Field-collected Strains Research Articles

Comparative acaricide susceptibility and detoxifying enzyme activities in field‐collected resistant and susceptible strains of Tetranychus urticae

A field-collected strain (MR-VL) of the two-spotted spider mite, Tetranychus urticae Koch, exhibited strong resistance to bifenthrin, dicofol and fenbutatin oxide in comparison with a susceptible laboratory strain (LS-VL). The MR-VL strain was screened for cross-resistance with several currently used acaricides. Cross-resistance was detected with clofentezine (RR = 2631), dimethoate (RR = 250), chlorfenapyr (RR = 154), bromopropylate (RR = 25), amitraz (RR = 17), flucycloxuron (RR = 15) and azocyclotin (RR = 7). Abamectin, acequinocyl, bifenazate, tebufenpyrad and spirodiclofen did not show any signs of cross-resistance. Enhanced detoxification by increased activity of mono-oxygenases (MO) and esterases is at least partially responsible for the observed resistance and cross-resistance. MO assays with 7-ethoxycoumarin (7-EC) were optimised and 7-ethoxy-4-trifluoromethylcoumarin (7-EFC), a new MO-substrate, was evaluated for the first time in T urticae and proved to be a good alternative to 7-EC. Approximately 3- and 4-fold higher MO activity was detected with 7-EFC and 7-EC respectively in the MR-VL strain. Kinetic parameters of general esterase assays with 4-nitrophenyl acetate and 1-naphthyl acetate as substrate indicated that more esterases were present in the MR-VL strain. A first attempt was made to classify the esterases present in T urticae. Acetyl-, aryl- and carboxyl-esterases were detected with the use of inhibitors after separation by native PAGE. Glutathione-S-transferases did not seem to play any role in the observed resistance and no differences were detected when the general oxidative capacities of the two strains were compared.

Reproductive Development of Northern and Southern Strains of Plum Curculio (Coleoptera: Curculionidae)

Laboratory-reared southern and field-collected northern strains of plum curculio, Conotrachelus nenuphar (Herbst), were sampled to examine the relationship between degree-day (DD) accumulation and female reproductive development, as measured by mating status, oocyte size, and number of oocytes. The overall goal was to generate an objective degree-day model for predicting damage potential that could be applied to various host commodities rather than relying on separate biofix models for each crop. Adult beetles were dissected to measure mating status, maximum oocyte size, and number of oocytes. Southern strain beetles reared at 25°C initiated mating 9 d after eclosion and did not require mating to induce oocyte development. By 20 d posteclosion, unmated females had significantly higher egg loads compared with mated females of the same age. Logistic regression analysis suggests that southern and northern strain beetles had a stable maximum oocyte length of 62 and 72 μm, respectively. Northern strain females mated after overwintering; with ≈95% of the female population mated after 134 DD (base 10°C), which is before fruit set in many host crops. Oocyte size was the only measured parameter of field reproductive progress that could be linked with confidence to degree-day accumulation. The other two parameters do not share an exclusive relationship with degree-days. Rapid assessment of field-caught female reproductive status could assist in determining the potential for plum curculio damage in high-value commodities and allow for more informed control decisions.

Origin and extent of resistance to fipronil in the German cockroach, Blattella germanica (L.) (Dictyoptera: Blattellidae).

Fipronil, a phenylpyrazole insecticide, was made available in 1999 in bait formulations for use against the German cockroach, Blattella germanica (L.). We have investigated resistance to fipronil in the descendants of cockroaches collected just before, or contemporaneously with, the introduction of fipronil baits. Cockroaches were obtained in two types of settings: homes that either had or had not been serviced by a pest management professional while occupied by their current residents. Thorough inspections by us turned up no evidence that fipronil had been used in any of the homes, and in addition, no residents claimed to have used baits containing fipronil. Resistance to fipronil was detected by topically dosing adult males with the LC99 of fipronil, the value of which was determined in a dose-response assay with males of an insecticide-susceptible strain. Fewer than 99 of 100 males of all field-collected strains died within 72 h of being treated. Moreover, substantial numbers of males survived doses three and 10-fold greater than the LC99. Regression analysis showed that 67% of the variation in the percentage of males that died after being treated with fipronil was explained by a linear relationship with the percentage that died after being treated with dieldrin. Therefore, it appears that resistance to fipronil in German cockroaches--whose ancestors had never been exposed to it--is attributable to enduring resistance to the cyclodienes, which were formerly used for cockroach control and have a similar mode of action as fipronil. Lastly, we found that insects resistant to topically administered fipronil were likewise resistant, and to a similar degree, to ingested fipronil.

Prevalence of insecticide resistance in field-collected populations of the German cockroach, Blattella germanica (Linnaeus) (Dictyoptera : Blattellidae) in Peninsular Malaysia

Thirty one field-collected strains of German cockroaches, Blattella germanica (L.), collected between February 1997 and March 2000 from various loca- tions in Peninsular Malaysia were screened for insecticide resistance using topical application method against three commonly used insecticides (propoxur, chlorpyrifos and deltamethrin). A discriminating dose (LDg9) obtained from bioassaying a labora- tory susceptible strain, was used to determine the prevalence of insecticide-resistant individuals among the field populations tested. Results indicated that both propoxur and deltamethrin resistance patterns were relatively similar, i.e. more than 50% of the strains screened show low to moderate level of tolerance, while chlorpyrifos resistance is still relatively low among the tested populations. Resistant gene frequencies estimated ranged between 0.01 to 0.78, 0.05 to 0.51 and 0 to 0.86 for propoxur, chlorpyrifos and deltamethrin, respectively.

Evaluation of metabolic detoxifying enzyme activities and insecticide resistance in Frankliniella occidentalis.

The western flower thrips Frankliniella occidentalis (Pergande) is a very significant pest of a number of different agricultural crops in the south-east of Spain. The importance of thrips as a pest is not due mainly to the direct damage inflicted on the plant, but to the loss in commercial value which occurs as a consequence of the development of dark spots caused by the tomato spotted wilt virus (TSWV) which they transmit. The economic threshold is therefore almost zero, which enhances the problems of resistance management. The present work is part of a global project that attempts to evaluate the status of insecticide resistance in field populations of thrips obtained from several agricultural crops. We have studied, in either individual or pooled insects, some enzyme systems classically related to detoxification of insecticides: esterase and glutathione-S-transferase (GST). The activity of these enzymes from laboratory populations selected with various classes of insecticides has also been measured using several appropriate substrates. An increase in GST mean activity was found in two field-collected strains. Differences in frequency distributions of esterase and GST activities were found for both field-collected strains and for a laboratory strain selected with acrinathrin. These activities were compared with those of a wild-type reference strain.

Development of a larval bioassay for susceptibility of cat fleas (Siphonaptera: Pulicidae) to imidacloprid.

Strategies for controlling cat fleas, Ctenocephalidesfelisfelis (Bouché), have undergone dramatic changes in the past 5 yr. With the advent of on-animal treatments with residual activity the potential for the development of insecticide resistance increases. A larval bioassay was developed to determine the baseline susceptibility of field-collected strains of cat fleas to imidacloprid. All four laboratory strains tested showed a similar level of susceptibility to imidacloprid. Advantages of this bioassay are that smaller numbers of fleas are required because flea eggs are collected for the test. Insect growth regulators and other novel insecticides can also be evaluated. Using a discriminating dose, the detection of reduced susceptibility in field strains can be determined with as few as 40 eggs.

Estandarización del método propuesto por la Organización Mundial de la Salud para determinar los niveles de susceptibilidad de los vectores de leishmaniasis a insecticidas

The WHO method for determining insecticide resistance was standardized for several species of Lutzomyia sand flies under laboratory and field conditions. The biological assays were applied solely to optimize the conditions for the control, i.e., without insecticide, and to estimate mortality due to handling or other unfavorable conditions. Adult female flies from 3 laboratory colonies and one field strain were tested: two laboratory strains of Lutzomyia longipalpis, one laboratory strain of Lutzomyia serrana and one field-collected strain of Lutzomyia quasitownsendi. The WHO method was compared with one modified in which, during the post-exposure period, the recommended plain tube apparatus was replaced with a plastic container layered with damp plaster of Paris. Three paper substrate types were compared under each condition: olive oil additive, silicon oil additive and plain paper. The measured variable was percent mortality in 24 h. For the WHO protocol, the L. longipalpis strains indicated a 0-10% mortality, L. serrana 20-80% and L. quasitownsendi 10-50%. With the modified WHO apparatus, the average mortality was < 4% for all species. No significant differences were observed among the paper treatments. These results indicate a strong species-specific effect of post-exposure conditions on sand flies. To establish baseline levels of insecticide resistance in Lutzomyia sand flies, the WHO method is recommended only for L. longipalpis, and the modified method for L. serrana, L. quasitownsendi and closely related species.

Prey-Stage Preference in Generalist and Specialist Phytoseiid Mites (Acari: Phytoseiidae) When OfferedTetranychus urticae(Acari: Tetranychidae) Eggs and Larvae

We tested 13 species of phytoseiid mites for prey preferences between eggs and larvae of twospotted spider mite, Tetranychus urticae Koch. In general, oligophagous, specialized spider mite predators preferred eggs whereas more polyphagous, generalist predators showed no prey-stage preference or preferred larvae: Phytoseiulus macropilis (Banks), P. persimilis Athias-Henriot, Neoseiulus longispinosus (Evans), and Neoseiulus fallacis (Garman) preferred eggs; Kampimodromus aberrans (Oudemans), Galendromus occidentalis (Nesbitt), N. barkeri Hughes, N. californicus (McGregor), N. cucumeris (Oudemans), and Typhlodromus pyri Scheuten showed no prey-stage preference; and Amblyseius andersoni Chant, Euseius finlandicus (Oudemans) and E. hibisci (Chant) preferred larvae. When the ratio of spider mite eggs to larvae was altered, adult females of E. finlandicus, G. occidentalis, N. fallacis, and P. persimilis maintained relatively constant feeding ratios, resulting in slight negative prey-stage switching due mostly to the changes in prey-stage ratios offered. However, when offered a prey-stage ratio that was biased toward eggs, adult females of T. pyri exhibited positive switching, feeding disproportionately more on eggs than would be expected given the change in the ratio offered. There was no clear trend in the effect of starvation time on prey-stage preference for N. fallacis adult females. The egg preference of N. fallacis was also not affected when individuals were reared solely on eggs or larvae of T. urticae and subsequently tested as adult females. Individuals of field-collected strains of N. fallacis and P. persimilis showed no differences in prey-stage preferences when compared with individuals from long-term reared laboratory strains of the same species. However, individuals of one field-collected strain of G. occidentalis showed a stronger preference for eggs than individuals from three other strains (one laboratory and two field-collected) of the same species.

Resistance to Organophosphates and Biochemical Genotyping of Acetylcholinesterases in Tetranychus urticae (Acari: Tetranychidae)

Abstract Resistance to organophosphorus acaricides in the two-spotted spider mite Tetranychus urticae is largely due to the reduced sensitivity of the target enzyme acetylcholinesterase (AChE). The insensitivity of AChE to the organophosphates (OPs) demeton-S-methyl, ethyl paraoxon, and chlorpyrifos oxon and the carbamate carbofuran was identified in a German laboratory strain of T. urticae (WI) selected with oxydemeton-methyl and a field-collected strain from Florida, T. urticae (VB). Depending on the inhibitor used, AChE in the resistant strains was 34- to 380-fold less sensitive than the AChE of a German susceptible strain, T. urticae (GSS), as revealed by conventional microtiter plate assays. A modified, highly sensitive microfluorometric assay enabled us to measure the AChE activity in a quarter of a single mite. The assay is based on the reaction of thiocholine with a fluorogenic coumarin phenylmaleimide, yielding an intensely fluorescent product. This technique was used to genotype individual spider mites and to examine the frequency of sensitive and insensitive AChE alleles in several strains. The kinetic parameters (Km and Vmax) of AChE from susceptible (GSS) and resistant (WI and VB) strains of T. urticae were compared for different substrates. In a foliar spray application bioassay, larvae of strains WI and VB showed only moderate resistance levels in contrast to their high degree of AChE insensitivity relative to the GSS strain. While resistance ratios to ethyl parathion were comparable in strains WI and VB, the difference in chlorpyrifos resistance was remarkable. The field-collected strain VB and the laboratory selected strain WI exhibited 78- and 9.5-fold resistance against chlorpyrifos, respectively. As there is no obvious difference in AChE inhibition kinetics between the two strains, this result suggests an additional resistance mechanism in the VB strain against chlorpyrifos.

Development of diagnostic concentrations for insecticide resistance monitoring in soybean looper (Lepidoptera: Noctuidae) larvae using an artificial diet overlay bioassay.

Diagnostic concentrations for several standard and experimental insecticides were determined for a laboratory reference strain of soybean looper, Pseudoplusia includens (Walker), using an insecticide diet overlay bioassay to evaluate the relative susceptibility of field (P) and F1 generations of four field-collected strains of third-, fourth-, and fifth-instar soybean loopers in 1996 and 1997. Diagnostic concentrations were defined as concentrations that killed 90-95% of the susceptible individuals and were 5 ppm for permethrin, 1,300 ppm for thiodicarb, 60 ppm for chlorfenapyr, 5 ppm for emamectin benzoate, and 60 ppm for spinosad. Field strains exhibited significantly greater percentage survival than the laboratory reference strain in the permethrin bioassays in 1996 and 1997 in both the P and F1 generation bioassays and in the thiodicarb bioassays in 1997. Larvae exposed to diagnostic concentrations of the experimental insecticides chlorfenapyr, emamectin benzoate, and spinosad usually did not exhibit significantly higher percentage survival than the reference strain.

Development of Diagnostic Concentrations for Insecticide Resistance Monitoring in Soybean Looper (Lepidoptera: Noctuidae) Larvae Using an Artificial Diet Overlay Bioassay

Diagnostic concentrations for several standard and experimental insecticides were determined for a laboratory reference strain of soybean looper, Pseudoplusia includens (Walker), using an insecticide diet overlay bioassay to evaluate the relative susceptibility of field (P) and F1generations of four field-collected strains of third-, fourth-, and fifth-instar soybean loopers in 1996 and 1997. Diagnostic concentrations were defined as concentrations that killed 90–95% of the susceptible individuals and were 5 ppm for permethrin, 1,300 ppm for thiodicarb, 60 ppm for chlorfenapyr, 5 ppm for emamectin benzoate, and 60 ppm for spinosad. Field strains exhibited significantly greater percentage survival than the laboratory reference strain in the permethrin bioassays in 1996 and 1997 in both the P and F1generation bioassays and in the thiodicarb bioassays in 1997. Larvae exposed to diagnostic concentrations of the experimental insecticides chlorfenapyr, emamectin benzoate, and spinosad usually did not exhibit significantly higher percentage survival than the reference strain.

Biochemical characterization of insecticide resistance in the German cockroach, Blattella germanica, from Malaysia.

The possible insecticide resistance mechanisms of four Malaysian field-collected strains of the German cockroach, Blattella germanica (Linnaeus) (Dictyoptera: Blattellidae), were characterized with biochemical assays and native polyacrylamide gel electrophoresis (PAGE). Elevated esterase activity (at low to moderate frequency) and altered acetylcholinesterase (low frequency) were detected in all field strains, while elevated glutathione S-transferase levels were present in only two strains. Seven esterase bands were separated by native PAGE; a greater intensity occurred in three bands in the resistant strains compared to the susceptible strain. Inhibition studies using specific inhibitors on polyacrylamide gels suggested that the slowest of these three esterases is a cholinesterase, while the other two are carboxylesterases with a preference for beta- over alpha-naphthyl acetate.

Inhibition of hatching of Tribolium castaneum by phosphine

Hatching was delayed and lower when 0–1-day-old eggs of a susceptible strain of red flour beetle, Tribolium castaneum, were exposed to 30–50 ppm of phosphine for 72–120 h causing 70% or more kill, and in a field-collected phosphine-resistant strain exposed to 100–300 ppm for 72 h producing 30% or more mortality. Inhibition of hatching was established by noting a relative increase in hatching in later days in the fumigated batches which was significant for the eggs of the susceptible strain exposed to 30 ppm of phosphine for 120 h and the eggs of the resistant strain exposed to 300 ppm for 72 h. The practical implications of the inhibition of hatching of T. castaneum eggs by phosphine treatment are discussed.

Characterization of esterases associated with profenofos resistance in the tobacco budworm, Heliothis virescens (F.).

The utility of microplate and electrophoretic assays for detection of biochemical and physiological mechanisms underlying resistance to profenofos in the tobacco budworm, Heliothis virescens (F.), was assessed. Esterase (EST) activities were significantly higher in profenofos-resistant than -susceptible larvae, and activities were highly correlated (r(2) = 0.87) with resistance to profenofos. Both qualitative and quantitative variation was observed in electrophoretic gels stained with alpha- and beta-naphthyl acetates. Staining of ESTs was more intense with resistant larvae than those from a susceptible strain. In addition, a band (designated A') was expressed in larvae from profenofos-resistant strains, but not in larvae from an insecticide-susceptible strain. The frequency of expression of A' increased following selection with profenofos and was detected in 100% of the individuals from a profenofos-selected strain. The appearance of this band coincided with the decreased expression of a second band (designated A). A similar pattern (overexpression of A' and underexpression of A) also was observed in larvae from field-collected strains. Finally, reduction in the activity or the sensitivity of acetylcholinesterase to inhibition by chlorpyrifos oxon was observed in laboratory-selected and field-collected larvae that expressed resistance to profenofos. These results suggest that microplate and electrophoretic assays can be utilized as complementary tools for detecting and monitoring profenofos resistance in H. virescens.

Local Dynamics, Overexploitation and Predator Dispersal in an Acarine Predator-Prey System

We studied the impact of dispersal of predatory arthropods on local predator-prey dynamics, Two types of dispersal strategies are distinguished: the Killer-strategy in which predators only disperse after a local prey population went extinct, and the Milker-strategy in which predators also disperse in the presence of prey. Theory predicts that Killers are favoured by selection when local predator populations are started by multiple, unrelated foundresses, whereas Milkers are favoured by selection when the number of foundresses is sufficiently low. To determine which of these strategies prevails in wild populations of predatory arthropods, we did replicated experiments on local dynamics and dispersal in a system of bean leaves, herbivorous mites (Tetranychus urticae) and various field-collected strains of predatory mites (Phytoseiulus persimilis). The experiments showed that all strains tend to drive local prey populations to extinction. Furthermore, we found two isofemale lines that showed consistent differences with regard to the onset of dispersal. In one linc predators did not disperse until after extermination of the local prey population, but in another some adult females dispersed when there were still prey present. The earlier predator dispersal was associated with a longer interaction period and a larger overall prey population. Interestingly, the Milker-type line originated from a population that was much more isolated from other populations than the one from which the Killer-type line was obtained. This result provides qualitative support for the so-called Milker-Killer hypothesis. Another observation was that near to prey elimination there is first a peak of dispersing juvenile predators, followed 1-3 d later by a peak of dispersing adult males and females. Dispersal continued up to 10 d after prey extermination. To explain why the predators stay so long in an area devoid of prey it is hypothesised that cannibalism on the younger and weaker stages motivates adult predators to delay dispersal, and or provides juveniles the means to reach maturity.

Abr1, a transposon-like element in the genome of the cultivated mushroom Agaricus bisporus (Lange) Imbach.

A 300-bp repetitive element was found in the genome of the white button mushroom, Agaricus bisporus, and designated Abr1. It is present in approximately 15 copies per haploid genome in the commercial strain Horst U1. Analysis of seven copies showed 89 to 97% sequence identity. The repeat has features typical of class II transposons (i.e., terminal inverted repeats, subterminal repeats, and a target site duplication of 7 bp). The latter shows a consensus sequence. When used as probe on Southern blots, Abr1 identifies relatively little variation within traditional and present-day commercial strains, indicating that most strains are identical or have a common origin. In contrast to these cultivars, high variation is found among field-collected strains. Furthermore, a remarkable difference in copy numbers of Abr1 was found between A. bisporus isolates with a secondarily homothallic life cycle and those with a heterothallic life cycle. Abr1 is a type II transposon not previously reported in basidiomycetes and appears to be useful for the identification of strains within the species A. bisporus.

Pyrethroid Encapsulation for Conservation of Acarine Predators and Reduced Spider Mite (Acari: Tetranychidae) Outbreaks in Apple Orchards

We examined the effects of a microencapsulated formulation of the pyrethroid cypermethrin on the predator Amblyseius fallacis (Garman) and the biological control of tetranychid mites. In functional response analysis, susceptible field-collected and mass-reared pyrethroid resistant strains of A. fallacis consumed similar numbers of Tetranychus urticae Koch eggs on unsprayed leaf-disks (14–16 eggs per day). Fewer prey were consumed on microencapsulated cypermethrin sprayed disks, but consumption was similar between strains (10.3–11.3 eggs per day). The functional response on cypermethrin EC (emulsifiable concentrate)-sprayed disks was associated with resistance. Susceptible A. fallacis mites (cypermethrin LC50 = 7.2 ppm) consumed a maximum of 1.0 eggs per day; field-collected predators (LC50 = 13.3 ppm) consumed a maximum of 4.0 eggs per day, and mass-reared A. fallacis (LC50 = 28.2 ppm) consumed a maximum of 4.33 eggs per day. Encapsulated cypermethrin was much less toxic to predators, and was similar in toxicity between strains (LC50 range = 87.0–100.4 ppm). In an experimental orchard, a commercial formulation of cypermethrin EC almost eliminated A. fallacis. On trees sprayed with the microencapsulated formulation, mean A. fallacis densities were reduced from 0.81 ± 0.26 per leaf to 0.26 ± 0.12 per leaf. On microencapsulated sprayed trees this predator density, together with the stigmaeid predator Zetzellia mali (Ewing), appeared to regulate Panonychus ulmi Koch to a peak density of 21.7 ± 11.7 per leaf, compared to 87.2 ± 15.6 per leaf on cypermethrin EC treated trees, and 10.4 ± 4.6 per leaf on unsprayed trees. The microencapsulated cypermethrin formulation conserved natural biological control in pyrethroid sprayed orchards better than the cypermethrin EC formulation.

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.

Target-Site Resistance to Pyrethroids inHeliothis virescens(F.) andHelicoverpa zea(Boddie)

A neurophysiological assay was used to record and compare target-site sensitivities to the pyrethroid insecticide, allethrin, in larvae and adults of the tobacco budworm,Heliothis virescens(F.) and larvae of the bollworm,Helicoverpa zea(Boddie). High frequencies of target-site resistance were measured in larvalH. virescensand expression was greater in early- than late-season collections. AdultH. virescensalso expressed reduced sensitivity to allethrin, but frequency distributions were not statistically different from that of a laboratory-reared susceptible strain. Target-site sensitivity was similar between larvae from laboratory-susceptible and field-collected strains ofH. zeaduring 1995, but low frequencies of target-site resistance were detected in larvae collected from conventional and transgenic cotton during 1996. The significance of these findings with respect to the utility of the adult vial test as a monitoring tool and the continued control ofH. zeawith pyrethroids is discussed.

Susceptibility of Field Populations of Beet Armyworm (Lepidoptera: Noctuidae) to Commercial and Experimental Insecticides

Susceptibility of field-collected strains of beet armyworm, Spodoptera exigua (Hubner), to commercial and experimental insecticides was evaluated in a diet overlay bioassay by using 2-d-old larvae. Larvae were collected from cotton, Gossypium hirsutum L., fields in Alabama, California, Louisiana, Mississippi, and Texas, and from Rio Bravo, Mexico. Several field strains exhibited reduced susceptibility to chlorpymos and thiodicarb, which are the only 2 labeled insecticides recommended for beet armyworm control in Louisiana. In chlorpyrifos bioassays, 7 of the 11 field strains had significantly higher LC50s than the reference strain from the Southern Insect Management Laboratory at Stoneville, MS. In thiodicarb bioassays, 3 of the 10 field strains had significantly higher LC50s than the reference strain. Generally, LC50s of field strains to the experimental insecticides chlorfenapyr, spinosad, and tebufenozide were comparable to those of the reference strain. In chlorfenapyr bioassays, a field strain from Red Cross, LA, had a significantly higher LC50 than the reference strain. In spinosad bioassays, 3 field strains had LC50s that were significantly lower than the reference strain. In tebufenozide bioassays, a field strain from Starkville, MS, had a significantly higher LC50 than the reference strain.