Assessing transmission of crop diseases by insect vectors in a landscape context

Theory indicates that landscape composition affects transmission of vector-borne crop diseases, but few empirical studies have investigated how landscape composition affects plant disease epidemiology. Since 2006, Bemisia tabaci (Gennadius) has vectored the cucurbit yellow stunting disorder virus (CYSDV) to cantaloupe and honeydew melons (Cucumis melo L.) in the southwestern United States and northern Mexico, causing significant reductions in yield of fall melons and increased use of insecticides. Here, we show that a landscape-based approach allowing simultaneous assessment of impacts of local (i.e., planting date) and regional (i.e., landscape composition) factors provides valuable insights on how to reduce crop disease risks. Specifically, we found that planting fall melon fields early in the growing season, eliminating plants germinating from seeds produced by spring melons after harvest, and planting fall melon fields away from cotton and spring melon fields may significantly reduce the incidence of CYSDV infection in fall melons. Because the largest scale of significance of the positive association between abundance of cotton and spring melon fields and CYSDV incidence was 1,750 and 3,000 m, respectively, reducing areas of cotton and spring melon fields within these distances from fall melon fields may decrease CYSDV incidence. Our results indicate that landscape-based studies will be fruitful to alleviate limitations imposed on crop production by vector-borne diseases.

Effects of local and landscape factors on population dynamics of a cotton pest

BACKGROUND

Many polyphagous pests sequentially use crops and uncultivated habitats in landscapes dominated by annual crops. As these habitats may contribute in increasing or decreasing pest density in fields of a specific crop, understanding the scale and temporal variability of source and sink effects is critical for managing landscapes to enhance pest control.

METHODOLOGY/PRINCIPAL FINDINGS

We evaluated how local and landscape characteristics affect population density of the western tarnished plant bug, Lygus hesperus (Knight), in cotton fields of the San Joaquin Valley in California. During two periods covering the main window of cotton vulnerability to Lygus attack over three years, we examined the associations between abundance of six common Lygus crops, uncultivated habitats and Lygus population density in these cotton fields. We also investigated impacts of insecticide applications in cotton fields and cotton flowering date. Consistent associations observed across periods and years involved abundances of cotton and uncultivated habitats that were negatively associated with Lygus density, and abundance of seed alfalfa and cotton flowering date that were positively associated with Lygus density. Safflower and forage alfalfa had variable effects, possibly reflecting among-year variation in crop management practices, and tomato, sugar beet and insecticide applications were rarely associated with Lygus density. Using data from the first two years, a multiple regression model including the four consistent factors successfully predicted Lygus density across cotton fields in the last year of the study.

CONCLUSIONS/SIGNIFICANCE

Our results show that the approach developed here is appropriate to characterize and test the source and sink effects of various habitats on pest dynamics and improve the design of landscape-level pest management strategies.

Fitness cost of resistance to Bt cotton linked with increased gossypol content in pink bollworm larvae

Fitness costs of resistance to Bacillus thuringiensis (Bt) crops occur in the absence of Bt toxins, when individuals with resistance alleles are less fit than individuals without resistance alleles. As costs of Bt resistance are common, refuges of non-Bt host plants can delay resistance not only by providing susceptible individuals to mate with resistant individuals, but also by selecting against resistance. Because costs typically vary across host plants, refuges with host plants that magnify costs or make them less recessive could enhance resistance management. Limited understanding of the physiological mechanisms causing fitness costs, however, hampers attempts to increase costs. In several major cotton pests including pink bollworm (Pectinophora gossypiella), resistance to Cry1Ac cotton is associated with mutations altering cadherin proteins that bind this toxin in susceptible larvae. Here we report that the concentration of gossypol, a cotton defensive chemical, was higher in pink bollworm larvae with cadherin resistance alleles than in larvae lacking such alleles. Adding gossypol to the larval diet decreased larval weight and survival, and increased the fitness cost affecting larval growth, but not survival. Across cadherin genotypes, the cost affecting larval growth increased as the gossypol concentration of larvae increased. These results suggest that increased accumulation of plant defensive chemicals may contribute to fitness costs associated with resistance to Bt toxins.

Human and environmental factors affecting Aedes aegypti distribution in an arid urban environment

Aedes aegypti has reappeared in urban communities in the southwestern U.S.A. in the 1990s after a 40-year absence. In 2003 and 2004, a systematic survey was conducted throughout metropolitan Tucson, AZ, to identify human and environmental factors associated with Ae. aegypti distribution within an arid urban area. Aedes aegypti presence and abundance were measured monthly using the Centers for Disease Control and Prevention enhanced oviposition traps at sampling sites established in a grid at 3- to 4-km intervals across the city. Sampling occurred in the summer rainy season (July through September), the peak of mosquito activity in the region. Multiple regression analyses were conducted to determine relationships between mosquito density and factors that could influence mosquito distribution. House age was the only factor that showed a consistent significant association with Ae. aegypti abundance in both years: older houses had more mosquito eggs. This is the 1st study of Ae. aegypti distribution at a local level to identify house age as an explanatory factor independent of other human demographic factors. Further research into the reasons why mosquitoes were more abundant around older homes may help inform and refine future vector surveillance and control efforts in the event of a dengue outbreak in the region.

Pollen- and seed-mediated transgene flow in commercial cotton seed production fields

Background

Characterizing the spatial patterns of gene flow from transgenic crops is challenging, making it difficult to design containment strategies for markets that regulate the adventitious presence of transgenes. Insecticidal Bacillus thuringiensis (Bt) cotton is planted on millions of hectares annually and is a potential source of transgene flow.

Methodology/Principal Findings

Here we monitored 15 non-Bt cotton (Gossypium hirsutum, L.) seed production fields (some transgenic for herbicide resistance, some not) for gene flow of the Bt cotton cry1Ac transgene. We investigated seed-mediated gene flow, which yields adventitious Bt cotton plants, and pollen-mediated gene flow, which generates outcrossed seeds. A spatially-explicit statistical analysis was used to quantify the effects of nearby Bt and non-Bt cotton fields at various spatial scales, along with the effects of pollinator abundance and adventitious Bt plants in fields, on pollen-mediated gene flow. Adventitious Bt cotton plants, resulting from seed bags and planting error, comprised over 15% of plants sampled from the edges of three seed production fields. In contrast, pollen-mediated gene flow affected less than 1% of the seed sampled from field edges. Variation in outcrossing was better explained by the area of Bt cotton fields within 750 m of the seed production fields than by the area of Bt cotton within larger or smaller spatial scales. Variation in outcrossing was also positively associated with the abundance of honey bees.

Conclusions/Significance

A comparison of statistical methods showed that our spatially-explicit analysis was more powerful for understanding the effects of surrounding fields than customary models based on distance. Given the low rates of pollen-mediated gene flow observed in this study, we conclude that careful planting and screening of seeds could be more important than field spacing for limiting gene flow.

Cadherin gene expression and effects of Bt resistance on sperm transfer in pink bollworm

Cadherin proteins bind Bacillus thuringiensis (Bt) toxins in lepidopteran midguts but their inherent function remains unclear. In pink bollworm, Pectinophora gossypiella, three recessive mutations in a cadherin gene (BtR) are tightly linked with resistance to Bt toxin Cry1Ac. Here we examined patterns of transcription of this gene and the association between cadherin genotype and sperm transfer in pink bollworm. Cadherin RNA was most abundant in larvae, but was also found in adults and embryos. In fourth instar larvae, cadherin RNA was most abundant in the gut, yet its presence in the testes indicates a potential role in sperm production. Previously, we found reduced first-male paternity in pink bollworm males homozygous for cadherin mutations conferring resistance to Bt, when a resistant and susceptible male competed for access to a female. However, the number of offspring sired by resistant and susceptible males was similar without competition. Male Lepidoptera produce both fertile eupyrene sperm and anucleate, non-fertile apyrene sperm, suggesting that apyrene sperm may contribute to male reproductive success when sperm competition occurs. Accordingly, we hypothesized that cadherin-based resistance to Bt entails fitness costs that reduce apyrene sperm transfer. To test this hypothesis, we compared apyrene and eupyrene sperm transfer in males from four strains of pink bollworm. Transfer of apyrene and eupyrene sperm was lower in homozygous resistant than in susceptible males. Furthermore, homozygous resistant males weighed less than susceptible males, which could have diminished sperm transfer by resistant males directly, or via a positive association between male weight, spermatophore weight and sperm transfer. While data suggest that cadherin mutations induced a recessive fitness cost affecting apyrene sperm transfer, these mutations also generated recessive costs that affected other traits and could have lowered first-male paternity.

Lack of fitness costs associated with pyriproxyfen resistance in the B biotype of Bemisia tabaci

BACKGROUND

The insect growth regulator pyriproxyfen has provided effective control of the whitefly Bemisia tabaci Gennadius in many countries. Here, whether or not fitness costs were associated with pyriproxyfen resistance in a laboratory-selected resistant strain (QC02-R) of the B biotype was determined.

RESULTS

Mortality caused by pyriproxyfen and fitness traits over time were measured in unselected and selected hybrid strains, which were created by crossing individuals of the resistant strain with individuals of a susceptible strain. Fitness costs were not associated with resistance in QC02-R, as mortality caused by pyriproxyfen did not increase over time in unselected hybrid strains and fitness traits were similar in unselected and selected hybrid strains. Using a new method to examine the inheritance of resistance, based on data from fitness cost experiments, it was estimated that pyriproxyfen resistance is controlled by two loci in the QC02-R strain.

CONCLUSION

The lack of fitness costs associated with pyriproxyfen resistance could promote the evolution of resistance in field populations with similar traits to QC02-R.

Inheritance of resistance to pyriproxyfen in Bemisia tabaci (Hemiptera: Aleyrodidae) males and females (B biotype)

We evaluated effects of the insect growth regulator pyriproxyfen on Bemisia tabaci (Gennadius) (B biotype) (Hemiptera: Aleyrodidae) males and females in laboratory bioassays. Insects were treated with pyriproxyfen as either eggs or nymphs. In all tests, the LC50 for a laboratory-selected resistant strain was at least 620 times greater than for an unselected susceptible strain. When insects were treated as eggs, survival did not differ between males and females of either strain. When insects were treated as nymphs, survival did not differ between susceptible males and susceptible females, but resistant males had higher mortality than resistant females. The dominance of resistance decreased as pyriproxyfen concentration increased. Resistance was partially or completely dominant at the lowest concentration tested and completely recessive at the highest concentration tested. Hybrid female progeny from reciprocal crosses between the susceptible and resistant strains responded alike in bioassays; thus, maternal effects were not evident. Rapid evolution of resistance to pyriproxyfen could occur if individuals in field populations had resistance with traits similar to those of the laboratory-selected strain examined here.

Effects of refuge contamination by transgenes on Bt resistance in pink bollworm (Lepidoptera: Gelechiidae)

Refuges of non-Bacillus thuringiensis (Bt) cotton, Gossypium hirsutum L., are used to delay Bt resistance in pink bollworm, Pectinophora gossypiella (Saunders) (Lepidoptera: Gelechiidae), a pest that eats cotton seeds. Contamination of refuges by Bt transgenes could reduce the efficacy of this strategy. Previously, three types of contamination were identified in refuges: 1) homozygous Bt cotton plants, with 100% of their seeds producing the Bt toxin Cry1Ac; 2) hemizygous Bt plants with 70-80% of their seeds producing Cry1Ac; and 3) non-Bt plants that outcrossed with Bt plants, resulting in bolls with Cry1Ac in 12-17% of their seeds. Here, we used laboratory bioassays to examine the effects of Bt contamination on feeding behavior and survival of pink bollworm that were resistant (rr), susceptible (ss), or heterozygous for resistance (rs) to Cry1Ac. In choice tests, rr and rs larvae did not differ from ss in preference for non-Bt versus Bt seeds. Survival of rr and rs also did not differ from ss on artificial outcrossed bolls (a mixture of 20% Bt and 80% non-Bt cotton seeds). On artificial hemizygous Bt bolls (70% Bt seeds) and homozygous Bt bolls (100% Bt seeds), rr had higher survival than ss, although rs and ss did not differ. In a simulation model, levels of refuge contamination observed in the field had negligible effects on resistance evolution in pink bollworm. However, in hypothetical simulations where contamination conferred a selective advantage to rs over ss individuals in refuges, resistance evolution was accelerated.

Field evaluation of resistance to pyriproxyfen in Bemisia tabaci (B biotype)

We determined effects of aerial sprays of the insect growth regulator pyriproxyfen on sweetpotato whitefly, Bemisia tabaci (Gennadius) (B biotype), in Arizona cotton (Gossypium spp.) fields. We measured survival for males and females from a susceptible strain and a laboratory-selected resistant strain, as well as for hybrid female progeny from crosses between the strains. Insects were exposed directly to pyriproxyfen sprays in the field or indirectly in the laboratory by rearing them on sprayed leaves collected from the field. In all tests, survival was higher for the resistant strain than the susceptible strain, but did not differ between sexes in each strain. Survival to the adult stage did not differ between eggs and nymphs directly exposed to sprays. For susceptible and hybrid individuals, survival was lower on leaves collected the day of spraying than on leaves collected 2 wk after spraying. In contrast, survival of resistant individuals did not differ based on the timing of exposure. Dominance of resistance to pyriproxyfen depended on the type of exposure. Resistance was partially or completely dominant in direct exposure bioassays and on leaves collected 2 wk after spraying (h > 0.6). Resistance was partially recessive on leaves collected the day of spraying (mean h = 0.34). Rapid evolution of resistance to pyriproxyfen could occur if individuals in field populations with traits similar to those of the laboratory-selected strain examined here were treated intensively with this insecticide.

Effects of resistance to Bt cotton on diapause in the pink bollworm, Pectinophora gossypiella

Fitness costs associated with resistance to Bacillus thuringiensis (Bt) crops are expected to delay the evolution of resistance. In a previous study where pink bollworm, Pectinophora gossypiella (Lepidoptera: Gelechiidae), larvae overwintered in outdoor insectaries, individuals from Bt-resistant strains had lower survival than individuals from Bt-susceptible strains or F1 progeny from crosses between resistant and susceptible adults. To investigate the physiological basis of such recessive cost, diapause duration was experimentally manipulated in the laboratory. Compared to a Bt-susceptible strain and F1 progeny, we hypothesized that Bt-resistant strains could exhibit a lower propensity or intensity of diapause, faster weight loss during overwintering, lower initial weight of diapausing larvae, and reduced longevity of moths emerging from diapause. Results were as expected for initial weight of diapausing larvae and longevity of overwintered male moths or female moths remaining in diapause for a short period. However, a higher diapause induction and intensity and slower weight loss occurred in F1 progeny and Bt-resistant strains than in a Bt-susceptible strain. Moreover, F1 progeny had greater overwintering survival than the Bt-resistant and Bt-susceptible strains, and F1 female moths had the greatest longevity after sustaining long diapausing periods. All of these unexpected results may be explained by pleiotropic effects of resistance to Bt cotton that increased the strength of diapause in the F1 progeny and Bt-resistant strains. Incomplete resistance was reflected in disadvantages suffered by Bt-resistant individuals feeding on a Bt diet instead of a non-Bt diet, including lower diapause propensity, lower diapause intensity and reduced longevity of overwintered male moths. While this study suggests that the evolution of resistance to Bt cotton and feeding on a Bt diet in Bt-resistant individuals have pervasive effects on several traits associated with diapause, further field experiments are needed to elucidate the basis of the overwintering cost in the pink bollworm.

Cadherin-based resistance to Bacillus thuringiensis cotton in hybrid strains of pink bollworm: fitness costs and incomplete resistance

Recessive resistance to Bacillus thuringiensis (Bt) cotton, Gossypium hirsutum L., in laboratory-selected strains of pink bollworm, Pectinophora gossypiella (Saunders), is associated with three resistance alleles (r1, r2, and r3) of a cadherin gene. Previous experiments based on measurement of fitness components in Bt-resistant and Bt-susceptible strains revealed that fitness costs and incomplete resistance are associated with resistance. Here, we used two hybrid strains of pink bollworm, each containing a mixture of susceptible and resistant individuals, and polymerase chain reaction (PCR) amplifications to test the association between cadherin genotype and fitness components for individuals sharing a common genetic background. All survivors on Bt cotton had two r alleles, confirming that recessive cadherin alleles are tightly linked with resistance to Bt cotton. On non-Bt cotton, significantly greater developmental time for rr than ss larvae indicated a recessive fitness cost, but costs did not affect survival or pupal weight. Incomplete resistance was manifested as longer developmental time, lower survival, and smaller pupal weight in rr individuals developing on Bt cotton compared with non-Bt cotton. As in previous experiments, no significant variation in performance on Bt cotton was detected among rr genotypes. However, a meta-analysis of data from seven experiments revealed that survival on Bt cotton relative to non-Bt cotton was lower in r2r3 and higher in r1r2 compared with the other rr genotypes. Assessment of fitness components associated with cadherin genotypes in hybrid strains of pink bollworm confirms that recessive resistance to Bt cotton is associated with recessive fitness costs and incomplete resistance.

Cadherin-based resistance to Bacillus thuringiensis cotton in hybrid strains of pink bollworm: fitness costs and incomplete resistance

Recessive resistance to Bacillus thuringiensis (Bt) cotton, Gossypium hirsutum L., in laboratory-selected strains of pink bollworm, Pectinophora gossypiella (Saunders), is associated with three resistance alleles (r1, r2, and r3) of a cadherin gene. Previous experiments based on measurement of fitness components in Bt-resistant and Bt-susceptible strains revealed that fitness costs and incomplete resistance are associated with resistance. Here, we used two hybrid strains of pink bollworm, each containing a mixture of susceptible and resistant individuals, and polymerase chain reaction (PCR) amplifications to test the association between cadherin genotype and fitness components for individuals sharing a common genetic background. All survivors on Bt cotton had two r alleles, confirming that recessive cadherin alleles are tightly linked with resistance to Bt cotton. On non-Bt cotton, significantly greater developmental time for rr than ss larvae indicated a recessive fitness cost, but costs did not affect survival or pupal weight. Incomplete resistance was manifested as longer developmental time, lower survival, and smaller pupal weight in rr individuals developing on Bt cotton compared with non-Bt cotton. As in previous experiments, no significant variation in performance on Bt cotton was detected among rr genotypes. However, a meta-analysis of data from seven experiments revealed that survival on Bt cotton relative to non-Bt cotton was lower in r2r3 and higher in r1r2 compared with the other rr genotypes. Assessment of fitness components associated with cadherin genotypes in hybrid strains of pink bollworm confirms that recessive resistance to Bt cotton is associated with recessive fitness costs and incomplete resistance.

Effect of resistance to Bacillus thuringiensis cotton on pink bollworm (Lepidoptera: Gelechiidae) response to sex pheromone

Fitness costs associated with resistance to transgenic crops producing toxins from Bacillus thuringiensis (Bt) could reduce male response to pheromone traps. Such costs would cause underestimation of resistance frequency if monitoring was based on analysis of males caught in pheromone traps. To develop a DNA-based resistance monitoring program for pink bollworm, Pectinophora gossypiella (Saunders) (Lepidoptera: Gelechiidae), we compared the response to pheromone traps of males with and without cadherin alleles associated with resistance to Bt cotton (Gossypium hirsutum L.). When irradiated males from two hybrid laboratory strains with an intermediate frequency of resistance alleles were released in large field cages, the probability of capture in pheromone traps was not lower for males with resistance alleles than for males without resistance alleles. These results suggest that analysis of trapped males would not underestimate the frequency of resistance. As the time males spent in traps in the field increased from 3 to 15 d, the success of DNA amplification declined from 100 to 30%. Thus, the efficiency of a DNA-based resistance monitoring program would be improved by analyzing males remaining in traps for 3 d or less.

Farm-scale evaluation of the impacts of transgenic cotton on biodiversity, pesticide use, and yield

Higher yields and reduced pesticide impacts are needed to mitigate the effects of agricultural intensification. A 2-year farm-scale evaluation of 81 commercial fields in Arizona show that use of transgenic Bacillus thuringiensis (Bt) cotton reduced insecticide use, whereas transgenic cotton with Bt protein and herbicide resistance (BtHr) did not affect herbicide use. Transgenic cotton had higher yield than nontransgenic cotton for any given number of insecticide applications. However, nontransgenic, Bt and BtHr cotton had similar yields overall, largely because higher insecticide use with nontransgenic cotton improved control of key pests. Unlike Bt and BtHr cotton, insecticides reduced the diversity of nontarget insects. Several other agronomic and ecological factors also affected biodiversity. Nevertheless, pairwise comparisons of diversity of nontarget insects in cotton fields with diversity in adjacent noncultivated sites revealed similar effects of cultivation of transgenic and nontransgenic cotton on biodiversity. The results indicate that impacts of agricultural intensification can be reduced when replacement of broad-spectrum insecticides by narrow-spectrum Bt crops does not reduce control of pests not affected by Bt crops.

Association between resistance to Bt cotton and cadherin genotype in pink bollworm

Two strains of pink bollworm, Pectinophora gossypiella (Saunders), each derived in 1997 from a different field population, were selected for resistance to Bacillus thuringiensis (Bt) toxin Cry1Ac in the laboratory. One strain (MOV97-R) originated from Mohave Valley in western Arizona; the other strain (SAF97-R) was from Safford in eastern Arizona. Relative to a susceptible laboratory strain, Cry1Ac resistance ratios were 1700 for MOV97-R and 520 for SAF97-R. For the two resistant strains, larval survival did not differ between non-Bt cotton and transgenic cotton producing CrylAc. In contrast, larval survival on Bt cotton was 0% for the two unselected parent strains from which the resistant strains were derived. Previously identified resistance (r) alleles of a cadherin gene (BtR) occurred in both resistant strains: r1 and r3 in MOV97-R, and r1 and r2 in SAF97-R. The frequency of individuals carrying two r alleles (rr) was 1.0 in the two resistant strains and 0.02 in each of the two unselected parent strains. Furthermore, in two hybrid strains with a mixture of susceptible (s) and r alleles at the BtR locus, all survivors on Bt cotton had two r alleles. The results show that resistance to Cry1Ac-producing Bt cotton is associated with recessive r alleles at the BtR locus in the strains of pink bollworm tested here. In conjunction with previous results from two other Bt-resistant strains of pink bollworm (APHIS-98R and AZP-R), results reported here identify the cadherin locus as the leading candidate for molecular monitoring of pink bollworm resistance to Bt cotton.

Effects of cotton cultivar on fitness costs associated with resistance of pink bollworm (Lepidoptera: Gelechiidae) to Bt cotton

Fitness costs associated with insect resistance to transgenic crops producing toxins from Bacillus thuringiensis (Bt) reduce the fitness on non-Bt refuge plants of resistant individuals relative to susceptible individuals. Because costs may vary among host plants, choosing refuge cultivars that increase the dominance or magnitude of costs could help to delay resistance. Specifically, cultivars with high concentrations of toxic phytochemicals could magnify costs. To test this hypothesis, we compared life history traits of three independent sets of pink bollworm, Pectinophora gossypiella (Saunders), populations on two cotton cultivars that differed in antibiosis against this cotton pest. Each set had an unselected susceptible population, a resistant population derived by selection from the susceptible population, and the F1 progeny of the susceptible and resistant populations. Confirming previous findings with pink bollworm feeding on cotton, costs primarily affected survival and were recessive on both cultivars. The magnitude of the survival cost did not differ between cultivars. Although the experimental results did not reveal differences between cultivars in the magnitude or dominance of costs, modeling results suggest that differences between cultivars in pink bollworm survival could affect resistance evolution. Thus, knowledge of the interaction between host plants and fitness costs associated with resistance to Bt crops could be helpful in guiding the choice of refuge cultivars.

Long-term evaluation of compliance with refuge requirements for Bt cotton

The success of the refuge strategy for delaying pest resistance to Bt crops depends on compliance by farmers. However, the accuracy of previous estimates of compliance has been questioned. We have applied a novel approach based on the use of Geographical Information System (GIS) technology to measure compliance with refuge requirements for Bt cotton in six Arizona regions from 1998 to 2003. Although compliance varied among regions, overall compliance was above 88% in five of six years. With the cooperation of farmers, our approach allows precise and economical assessment of compliance with the refuge strategy.

Effects of gossypol on fitness costs associated with resistance to Bt cotton in pink bollworm

Fitness costs associated with insect resistance to Bacillus thuringiensis (Bt) crops may help to delay or prevent the spread of resistance alleles, especially when refuges of non-Bt host plants are present. The potential for such delays increases as the magnitude and dominance of fitness costs increase. Here, we examined the idea that plant secondary chemicals affect expression of fitness costs associated with resistance to Bt cotton in Pectinophora gossypiella (Saunders). Specifically, we tested the hypotheses that gossypol affects the magnitude or dominance of fitness costs, by measuring performance of three independent sets of pink bollworm populations fed artificial diet with and without gossypol. Each set had an unselected susceptible population, a resistant population derived by selection from the susceptible population, and the F1 progeny of the susceptible and resistant populations. No individuals completed development on diets with gossypol in one set, suggesting that these individuals partially lost the ability to detoxify this chemical. In the other two sets, costs affecting survival did not support the hypotheses, but costs affecting pupal weight did. Adding gossypol to diet increased the magnitude and dominance of costs affecting pupal weight. In one of the two sets with survivors on diet with gossypol, costs affecting development time were less recessive when gossypol was present in diet. These results indicate that gossypol increased the magnitude and dominance of some fitness costs. Better understanding of the effects of natural plant defenses on fitness costs could improve our ability to design refuges for managing insect resistance to Bt crops.

Effects of insect population size on evolution of resistance to transgenic crops

Models of the evolution of insect resistance to transgenic crops have often assumed that population size is infinite or that carrying capacity is fixed. To evaluate potential effects of population size on resistance evolution, we conducted sensitivity analyses by using a stochastic, spatially explicit model based partly on the interaction between pink bollworm and Bacillus thuringiensis (Bt) cotton. We examined interactions of carrying capacity, region size, dispersal, and percentage of fields planted with Bt cotton. The median and variance in the time to resistance decreased as region size increased, regardless of carrying capacity. This occurred because larger regions were more likely to have at least one field in which resistance evolved rapidly and served as a source from which resistance spread throughout the region. Carrying capacity significantly affected the median time to resistance with 75% of fields planted with Bt cotton, but not with 50% Bt cotton. In contrast, carrying capacity significantly influenced the variance in the time to resistance with 50% Bt cotton, but not with 75% Bt cotton. We also found resistance evolution was affected by interactions between carrying capacity, dispersal, and the percentage of fields planted with Bt cotton. The high variability observed in our simulations indicates that factors affecting stochastic events can play an important role in the evolution of resistance. Because population size determines the extent to which stochastic events are important, reasonable estimates of population size are essential for devising robust models of resistance evolution.

Three cadherin alleles associated with resistance to Bacillus thuringiensis in pink bollworm

Evolution of resistance by pests is the main threat to long-term insect control by transgenic crops that produce Bacillus thuringiensis (Bt) toxins. Because inheritance of resistance to the Bt toxins in transgenic crops is typically recessive, DNA-based screening for resistance alleles in heterozygotes is potentially much more efficient than detection of resistant homozygotes with bioassays. Such screening, however, requires knowledge of the resistance alleles in field populations of pests that are associated with survival on Bt crops. Here we report that field populations of pink bollworm (Pectinophora gossypiella), a major cotton pest, harbored three mutant alleles of a cadherin-encoding gene linked with resistance to Bt toxin Cry1Ac and survival on transgenic Bt cotton. Each of the three resistance alleles has a deletion expected to eliminate at least eight amino acids upstream of the putative toxin-binding region of the cadherin protein. Larvae with two resistance alleles in any combination were resistant, whereas those with one or none were susceptible to Cry1Ac. Together with previous evidence, the results reported here identify the cadherin gene as a leading target for DNA-based screening of resistance to Bt crops in lepidopteran pests.

Long-term regional suppression of pink bollworm by Bacillus thuringiensis cotton

Despite the potentially profound impact of genetically modified crops on agriculture and the environment, we know little about their long-term effects. Transgenic crops that produce toxins from Bacillus thuringiensis (Bt) to control insects are grown widely, but rapid evolution of resistance by pests could nullify their benefits. Here, we present theoretical analyses showing that long-term suppression of pest populations is governed by interactions among reproductive rate, dispersal propensity, and regional abundance of a Bt crop. Supporting this theory, a 10-year study in 15 regions across Arizona shows that Bt cotton suppressed a major pest, pink bollworm (Pectinophora gossypiella), independent of demographic effects of weather and variation among regions. Pink bollworm population density declined only in regions where Bt cotton was abundant. Such long-term suppression has not been observed with insecticide sprays, showing that transgenic crops open new avenues for pest control. The debate about putative benefits of Bt crops has focused primarily on short-term decreases in insecticide use. The present findings suggest that long-term regional pest suppression after deployment of Bt crops may also contribute to reducing the need for insecticide sprays.

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.

Predicting spring moth emergence in the pink bollworm (Lepidoptera: Gelechiidae): implications for managing resistance to transgenic cotton

Cultural control methods have been central in the southwestern United States for reducing pink bollworm, Pectinophora gossypiella (Saunders), damage to cotton. Nevertheless, it is not clear at present how such methods could be integrated within the novel pest management framework allowed by introduction of cotton producing a toxin from Bacillus thuringiensis (Bt) for pink bollworm control. Using statewide pheromone trapping and climatic data in conjunction with deterministic simulation models, we investigated whether manipulation of cotton planting date and use of other cultural control methods could represent valuable tactics for control of the pink bollworm in Arizona. Accumulation of heat units from one January accurately predicted the rate of pink bollworm emergence from diapause in 15 cotton-producing regions. Significant variation in rate of emergence from diapause was present among regions, with earlier emergence at higher altitudes. Most adults emerge from diapause too early to reproduce successfully on cotton, a phenomenon known as suicidal emergence. A method for prediction of the fraction of suicidal emergence resulting from adoption of a given cotton planting date is presented. Results from simulation models suggest that manipulation of planting date and implementation of other control cultural methods reduce the rate of application of insecticides and delay the evolution of resistance to Bt cotton in the pink bollworm.

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.