Pest resistance to Cry1Ab Bt maize: Field resistance,contributing factors and lessons from South Africa

This paper documents the historical development of resistance of the African maize stem borer, Busseola fusca (Fuller) (Lepidoptera: Noctuidae) to Bt maize (Zea mays L.). This pest was one of the first to evolve resistance to Bt maize expressing Cry1Ab protein. A time-line of events and contributing factors are presented, from the commencement of efficacy testing through to the present situation, where the Cry1Ab toxin has lost its efficacy against B. fusca at many localities throughout the maize producing region, and single-gene Bt maize events often require insecticide treatments for which farmers are compensated. Significant levels of pest survival on Bt maize was observed in the first season after commercial release in 1998 and confirmed seven years later. Reduced selection pressure on the target pest is the objective of insect resistance management (IRM), and strategies to accomplish this should receive highest priority. Where resistance is prevalent, the only viable options to reduce selection pressure are withdrawal of the product and/or enforcement of high-dose/refuge requirements. The latter action may however be of no value under conditions where resistance is prevalent, since the value of refugia to an IRM strategy may be compromised. Remedial actions taken in South Africa included the propagation and enforcement of refuge compliance followed by the release of pyramided maize hybrids in 2011. These pyramids combine Cry1A.105 and Cry2Ab2 toxin-producing transgenes, replacing the ineffective single-transgene. However, it remains uncertain if cross-resistance occurs between Cry1A.105/Cry2Ab2 and the closely related Cry1Ab toxin, and for how long this pyramided event will endure. Cultivation of Cry1Ab-expressing hybrids continues in areas where resistance levels have been confirmed to be high. In retrospect, this case provides lessons regarding IRM, not only in South Africa, but wherever Bt crops are being introduced.     

Prevalence of Helicoverpa zea (Lepidoptera: Noctuidae) on late season volunteer corn in Mississippi: Implications on Bt resistance management

The southern United States has a long growing period between corn, Zea mays L., harvest and first winter frost, so volunteer corn which germinates after harvest has a growing period sufficient for corn earworm, Helicoverpa zea (Boddie) and fall armyworm, Spodoptera frugiperda (J. E. Smith) to feed on these plants. However, lower air temperatures can limit larval development on late season volunteer corn and thereby successful pupation. Here we explore the suitability of late season volunteer corn for larval development and the potential contribution of H. zea larvae to the overwintering population. Our survey revealed the occurrence of volunteer corn in high densities, with monthly mean densities ranging from 56,000 to 143,000 plants ha⁻¹. H. zea larvae were found feeding on both vegetative and reproductive stage plants while S. frugiperda were only found on vegetative stage plants. An analysis of H. zea growing degree day (GDD) accumulations based on Mississippi weather data from 1980 to 2010 revealed that sufficient GDD to reach prepupation would always be accumulated before first frost if oviposition occurred by 9 September, with the probability of successful pupation decreasing rapidly thereafter. However, most of the H. zea larvae were oviposited after this, and could not reach pupation. Because S. frugiperda cannot overwinter in Mississippi, their ability to pupate was not examined. Low suitability of whorl stage corn for H. zea development coupled with low larval densities during this stage effectively diminish the number of larvae that complete development on late season volunteer transgenic corn expressing genes from the soil bacterium, Bacillus thuringiensis (Bt). This limits the Bt resistance risk posed by larvae developing on late season volunteer corn in all but the most southern locations in the US.     

Functional dominance of different aged larvae of Bt-resistant Spodoptera frugiperda (Lepidoptera: Noctuidae) on transgenic maize expressing Vip3Aa20 protein

Fall armyworm (FAW), Spodoptera frugiperda (J. E. Smith), is the main target pest of transgenic maize expressing insecticidal proteins from Bacillus thuringiensis Berliner (Bt) in Brazil. To optimize resistance management strategies, we evaluated the functional dominance of different aged larvae of Bt-resistant FAW on Vip3Aa20 maize. We measured the survival and development of Vip3Aa20-resistant, -heterozygote, and -susceptible strains on MIR162 (expressing Vip3Aa20) and Bt11 × MIR162 × GA21 (expressing Vip3Aa20 and Cry1Ab) maize. The resistant strain, from neonate to sixth instar, showed more than 72% survival on Vip3Aa20 maize. From surviving larvae, more than 64 and 54% developed to pupae and adults, respectively. In contrast, heterozygote and susceptible strains showed no larval survival up to fourth instar, and less than 25% larval survival in the fifth and sixth instar on Vip3Aa20 maize. These larvae produced less than 21% of pupae and adults. The development time of FAW strains from neonate-to-adult exposed to Vip3Aa20 maize was similar; however, the resistant strain showed an increase of ∼ 2 d when compared to those fed only non-Bt maize. In summary, the resistance of S. frugiperda to Vip3Aa20 maize is functionally recessive from neonate up to fourth instar larvae. However, high larval survival of resistant strain and some survival of heterozygote larvae in advanced instars on Vip3Aa20 maize were observed. These results will be important for designing insect resistance management to Bt maize plants expressing Vip3Aa20 protein in Brazil.     

Performance of Cry1A.105-selected fall armyworm (Lepidoptera: Noctuidae) on transgenic maize plants containing single or pyramided Bt genes

Cry1A.105 is a Cry protein expressed in some transgenic Bacillus thuringiensis (Bt) maize products. In this study, performance of five populations of fall armyworm, Spodoptera frugiperda (J.E. Smith), were evaluated on four non-Bt and eight commercial and experimental Bt maize hybrids/lines (hereafter referred as maize products). The five insect populations included one Cry1A.105-susceptible strain, two Cry1A.105-resistant strains, and two F₁ heterozygous genotypes. The eight Bt maize hybrids/lines consisted of five single-gene Bt maize products containing Cry1A.105, Cry2Ab2, Cry1F, or Cry1Ab protein, and three pyramided Bt maize products expressing Cry1A.105/Cry2Ab2, Cry1A.105/Cry2Ab2/Cry1F, or Cry1Ab/Vip3A for targeting aboveground lepidopteran maize pests. In the study, neonates of each population were tested on leaf tissues in the laboratory and whole plants in the greenhouse. Cry1A.105 and Cry1F maize killed 92.2–100% susceptible larvae in both test methods, while resistant larvae survived well on these two maize products. Performance of the two F₁ populations on Cry1A.105 and Cry1F maize varied between the two test methods. In leaf tissue bioassay, Cry1Ab maize was marginally effective against the susceptible population. In contrast, few live larvae and little leaf injury from any of the five populations were observed on Cry2Ab2 and the three pyramided Bt maize products. The results of this study showed evidence of cross resistance of the Cry1A.105-resistant S. frugiperda to Cry1F and Cry1Ab maize, but not to the Bt maize products containing Cry2Ab2 or Vip3A. Data generated from this study will be useful in developing resistance management strategies for the sustainable use of Bt maize technology.     

Larval survival and plant injury of Cry1F-susceptible, -resistant,and -heterozygous fall armyworm (Lepidoptera: Noctuidae) on non-Bt and Bt corn containing single or pyramided genes

The fall armyworm, Spodoptera frugiperda (J.E. Smith), is a major target of transgenic corn, Zea mays L., expressing Bacillus thuringiensis (Bt) proteins in both North and South America. A highly Cry1F-resistant strain of S. frugiperda was established from a field collection in Puerto Rico in 2011. In this study, three greenhouse trials were conducted to evaluate larval survival and leaf injury of Cry1F-susceptible, -resistant, and -heterozygous genotypes of S. frugiperda on whole plants of five non-Bt and eight Bt corn hybrids. The Bt corn products included two single-gene Bt corn hybrids containing Herculex^®I (Cry1F) and YieldGard^® (Cry1Ab) traits and six pyramided Bt corn hybrids representing four traits: Genuity^® VT Double Pro™, Genuity^®VT Triple Pro™, Genuity^® SmartStax™, and Agrisure^® Viptera™ 3111. In each trial, neonates of S. frugiperda were placed into the plant whorls at vegetative plant stages (V6–V10). Larvae of the three insect genotypes on non-Bt corn hybrids survived well and caused serious plant injury. Cry1Ab corn was ineffective against all three insect genotypes. On Cry1F corn plants, resistant larvae survived on 72.9% plants after 12–15 d and caused a leaf injury rating (Davis' 1 to 9 scales) of 5.7 after 7 d and 7.6 after 12–15 d. Both the larval survivorship and leaf injury rates of the resistant larvae on Cry1F corn plants were not significantly different from those observed on non-Bt corn hybrids. In contrast, no live larvae and little or no leaf injury were observed on the Cry1F corn plants that were infested with susceptible or heterozygous genotypes, or on the pyramided Bt plants. The results demonstrated that the Cry1F-resistant S. frugiperda was highly resistant to whole plants of Cry1F corn and the resistance was recessive. Hybrids that contained one of the four pyramided Bt traits were effective for managing the Cry1F resistance in S. frugiperda.     

Resistance of Spodoptera litura (Lepidoptera: Noctuidae) to profenofos: Relative fitness and cross resistance

Profenofos, an acetylcholine esterase inhibitor insecticide, has been used for the management of various lepidopteron pests of many crops in Pakistan. In the present study, we investigated the impact of insecticide resistance on fitness cost in Spodoptera litura, and evaluated cross resistance to other insecticides. The effect of profenofos on different life history parameters including survival rate, female ratio, fecundity and hatchability, intrinsic rate of population increase and biotic potential was determined. Significant differences associated with fitness costs were revealed. A field collected S. litura strain was selected by exposure to profenofos in the laboratory (Profen-SEL) and after 14 generations of selection it developed a 52-fold resistance to this insecticide. The Profen-SEL strain showed high cross-resistance to chlorpyrifos (62-fold), but very low to no cross resistance to lambda-cyhalothrin (2.34-fold) or methomyl (0.80-fold), respectively. The resistant strain had a relative fitness of 0.38 with a low larval survival rate, longer larval duration, longer male pupal duration, longer development time, low emergence rate of healthy adults, fecundity and hatchability compared with an unselected strain. The intrinsic rate of natural population increase, mean relative growth rate and biotic potential were lower for the selected strain compared with an unselected strain. Development of resistance may cause fitness costs for the resistant strain. This study provided important information for understanding profenofos resistance and facilitating a better strategy for the management of resistance.     

Selection,mechanism, cross resistance and stability of spinosad resistance in Spodoptera litura (Fabricius) (Lepidoptera: Noctuidae)

Spodoptera litura is one of the most destructive pests in Pakistan and in many other regions of the world. A field collected population of S. litura was selected with spinosad for eleven generations under controlled laboratory conditions to study the cross resistance, mechanism and stability of spinosad resistance in S. litura. The resistance to spinosad in S. litura increased 3921-fold (after eleven generations of selection with spinosad) as compared to a susceptible population of S. litura. No cross resistance between spinosad and emamectin benzoate, methoxyfenozide, fipronil, indoxacarb, profenofos, lufenuron or deltamethrin was found in the spinosad-selected population of S. litura. To find the possible mechanism of spinosad resistance in S. litura two synergists, Piperonyl butoxide (PBO), S, S, S-tributyl phosphorotrithioate (DEF) were tested on the susceptible and resistant strains and on the un-selected field population. The values of the synergism ratios of PBO and DEF were 2.33 and 1.06 for the spinosad-selected strain, 1.36 and 1.06 for the un-selected field population and 1.14 and 1.00 for the susceptible strain, respectively. As high PBO ratio indicates the role of microsomal O-demethylase in causing spinosad resistance in S. litura. The spinosad-resistant and field populations of S. litura were reared without any selection pressure from the 12th to the 16th generation (G₁₂–G₁₆). The spinosad resistance decreased from 3921 to 678-fold in the spinosad-resistant population and from 31.1 to 15.1-fold in the un-selected population of S. litura as compared to the susceptible strain. Spinosad resistance in S. litura has a high reversion rate (−0.15) which indicates that spinosad resistance in S. litura is unstable and can be easily managed by switching off the selection pressure for a few generations or alternating with insecticides having different modes of action.     

Corn earworm (Lepidoptera: Noctuidae) damage at various kernel development stages on food maize

Feeding by corn earworm, Heliocoverpa zea (Boddie), larvae lowers the quality of food maize by causing damage to kernels that results in discoloration of food maize products during cooking. This damage includes kernel cracking or nipping of kernels by larvae. Larval infestations were highest from first silk to early dough growth stages. Infestations during the silk and blister-dough stages caused the least amount of kernel damage on an ear. The area on an ear exposed to corn earworm damage during the blister, dough, or dent to harvest treatments was similar. Kernels were most susceptible to quality loss when they were hardening. Yield was not affected by corn earworm feeding, but the highest quality loss occurred on ears exposed to earworms during the dent-harvest period and throughout the season. Consequently, late season control actions (during the dough stage) would be more effective for reducing quality loss from corn earworm feeding on food maize, than is the present practice of spraying during the green silk stage.     

Stability,cross-resistance and fitness costs of resistance to emamectin benzoate in a re-selected field population of the beet armyworm,Spodoptera exigua (Lepidoptera: Noctuidae)

Spodoptera exigua (Hübner) has a worldwide distribution with a high capacity for damaging a wide range of food, forage and fiber crops. It has been reported extensively from all over the world that populations of this pest species have developed field resistance against many insecticides. The objectives of this study were to determine whether an emamectin benzoate resistant field population of S. exigua re-selected with emamectin benzoate in the laboratory (Ema-SEL) showed cross-resistance to other insecticides, whether resistance was stable under laboratory conditions, and whether there were fitness costs associated with emamectin benzoate resistance. Bioassays at G₁ for the field population, gave resistance ratios (RRs) of 220, 149 and 38-fold for emamectin benzoate, spinosad and lufenuron, respectively, compared with a susceptible laboratory population (Lab-PK). Resistance ratios were increased by 526-fold and 6-fold compared with Lab-PK and the unselected field population (Ema-UNSEL, G₆), respectively after selection with emamectin benzoate (Ema-SEL) for five generations (G₆). Selection with emamectin benzoate had no apparent effect on susceptibility of Ema-SEL to spinosad and lufenuron, instead toxicity to the latter insecticides reduced, suggesting there was no cross-resistance between these compounds. Analysis of various life history traits suggested that the Ema-SEL population had a lower overall fitness (0.38) compared with the Lab-PK (1.0). Lack of cross-resistance and the apparent instability of resistance to emamectin benzoate suggest that spinosad and lufenuron are suitable alternatives for use with emamectin benzoate in resistance management. In addition, the high relative fitness costs observed suggests that emamectin benzoate-resistant insects are at a considerable disadvantage to susceptible populations in the absence of selection pressure although this remains to be tested under field conditions.     

Helicoverpa zea (Lepidoptera: Noctuidae) and Spodoptera frugiperda (Lepidoptera: Noctuidae) Responses to Sorghum bicolor (Poales: Poaceae) Tissues From Lowered Lignin Lines

The presence of lignin within biomass impedes the production of liquid fuels. Plants with altered lignin content and composition are more amenable to lignocellulosic conversion to ethanol and other biofuels but may be more susceptible to insect damage where lignin is an important resistance factor. However, reduced lignin lines of switchgrasses still retained insect resistance in prior studies. Therefore, we hypothesized that sorghum lines with lowered lignin content will also retain insect resistance. Sorghum excised leaves and stalk pith Sorghum bicolor (L.) Moench (Poales: Poaceae) from near isogenic brown midrib (bmr) 6 and 12 mutants lines, which have lowered lignin content and increased lignocellulosic ethanol conversion efficiency, were examined for insect resistance relative to wild-type (normal BTx623). Greenhouse and growth chamber grown plant tissues were fed to first-instar larvae of corn earworms, Helicoverpa zea (Boddie) and fall armyworms Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae), two sorghum major pests. Younger bmr leaves had significantly greater feeding damage in some assays than wild-type leaves, but older bmr6 leaves generally had significantly less damage than wild-type leaves. Caterpillars feeding on the bmr6 leaves often weighed significantly less than those feeding on wild-type leaves, especially in the S. frugiperda assays. Larvae fed the pith from bmr stalks had significantly higher mortality compared with those larvae fed on wild-type pith, which suggested that bmr pith was more toxic. Thus, reducing lignin content or changing subunit composition of bioenergy grasses does not necessarily increase their susceptibility to insects and may result in increased resistance, which would contribute to sustainable production.     

Field-evolved resistance to Cry1F maize by Spodoptera frugiperda (Lepidoptera: Noctuidae) in Brazil

The Cry1F protein from Bacillus thuringiensis Berliner expressed in event TC1507 maize (Zea mays L.) was one of the most effective ways to control Spodoptera frugiperda (J. E. Smith) in Brazil. After reports of reduced effectiveness of this Bt maize event in some areas of Brazil, research was undertaken to investigate if damage to Cry1F maize was caused by resistant S. frugiperda. Additional investigations were conducted to evaluate the genetic basis of the resistance and to test if Cry1F resistant S. frugiperda selected from populations of different regions of Brazil share the same resistance locus by using complementation tests. Neonate larvae of S. frugiperda collected from TC1507 maize fields with damage in Western Bahia region in 2011 were able to survive on Cry1F maize plants under laboratory conditions and subsequently produced normal adults. Survival of Cry1F-susceptible S. frugiperda on non-Bt maize was significantly higher in leaf than plant bioassays. Resistance ratio in diet overlay bioassays was >5000-fold. A discriminating concentration of 2000 ng cm⁻² of Cry1F protein was defined for monitoring the frequency of resistance of S. frugiperda to Cry1F. Cry1F resistant S. frugiperda showed a recessive autosomal inheritance for alleles involved in resistance to Cry1F protein. In complementation tests, the resistant population from Western Bahia was crossed with the other seven resistant populations collected from different States of Brazil. F₁ larvae from each cross had the same survival at discriminating concentration of 2000 ng cm⁻² of Cry1F protein, indicating that the resistance alleles in each population were likely at the same locus. Therefore, implementation of resistance management strategies is urgent to prolong the lifetime of Cry1F for controlling S. frugiperda in Brazil.     

Sequential sampling plan for assessing corn rootworm (Coleoptera: Chrysomelidae) larval injury to Bt maize

In-field product performance assessments are an essential component of corn rootworm (Diabrotica spp.; CRW) resistance management plans for transgenic maize (Zea mays L.) products expressing proteins derived from the bacterium Bacillus thuringiensis (Bt). The goal of a successful field sampling program is to accurately characterize in-field product performance while also minimizing resource demand, as collection of maize root samples to evaluate CRW injury can present resource challenges such as labor intensiveness, potential safety issues, and a limited time window available for sampling. A resource-efficient sequential sampling plan was developed that utilizes data-driven root injury threshold values derived from benchmark product performance data for both single and pyramided Bt maize traits for CRW control. This sequential sampling methodology incorporates unbiased sampling and controlled false positive and false negative error rates, enabling accurate assessment decisions to be made with efficient resource use. Our proposed approach enables systematic and effective classification of in-field Bt maize product performance, with applications to other CRW control technologies besides Bt maize products.     

Monitoring of insecticide resistance in Spodoptera litura (Lepidoptera: Noctuidae) from four districts of Punjab,Pakistan to conventional and new chemistry insecticides

Studies were carried out to evaluate the resistance of Pakistani populations of the beet armyworm, Spodoptera litura (F) to several commonly used insecticides. Different field populations of S. litura from four districts of the Punjab were monitored from 2009 to 2011 for resistance to insecticides using a standard leaf dip bioassay method. For organophosphates and pyrethroids, resistance ratios compared with a susceptible Lab-Pk population were in the range of 8–109 fold for deltamethrin, 11–139 fold for cypermethrin, 19–143 fold for chlorpyrifos and 39–162 fold for profenofos. For new chemistry insecticides, resistance levels were 2–74 fold for spinosad, 4–216 fold for abamectin, 7–87 fold for indoxacarb, 2–77 fold for emamectin benzoate, 1.9–58 fold for lufenuron and 4–43 fold for methoxyfenozide. Pairwise correlation coefficients of LC₅₀ values showed a positive correlation with cross-resistance among deltamethrin, cypermethrin and chlorpyrifos, while resistance to profenofos showed correlations with resistances to other insecticides except chlorpyrifos. New chemistry insecticides showed no correlations between any of the tested insecticides. There were high to very high levels of resistance to organophosphates in most of the population, which suggested that the use of these should be avoided against this pest. Selective use of pyrethroids in several areas, including Bahawalpur and Lodhran, where the pest showed a low level of resistance, would appear to be acceptable, the new chemistry insecticides, lufenuron, methoxyfenozide, emamectin and indoxacarb had no, very low, low and moderate resistance levels against populations, respectively. These are considered to be safe to the environment and safer to natural enemies.     

Comparative assessment of the field-susceptibility of Sesamia nonagrioides to the Cry1Ab toxin in areas with different adoption rates of Bt maize and in Bt-free areas

A three year (2003-2005) field study compared the susceptibility to the Cry1Ab toxin, expressed in Bt maize, of Mediterranean corn borer (MCB) Sesamia nonagrioides populations collected from areas with different adoption rates of Bt maize in Spain with Bt-free areas in Greece. Spain is the only European country where the cultivar Compa CB derived from the event Bt176 was commercially grown, from 1998 to 2005. The large decrease of the titer of the toxin in this cultivar at later growth stages represented the worst-case scenario for resistance development of MCB, since larvae of the second and third generations were exposed to sublethal concentrations of Cry1Ab toxin. Our data revealed that the variation in susceptibility to Cry1Ab for the MCB Spanish field populations analyzed in the three years was very low, with LC₅₀ values fluctuating between 12 and 30 ng Cry1Ab/cm², regardless of the region of origin, the type of maize (Bt or non-Bt) and the year. Furthermore, no significant differences were found when comparisons were made with a laboratory population (LC₅₀ values: 18-26 ng Cry1Ab/cm²) or with field populations from Greece (Bt-free areas), which displayed LC₅₀ values ranging between 22 and 27 ng Cry1Ab/cm². Standardizing bioassay protocols proved to be essential for obtaining comparable results. These findings suggest that resistant MCB populations did not evolve in those Spanish maize areas where Compa CB was largely cultivated for eight years, contradicting the expected rapid development of resistance under these unfavourable conditions. Additionally, our results can be used as baseline indices in post-market resistance monitoring programs if Bt maize is introduced in Greece. Further studies should continue, since the insights gained from a resistance monitoring program may help to enhance the durability of Bt maize.     

High level of metaflumizone resistance and multiple insecticide resistance in field populations of Spodoptera exigua (Lepidoptera: Noctuidae) in Guangdong Province,China

Spodoptera exigua is a phytophagous pest that causes critical economic losses in vegetable crops, and insecticides are commonly used against it in vegetable growing areas. However, excessive and frequent applications of insecticides cause resistance in S. exigua. The current resistance in field populations of S. exigua collected from Huizhou, Guangdong Province, China to 12 insecticides was investigated. S. exiguahad developed very high resistance to lambda cyhalothrin (2925- to 3449-fold), chlorpyrifos (>1786-fold), emamectin benzoate (174- to 867-fold), and metaflumizone (60.3- to 942-fold). High resistance to tebufenozide (51.5- to 75.4-fold) and chlorfluazuron (60.4- to 63.0-fold) was also found. Synergism assays revealed that the resistance to metaflumizone and lambda cyhalothrin was associated with esterase and microsomal oxidases, respectively. The resistance to emamectin benzoate was not affected by detoxification enzymes inhibitors and might be conferred by other mechanisms. The selection of the field population by metaflumizone for 10generations in the laboratory resulted in a 6.1-fold increase in metaflumizone resistance but did not lead to increases in resistance to other insecticides. After metaflumizone selection, susceptibilities to spinosad and endosulfan did not change, and the susceptibilities to indoxacarb, methomyl, pyridalyl, tebufenozide, chlorfluazuron, emamectin benzoate and lambda cyhalothrin decreased slightly. However, no statistically significant differences in the resistance levels were observed among the selected population, its starting strain and the unselected strain. The resistance to chlorantraniliprole noticeably decreased in unselected strain and the strain subjected to selection for 10 generations compared with their starting strains. Lack of cross-resistance to tested insecticides suggested the involvement of multiple mechanisms of resistance and the need for wise application of these insecticides for the management of S. exigua.     

Low susceptibility of Spodoptera cosmioides,Spodoptera eridania and Spodoptera frugiperda (Lepidoptera: Noctuidae) to genetically-modified soybean expressing Cry1Ac protein

Spodoptera cosmioides (Walker), Spodoptera eridania (Stoll) and Spodoptera frugiperda (J. E. Smith) have caused significant damage on soybean Glycine max (L.) Merrill in Brazil. Genetically-modified MON 87701 × MON 89788 soybean that expresses the Cry1Ac protein is potentially an alternative tool for the management of these species. Purified protein bioassays were done to evaluate the susceptibility of S. cosmioides, S. eridania and S. frugiperda to Cry1Ac protein. The level of efficacy of the Bt soybean plants in controlling these species was measured through laboratory and greenhouse trials under high artificial insect infestations. The biology of these insects was evaluated over their development cycles to understand their life history when fed on Bt soybean. Purified Cry1Ac protein at the maximum concentration tested (100 μg Cry1Ac mL⁻¹ diet) resulted in low mortality of S. cosmioides and S. eridania (<13%) and intermediate mortality of S. frugiperda (50%). No significant effects of the Bt soybean plants were observed in the life table parameters of S. cosmioides and S. eridania. However, S. frugiperda fed on Bt soybean plants had a prolonged larval stage (by 5 days), reduced larvae viability, increased mean generation time (by 8 days) and reduced intrinsic rate of increase. In general, the Bt soybean plants showed poor control of Spodoptera species when evaluated by leaf-disc bioassay and greenhouse trials. Consequently, other control tactics must be used in combination with MON 87701 × MON 89788 soybean in the field for the efficient management of S. cosmioides, S. eridania and S. frugiperda.     

Survival of cabbage looper, Trichoplusia ni (Lepidoptera: Noctuidae), through winter cleanups of commercial vegetable greenhouses: Implications for insecticide resistance management

Cabbage looper, Trichoplusia ni (Hübner) (Lepidoptera: Noctuidae), is a highly migratory and destructive pest of various crops in North America. Its infestations in vegetable greenhouses in western Canada are generally attributed to annual immigrations from southern overwintering areas, but whether it survives overwinter in western Canada is uncertain. Bioinsecticides based on Bacillus thuringiensis (Bt) toxins are commonly used in southwestern British Columbia where some T. ni populations have developed strong resistance to Bt-insecticides. In this study, overwintering success of T. ni populations was monitored outside and inside three commercial greenhouses in British Columbia. In both years, T. ni pupae placed outside greenhouses did not survive although pupae placed inside the greenhouses survived the unheated cleanup period and exposure to organophosphate insecticides commonly used in the cleanup procedure in the greenhouses. During the cleanup period, no moths were caught inside the unheated greenhouses, but once the heating was restored for the new growing season, adults were captured in pheromone traps. No developmental stages of T. ni were detected on nursery seedlings and visual monitoring of transplants at the beginning of the season found no signs of previous feeding damage. This strongly suggests that T. ni were not introduced with the seedlings. Our findings are important for understanding the potential risks of inadequate winter cleanup and the potential carryover of resistance to Bt-insecticides in populations from one season to the next that could complicate pest management.     

Selection of bifenthrin resistance in cotton mealybug Phenacoccus solenopsis Tinsley (Homoptera: Pseudococcidae): Cross-resistance,realized heritability and possible resistance mechanism

Cotton mealybug Phenacoccus solenopsis Tinsley is an important pest of cotton in Pakistan, and its management is difficult due to the development of insecticide resistance. This research was conducted to characterize the bifenthrin resistance in populations of P. solenopsis and different parameters such as cross-resistance, realized heritability and possible resistance mechanisms were studied to improve the management of this important pest. A field-collected population was selected with bifenthrin in the laboratory for 14 generations and developed a resistance of 178-fold. The realized heritability of bifenthrin resistance was 0.54 in the selected population. The toxicity of bifenthrin was synergized by the addition of either piperonylbutoxide (PBO) or S,S,S tributylphosphorotrithioate (DEF) which suggests a general metabolic resistance due to possible involvement of mono-oxygenases or esterases. However, the resistant population did not develop a significant cross-resistance to either buprofezin, chlorpyrifos or lambda-cyhalothrin. These data suggest that alternative insecticide-based management programs can be developed for this pest in the short-term, but resistance management strategies which can reduce the sole reliance on insecticides are still needed.     

草地贪夜蛾对不同品种玉米室内危害级别及种群适合度比较

草地贪夜蛾[Spodoptera frugiperda(J.E.Smith)]是2019年由缅甸开始入侵我国的一种迁飞性农业害虫,具有取食范围广、极强的迁飞性、适应性及繁殖能力等特点.选育抗性品种是一项重要的农业防治手段,而评价不同玉米品种的危害级别对后续利用多组学分析筛选出特定的抗性基因,培育抗性品种具有指导作用.本...     

Adaptation of Habrobracon hebetor (Hymenoptera: Braconidae) to Rearing on Ephestia kuehniella (Lepidoptera: Pyralidae) and Helicoverpa armigera (Lepidoptera: Noctuidae)

Food characteristics strongly regulate digestive enzymatic activity of insects through direct influences on their midgut mechanisms. Insect performance is better on diets that contain nutrients in proportions that fit its digestive enzymes. Little is known about the influences of rearing history on parasitism success of Habrobracon hebetor Say. This research focused on the effect of nutrient regulation on survival, development, and parasitism of H. hebetor. Life history and digestive enzyme activity of fourth-stage larvae of H. hebetor were studied when reared on Ephestia kuehniella Zeller. This parasitoid was then introduced to Helicoverpa armigera (Hübner), and above-mentioned parameters were also studied in the first and fourth generations after transfer. In term of parasitism success, H. hebetor preferred E. kuehniella over He. armigera. When the first and fourth generations of He. armigera-reared H. hebetor were compared, the rearing history affected the life history and enzymatic activity of the parasitoid. A better performance of H. hebetor was achieved after it was reared on He. armigera for the four generations. Because, digestive α-amylase and general protease of the parasitoid were matched with the new host, it used reserve energy for a better performance. Thus, a better performance of H. hebetor could be obtained when the parasitoid was reared on its original host for at least four generations.