Pyriproxyfen resistance of Bemisia tabaci (Homoptera: Aleyrodidae) biotype B: metabolic mechanism.

Juvenile hormone (JH) analog insecticides are relatively nontoxic to vertebrates and provide efficient control of key arthropod pests. One JH analog, pyriproxyfen, has provided over a decade of exceptional management of whiteflies in cotton of the southwestern United States. Thwarting resistance to pyriproxyfen in Bemisia tabaci (Gannadius) (a.k.a. Bemisia argentifolii Bellows and Perring) has been the focus of an integrated resistance management program because this insecticide was first registered for use in Arizona cotton in 1996. Resistance levels have increased slowly in field populations in recent years but have not demonstrably affected field performance of pyriproxyfen. Resistant strains have been isolated and studied in the laboratory to determine the mechanism of resistance and identify optimal strategies for controlling resistant whiteflies. Synergism bioassays showed that resistance in a laboratory-selected strain QC02-R, was partially suppressible with piperonyl butoxid (PBO) and diethyl maleate (DEM) but not with S, S, S-tributyl phosphorotrithioate (DEF). Consistent with the synergism bioassay results, enzymatic assays revealed that the enzyme activities of cytochrome P450 monooxygenases (P450) and glutathione S-transferases (GST) but not esterases were significantly higher in the pyriproxyfen-resistant QC02-R strain than in the susceptible strain. These results indicate that both P450 and GST are involved in whitefly resistance to pyriproxyfen.

Analyzing haplodiploid inheritance of insecticide resistance in whitefly biotypes.

We developed new methods for analyzing inheritance of insecticide resistance in haplodiploid arthropods and applied them to elucidate resistance of the whitefly Bemisia tabaci (Gennadius) to an insect growth regulator, pyriproxyfen. Two invasive biotypes of this devastating crop pest, the B biotype in Arizona and the Q biotype in Israel, have evolved resistance to pyriproxyfen. Here, we incorporated data from laboratory bioassays and crossing procedures exploiting haplodiploidy into statistical and analytical models to estimate the number of loci affecting pyriproxyfen resistance in strains of both biotypes. In tests with models of one to ten loci, the best fit between expected and observed mortality occurred with a two-locus model for the B biotype strain (QC-02) and for one- and two-locus models for the Q biotype strain (Pyri-R). The estimated minimum number of loci affecting resistance was 1.6 for the B biotype strain and 1.0 for the Q biotype strain. The methods used here can be applied to insecticide resistance and other traits in haplodiploid arthropods.

Mutations in the Bemisia tabaci para sodium channel gene associated with resistance to a pyrethroid plus organophosphate mixture.

The voltage-gated sodium channel is the primary target site of pyrethroid insecticides. In some insects, super knockdown resistance (super-kdr) to pyrethroids is caused by point mutations in the linker fragment between transmembrane segments 4 and 5 of the para-type sodium channel protein domain II (IIS4-5). Here, we identify two mutations in the IIS4-5 linker of the para-type sodium channel of the whitefly, BEMISIA TABACI: methionine to valine at position 918 (M918V) and leucine to isoleucine at position 925 (L925I). Although each mutation was isolated independently from strains >100-fold resistant to a pyrethroid (fenpropathrin) plus organophosphate (acephate) mixture, only L925I was associated with resistance in strains derived from the field in 2000 and 2001. The L925I mutation occurred in all individuals from nine different field collections that survived exposure to a discriminating concentration of fenpropathrin plus acephate. Linkage analysis of hemizygous male progeny of unmated heterozygous F1 females (L925Ixwild-type) shows that the observed resistance is tightly linked to the voltage-gated sodium channel locus. The results provide a molecular tool for better understanding, monitoring and managing pyrethroid resistance in B. tabaci.

Effects of Bt cotton and crylac toxin on survival and development of pink bollworm (Lepidoptera: Gelechiidae).

We evaluated the effects of Bacillus thuringiensis (Bt) toxin CrylAc on survival and development of a susceptible strain and laboratory-selected resistant strains of pink bollworm, Pectinophora gossypiella (Saunders). For susceptible and resistant strains tested on artificial diet, increases in CrylAc concentration reduced developmental rate and pupal weight. In greenhouse tests, survival of resistant larvae on transgenic cotton that produces CrylAc (Bt cotton) was 46% relative to their survival on non-Bt cotton. In contrast, Bt cotton killed all susceptible larvae tested. F1 hybrid progeny of resistant and susceptible adults did not survive on Bt cotton, which indicates recessive inheritance of resistance. Compared with resistant or susceptible larvae reared on non-Bt cotton, resistant larvae reared on Bt cotton had lower survival and slower development, and achieved lower pupal weight and fecundity. Recessive resistance to Bt cotton is consistent with one of the basic assumptions of the refuge strategy for delaying resistance to Bt cotton. Whereas slower development of resistant insects on Bt cotton could increase the probability of mating between resistant adults and accelerate resistance, negative effects of Bt cotton on the survival and development of resistant larvae could delay evolution of resistance.

Overwintering cost associated with resistance to transgenic cotton in the pink bollworm (Lepidoptera: Gelechiidae).

Fitness costs associated with resistance to transgenic crops producing toxins from Bacillus thuringiensis (Bt) may have important effects on the evolution of resistance. We investigated overwintering costs in pink bollworm, Pectinophora gosypiella (Saunders), strains with different degrees of resistance to Bt cotton. Frequency of resistant individuals in a strain was not associated with induction of diapause or emergence from diapause in early winter. Emergence from diapause in the spring was 71% lower in three highly resistant strains than in two heterogeneous strains from which the resistant strains were derived. This underestimates the overwintering cost because the frequency of the resistance allele was relatively high in the heterogeneous strains. Emergence in the spring in hybrid progeny from crosses between the resistant and heterogeneous strains was greater than in resistant strains but did not differ from susceptible strains, showing that the overwintering cost was recessive to some extent.

Large-scale management of insect resistance to transgenic cotton in Arizona: can transgenic insecticidal crops be sustained?

A major challenge for agriculture is management of insect resistance to toxins from Bacillus thuringiensis (Bt) produced by transgenic crops. Here we describe how a large-scale program is being developed in Arizona for management of resistance to Bt cotton in the pink bollworm, Pectinophora gossypiella (Saunders) (Lepidoptera: Gelechiidae), and other insect pests of cotton. Financial support from growers makes this program possible. Collaboration between the Arizona Cotton Research and Protection Council, the University of Arizona, and government agencies has led to development of resistance management guidelines, a remedial action plan, and tools for monitoring compliance with the proposed guidelines. Direct participation in development of resistance management policies is a strong incentive for growers to invest in resistance management research. However, more research, regularly updated regulations, and increased collaboration between stakeholders are urgently needed to maintain efficacy of Bt toxins in transgenic crops.

Fitness costs and maternal effects associated with resistance to transgenic cotton in the pink bollworm (Lepidoptera: Gelechiidae).

Transgenic cotton producing a Bacillus thuringiensis (Bt) toxin is widely used for controlling the pink bollworm, Perctinophora gossypiella (Saunders). We compared performance of pink bollworm strains resistant to Bt cotton with performance of their susceptible counterparts on non-Bt cotton. We found fitness costs that reduced survival on non-Bt cotton by an average of 51.5% in two resistant strains relative to the susceptible strains. The survival cost was recessive in one set of crosses between a resistant strain and the susceptible strain from which it was derived. However, crosses involving an unrelated resistant and susceptible strain indicated that the survival cost could be dominant. Development time on non-Bt cotton did not differ between the two related resistant and susceptible strains. A slight recessive cost affecting development time was suggested by comparison of the unrelated resistant and susceptible strains. Maternal effects transmitted by parents that had eaten Bt-treated artificial diet as larvae had negative effects on embryogenesis, adult fertility, or both, and reduced the ability of neonates to enter cotton bolls. These results provide further evidence that fitness costs associated with the evolution of resistance to Bt cotton are substantial in the pink bollworm.

Frequency of resistance to Bacillus thuringiensis in field populations of pink bollworm.

Strategies for delaying pest resistance to genetically modified crops that produce Bacillus thuringiensis (Bt) toxins are based primarily on theoretical models. One key assumption of such models is that genes conferring resistance are rare. Previous estimates for lepidopteran pests targeted by Bt crops seem to meet this assumption. We report here that the estimated frequency of a recessive allele conferring resistance to Bt toxin Cry1Ac was 0.16 (95% confidence interval = 0.05-0.26) in strains of pink bollworm (Pectinophora gossypiella) derived from 10 Arizona cotton fields during 1997. Unexpectedly, the estimated resistance allele frequency did not increase from 1997 to 1999 and Bt cotton remained extremely effective against pink bollworm. These results demonstrate that the assumptions and predictions of resistance management models must be reexamined.

Cross-resistance of pink bollworm (Pectinophora gossypiella) to Bacillus thuringiensis toxins.

Two strains of pink bollworm (Pectinophora gossypiella) selected in the laboratory for resistance to Bacillus thuringiensis toxin Cry1Ac had substantial cross-resistance to Cry1Aa and Cry1Ab but not to Cry1Bb, Cry1Ca, Cry1Da, Cry1Ea, Cry1Ja, Cry2Aa, Cry9Ca, H04, or H205. The narrow spectrum of resistance and the cross-resistance to activated toxin Cry1Ab suggest that reduced binding of toxin to midgut target sites could be an important mechanism of resistance.

Whitefly (Hemiptera: Aleyrodidae) binding site for imidacloprid and related insecticides: a putative nicotinic acetylcholine receptor.

Imidacloprid is used extensively to control sweetpotato whiteflies, Bemisia argentifolii Bellows & Perring [also known as B. tabaci (Gennadius) biotype B]. As a radioligand, [3H]imidacloprid binds rapidly to a single class of high-affinity sites in membrane preparations from whole adult whiteflies with an apparent dissociation constant of 2 nM and maximal binding capacity of 101 fmol/mg protein. Three related compounds (the nitromethylene analog of imidacloprid, acetamiprid, and nitenpyram) inhibit [3H]imidacloprid binding by 50% at 0.40, 2.9, and 57 nM, respectively. The pharmacological profile of the binding site (examined with imidacloprid and the analogs listed above, and nicotine, alpha-bungarotoxin, carbachol, acetylcholine [with paraoxon], and atropine) is consistent with that anticipated for a nicotinic acetylcholine receptor and correlates well with binding results for house fly, Musca domestica L., head membranes under the same conditions. Thus, [3H]imidacloprid is a suitable radioligand to investigate the putative nicotinic acetylcholine receptor of Bemisia and the possible modifications of this target site associated with selection of resistant strains.

Prior exposure to carbaryl alters behavior ofTetranychus urticae Koch on acaricide-treated leaf surfaces.

Preadult exposure to carbaryl affected the subsequent behavior of two-spotted spider mites (Tetranychus urticae Koch) on leaf surfaces with discontinuous acaricide residues. In dicofol bioassays, preexposure to carbaryl caused a loss of avoidance behavior (eliminating the tendency of mites to feed and stand longer off treated areas). In amitraz bioassays, preexposure to carbaryl had the opposite effect. It increased the propensity of mites to feed and stand off amitraz-treated areas, resulting in increased avoidance of amitraz. Carbaryl preexposure therefore resulted in diametrically opposed behavioral changes in subsequent encounters with two acaricides. These effects provide additional evidence of the unpredictable nature of interactions between pesticides and show how a chemical, irrespective of degree of toxicity, can alter the behavior of arthropods in response to subsequent chemical encounters.

Enhanced activation is the mechanism of negative cross-resistance to Chlorpyrifos in the Dicofol-IR strain of Tetranychus urticae (Acari: Tetranychidae).

The mechanism responsible for negative cross-resistance to chlorpyrifos was examined in isogenic dicofol susceptible (Orchard-12) and resistant (Dicofol-IR) two-spotted spider mites, Tetranychus urticae Koch. The acetylcholinesterase of both strains was equally sensitive to inhibition by chlorpyrifos oxon. However, the Dicofol-IR strain showed increased oxidative activation of chlorpyrifos to chlorpyrifos oxon relative to the Orchard-12 strain, suggesting this mechanism is responsible for the observed negative cross-resistance.