Management of Tuta absoluta (Lepidoptera, Gelechiidae) with Insecticides on Tomatoes

Tomato, Lycopersicon esculentum Mill is a vegetable crop of large importance throughout the world. Its annual production accounts for 107 million metric tons with fresh market tomato representing 72 % of the total (FAO, 2002). It is the first horticultural crop in Tunisia with a production area of 25,000 hectares and a total harvest of 1.1 million metric tons (DGPA, 2009) of which nearly 70 % are processed (Tomatonews, 2011). Tomatoes are grown both under plastic covered greenhouses and in open field. The tomato leafminer, Tuta absoluta Meyrick, (Lepidoptera : Gelechiidae) is a serious pest of both outdoor and greenhouse tomatoes. The insect deposits eggs usually on the underside of leaves, stems and to a lesser extent on fruits (photo 1). After hatching, young larvae penetrate into tomato fruits (photo 2), leaves (photo 3) on which they feed and develop creating mines and galleries. On leaves, larvae feed only on mesophyll leaving the epidermis intact (OEPP, 2005). Tomato plants can be attacked at any developmental stage, from seedlings to mature stage.


Introduction
Tomato, Lycopersicon esculentum Mill is a vegetable crop of large importance throughout the world. Its annual production accounts for 107 million metric tons with fresh market tomato representing 72 % of the total (FAO, 2002). It is the first horticultural crop in Tunisia with a production area of 25,000 hectares and a total harvest of 1.1 million metric tons (DGPA, 2009) of which nearly 70 % are processed (Tomatonews, 2011). Tomatoes are grown both under plastic covered greenhouses and in open field. The tomato leafminer, Tuta absoluta Meyrick, (Lepidoptera : Gelechiidae) is a serious pest of both outdoor and greenhouse tomatoes. The insect deposits eggs usually on the underside of leaves, stems and to a lesser extent on fruits (photo 1). After hatching, young larvae penetrate into tomato fruits (photo 2), leaves (photo 3) on which they feed and develop creating mines and galleries. On leaves, larvae feed only on mesophyll leaving the epidermis intact (OEPP, 2005). Tomato plants can be attacked at any developmental stage, from seedlings to mature stage. Photo 1. T. absoluta egg Photo 2. Larvae on fruit Photo 3. Larva of T. absoluta Originated from South America, T. absoluta was reported since the early 1980s from Argentina, Brazil and Bolivia (Estay, 2000); the insect rapidly invaded many European and Mediterranean countries. It was first recorded from eastern Spain in late 2006 (Urbaneja, 2007), then Morocco, Algeria, France, Greece, Malta, Egypt and other countries (for a complete list see www.tutaabsoluta.com; Roditakis et al., 2010, Mohammed, 2010. Chemical control using synthetic insecticides is the primary method to manage the pest, but it has serious drawbacks, including reduced profits from high insecticide costs, destruction of natural enemy populations (Campbell et al., 1991), build-up of insecticide residues on tomato fruits  and in the environment and fundamentally the rapid development of insecticide resistance. For example, resistance development has been reported against abamectin, cartap, methamidophos and permethrin in Brazil (Siqueira et al., 2000a, Siqueira et al., 2000b and against deltamethrin and abamectin in Argentina (Lietti et al., 2005). Thus, in order to avoid selection of resistant biotypes, a careful management with frequent changes of active ingredients is desirable. Furthermore, modern integrated pest management recommends effective pesticides that have low mammalian toxicity, low persistence in the environment and high degree of selectivity. Since insecticide control currently remains an indispensable tool, the goal is to minimize the amount and impact of pesticides through the diversification of active ingredients used.
In this paper, we present the data from insecticides trials conducted in 2009 and 2010 under laboratory and field conditions, in which the efficacy of several hitherto untested insecticides and natural products was compared with that the widely used insecticides to manage T. absoluta in Tunisia such as spinosad, indoxacarb and pyrethroids compounds.

Laboratory trials 2.1.1 Laboratory assays in 2009
Tomato seeds (cv Topsun) were sown on 30 January 2009. Seeds were deposited in 110 cm3 cells in a rectangular polyester tray of 60 cm x 40 cm x 5 cm filled with peat (Potgrond H, Germany). On March 3, 2009, seedlings were transplanted into 1 liter plastic flowerpot (bottom diameter =8 cm, top diameter = 12 cm and height = 12 cm) filled with peat without fertilization and watered as required. The tomato plants were maintained in the laboratory until use. Three days before the assay, plants (having four to six true leaves) were deposited in a tomato crop situated in the vicinity of the laboratory to permit T. absoluta egg-laying then transferred to the laboratory. Leaves were examined under binocular microscope and T. absoluta larvae were counted. Insecticides were sprayed using a hand sprayer (1 liter of capacity). After drying, the treated plants were kept in an unsealed empty greenhouse bordering the laboratory. There were four replications (plants) for each product and an untreated plant was used as a check. The efficacies of the products were tested twice: 48 hours following sprays and 12 days later. The Insecticides and natural plant extracts used are given in table 1.

Laboratory assays in 2010
A colony of T. absoluta was established from larvae and pupae collected from tomato infested field in the Chott-Mariem region. The insect was reared and maintained in a small greenhouse (10*6 m). From time to time, tomato leaves harboring T. absoluta pre-imaginal stages collected in the field were introduced in the rearing greenhouse. Tomato seeds (cultivar Riogrande) were sown on February 13, 2010 in a rectangular polyester tray as mentioned before. Plants having four to six true leaves were transferred to the rearing greenhouse and remained there for 2 to 3 days to allow egg-laying. Thereafter  Table 3. Natural products experimented in 2010.
Plots measured 4 m2 each (10 plants) arranged in a randomized block design with four replications. The active ingredients, the trade name and doses of the natural products are given in table 3. The products were diluted with tap water and applied at field rates based on the recommended label dilutions without surfactants.

Trials using insecticides
Trials using insecticides were undertaken during the same period in the second half greenhouse. Plot measured 8 square meters each (20 plants) arranged in a randomized block design with four replications. Three chemical compounds were used (

Insect monitoring
To assess the T. absoluta infestation prior to the trial, thirty leaf samples, taken from about 30 different plants were weekly collected (from January to March 2010) at random from the entire greenhouse. The sample was placed in a plastic bag and taken to the laboratory. Leaves were examined under binocular microscope (Leica MZ12.5); eggs, larvae pupae, of T. absoluta live or dead as well as mines were recorded. However, only larvae (live or dead) were presented in this study.

Statistical analysis
Data on the effectiveness of various insecticides were analyzed using the Minitab Software for Windows (Minitab 13.0). The mean number of live larvae per plant or per leaf was tested for Normality assumption by Kolmogorov-Smirnov test then the data were square root transformed. General linear model procedures w e r e u s e d t o p e r f o r m t h e a n a l y s i s o f variance. Wherever significant difference occurred, Tukey's multiple comparison test was applied for mean separation.
In the laboratory trial of 2010, due to the low number of live larvae in the control, a one way-ANOVA percentage of mortality was used instead of corrected mortality. The percentages of efficacies of insecticides were evaluated either: i. Abbott formula : the percentage of efficacy = (Ca-Ta)/Ca*100 where Ca is the average live larvae in the control and Ta is the mean survival score in the treatment. ii. The percentage of larval mortality = mean number of dead larvae/( mean number of dead larvae + mean number of live larvae)*100.

Laboratory trials 3.1.1 Assays in 2009
One day before the assay, the mean number of total live larvae (L1 to L4 instars) per plant varied from 0.75 to 3. There is no significant difference between treatments (GLM-ANOVA. F= 0.99, df= 9,30; P = 0.47, table 5). Three days after the first application, the mean number of live larvae per plant decreases in all treatments except in the control (Table 5). All insecticides significantly reduced T. absoluta larvae when compared with non treated control (F= 4.24, df = 9,30; P= 0.001, Table 5). However, the level of suppression by acetamiprid and bifenthrin did not differ significantly from the control (  Five days following the first application, all the products performed well except acetamiprid and bifenthrin which show no significant difference compared with the control (Table 5). Eight days after the first application, the mean number of total live larvae per plant varied from 0.5 to 4. All the tested products reduced significantly the density of live larvae per plant compared with the control (F= 4.20; df = 9,30; P= 0.001). Still, acetamiprid and bifenthrin showed mild efficacy (  Table 6. Mean number of total T. absoluta live larvae/plant the day of the second treatment and thereafter (DAT2) (the treatment was undertaken on April 21) Just before the second application, the mean number of live larvae in treated plants remained low compared with the control. It varied between zero (Tracer) and 3.5 (control) (table 6). Two days following the second insecticide application, all tested compounds show good efficacy compared with control (F=4.24; df = 9,30; P<0.001). Spinosad (Tracer) performed well (100 % efficacy according to Abbott corrected mortality formula). However, bifenthrin (Talstar) shows mild efficacy (table 6). The same conclusion can be formulated four days following treatments (table 6). At eight days after trial, the insecticide spinosad remains active and performed well (83.33 % efficacy) (table 6). The overall efficacy according to Abbott formula (1925) shows the good performance of spinosad (Tracer), rotenone (Rotargan), methomyl (Lannate), abamectin (Vertimec) (Fig. 1.).

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However, the percentage of larval mortality (number of dead larvae/sum of dead and live larvae) following the first and second insecticide application shows the best performance of spinosad (91 %), neem extract (71 %) and abamectin (71%). www.intechopen.com

Assays in 2010
Just before the first spray (April 3, 2010), the mean number of live larvae (first to fourth instars) per leaf varied from 0.12 (Control) to 0.52 (Proclaim ® ). Although there is no significant difference between treatments (ANOVA-GLM F= 1.37, df = 4, 116; P=0.24), the control plants harboured less live larvae (table 7). There is no larval mortality. Two days following the first spray (April 5), there is no significant difference between treatments regarding live larvae (GLM; F= 0.93, df = 4, 116; P= 0.46. Table 7). However, the percentage of larval mortality did vary (ANOVA, 1 factor, F = 4.17; df = 4, 120; P= 0.003) showing the best performance of Proclaim ® (57.14 %; Table 7). Nine days after the first insecticide application (April 12), the mean number of live larvae per leaf did not significantly vary between treatments (ANOVA-GLM procedure Table 7). However, the percentage of mortality significantly varies between treated and untreated plants (ANOVA 1 factor, F= 3.07; df = 4, 120; P= 0.021). The maximum percentage of mortality is given by Proclaim ® (45.70%, table 7). At 11 days after the first insecticide application (on April 14), the mean number of live larvae did not significantly vary among treated and untreated plants (ANOVA -GLM procedures Table 7). However, the percentage of mortality did vary according to treatments (F = 3.16, df = 4, 120; P= 0.017) showing the good efficacy of Proclaim ® (52.93 % Table 7).

Field trials 3.2.1 Natural products experimented in 2010 under greenhouse
The first spray was undertaken on March 26, 2010, then on April 8 and on April 19, 2010. At three days following the first application, the mean live larvae (small and old larvae) per leaf did not significantly vary between treated and control plots (GLM-ANOVA Procedure, P= 0.09). Although, plots treated with spinosad show the minimum live larvae as demonstrated by 70% efficacy according to Abbott formula (Table 8). The details of larval instars (small larvae: first and second instars and old larvae: three and fourth instars) show a significant difference between insecticides tested. The compounds Tracer, Armorex and Deffort performed well (table 9).
At 10 days after the first natural products applications, the ANOVA-GLM procedure shows no significant difference between treatments regarding the mean number live larvae (Table  8). The Abbott's percentages of efficacy show the performance of spinosad (84.61 %) and the plant extract (Armorex; 69.23%). At two days after the second spray, (April 10) there is a significant difference between treated plots (AVOVA-GLM procedure, P= 0.008, table 8). The plots treated with Deffort show the maximum density of mean live larvae per leaf (table 8). However, there is no significant difference between the other products and control. The details of larval stages confirm the low efficacy of Deffort compared with the other products and control (small larvae : P= 0.026; Old larvae P= 0.019; table 9). Six days following the second application (April 14), the mean number of live larvae shows no significant difference between treated and untreated plots (Table 8).
At eleven days after the second spray, the mean number of live larvae per leaf is relatively similar among treatments and did not significantly vary (ANOVA-GLM procedure P= 0.211) varying from 0.1 to 0.9. Plots treated with Kaolin (Surround) harbour the minimum density. Four days after the third spray (April 23, 2010), the treated plot differed significantly showing the good performance of the compounds neem extract, Tracer and Konflic (table  8). This is confirmed by the analysis of detailed larval instars (table 9).
At nine days after the third spray, the mean number of total larvae varied between 0.2 and 2.05. The ANOVA-GLM procedure showed a significant difference between treatments. The products Tracer, Armorex and Deffort were effective in reducing T. absoluta larval densities (     Three days following the first insecticide application, the mean number of live larvae (small and large) did not vary significantly between treated and untreated plots (ANOVA-GLM Procedure F= 1.94, df = 3, 309 P= 0.063). However, the plants treated with spinosad (Tracer) harbor the minimal larval density (Table 9).

Insecticides compounds experimented under tomato greenhouse in 2010
Four days before the first insecticide application, the mean number of live larvae per leaf varied between 0.6 and 0.97 showing no significant difference between treatments and control (ANOVA. GLM, F= 0.82, df =3, 156; P=0.82). Two days following the first treatment (March 24), the mean number of live larvae remains relatively low and did not significantly vary between treatment and control (F = 0.34; df = 3, 153; P= 0.79). The corrected mortality according to Abbott formula shows slight efficacy of tested products (Table 10). At 12 days following the first application, the mean number of live larvae significantly differed between treatments (GLM, F=2.90, df = 3, 156; P= 0.037). The Tukey multiple comparisons showed the good performance of indoxacarb (Avaunt) (   At 18 days following the second application, the mean number of live larvae significantly varies between treated and control plots (GLM F= 40.88; df = 3, 153; P= 0.000). The efficacy of tested insecticide remains high compared with the control. At 3 and 12 days following the third insecticide application all tested insecticides continue to be effective compared with the control (F= 20.91 df =3, 153; P= 0.00 ; F=10.87; df =3, 153; P= 0.00). Nevertheless, indoxacarb (Avaunt) tend to be a powerful suppressor of T. absoluta larvae (table 10).

Discussion
In Argentina, the primary T. absoluta management tactic was chemical sprays (Lietti et al., 2005). Organophosphates were initially used for T. absoluta control then were gradually replaced by pyrethroids during the 1970s. During the early 1980s, cartap which alternates with pyrethroids and thiocyclam were sprayed showing the good effectiveness of the former. During the 1990s, insecticides with novel mode of actions were introduced such as abamectin, acylurea, insect growth regulators, tenbufenozide and chlorfenapyr (Lietti et al., 2005). Our laboratory results demonstrate the efficacy of spinosad (Tracer), rotenone (Rotargan), methomyl (Lannate) and abamectin (Vertimec). Methomyl was only tried due to its highly used frequency in tomato production against Noctuid larvae in Tunisia. Spinosad, a mixture of spinosyns A and D, is derived from the naturally occurring actionomycete, Saccharopolyspora spinosa (Sparks et al., 1998). Because of its unique mode of action, involving the postsynaptic nicotinic acetylcholine and Gamma-aminobutyric (GABA) receptors, spinosad has strong insecticidal activity against insects (Salgado, 1998) especially Lepidoptera (e.g. Helicoverpa armigera (Wang et al., 2009), Spodoptera frugiperda (Méndez et al., 2002), Diptera (King and Hennesey 1996;Collier and Vanstynwyk , 2003 ;Bond et al., 2004), some Coleoptera (Elliott et al., 2007) as well as stored grains (Hertlein et al, 2011). To date, spinosad is considered a good alternative control of Lepidopteran pests due to its high activity at low rates and its use in integrated pest management programs. The product possesses advantages in term of safety for farm workers and consumers due to its low mammalian toxicity and rapid breakdown in the environment (Sparks et al., 1998). The compound is considered as a standard product for the control of T. absoluta in Brazil (Maraus et al., 2008) showing, however low efficacy compared with the insecticide novaluron.
Rotenone has been reported to be an excellent insecticide against a wide range of insect pests. Davidson (1930) found that rotenone was a toxic and effective contact insecticide against several species of whiteflies, aphids, caterpillars and mites. Also, Turner (1932) reported a high toxicity of rotenone to larvae of the Colorado potato beetle Leptinotarsa decemlineata (Say). Azadirachtin, a tetranortriterpenoid isolated from the seeds of neem tree, Azadirachta indica (Meliaceae), and the fruit of chinaberry, Melia azaderach (Meliaceae) acts as an antifeedant and inhibits the growth and the development of several insects (Meisner et al., 1981, Raffa, 1987McMillian et al., 1969). The antifeefant effects of azadirachtin are partly due to sensory detection and avoidance by insects (Simmonds and Blaney 1984). Acetamiprid (Mospilan) is a neonicotinoid insecticide that is formulated for both soil and foliar application. It is a broad-spectrum insecticide effective against several groups of insects including Lepidopterans, Coleopterans, Hemipterans and Thysanopterans. The insecticide has an ingestion and stomach action and has a strong osmotic and systemic action (Takahashi et al., 1998). The compounds interact with Acetylcholine receptors (AChRs) in a structure-activity relationship, resulting in excitation and paralysis followed by death (Ishaaya et al., 2007). Abamectin a mixture of avermectins is extracted by the fermentation of the soil bacterium Streptomyces avermitilis (Strong & Brown 1987). The insecticide acts on the GABA receptor activating the chloride channel (nerve and muscles) (Aliferis and Jabaji, 2011). Throughout the assay, the product emamectin benzoate (Proclaim ® ) showed the best efficacy strongly suppressed T. absoluta larval populations. Indeed, several authors reported the performance of this product against several insects, for example, Seal (2005), reported the efficacy of emamectin benzoate at various rates in reducing the densities of the melon thrips, Thrips palmi adults and larvae. Stanley et al., (2005) reported the high acute toxicity of emamectin benzoate to Helicoverpa armigera under laboratory conditions. Cook et al., (2004) conducted field and laboratory trials on cotton and soybean for the control of the beet armyworm Spodoptera exigua (Hübner) and the fall armyworm Spodoptera frugiperda using indoxacarb, pyridalyl, spinosad methoxyfenozide and emamectin benzoate demonstrated the good efficacy of tested products compared with the control. Plots treated with indoxacarb, spinosad and emamectin benzoate had significantly fewer beet armyworm larvae.
Avermectins are a family of 16-membered macrocyclic lactone natural product homologues produced by the soil microorganisms, Streptomyces avermitilis. They act as agonists on GABA and glutamate gated chloride channels. The chloride ion flux produced by the direct opening of channels into neuronal cells results in loss cell function and disruption of nerve impulses. Consequently, arthropods are paralyzed irreversibly and stop feeding. Maximum mortality is achieved within four days (Jansson et al., 1997). Emamectin benzoate (Proclaim) is a novel semi-synthetic derivative of the natural product abamectin in the avermectin family. This insecticide has a high potency against a broad spectrum of lepidopterous pests with an efficacy of about 1,500-fold more potent against certain armyworm species (Jansson et al., 1996) Insect growth regulators like triflumuron, lufenuron are claimed to be safe and have little impact on beneficial arthropods compared with conventional insecticides and thus attracted considerable attention for their inclusion in IPM programs (Ishaaya et al., 2007). In this study, triflumuron showed low efficacy against T. absoluta larvae. These results are in accordance with data reported by El-Sheikh and Aamir (2011) suggesting the greater efficiency of lufenuron in controlling Spodoptera littoralis Boisd compared with triflumuron or flufenoxuron. Similarly, low effectiveness of triflumuron (Alystin SC48) for the control of Cactoblastis cactorum (Lepidoptera: Pyralidae) was reported in Argentina by Labos et al., (2002). Yet the concentration used was lower (30 cc/ hl). Regarding the control of the Mediterranean fruitfly, Ceratitis capitata, triflumuron (Alystin 25) failed to give satisfactory results (a concentration of 150 ppm did not kill adults, Zapata et al., (2006)). Diafenthiuron (Pegasus) is a new type of thiourea derivative that affects respiration in insects. It disrupts oxidative phosphorylation by inhibition of the mitochondrial ATP synthase, an enzyme with essential role in cellular bioenergetics (Ishaaya, 2010). It is an insecticide and acaricide which kills larvae, nymphs and adults by contact and/or stomach action, showing also some ovicidal action (e-pesticide manual, 2005). In our laboratory trial, diafenthiuron (Pegasus) shows little efficacy in T. absoluta larval suppression (table 10).
Tutafort (plant extract) shows little efficacy after the first application but increases effectiveness after the second application engendering about 80 % of larval mortality (table  7.Cont.). Yet according to manufacturer, (Altinco, 2011), the product has a preventive action and should be applied against eggs and adults. The compound acts by contact penetrating the insect cuticle and dissolves the cell membranes causing the insect dehydrate and its death (Altinco, 2011). Management of resistance to prevent or delay the development of resistance to an insecticide and cross resistance to additional insecticides is necessary for increasing the chance of chemical control of T. absoluta. Thus, the avoidance of resistance requires the development of pest management programs in which efforts are made to take advantages of natural enemies of pests, plant resistant cultivars, if available, appropriate cultural and physical methods. Accordingly, diversification of control tactics should be implemented with the minimum use of chemicals. Insecticides should be applied only as needed basis and only used as the last form of control. When insecticides are applied, the way that they are used should be rationalized and optimized to exploit the full diversity of synthetic chemicals and natural products mostly used at rotational basis. Development of resistance in T. absoluta is an important problem in regions where the insect is established. The expanding international trade of plant material not only spread the pest but also spreads the resistance genes associated with the pest (Denholm and Jespersen, 1998). It is possible that the Mediterranean populations of T. absoluta already carried gene resistance from South American counterpart populations and thus, may already express high level of resistance to one or multiple insecticide. Indeed, Cifuentes et al., (2011), demonstrated high genetic homogeneity of T. absoluta populations came from Mediterranean basin and from South America countries using ribosomal and mithochondrial markers. Our field results (tomato greenhouse) suggest the good performance of the tested compounds (indoxacarb, triflumuron and diafenthiuron). So far, the product indoxacarb tend to be a powerful suppressor of T. absoluta larvae. Indoxacarb is reported by several authors as a powerful insecticide in managing many Lepidopteran pests. Wakil et al. (2009) in their study for the management of the pod borer, Helicoverpa armigera Hubner (Lepidoptera : Noctuidae) in Pakistan showed the integration of weeding, larvae hand picking and indoxacarb sprays was the most effective in reducing the larval population, pod infestation and maximum grain yield. Also, in Cameroon, Brévault et al., (2008) reported a good efficacy of indoxacarb as a larval insecticide of H. armigera.
In the United Kingdom, three insecticides were registered for the control of T. absoluta under protected tomato, pepper and aubergine: Bacillus thuringiensis var. kurstaki, indoxacarb and spinosad (FERA, 2009). Indoxacarb belongs to a novel class of insecticides, the oxadiazines. It a broad spectrum non-systemic insecticide active especially against Lepidoptera. Indoxacarb affects insect primarily through ingestion but also by contact with treated plant surface. It kills by binding to a site of sodium channels and blocking the flow of sodium ions into nerve cells.
The result is impaired nerve function, feeding cessation, paralysis and death (Wing et al, 2000).

Conclusions
T. absoluta has been a serious pest of tomatoes in Tunisia since the autumn 2008. Farmers have gradually come to understand that conventional insecticides such as organophosphates and carbamates are not effective against the insect. Even though more expensive compared with other insecticides, spinosad (Tracer) is now the widely used bioinsecticide to manage the insect. It is not the intent in this study to advocate one insecticide over another but to enlarge the array of effective insecticide and bio-insecticides with different modes of action. These studies clearly demonstrated the efficacious of several chemicals such as spinosad, abamectin, emamectin benzoate, triflumuron and diafenthiuron. Although, plant extracts such as Armorex and Deffort show mild efficacy in controlling T. absoluta larvae, they can be used in conjunction with chemical products and integrated in a whole program of control. The efficacies of sprayings using mixtures of natural products and synthetic chemicals for the control of the pest are planned in our laboratory studies. Indeed, insecticides that work in synergy when mixed together are an avenue to explore in T. absoluta control. It has been proposed that pesticides mixtures with different modes of action may delay the onset of resistance developing in pest populations (Bielza et al., 2009). However, some problems need to be considered when two or more insecticides are mixed together especially phytotoxicity. The use of insecticides to control T. absoluta must not divert attention from the implementation of alternative pest management strategies including cultural, mass-trapping and biological control that can reduce reliance to chemical products. Chemical pesticides continue to be an important component of insect pest management even with the development of other control methods (mass-trapping, plant resistance…). The use of insecticides based on different chemistries and with varying modes of action is an important component of an integrated pest management strategy. Hence, insecticides will continue to be an integral component of pest management programs due mainly to their effectiveness and simple use. However, the principal factor account for the possible reluctance to shift to the newer insecticides is the high cost.