Topical Application Bioassays Research Articles

Survey of Permethrin and Malathion Resistance in Human Head Lice Populations from Denmark

Head lice, Pediculis capitis De Geer, populations were investigated for permethrin and malathion resistance after initial establishment of a discriminating dose of topical application bioassay with body lice, Pediculus humanus L. For both insecticides, approximately 2 times the lethal dose (LD)95 at 4 h was selected, 2 ng of permethrin and 100 ng of malathion per head louse, respectively. Head lice were collected from heads of infested children in Denmark at 33 primary schools, one kindergarten, and seven boarding schools. The lice were collected by combing of dry hair, with a fine-toothed antilouse comb attached to a vacuum cleaner. A resistance survey covers head lice collected from 208 of 1,441 persons combed. The frequency of permethrin- and malathion-resistant head lice is high in Danish head lice populations. In 17 of 24 samples tested for permethrin resistance, all head lice survived the discriminating dose. Six samples had between 3 and 25% dead head lice, whereas one sample had 60% mortality. In nine of 25 samples tested for malathion resistance, all head lice survived the discriminating dose. Seven samples had <25% dead head lice, and four samples had a mortality of 50% or more at the discriminating dose. The connection between permethrin resistance and kdr-like mutations is confirmed by our findings. The frequency of the double mutation T929I-L932 F in the voltage-sensitive sodium channel gene associated with permethrin resistance was 0.95 in Danish head lice populations.

Insecticidal activity and antifeedant effect of a new type biocide GCSC-BtA against Plutella xylostella L. (Lep., Plutellidae)

Insecticidal activity and antifeedant effect of a new type biocide GCSC-BtA and its two reactants Bacillus thuringiensis (B.t.) crystal and Abamectin against third instar larvae of Plutella xylostella L. (Lep., Plutellidae) were investigated using bioassays. GCSC-BtA showed significantly higher toxicity to P. xylostella with LC50 of 0.021 mg/ml than B.t. crystal with 0.060 mg/ml and Abamectin with 0.139 mg/ml in leaf-dip bioassay. Net leaf-dip test gave LC50s of 0.073, 0.071 and 0.670 mg/ml for GCSC-BtA, B.t. crystal and Abamectin, respectively. The significant differences of the biocide toxicities between leaf-dip and net leaf-dip methods showed the potential of net leaf-dip method for separating ingestion and contact poisons. Variation in toxicity of the biocide and two reactants in net leaf-dip method and in mortality in topical application bioassay showed that GCSC-BtA possessed a combination of strong stomach as well as contact poison property higher than B.t. or Abamectin. GCSC-BtA indicated no fumigant and systemic toxicities, which the two reactants lacked. In the antifeedant bioassay, GCSC-BtA acted as a strong antifeedant to the larvae of P. xylostella. Leaf consumption by the larvae after GCSC-BtA treatment was 0.04–0.60 mm2 with a concentration range of 0.04–4.00 mg/ml, which was lower than 0.26–1.02 and 0.04–6.47 mm2 for B.t. crystal and Abamectin, respectively. DC50 (concentration resulting in a deterrence index of 50%) value for GCSC-BtA was 0.089 mg/ml, which was also significantly lower than 0.275 and 0.736 mg/ml for B.t. crystal and Abamectin, respectively.

Cross-resistance potential of fipronil in Musca domestica.

The toxicity of fipronil to insecticide-susceptible houseflies and the cross-resistance potential of fipronil were determined for six insecticide-resistant laboratory housefly strains by topical application and feeding bioassay. The insecticide-resistant strains represented different levels and patterns of resistance to pyrethroids, organophosphates, carbamates and organochlorines. Five strains were almost susceptible to fipronil in feeding bioassay with resistance factors at LC50 between 0.36 and 3.0. Four of these strains were almost susceptible to topically applied fipronil (resistance factors at LD50 were 0.55, 0.83, 3.3 and 2.5, respectively), whereas one strain was 13-fold resistant to topically applied fipronil. A highly gamma-HCH-resistant strain, 17e, was 430-fold resistant to fipronil in topical application bioassay and 23-fold resistant in feeding bioassay at LD50/LC50. We also tested the toxicity of fipronil in a feeding bioassay and gamma-HCH in topical application bioassay on thirteen housefly field populations. Eleven of the field populations had resistance factors for fipronil ranging from 0.98 to 2.4 at LC50, whereas two populations were 4.0- and 4.6-fold resistant to fipronil. The resistance level to gamma-HCH at LD50 in the field populations ranged from 1.8- to 8.1-fold. The two strains showing fipronil resistance were 3.4- and 8.1-fold resistant to gamma-HCH. Fipronil and gamma-HCH toxicities were positively correlated in the field populations. Biochemical assays of esterase, glutathione S-transferase and cytochrome P450 monooxygenase indicated that the low fipronil resistance observed in laboratory and field strains could be caused by elevated detoxification or be due to a target-site resistance mechanism with cross-resistance to gamma-HCH.

Susceptibility of Spinosad in Musca domestica (Diptera: Muscidae) Field Populations

The toxicity of spinosad was determined in one susceptible and five insecticide-resistant laboratory strains of house fly, Musca domestica L. Spinosad was relatively slow-acting, but highly toxic to house flies. In a feeding bioassay, spinosad LC50 at 72 h was 0.51 microg of spinosad per gram of sugar, making it 6.3- and 3.5-fold more toxic to house flies compared with azamethiphos and methomyl, respectively. In topical application bioassay, the LD50 at 48 h of spinosad in susceptible house flies was 40 ng per 20 mg of house fly, making spinosad less toxic than the pyrethroid bioresmethrin synergized by piperonyl butoxide and the organophosphate dimethoate. The insecticide-resistant laboratory strains had resistance factors to spinosad at LC50 in feeding bioassay from 1.5 to 5.5 and at LD50 in topical application bioassay from 2.5 to 4.7, indicating that in house fly cross-resistance to the major insecticide classes will not initially be of major concern for the use of spinosad for house fly control. The toxicity of spinosad was also evaluated against 31 field populations of house flies collected from livestock farms across Denmark. The field populations were 2.2- to 7.5-fold resistant to spinosad at 72 h in feeding bioassay, but based on steep slopes in the bioassay and the limited variation of spinosad toxicity against the various field populations, we consider the field populations to be spinosad-susceptible. We propose a diagnostic dose of 12 microg of spinosad per gram of sugar in feeding bioassay with impregnated sugar for determination of resistant house flies, which is 10x the LC95 of the susceptible strain WHO and approximately = 2x the LD95 of the field populations. Spinosad showed no substantial cross-resistance to the pyrethroid bioresmethrin synergized by piperonyl butoxide, the anticholinesterases dimethoate, azamethiphos, methomyl, and spinosad in house fly field populations.

Susceptibility of Spinosad in &lt;I&gt;Musca domestica&lt;/I&gt; (Diptera: Muscidae) Field Populations

The toxicity of spinosad was determined in one susceptible and five insecticide-resistant laboratory strains of house fly, Musca domestica L. Spinosad was relatively slow-acting, but highly toxic to house flies. In a feeding bioassay, spinosad LC50 at 72 h was 0.51 microg of spinosad per gram of sugar, making it 6.3- and 3.5-fold more toxic to house flies compared with azamethiphos and methomyl, respectively. In topical application bioassay, the LD50 at 48 h of spinosad in susceptible house flies was 40 ng per 20 mg of house fly, making spinosad less toxic than the pyrethroid bioresmethrin synergized by piperonyl butoxide and the organophosphate dimethoate. The insecticide-resistant laboratory strains had resistance factors to spinosad at LC50 in feeding bioassay from 1.5 to 5.5 and at LD50 in topical application bioassay from 2.5 to 4.7, indicating that in house fly cross-resistance to the major insecticide classes will not initially be of major concern for the use of spinosad for house fly control. The toxicity of spinosad was also evaluated against 31 field populations of house flies collected from livestock farms across Denmark. The field populations were 2.2- to 7.5-fold resistant to spinosad at 72 h in feeding bioassay, but based on steep slopes in the bioassay and the limited variation of spinosad toxicity against the various field populations, we consider the field populations to be spinosad-susceptible. We propose a diagnostic dose of 12 microg of spinosad per gram of sugar in feeding bioassay with impregnated sugar for determination of resistant house flies, which is 10x the LC95 of the susceptible strain WHO and approximately = 2x the LD95 of the field populations. Spinosad showed no substantial cross-resistance to the pyrethroid bioresmethrin synergized by piperonyl butoxide, the anticholinesterases dimethoate, azamethiphos, methomyl, and spinosad in house fly field populations.

Resistance of pear psylla (Cacopsylla pyri L.; Hom., Psyllidae) to deltamethrin and synergism with piperonyl butoxide

Abstract: Topical application bioassays, conducted on French populations of Cacopsylla pyri, showed resistance rates to deltamethrin ranging from 31‐fold at the adult stage to 135‐fold at the last larval stage. Synergism studies between deltamethrin and piperonyl butoxide (PBO), applied either topically or by tarsal contact with PBO, showed (i) that the rates of synergism were more than 10‐fold in all the populations and that they could increase to a thousand‐fold in some populations, (ii) that the synergistic effect was present to a similar extent at both the larval and adult stages, (iii) that there was no correlation between the synergistic effect and the resistance rates, and (iv) that no correlation was observed between the dates of collection and the rates of synergism. The results makes it impossible to determine whether resistance to deltamethrin is of genetic origin or results only from the induction of monooxygenases by allelochemical substances present in the food ingested by the species.

An insensitive acetylcholinesterase confers resistance to methomyl in the beet armyworm Spodoptera exigua (Lepidoptera: Noctuidae).

Two forms of acetylcholinesterase were identified in field populations of the beet armyworm, Spodoptera exigua (Hübner), collected from cotton in San Joaquin Valley, CA. Strains (BESS and BKRR) homogeneous for each variant were isolated and their relative susceptibilities to methomyl, chlorpyrifos, and chlorpyrifos-oxon assessed by topical application bioassay. In comparisons with a laboratory susceptible strain (DOW), BKRR and BESS expressed 68-fold and sevenfold resistance, respectively, to the carbamate methomyl. Neither strain was cross-resistant to chlorpyrifos or its oxygen analog (chlorpyrifos-oxon). In biochemical studies, the BKRR AChE enzyme was approximately 30-fold and sixfold more insensitive to methomyl and chlorpyrifos-oxon, respectively, compared with the DOW enzyme. The correlation between the toxicological and biochemical studies provides strong evidence that target-site insensitivity is an important mechanism of resistance to methomyl. The lack of significant cross-resistance to chlorpyrifos suggests that the insensitive AChE in these field populations was selected by methomyl alone and not by the organophosphate.

Cloning of a horn fly cDNA, HiαE7, encoding an esterase whose transcript concentration is elevated in diazinon-resistant flies

Reverse transcriptase-polymerase chain reaction (PCR) was used to clone two esterase cDNAs from a diazinon-resistant field population of horn flies that expresses qualitative and quantitative differences in esterases compared with a susceptible population. The open reading frame from one of the esterase cDNAs, HiαE7, exhibits substantial amino-acid identity to an esterase associated with diazinon resistance in Lucilia cuprina. RNA Northern blots showed that HiαE7 mRNA was more abundant in the diazinon-resistant population than the susceptible population. DNA copy number analysis did not reveal major differences in HiαE7 gene copy number between the two populations. The full-length cDNA to HiαE7 was cloned and sequenced, and found to contain all of the highly conserved sequence elements associated with carboxyl/cholinesterases. The HiαE7 homologs in diazinon-resistant strains of L. cuprina and Musca domestica have been shown to possess an amino-acid substitution conferring diazinon hydrolytic activity to the esterase enzyme. This amino-acid substitution was not found in diazinon-resistant horn flies examined by allele-specific PCR. Individual flies from the resistant field population were phenotyped as diazinon-resistant or diazinon-susceptible by topical diazinon application bioassays and total RNA isolated and hybridized to HiαE7 probe in ribonuclease protection assays. HiαE7 transcript was expressed at a five-fold higher level in resistant female individual flies than in susceptible female individuals.

Insecticide resistance in Brazilian Triatoma infestans and Venezuelan Rhodnius prolixus.

Resistance levels to insecticides used in control of Chagas Disease vectors were assessed in two species of bugs (Hemiptera: Reduviidae): Triatoma infestans (Klug) from Brazil and Rhodnius prolixus Stål from Venezuela. The resistance ratios, compared to susceptible laboratory strains, were determined by topical application bioassays. The T. infestans PA strain exhibited resistance ratios of 7x to deltamethrin, 3.6x to beta-cyfluthrin and 3.3x to cypermethrin, but was susceptible to beta-cypermethrin and lambda-cyhalothrin. Rhodnius prolixus CA strain showed resistance to all the pyrethroids evaluated, the resistance ratios ranging between 4.5x to lambda-cyhalothrin and 12.4x to cypermethrin. Deltamethrin resistance in both strains was decreased by piperonyl butoxide, suggesting oxidative metabolism as cause of resistance.

Survey of insecticide resistance in Mexican populations of maize weevil, Sitophilus zeamais Motschulsky (coleoptera: curculionidae)

Topical application bioassays of DDT, lindane, malathion, pirimiphos-methyl, deltamethrin, and permethrin were undertaken with 11 field strains of maize weevil, Sitophilus zeamais Motschulsky, collected from nine states in Mexico. Concentration-mortality regression lines were estimated to compare the resistance ratios for each insecticide and strain. For each insecticide, resistance ratios were determined relative to the most susceptible strain. The field-collected strains showed low to moderate levels of resistance to DDT (1.3- to 14.1-fold), moderate to high levels of resistance to lindane (4.7- to 20.9-fold), low to high levels of resistance to malathion (1.6- to 31.4-fold), and low levels of resistance to pirimiphos-methyl (3.0- to 3.7-fold), deltamethrin (1.2- to 1.8-fold), and permethrin (2.3- to 3.5-fold). This is the first report of high levels of lindane and malathion resistance in strains of S. zeamais in Mexico. Given this information, I conclude that insecticide resistance in S. zeamais in Mexico is important, and may become a more serious problem in the future.

Toxicity of Imidacloprid in the German Cockroach (Dictyoptera: Blattellidae), and the Synergism Between Imidacloprid and Metarhizium anisopliae (Imperfect Fungi: Hyphomycetes)

German cockroaches, Blattella germanica (L.), orally exposed to solid imidacloprid baits exhibited toxic symptoms rapidly (within 30 min). However, cockroaches recovered from this toxicity. The percentage of knockdown was positively correlated and the rate of recovery was negatively correlated with concentration. In the liquid bait tests, imidacloprid produced rapid moribundity (within minutes) in all exposed cockroaches, but this moribundity was transient. Topically applied imidacloprid also produced rapid but transient moribundity in both in the laboratory-snsceptible (JWax-S) and the field-collected resistant (Muncie'86) strains. In the topical application bioassay, recovery time was dependent on cockroach strain and presence of piperonyl butoxide (PBO). Resistant cockroaches recovered faster than susceptible cockroaches; cockroaches pretreated with PBO recovered faster than those not pretreated. The JWax and Lafayette (field-collected) strains were killed significantly faster with The entomopathogenic fungus, Metarhizium anisopliae alone, or Metarhizium + solid imidacloprid bait than imidacloprid bait alone in both the choice and no-choice bioassays. Combinations of direct contact to the conidia of Metarhizium and feeding on imidacloprid bait indicated synergy when compared with the activity of the individual components. We found no differences in mortality when the JWax cockroaches fed on either 500 or 1,000 ppm imidacloprid baits. The presence of alternate food slowed the lethal time for imidacloprid against the JWax and Lafayette strains. Both strains were killed significantly faster when cockroaches lind on imidacloprid after a topical application of spore suspensions of Metarhizium than when insects fed on imidacloprid bait alone, indicating possible synergistic interaction (i.e., greater than expected mortality) between imidacloprid bait and Metarhizium The mortality of cockroaches inoculated topically with spore suspensions of Metarhizium increased significantly with an increase in the number of fungal spores. No significant differences in daily cockroach mortality were detected among imidacloprid concentrations for each Metarhizium dose. Imidacloprid; d bait alone has a limited potential for use in controlling cockroaches, but it may have a potential if used in combination with a fungal pathogen such as Metarhizium.

Toxicological Responses of Tobacco Budworm (Lepidoptera: Noctuidae) from Louisiana, Mississippi, and Texas to Selected Insecticides

Topical application bioassays of cypermethrin, methomyl, profenofos, and sulprofos were done with the Louisiana State University laboratory strain (LSU-LAB) and field-collected strains of tobacco bud worm, Heliothis virescens (F.), from Louisiana, Mississippi, and Texas. The field-collected strains showed low to high levels of resistance to cypermethrin (2- to 41-fold), low to moderate levels of resistance to profenofos and sulprofos (2- to 8-fold) and low to moderate levels of resistance to methomyl (2- to 8-fold) compared with the LSULAB strain. Spray chamber bioassays indicated reduced efficacy of cypermethrin, endosulfan, profenofos, and thiodicarb against the field-collected strains of H. virescens when their responses were compared with responses of the Stoneville laboratory strain.

Field Evaluation of Insecticide Application Strategies on Development of Insecticide Resistance by Colorado Potato Beetle (Coleoptera: Chrysomelidae)

Five insecticide application regimes were evaluated to investigate their influence on development of insecticide resistance in field populations of Colorado potato beetle, Leptinotarsa decemlineata (Say). These regimes included season-long sequential esfenvalerate applications; two alternate uses of esfenvalerate, azinphosmethyl, endosulfan, oxamyl, and Bacillus thuringiensis ssp. tenebrionis ; an integrated pest management (IPM) program; and one early season esfenvalerate application. Response of Colorado potato beetle from each treatment to four insecticides was determined at the beginning and end of the growing season. Afilter-paper technique was used to estimate the concentration response relationship for first instars, and a topical application bioassay was used to measure response of adults. Differences in esfenvalerate susceptibility existed among treatments in the early season; these differences were correlated with frequency of esfenvalerate applications during the previous season. Populations from all field treatments at the end of season showed an increase in resistance to esfenvalerate and endosulfan but not to azinphosmethyl or oxamyl. Increases in resistance generally were related to the frequency that a specific chemical was applied. We observed significant correlations between increase in esfenvalerate resistance and number of esfenvalerate applications in a given regime; 10 applications resulted in a 3.6-fold increase. Direct selection by esfenvalerate appeared to be the primary factor in development of resistance in these populations. Consequently, we suggest that reduction of repeated pyrethroid use is critical for limiting resistance development. The lowest increase in esfenvalerate resistance was found in the IPM treatment, indicating that this management strategy was also effective in retarding resistance development.

Activity of two novel insecticides against permethrin-resistantPseudoplusia includens

Two experimental insecticides, AC 303,630 and MK 244, were tested against a laboratory colony and three field strains of Pseudoplusia includens (Walker). Topical application bioassays indicated that permethrin resistance in the field strains ranged from 3.9 to 489.0-fold. In leaf dip bioassays, LC50 and LC90 values for AC 303,630 ranged from 6.7 to 15.1 mg litre −1 and 8.7 to 28.2 mg litre −1, respectively, for third-instar larvae. The Louisiana 1991 field strain was significantly more susceptible to AC 303,630 than the laboratory and other field strains. The LC50 (but not LC90) for the Louisiana 1992 field strain was significantly higher than that of the laboratory strain. However, there was no difference in toxicity of AC 303,630 between the field strain with the highest level of permethrin resistance and the laboratory strain. LC50 and LC90 values for MK 244 in leaf dip bioassays ranged from 0.014 to 0.023 mg litre −1 and 0.079 to 0.174 mg litre −1, respectively. There were no significant differences in LC 50 or LC 90 among any of the strains for MK 244. Field trials in soybean were also conducted in 1991 and 1992 in an area of Louisiana where permethrin efficacy against P. includens has declined. In both years, AC 303,630 at 0.11–0.22 kg ha −1, and MK 244 at 0.0042–0.0084 kg ha −1, provided significantly better control than permethrin at 0.11 kg ha −1, and control equal to the recommended standard, thiodicarb. These studies indicate no cross-resistance exists between the experimental insecticides and permethrin.

Assessment of Different Bioassay Techniques for Resistance Monitoring in the Diamondback Moth (Lepidoptera: Plutellidae)

Topical application, leaf residue, and filter paper residue bioassay methods were used to assess the toxicity of permethrin and methomyl to larvae of the diamondback moth, Plutella xylostella (L.). The objective was to develop an on-farm insecticide resistance monitoring technique for diamondback moth that required only simple, inexpensive equipment and was easy to use. The results of tests using the three bioassay methods led to similar resistance ratios for three diamondback moth strains. Leaf residue and filter paper residue tests using discriminating concentrations could detect the relative susceptibility to permethrin and methomyl in three strains, indicating that both techniques were practical for on-farm resistance monitoring. Third instars were more suitable for the detection of resistance than fourth instars. A monitoring system incorporating concentrations f both the LC90 for susceptible diamondback moth and the recommended field concentration of insecticide is suggested to detect resistance at low levels and to help choose insecticides for the control of diamondback moth strains with higher resistance levels.

Resistance to Pyrethroid, Carbamate, and Organophosphate Insecticides in Field Populations of Tobacco Budworm (Lepidoptera: Noctuidae) in 1990

Responses of tobacco budworm, Heliothis virescens (F.), to several classes of insecticides were determined using four types of laboratory bioassays. Topical application bioassays on third instars showed 13- to IS-fold resistance to cypermethrin, 7- to 12-fold resistance to profenofos, and 7- to 10-fold resistance to sulprofos in strains collected from Louisiana in 1990. A bioassay with neonate larvae showed 139- to 173-fold resistance levels to cypermethrin in the Louisiana strains. An adult vial test showed 9- to 12-fold resistance to cypermethrin. Resistance to pyrethroid, carbamate, and organophosphate insecticides was found in strains from Louisiana and Mississippi using a spray chamber bioassay. Multiple resistance to insecticides has not been reported in H. virescens since the late 1970s. Implications for control of tobacco budworm with currently available insecticides are discussed.

Responses of the alien leaf miners Liriomyza trifolii and Liriomyza huidobrensis (Diptera: Agromyzidae) to some pesticides scheduled for their control in the UK

Baseline responses to a range of pesticides scheduled for the statutory eradication of outbreaks of Liriomyza trifolii, a leaf miner alien to the UK, were established using a topical application bioassay against a susceptible laboratory population. Using the same technique the responses of outbreak populations of L. trifolii, and also the related species L. huidobrensis, have been investigated. Field populations of L. trifolii showed resistance to the pyrethroids tested but no evidence of resistance to organophosphates was found. There was some evidence of resistance to gamma-HCH. L. huidobrensis showed a response to pyrethroids similar to that of resistant L. trifolii, and levels of tolerance to organophosphates approximately eight times that of susceptible L. trifolii. The response to gamma-HCH was similar to that of susceptible L. trifolii.

Relationship Between Behavioral and Physiological Responses of Plutella xylostella (Lepidoptera: Plutellidae) Larvae to Permethrin Deposits within Two Ohio Populations

Variation in behavioral and physiological responses to permethrin (and the relationship between the two) were examined within two Ohio populations of diamondback moth, Plutella xylostella (L.). Groups of larvae that differed in behavioral response to a graded series of permethrin droplet density were selected from each population and reared separately. Behavioral responses of the F, progeny of each group to the same series of droplet densities were measured. The physiological response of each group was measured with a topical application bioassay. Progeny of groups selected from the Fremont population did not show significant differences in either behavioral or physiological responses; progeny of Celery ville groups showed significant differences in both types of response. Within the Celeryville population, more behaviorally responsive groups were also more physiologically susceptible to permethrin. The relationship of behavioral and physiological responses could have an important effect on how behavior affects selection for physiological resistance.

Field Monitoring for Insecticide Resistance in Beet Armyworm (Lepidoptera: Noctuidae)

A technique for monitoring insecticide resistance in field populations of beet armyworm, Spodoptera exigua (Hubner), was developed and compared with conventional topical bioassay. Insecticides were incorporated into the adhesive of a pheromone trap, thereby combining insect collection with insecticide application. This attractant trap technique (ATT) provided stable LC 50 's with low control mortality when traps were incubated at 21 o C for 36-36 h after insects were captured. LC 50 's of a laboratory colony tested with fenvalerate, permethrin, and methomyl were 95.6, 142, and 11.5 μg/g of adhesive, respectively. Slopes of probit regression for males were similar for topical and ATT bioassay, indicating a parallel response of the insect to the two methods. When larval and adult susceptibility were compared with topical application bioassay, toxicity differed less than 3-fold (adult LD 50 >larval LD 50 ) based upon body weight, and slopes of the probit lines were nearly equal for fenvalerate and permethrin (pyrethroids) Larvae exposed to methomyl (a carbamate) were more tolerant (20 times at LD 50 ) than adult males, and the slope of their probit regression was lower. Use of the ATT was further evaluated by monitoring a fresh market tomato field in southern California. Resistance ratios at LC 50 ranged from 3.0 to 7.3 for fenvalerate, 1.5 to 2.5 for permethrin, and 7.1 to 33.0 for methomyl

Expression of Resistance to Pyrethroid Insecticides in Adults and Larvae of Tobacco Budworm (Lepidoptera: Noctuidae): Implications for Resistance Monitoring

Resistance to pyrethroid insecticides was diagnosed in Heliothis virescens (F.) from cotton fields in Texas in 1985 and in Mississippi in 1986. Susceptible third instars on cotton terminals sprayed with recommended rates of pyrethroids suffered 90-95% mortality; however, only 20-71% of the field-collected, resistant larvae died. We confirmed the presence of pyrethroid resistance in Texas in 1985. Topical application bioassays in the laboratory failed to show more than sixfold resistance levels in larvae of some of these strains from Texas and Mississippi, indicating that standard topical bioassays do not always accurately diagnose resistance in this species. However, the frequency of resistant phenotypes in the cotton terminal assays and results with an adult bioassay technique were highly correlated.