides and miticides [52, 54].
1.3.3. Efficacy as insecticides or miticides
Most reports of pest control with D + S state relatively high levels of control (measured as either density reduction or mortality) against target pests. Those levels were usually achieved with the highest concentration tested, in most cases under or equal to 2%, either w/v or v/v, and considering the largest number of sprays [31]. The efficacy was directly related to coverage (the volume of water/ha used) and the stage of the pest (younger instars, except eggs, are the more susceptible ones, see Table 4) [11, 55]. In a few reports, however, the level of control obtained with soaps was poor [31, 56] or not significant when compared to some standard treatments (a recommended conventional pesticide). Maximum control was frequently measured when evaluations were conducted about a week after application, presumably due to a slower activity on arthropods than conventional pesticides [9], but some rapid stop-feeding response was also reported for insecticidal soaps, although mortality was achieved more slowly [12]. A few formulations include insecticides (e.g., pyrethrins are added in small amounts, [12]) for uses as agriculture soaps or louse shampoos [45], increasing their biocidal activity because of the addition of the natural neurotoxicant, but this is not the case of the products reported herein.
Detergents | LC50 on | LC903 on |
---|---|---|
Tecsa fruta | 1.4 b2 (nymphs) | 4.2 (nymphs) |
2.5 a (adults) | 9.7 (adults) | |
SU 120 | 1.2 c (nymphs) | 7.5 (nymphs) |
1.4 b (adults) | n/d4 (adults) |
1.4. Challenges and opportunities of detergents and soaps for pest control
1.4.1. Phytotoxicity
Toxicity to plants is a risk associated to the use of D + S, particularly at concentrations above 1–2% (v/v), but this effect should be a function of the proportion and type of surfactant(s) in the commercial formulation. It also depends on the plant species (its specific susceptibility or tolerance), their physiological condition, morphology, and growth stage. Phytotoxicity affects mainly leaves, flowers, and fruits [27, 57]; symptoms on the foliage range from yellowing to bronzing, and wilting or curling, up to necrosis and defoliation, whereas in fruits they range from small brown spots or massive epidermal browning to fruit dropping (Figure 2). Petal flowers can become brown or even necrotic when D + S are applied during flower bud appearance and blooming. These symptoms are also observed after repeated sprays with high concentrations (usually above 1%) of detergents [58] or when plants are under some type of stress (e.g., shaded plants, see below the case of
Treatments | # sprays Dafs3 | % mortality |
---|---|---|
TS 2035 0.5% | 1 (0) | 15.2% e4 |
“ | 2 (0 and 7) | 38.0% de |
“ | 3 0, 7, and 14 | 62.8% bcd |
TS 2035 1.0% | 1 (0) | 61.1% cd |
“ | 2 (0 and 7) | 84.4% abc |
“ | 3 (0, 7, and 14) | 90.5% ab |
Chlorpyrifos1 | 1 (0) | 94.4% a |
Control2 | 3 (0, 7, and 14) | 0.0% f |
1.4.2. Lack of residual activity
Some reports state that insecticidal soaps are not persistent since they suffer rapid degradation [12]. However, some other studies on detergents or surfactants have demonstrated that their residues persist on the substrate after application. Triton X and Tween 80 (see Table 3 for details), two surfactants used as co-adjuvants, produced persistent residues, at least a week after the spray on tomato fruits or tobacco leaves, respectively [60, 62]. Despite that, D + S residues do not have residual activity in terms of protection over time [31], which occurs only in solution [45], thus they are considered strictly contact pesticides (spray or topic exposure), some affecting the pest quickly [12]. Some soaps have been incorporated into a diet causing a slight mortality in the laboratory [56], showing some ingestion activity, but only at high concentrations (5× the recommended field rate). There is, however, some “residual” activity shortly after the application of D + S, if the solution lasts as either droplets or a liquid layer on the foliage and contacts the arthropod [47]. There is also the possibility of re-hydration if, for instance, relative humidity increases enough and shortly (after the spray) during fog events, to re-dilute D + S residues. It has been proposed to conduct repeated and frequent sprays of D + S to counteract their lack of residual activity on recurrent pests (see Tables 5 and 6 for successful examples), but some concerns have been mentioned about the potential buildup of surfactants in the soil [63], although specific studies have not been conducted, except for some co-adjuvants [64]. On the other hand, the lack of residual effect turns out to be an advantage, preventing mortality of beneficial arthropods released after residues, which are dry, making D + S compatible with biological control and IPM programs.
Treatments | # sprays3 | Dafs4 | % mortality5 |
---|---|---|---|
TS 203511 | 1 | 0 | 29.0 cde |
2 | 0 and 10 | 23.7 de | |
3 | 0, 10, and 20 | 51.7 abc | |
4 | 0, 10, 20 and 30 | 54.2 ab | |
1 | 30 | 49.6 cd | |
Imidacloprid2 | 1 | 0 | 78.8 a |
Control | 0 | 0 | 12.0 e |
1.4.3. Legal restrictions and registration
Authorization is an obligatory requirement to legally utilize D + S as pesticides in agriculture. It implies the demonstration of no toxicological risks (including ecotoxicology) and agronomic efficiency, based on science, excluding compounds that do not comply. The process requires a large effort, and it is slow and expensive, making the agrochemical industry to proceed only when the economic return is attractive. There are a few cases of registered D + S as insecticides and/or miticides for agriculture, a few in the United States [30, 65]. In Chile, there has been one registration (Disolkyn, see Table 3) for a few years during the mid-2000s [66], but it was not renewed, so there are no legally available D + S for pest control currently in this country. Despite that, non-registered D + S have been used in Chile for pest control, suggesting that they do not cause problems. Their use with no sanctions has occurred because this is an issue not regulated specifically, since the products can be declared as used, for instance, as tree cleaners (an authorized use in some agricultural detergents), pest control being the real purpose [11]. However, growers subjected to the certification process do not use D + S. This causes a serious bottleneck for registration and development for these compounds as tools for pest management. Besides, the chemical and agrochemical industry have not made large efforts for detergent registration as pesticides, in part for a low market expectative in economic terms (low profit), and also due to the difficulty and elevated costs involved. For D + S, government agencies require the same requisite used for the registration of conventional pesticides, making even more difficult for the industry to spend efforts in a registration process for these types of compounds. However, as mentioned before, many surfactants, detergents, and soaps are safe for the environment and the users, and some are even food additives or edible surfactants, so there is room for pesticide development to identify and select those D + S with very low risks. Similar to the case of horticulture oils, pheromones, or biological pesticides [12, 13, 18], D + S should be developed as safe products, obviously excluding those questioned and dangerous [15, 17]. Therefore, authorization for D + S must be addressed by all the actors involved: government (registration agency, Departments of Health and of Agriculture), producers (the surfactants industry and agrochemical companies, suppliers, and distributors), the academic sector (researchers from the agronomic, chemistry, and toxicology areas), and even grower and consumer organizations (particularly those advocated to consumption of safer foods). Only by acting jointly, the analysis, selection, and development will lead to register and use D + S in pest management. Once available, these compounds will serve in IPM, but also to conventional or organic production schemes, and serve in many complex scenarios (e.g., used very close to harvest with no other management options).
1.4.4. Spray conditions
Since D + S work strictly by direct contact, application should maximize the exposure of the pest as much as possible. Spray equipment must be adapted, for instance, modifying nozzles orientation in order to apply from underneath the leaves or fruits, where mealybugs, spider mites, or whiteflies use to feed [9]. Air-blast or powered backpack sprayers have been preferred for D + S applications, since better coverage and smaller droplets are achieved [9, 27]. If possible, trees might be pruned before spraying surfactants in order to increase pest exposure and air circulation that will help in the dehydration of treated insects and mites [9]. Solutions should be applied considering whole coverage of infested organs, using high volumes of water/ha and high-pump pressure during the spray [8, 63]. Besides, sprays should be done early in the morning or late in the evening to increase the duration of the wet layer and extend their insecticide lifetime [31].
1.4.5. Pest biology and ecology
The habits, biology, and morphology of the pest should also be considered to maximize exposure by D + S sprays. For instance, nocturnal pests (armyworms (Lepidoptera: Noctuidae) or snails (Mollusca: Pulmonata, Helicidae)) should be sprayed at night for direct exposure. In fact, some noctuids have not been controlled efficiently by diurnal soap sprays in the field [56]. For the greenhouse whitefly
2. Review of agriculture pests controlled with detergents and soaps
2.1. Hemiptera
Most examples of pest species controlled with D + S belong to this insect Order. They are the main target group because of their (a) size, being small (most), therefore highly dependent on their protective wax layer; (b) exposure on plant tissues, many being relatively easy to reach and/or remove from the foliage by the spray; (c) type of cuticle, being either soft or thin, thus more susceptible to D + S; (d) damage, as most species cause it when reaching high populations, thus, a significant reduction (but maybe not eradication) is enough to secure satisfactory yields, as expected for surfactants; and (e) null development of resistant populations as with conventional insecticides, thus, management with multisite D + S helps to avoid or reverse the problem, etc. The following review presents the most important hemipteran groups controlled with these types of compounds.
2.1.1. Aleyrodidae
Whiteflies are plant-sucking pests, having many generations per crop cycle, which infest mainly the foliage (usually the underside of leaves) of vegetables, tree fruit orchards, and ornamentals. They affect plant growth and yield by sap sucking, transmission of some diseases during feeding, and release of honeydew on the foliage and fruits, allowing the colonization by sooty mold. This fungus reduces both photosynthetic capacity and the value of the produce (downgrading the price of fruits and vegetables). Honeydew also serves as food for attendant ants that disturb biological control agents. Whiteflies have externally a conspicuous white-dusting wax layer to protect them from dehydration, also serving to reduce insecticide exposure. Detergent and soap sprays have been widely used to target the underside of the leaves and control whiteflies, despite some limitations against these pests as the lack of both systemic activity and residual effect. To counteract these narrowing factors, sprays require to be frequent, to cover the whole population. Besides, as whiteflies have several generations lasting about a month per crop cycle, each one should receive sprays. Butler et al. [30] were one of the first researchers in testing 16 D + S (e.g., M-Pede, Palmolive, etc.; see details in Table 3) on the control of the sweet-potato whitefly,
2.1.2. Aphidoidea
Aphids (
Woolly aphids (
2.1.3. Coccidae
Coccids or “soft scales” are important plant-sucking pests that infest mainly leaves and branches, and occasionally fruits, affecting plants similarly than whiteflies and aphids. Scales are relatively exposed to sprays, but their bodies are protected by a thick and hard shield. Because of that, sprays with contact insecticide target mainly young nymphs that have a poorly developed shield. Since coccids have usually one or two generations/year, the timing for insecticide contact sprays must be precisely defined by monitoring. Detergents and soaps have been informed to control coccid pests since several decades ago (e.g., Singh and Rao, 1979, on the green scale,
It is worth noting that other coccid species have been reported to be satisfactorily controlled by D + S: the soft scale
2.1.4. Diaspididae
Armored scales are also sucking pests, but have a dorsal and protective shield not glued to the body. They colonize mainly branches and fruits (and eventually the leaves) of tree fruit orchards and ornamentals, but do not produce honeydew. Only nymphs I are mobile (crawlers), but once they set on a structure, they lose their legs and become sessile. Diaspidids have usually two to four generations/year. Reports on armored scale control with D + S are less frequent. For instance, the mortality of the oleander scale
2.1.5. Pseudococcidae
Mealybugs are, in general, similar to soft scales regarding the effect on infested plants. However, mealybugs do not significantly reduce plant growth and tend to infest fruits and branches instead of leaves, wood crevices and cuts, zones of fruit contact and calyx cavities, where they can stay even after harvest. Some are serious quarantine problems for exports. Because of that, detergents or soaps are usually not used for mealybug control in orchards oriented to export (however, see the use as postharvest treatment below). Consequently, an intense chemical control program is applied in Chilean orchards exporting fresh fruit, using conventional insecticides (preferring systemic and/or residual products), but their efficacy is still relatively low. This is due mainly to the insect’s habits (see Section “Pest biology and ecology”), its phenology (having three to four generations/season they infest the plant the entire season), and morphology (mealybugs are superficially covered and protected by a layer of waxes and woolly filaments). When exposed to sprays, however, mealybugs are highly susceptible to contact insecticides, including D + S. For instance, two agriculture detergents, Tecsa fruta and SU 120, were compared to control
2.2. Thysanoptera
Thrips are serious pests of vegetables, flowers, and fruit orchards, mainly affecting cut flowers and the skin of fruits (causing russet). They can produce silvering on flowers, leaves, and fruits, downgrading their value. Adults and nymphs are not sessile but tend to stay inside flower structures, under sepals, or at the contact point between either fruits or leaves and fruits. Therefore, D + S can be useful resources to reach them at those protected sites, by being used alone or as co-adjuvants (as surfactants) for conventional insecticides. However, trials evaluating thrips control have achieved different results in terms of mortality or density reduction when D + S have been used alone. For instance, the use of an agricultural soap (Acco Highway plant spray soap at 1% v/v) on the greenhouse thrips,
2.3. Acari
Spider mites feed mainly on the content of epidermal and parenchymal plant cells. While feeding, they do not reach vascular vessels; therefore, they do not produce honeydew. However, high populations can quickly develop on leaves causing bronzing, necrosis, and defoliation due to cell damage and the release of toxic substances. Mites tend to colonize the underside of leaves, where they need to be sprayed with contact and residual miticides, since colonization (for instance, from overwintering sites to the foliage developing during the spring) can last several weeks. During their development, they have sessile phases (proto- and deuto-nymphs), otherwise they are considered mobile arachnids. Besides, spider mites have several generations a year, being necessary to repeatedly control them along the season when populations reach dangerous densities. Some of the first modern reports of D + S used to control agricultural pests are related to spider mites [63, 65].
2.3.1. Tetranychidae
Osborne and Petit [65] found that the lowest insecticidal soap concentration (Safer at 1.25%, v/v) was effective in controlling adults and eggs of
Treatments1 | Days after a spray | ||
---|---|---|---|
0 | 2 | 9 | |
M-Pede | 60.0 a3 | 17.5 ab | 20.8 b |
TS 2035 | 54.8 a | 11.5 ab | 9.3 b |
Horticulture oil | 56.0 a | 12.0 ab | 33.8 ab |
Pyridaben2 | 53.3 a | 1.8 b | 6.5 b |
Control | 127.3 a | 138.3 a | 199.3 a |
2.3.2. Tenuipalpidae
A recent report indicates that the detergent SU 120 at 1.5% (v/v) sprayed in an infested vineyard had a significant effect on reducing
2.4. Detergents and soaps used against other organisms
Other insects than those addressed herein, as armyworms (Lepidoptera: Noctuidae, [56]), cockroaches (Blattodea: Blatellidae [43]), and ants (Hymenoptera: Formicidae [44]) have been reported as controlled by D + S, or at least affected. Besides, the control of other organisms including mollusks [85] and fungi [86] with D + S or surfactants has also been reported. All this evidence demonstrates that the potential target for this type of control tactic is far beyond sessile, soft integument, and small insects or spider mites.
3. Costs and economic benefits of using D + S
Costs of detergents or soaps used against agricultural pests, in general, should be relatively low per spray (and it will become even lower if D + S increase their use in agriculture), but there are some exceptions (e.g., expensive insecticidal soaps sold in smaller containers for garden pests in the United States). Table 8 compares the direct costs of applying a detergent program versus a conventional insecticide, considering having a residual effect shorter or similar to the period of evaluation in the field, and conditions where both strategies have achieved statistically similar levels of control for two pests in either apples or vines (see Tables 5 and 6). When comparing the detergent program versus chlorpyrifos used against the apple woolly aphid, the TS 2035 program cannot outcompete the conventional insecticide, being more than 2× more expensive. If Lorsban 4E be used (another much inexpensive chlorpyrifos formulation recommended at 120 mL/hL, with a cost of US$9.7/L), the cost of the detergent program would be about 3× more expensive. However, if other insecticides as buprofezin (Applaud 25 WP used at 120 g p.c./hL, US$42.1/kg) or imidacloprid (Confidor 350 SC) are used (modern and less restricted insecticides, but also more expensive products), considering application conditions and assumptions as described for chlorpyrifos, the standard strategy/detergent program ratio would increase, to near 0.79 (the detergent program being now only 20% more expensive than Applaud) and 1.49, respectively. In the latter case, the detergent program was 49% cheaper (including costs of products, equipment, and workers) than the conventional neonicotinoid. Now, when comparing the use of a neonicotinoid in vines against scales versus the detergent program, results also become very competitive in favor of the detergent strategy (ratio = 1.63). Even considering increasing the detergent concentration to 1% (see discussion in Table 6), the detergent program (three sprays) would be 1% less expensive than the use of imidacloprid once. Thus, detergents tend to be competitive when new, more expensive molecules, are used as standard treatments, a trend expected in the next years. The two main factors increasing costs of detergent treatments have been (1) the need to re-apply in order to counteract the lack of residual effect to achieve a level of control similar to that of conventional (and residual) pesticides. Thus, the cost rises due to the increasing value of motorized equipment and drivers, used two to three times (against just one application of the standard); (2) the use of concentrations about 8× greater than conventional pesticides to obtain similar results (detergents need to be used at 0.5–1% c.p. vs. the standards used at 0.06% (imidacloprid) or 0.12% (v/v chlorpyrifos or w/v buprofezin). Besides, since D + S must be applied using high volumes at relatively high concentrations, the amount of product used is larger. The examples presented are based on particular conditions (see the Table 8 legend). However, the costs should vary among different countries, crops, management strategies, pesticide values, or pest species.
Pest species and crops | #1 of detergent sprays ≈ to standard2 control | A: US$ cost of standard (appl./ha)TF8-3 | B: US$ cost of detergent (appl./ha)4 | Ratio A/B5 |
---|---|---|---|---|
3 | 113.5 | 23.2 | 1.63 | |
2 | 74.4 | 77.0 | 0.48 |
It is important to point out that the exercise above does not consider other benefits of using D + S (used instead of conventional pesticides), as the avoidance of both pest resistance development to chemical pesticides or pest resurgence, or the relative improvement of the environment and the agro-ecosystem, or the reduction of risks of human intoxications (workers and consumers), and so on, because their costs are difficult to estimate. Therefore, if all those costs were valuable, it would probably make the figures much more favorable for D + S. Additionally, the access to markets preferring food not treated with conventional pesticides might also be considered an economic benefit. For instance, IPM or organic products can eventually achieve higher prices than conventional agriculture produce. Besides, foods treated with soaps or detergents will not have major restrictions to reach many different countries since they do not present questionable residues, making easier (and cheaper) the marketing process. In favor of conventional pesticides, an additional economic benefit of their use is their wider spectrum of action against some pest complexes in some crops, but D + S have also demonstrated an extended range of action on pests. Besides, some conventional products can protect for long periods against pests. However, some cannot be used during some phenological stages (Lorsban 4E is used today mainly as postharvest or winter treatment).
Among other examples in the literature, an IPM program was cost-effective at most of the studied sites where the majority of pest were controlled using spot sprays of insecticidal soap or horticultural oil versus the management with conventional pesticides applied on the whole plantation [87]. Another report showed that up to five detergent sprays could be applied before reaching the cost equivalent of controlling pests with conventional pesticides applied twice (only considering the value of the commercial product, but no other application costs) [11]. Similarly, a recommended mixture of a miticide plus the synergic surfactant co-adjuvant Silwet 77 was over 5× more expensive than the cost of using the surfactant alone, which provided most of the control. Unfortunately, the surfactant was not registered as miticide, and was not allowed as a legally authorized control method [53]. Reduced pest control costs, by the use of soaps, were also mentioned by Lee et al (2006) [88].
4. Detergents as insecticide co-adjuvants
The use of surfactants, including D + S, as adjuvant, improves both the active ingredient solubility in the formulation and its physical and biocidal performance (e.g., wetting properties on plant or insect cuticle). Co-adjuvants are added directly to the tank before applications with the same purposes [11]. The oldest report of using soaps (as co-adjuvant) in mixture with other pesticides in the tank was published in Australia in 1969 [74], as a part of the phytosanitary program in Citrus, providing a satisfactory degree of both, coccids and diaspidids control. Later, surfactants were described as co-adjuvants, particularly for cuticle penetration in insects [89]. Last year, an entomopathogen spore suspension (
Treatments1 | Time (h)3 | LC504 |
---|---|---|
24 | 8.8 × 106 ab5 | |
24 | 8.6 × 107 c | |
72 | 7.8 × 106 a | |
72 | 3.3 × 107 c | |
144 | 6.1 × 106 a | |
144 | 3.0 × 107 bc |
5. Postharvest control of pests with detergents
Immersion of the fruit in warm water has been used as postharvest pest control against several pests on diverse fruit species [90, 91]. Besides, several D + S are allowed for postharvest uses, including fruit cleaning. The combination of both approaches (warm detergent solution) was tested, finding that pomegranates infested with mealybugs and immersed in a 1% (v/v) TS 2035 solution (at 47°C) for 15 min, maintaining the pH at either 5.5 and 8.5, notably (but not totally) controlled
Temp.1 (°C) | Det. Conc.2 | pH3 | Exposure time (min)4 | Adult females | Nymphs II and III | Nymphs I | All mealybug stages |
---|---|---|---|---|---|---|---|
15 ± 2 | 0 | 5.5 | 15 | 2.755 | 8.50 | 8.75 | 20.00 |
15 ± 2 | 0 | 8.5 | 15 | 2.00 | 6.25 | 22.00 | 30.25 |
15 ± 2 | 1 | 5.5 | 15 | 3.50 | 3.25 | 11.00 | 17.75 |
15 ± 2 | 1 | 8.5 | 15 | 2.25 | 7.75 | 18.00 | 28.00 |
47 ± 2 | 0 | 5.5 | 15 | 1.25 | 7.50 | 9.50 | 18.25 |
47 ± 2 | 0 | 8.5 | 15 | 6.00 | 4.75 | 15.25 | 26.00 |
47 ± 2 | 1 | 5.5 | 15 | 0.50 | 0.25 | 12.75 | 13.50 |
47 ± 2 | 1 | 8.5 | 15 | 1.00 | 1.25 | 2.75 | 5.00 |
6. Conclusions and prospects
Many different agriculture pests (mainly hemipterans and spider mites) are efficiently controlled by detergents and soaps, provided they are directly covered by the spray. The knowledge of their biology and ecology must be used to improve their performance by increasing the pest exposure. The research on new potential targets and the combination of D + S with biological control agents should be studied. D + S can be used as well to avoid or even reverse pest resistance problems.
The modes of action of D + S as insecticides and/or miticides seem to be mainly wax removal, arthropod dislodgement, and drowning, but it is an unsolved issue in many situations yet. It is then necessary to keep researching on this issue to optimize the use of surfactants as pesticides.
Despite some environmental and toxicological concerns, the appropriate use of D + S, and the selection and formulation of surfactants with minimum risks (for instance, among the offer of new, safe, and low-cost surfactants), makes them potentially useful pesticides, but it is necessary to confirm their relatively safety (for mammals and the environment) and capacity for pest control, in food products.
There is a need to standardize the biocidal activity when comparing D + S, maybe based on the proportion of surfactants in the formulae or contrasting with some standard compound.
Detergents and soaps can be used as co-adjuvants (in the tank) for conventional or biological pesticides. D + S can also be applied first to debilitate pest insects and mites, spraying later insecticides and miticides. In both cases, a rate reduction for conventional (and more expensive and restricted products) is possible, but these issues need further research.
Detergents and soaps can be used in orchards, vegetables, or greenhouses, serving to conventional, IPM, or organic growers, making possible to reach highly selective markets and consumers willing to pay for foods free of insecticide residues and, at the same time, take advantage of their relative sustainable status, replacing conventional pesticides. D + S could be applied very close to harvest, when conventional pesticides cannot, due to the insufficient preharvest intervals.
However, in order to provide satisfactory control and become a greater tool for pest control, D + S need to solve the (a) lack of residual effect, (b) potential for plant toxicity, (c) legal status, and (d) cost. For multivoltine pests, or those infesting crops for long periods, their repeated use over relatively short periods has probed in several cases to provide a control equivalent to conventional (and residual) insecticides. Plant toxicity has been diminished by selecting tolerant crops, or tolerant phenology stages of the crops, excluding otherwise the use of D + S. This issue needs more research to identify tolerant crops and the conditions and mechanism causing plant toxicity, in order to develop safer D + S. The repeated applications of small concentrations of D + S have overcome these two problems, becoming useful tools for IPM productive schemes, particularly considering their multi-site action, selectivity to beneficial organisms, lack of residual effect, and relatively low environment and human toxicity. The facts that D + S are relatively quick to control, easy to produce and use, versatile, and lack major legal restrictions just improve their possibilities to be incorporated in pest programs.
The cost of efficient programs of control with D + S can be competitive with conventional pesticides, depending on the crop, pest, type of grower, and alternatives of pesticides, and it deserves a more detailed analysis, including the precise valorization of several benefits associated to the use of D + S, although some of them are difficult to measure, as lower probability of inducing insecticide resistance or pest resurgence, lower risks of intoxications to workers, etc.
Besides the cost issue, the authorization of D + S as pesticide products seems to be the next main challenge, being necessary that the industry (producers and suppliers), government agencies (regulatory apparatus), scientists (agronomists, entomologists, chemists, toxicologists), and even growers and consumers interact in order to develop a regulation process that allows to increase D + S registrations, particularly those safer compounds, that can be efficiently used with minimum risk (by far lower than conventional pesticides) at pre- and postharvest, becoming valuable tools for sustainable pest management.
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