Refuges in reverse: the spread of Bacillus thuringiensis resistance to unselected greenhouse populations of cabbage loopers Trichoplusia ni

1 The dispersal of susceptible insects between refuges and Bacillus thuringiensis (Bt) treated fields is the key to resistance management of Bt crops. Here we describe the opposite situation; the movement of Bt resistant Trichoplusia ni moths from over‐wintered, greenhouse populations in British Columbia (BC) exposed to high Bt use to neighbouring greenhouses where Bt sprays have not been used. 2 The spread of Bt resistance to non‐selected populations of T. ni, and the resulting increase in resistance, indicates a surprising level of dispersal of resistant moths among greenhouses even in the face of fitness costs. 3 Field populations of T. ni in BC are seasonal migrants from regions of California where Bt cotton is grown. In 2006, field populations surveyed along the migration path from California through Oregon were highly susceptible to Bt insecticides and, thus, showed no indication of selection for resistance among these source populations. 4 The arrival of the immigrant moths provides a potential source of susceptible individuals to dilute the levels of resistance in greenhouse populations in BC later in the summer, but this has not occurred. Thus, field populations in BC do not appear to serve as refuges to combat Bt resistance in greenhouse populations.     

The cost of resistance to Bacillus thuringiensis varies with the host plant of Trichoplusia ni

Selection for resistance to insecticides, diseases and parasitoids is assumed to be costly and often requires tradeoffs with reproductive fitness. The costs of resistance, however, are often difficult to measure. Cabbage looper, Trichoplusia ni, a generalist Lepidopteran herbivore, has become highly resistant following the extensive use of the microbial insecticide, Bacillus thuringiensis kurstaki (Bt) in vegetable greenhouses. We compared the growth rate, pupal size and survival of resistant, susceptible and hybrid T. ni larvae fed on tomato, bell pepper and cucumber. Performance was best on cucumber and worst on pepper, and the magnitude of fitness costs associated with Bt resistance increased with declining host plant suitability. This supports the hypothesis that in this system, resistance costs are condition dependent and are greatest in the most stressful environment. Management strategies that rely on the presence of fitness costs to reduce the frequency of resistance genes must consider this variation and should be more successful on crops that are less suitable food plants. In general, condition dependence should be considered in studies designed to measure the costs of resistance.     

The influences of host plant and genetic resistance to Bacillus thuringiensis on trade-offs between offspring number and growth rate in cabbage loopers, Trichoplusia ni

Abstract.  1. Variation in progeny size and quality is common among insects and this variation can strongly influence individual fitness. Larger progeny typically survive better and develop faster under adverse conditions and may have higher fecundity. Due to resource limitations, however, trade-offs may arise between having fewer large offspring or more smaller ones.
2. For cabbage loopers, Trichoplusia ni , pepper leaves are a poorer larval host than cucumber or tomato leaves as indicated by survival, development rate, and body size. Moths reared on cucumber produced more slower growing offspring than those that had been reared on pepper, which produced fewer, faster growing progeny.
3. Traits conferring resistance to Bacillus thuringiensis ( Bt ) generally are associated with strong deleterious effects that may influence resource allocation and reproductive trade-offs between progeny size and number.
4. Unlike the host-plant related trade-off between progeny size and fecundity observed among susceptible control moths, Bt -resistant parents had both the lowest fecundity and smallest progeny size on all host plants. This finding suggests that the progeny size–number relationship is constrained in resistant individuals.     

Fitness consequences of malathion-specific resistance in red flour beetle (Coleoptera: Tenebrionidae) and selection for resistance in the absence of malathion

Malathion resistance in the red flour beetle, Tribolium castaneum (Herbst), is a worldwide problem and is very stable once it becomes widespread in natural populations. In the absence of insecticide the proportion of resistant phenotypes may rapidly decline but the development of resistance does not always involve reduced fitness. Malathion-specific resistance in T. castaneum seems not to involve any loss of fitness in laboratory or field conditions. Susceptible beetles were in competition with resistant beetles at different initial frequencies and modifications of susceptible gene frequency were estimated in these laboratory populations over 10 generations. A significant decrease in susceptible gene frequency was observed in Tribolium populations over time. The selection coefficient of the susceptible allele was estimated and the fitness of susceptible alleles in all tests was observed to range from 0.89 to 0.93 compared with the fitness of resistant genotypes, which was assumed to be 1. Data provided evidence that the resistant strains exhibited fitness advantages in the absence of malathion. We also compared the biotic potential (fecundity and developmental time) of the susceptible strain, the homozygous malathion-specific resistant strain, and their hybrids. Malathion-specific resistant strains showed an 8 -23% increase in biotic potential relative to the susceptible strain. These findings are consistent with those of malathion-specific resistance in T. castaneum; the fitness of the insects seems independent of the genetic background and the fitness of the resistant insects is not affected by this resistance mechanism.     

Increased fecundity of malathion-specific resistant beetles in absence of insecticide pressure

Despite that resistance frequency is assumed to decline when selective pressure is relaxed, the stability of resistance frequency has been observed in some insects in the absence of insecticide. In the red flour beetle, Tribolium castaneum, the first case of malathion-resistance was reported in the early 1960s. The malathion-specific resistant phenotype has now almost completely replaced the susceptible one in red flour beetle populations. In the present study, several life-history traits that could influence the fitness of the insects were compared between insecticide-susceptible and malathion-specific resistant populations of the red flour beetle. On average, egg fertility and egg-to-adult development time did not differ between susceptible and resistant populations. However, the fecundity of resistant females was greater than that of susceptible ones. Generally, differences in development time between insecticide resistant and susceptible populations are considered as having more effect on fitness than do differences in fecundity. However, the observed increased female fecundity may participate, in combination with the previously observed increased male reproductive success, to the development and the stability of malathion-specific resistance in T. castaneum.     

Fitness costs of resistance to Bacillus thuringiensis in the Indianmeal moth, Plodia interpunctella

Genetic changes in insects that result in insecticide resistance can also affect their fitness. Here, we report measurements of development time and survival of the Indianmeal moth, Plodia interpunctella (Hübner), to compare the relative fitness of Bacillus thuringiensis (Bt)-susceptible and -resistant colonies. Measurements of larval development time and survival indicated that a fitness cost was associated with resistance to Bt in some Bt-resistant colonies but not others. Comparisons of geographically different populations revealed inherent differences in development time and survival. In most cases, Bt-resistant moths suffered no disadvantage when feeding on a Bt-treated diet. In many cases, the development of Bt-resistant moths on Bt-treated diet was slower than the unselected moths on untreated diet, but it is unclear whether these differences would affect the successful mating of susceptible and resistant moths.     

Relative fitness of Cry1A-resistant and -susceptible Helicoverpa armigera (Lepidoptera: Noctuidae) on conventional and transgenic cotton

Glasshouse and laboratory experiments were conducted to evaluate the relative fitness of Cry1A-susceptible and laboratory-selected resistant strains of Helicoverpa armigera (Hübner). Life history parameters of H. armigera larvae feeding on young cotton plants showed a significant developmental delay of up to 7 d for the resistant strain compared with the susceptible strain on non-Bacillus thuringiensis (Bt) cotton. This fitness cost was not evident on artificial diet. There was no developmental delay in the F1 hybrid progeny from the reciprocal backcross of the resistant and susceptible strains, indicating that the fitness cost is recessive. In two cohorts tested, survival to pupation of resistant larvae on Bt cotton expressing Cry1Ac was 54 and 51% lower than on non-Bt cotton, whereas all susceptible and F1 larvae tested on Cry1Ac cotton were killed. Mortality of susceptible larvae occurred in the first or second instar, whereas the F1 larvae were able to develop to later instars before dying, demonstrating that resistance is incompletely recessive. The intrinsic rate of increase was reduced by >50% in the resistant strain on Cry1Ac cotton compared with the susceptible strain on non-Bt cotton. There was a significant reduction in the survival of postdiapausal adults from the resistant strain and the F1 strains, indicating that there is a nonrecessive overwintering cost associated with Cry1A resistance in H. armigera.     

Host-plant effects the expression of resistance to Bacillus thuringiensis kurstaki in Trichoplusia ni (Hubner): an important factor in resistance evolution

Pathogens are thought to exert strong selection on their hosts leading to increased host resistance. Bacillus thuringiensis kurstaki (Bkt) is a ubiquitous entomopathogen that has become the mainstay of nonchemical control of Lepidopteran pests and thus, the potential exists for the evolution of resistance in targeted host insects. We have studied the expression of Btk resistance in the cabbage looper, Trichoplusia ni (Hubner). For this generalist insect herbivore, three common host plants, tomato, pepper and cucumber, vary in suitability for larval growth and development. Here we show that the host plant also affects the overall toxicity of Btk, the relative expression of resistance between a resistant and a susceptible line and their F(1) reciprocal crosses, and importantly, the dominance of the resistance trait. This study demonstrates that tri-trophic interactions involving an insect, host plants and a pathogen have the potential to strongly influence the evolutionary response of an insect host to a pathogen.     

Genetic Resistance to Bacillus thuringiensis Alters Feeding Behaviour in the Cabbage Looper,Trichoplusia ni

Evolved resistance to xenobiotics and parasites is often associated with fitness costs when the selection pressure is absent. Resistance to the widely used microbial insecticide Bacillus thuringiensis (Bt) has evolved in several insect species through the modification of insect midgut binding sites for Bt toxins, and reports of costs associated with Bt resistance are common. Studies on the costs of Bt-resistance restrict the insect to a single artificial diet or host-plant. However, it is well documented that insects can self-select appropriate proportions of multiple nutritionally unbalanced foods to optimize life-history traits. Therefore, we examined whether Bt-resistant and susceptible cabbage loopers Trichoplusia ni differed in their nutrient intake and fitness costs when they were allowed to compose their own protein:carbohydrate diet. We found that Bt-resistant T. ni composed a higher ratio of protein to carbohydrate than susceptible T. ni. Bt-resistant males exhibited no fitness cost, while the fitness cost (reduced pupal weight) was present in resistant females. The absence of the fitness cost in resistant males was associated with increased carbohydrate consumption compared to females. We demonstrate a sex difference in a fitness cost and a new behavioural outcome associated with Bt resistance.     

Susceptibility of Tuta absoluta (Meyrick) (Lep., Gelechiidae) reared on two species of Lycopersicon to Bacillus thuringiensis var. kurstaki

The mortality of Tuta absoluta (Meyrick) larvae that were fed on leaves of Lycopersicon hirsutum f. glabratum (PI 134417, insect resistant) and Lycopersicon esculentum (cultivar Santa Clara, susceptible) treated with Bacillus thuringiensis var. kurstaki (Btk) was evaluated. Feeding on untreated PI 134417 was detrimental to the survival of T. absoluta larvae. When Btk was applied to the two Lycopersicon plants, mortality occurred in all T. absoluta instars. Application of Btk on tomato leaves had synergistic or additive effects with the resistant genotype on larval survival. This effect was dependent on the instar at which the larvae were fed Btk-treated leaves. Delayed Btk application may cause higher insect mortality if the insects become more susceptible to the pathogen after a longer period of feeding on the resistant crop.     

褐飞虱抗吡虫啉品系生物适合度研究

毒力测定结果显示 ,虽然褐飞虱NilaparvatalugensSt l田间品系对吡虫啉还没有表现出明显的抗性 ,但室内筛选品系已经出现了一定水平的抗性 ,抗性品系 1 (R1)和抗性品系 2 (R2 )的抗性倍数分别为 2 5 3 8和 68 92。通过建立抗性品系、田间种群和敏感品系的生命表 ,发现抗性品系适合度显著下降 ,而且R2 品系下降的幅度明显大于R1品系。存在显著变化的因素有低龄若虫存活率、成虫羽化率、交配率、产卵量和孵化率下降 ,雌虫寿命缩短 ;R2 品系还表现为卵历期延长 ,3龄若虫至 5龄若虫存活率下降。R2 品系产卵高峰期迟 ,高峰期日虫量显著低于敏感品系和田间种群。用种群数量趋势指数 (I)来确定抗性品系的相对生物适合度 ,发现与敏感性品系相比 ,R1品系和R2 品系的相对适合度分别为0 60 9和 0 2 45。     

Inheritance of resistance to Bacillus thuringiensis Cry1Ac toxin in a greenhouse-derived strain of cabbage looper (Lepidoptera: Noctuidae)

A population of cabbage looper, Trichoplusia ni (Hübner), collected from commercial greenhouses in the lower mainland of British Columbia, Canada, in 2001 showed a resistance level of 24-fold to Dipel, a product of Bacillus thuringiensis (Bt) subspecies kurstaki. This population was selected with Cry1Ac, the major Bt Cry toxin in Dipel, to obtain a homogenous population resistant to Cry1Ac. The resulting strain of T. ni, named GLEN-Cry1Ac, was highly resistant to Cry1Ac with a resistance ratio of approximately 1000-fold. The larvae from the GLEN-Cry1Ac strain could survive on Cry1Ac-expressing transgenic broccoli plants that were highly insecticidal to T. ni and diamondback moth, Plutella xylostella (L.). The inheritance of Cry1Ac resistance in this T. ni strain was autosomal and incompletely recessive. The degree of dominance of the resistance was -0.402 and -0.395, respectively, for the neonates in reciprocal crosses between the GLEN-Cry1Ac and a laboratory strain of T. ni. Using chi2 goodness-of-fit test, we demonstrated that the inhibition of larval growth resulting from testing 12 toxin doses in the progeny of the backcross fit the predicted larval responses based on a monogenic inheritance model. Therefore, we conclude that the inheritance of the resistance to Cry1Ac in the T. ni larvae is monogenic.     

高温对抗性和敏感小菜蛾生命表参数及后代杀虫剂敏感性的影响

为研究高温对抗性和敏感小菜蛾Plutella xylostella生物学适合度及抗药性进化的影响,本研究测定了温度对杀虫剂抗性和敏感小菜蛾实验种群生命表参数及后代对阿维菌素敏感性的影响。 结果表明：与25℃相比,33.5℃条件下抗性和敏感种群的平均世代历期（D）、净生殖率（R₀）、内禀增长率（r_m）、周限增长率（λ）值、总产卵量、雌蛾平均寿命、日产卵量和孵化率均显著下降。 在25℃下,抗性和敏感种群的r_m,λ,D和日产卵量无显著差异,但抗性种群的R₀略低于敏感种群（为敏感种群的87%）。 在33.5℃下,抗性种群的r_m和R₀显著低于敏感种群（分别为敏感种群的75%和64%）,抗性种群雌成虫寿命、总产卵量和孵化率显著也低于敏感种群。 抗性种群在33.5℃下饲养一代后,后代对阿维菌素的抗性水平大幅下降,而相同处理下敏感种群对阿维菌素的抗性水平没有显著下降。 上述结果表明,无论在常温还是高温条件下,抗性种群生物学适合度均低于敏感种群,尤其是高温下抗性种群适合度远低于敏感种群。 抗性种群经高温饲养一代后对阿维菌素的抗性水平大幅下降可能与高温下抗性种群适合度显著较低有关。     

Laboratory selection for resistance to sulfoxaflor and fitness costs in the green peach aphid Myzus persicae

Sulfoxaflor is a newly released fourth-generation neonicotinoid insecticide for management of sap-feeding pests that have developed resistance to established insecticide groups. The risk of resistance developing to this pesticide in target pests is unclear. We selected a strain of the green peach aphid, Myzus persicae (Sulzer) (Hemiptera: Aphididae), for resistance to sulfoxaflor in the laboratory, which showed 199-fold resistance after 45 generations compared to the starting population. Life table analysis showed that the resistant strain had a fitness of 0.83 compared to the susceptible strain. Adult longevity of the resistant strain was reduced by 9.55% compared to the susceptible strain. The period when adults of the resistant strain produced offspring was reduced by 17.19%, while the mean fecundity of the resistant strain was reduced by 15%. These findings suggest that M. persicae can develop a high level of resistance to sulfoxaflor, but fitness costs may result in a recovery of sensitivity when field populations are no longer exposed to sulfoxaflor.     

Effects of temperature on fitness costs in chlorpyrifos‐resistant brown planthopper,Nilaparvata lugens (Hemiptera: Delphacidae)

Insecticide resistance is inevitable if an insecticide is widely used to control insect pests. Fortunately, the resistance‐associated fitness costs often give chances to manage resistances. In most cases, the fitness cost in resistant insects is often evaluated under laboratory conditions for insect development, which limits its practical application in pest control in the field. In a laboratory population R9 with 253‐fold resistance to chlorpyrifos after nine‐generation selection with chlorpyrifos, the relative fitness was only 0.206 under laboratory conditions (25°C, humidity 70%–80% and 16 h light/8 h dark photoperiod), when compared to S9, a susceptible counterpart (resistance ratio = 2.25‐fold) from the same origin as R9 but without any selection with insecticides. Temperatures varied the resistance‐associated fitness costs, with enhanced costs at high temperatures and reduced costs at low temperatures, such as 0.174 at 32°C and 0.527 at 18°C. The copulation rate and fecundity were two key factors for the reduced costs at low temperatures. Another finding was that R9 individuals needed much more time to recover from heat shock than that of S9, but R9 and S9 individuals were similarly sensitive to cold shock. The low fitness cost at low temperatures would increase the overwintering population, which might further increase risks of rapid development and widespread distribution of chlorpyrifos resistance in Nilaparvata lugens.     

桃蚜抗吡虫啉品系和敏感品系某些生物学特性比较

室内连续筛选15代,桃蚜Myzuspersicae(Sulzer)对吡虫啉产生了一定水平的抗性,抗性倍数是14.34倍。通过建立抗性品系和敏感品系的生命表,比较了抗性品系和敏感品系的发育历期和生殖力的差异。结果表明,抗性品系适合度显著下降,若蚜存活率降低,若蚜历期延长,成蚜寿命缩短;在繁殖上表现为产仔量下降,生殖期缩短。以净生殖率(Ro)和内禀增长率(rm)来评价抗性品系的相对生物适合度,与敏感品系相比,吡虫啉抗性品系的相对适合度分别为0.699和0.897。     

Relative developmental and reproductive fitness associated with pyrethroid resistance in the major southern African malaria vector, Anopheles funestus

The effect of pyrethroid resistance on the fitness of a laboratory strain of Anopheles funestus originating from southern Mozambique was evaluated by comparing the developmental and reproductive characteristics of a pyrethroid resistant strain with an insecticide susceptible strain. Fitness was evaluated in terms of fecundity, fertility, egg production, developmental time and life stage progression and survival. Of the eggs laid by females of the resistant strain, 81.5% hatched while only 66.9% were recorded in the susceptible strain. The time from egg hatch to adult emergence was longer for the resistant strain (15.9 days) than the susceptible strain (15.2 days). A significantly higher proportion of eggs from the resistant strain (61.6%) survived to adulthood compared with those of the susceptible strain (49%). Fecundity and larval and pupal survival did not differ significantly between strains. Of spermathecae dissected from females of the resistant strain, 56.8% were fertilized compared to 52.6% from the susceptible strain. The proportion of females that successfully produced eggs was 43.3% and 23.3% for the resistant and susceptible strains respectively. Complete failure of larval hatch was recorded in 28.6% of susceptible strain families compared to 7.7% of resistant families. Our results show that pyrethroid resistance in southern African An. funestus does not incur any loss of fitness under laboratory conditions. These results suggest that the removal of pyrethroid insecticide selection pressure may not lead to a regression of resistance alleles in pyrethroid resistant An. funestus populations in southern Africa.     

Evolutionary trade-offs of insect resistance to Bacillus thuringiensis crops: fitness cost affecting paternity

Evolution of resistance to insecticides provides a useful model for examining fitness trade-offs associated with adaptation to stress. Here, we examined male reproductive costs in pink bollworm (Pectinophora gossypiella) resistant to an insecticidal protein of Bacillus thuringiensis (Bt) produced by transgenic cotton, using contrasts between two pairs of related susceptible and resistant strains. Without competition for access to females, no costs affecting reproductive success of resistant males were observed. Resistant and susceptible males had similar mating frequency and fertility. Additionally, fecundity of females mated to resistant and susceptible males was comparable. In competition for matings with virgin females, resistant and susceptible males had comparable success in one strain, whereas susceptible males tended to mate more often than resistant males in the other. However, irrespective of strain origin, resistant males that mated first sired significantly less offspring than susceptible males that mated first. The reduced first-male paternity in resistant males may involve reduced sperm precedence caused by mutations in a cadherin gene linked with resistance to Bt cotton.     

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.