Baseline Susceptibility and Cross-Resistance of HearNPV in Helicoverpa armigera (Lepidoptera: Noctuidae) in Brazil.

The marked adoption of bioinsecticides in Brazilian agriculture in recent years is, at least partially, explained by the increasingly higher levels of insect pest resistance to synthetic insecticides. In particular, several baculovirus-based products have been registered in the last 5 years, including Helicoverpa armigera nucleopolyhedrovirus (HearNPV: Baculoviridae: Alphabaculovirus (Armigen®)). Understanding the susceptibility of Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) to HearNPV is an important step toward development of robust Integrated Pest Management (IPM) and Insect Resistance Management programs (IRM) aimed at managing this serious insect pest. In this study, droplet feeding bioassays were used to characterize the baseline susceptibility to HearNPV (Armigen®) in H. armigera populations collected from major soybean and cotton-growing regions in Brazil. We defined and validated a diagnostic concentration for susceptibility monitoring of H. armigera populations to HearNPV. Additionally, cross-resistance between HearNPV and the insecticides flubendiamide and indoxacarb was evaluated by testing HearNPV in a susceptible strain and in resistant strains of H. armigera to these insecticides. A low interpopulation variation of H. armigera to HearNPV was detected. The LC50 values ranged from 1.5 × 105 to 1.1 × 106 occlusion bodies (OBs) per mL (7.3-fold variation). The mortality rate at the identified diagnostic concentration of 6.3 × 108 OBs/mL, based on the calculated LC99, ranged from 98.6 to 100% in populations of H. armigera collected from 2018 to 2020. No cross-resistance was detected between HearNPV and flubendiamide or indoxacarb. These results suggest that HearNPV (Armigen®) can be an effective tool in IPM and IRM programs to control H. armigera in Brazil.

High frequency of CYP337B3 gene associated with control failures of Helicoverpa armigera with pyrethroid insecticides in Brazil.

Control failures with the use of pyrethroid insecticides have been reported frequently for populations of Helicoverpa armigera (Hübner) in Brazil, since its detection in 2013. Here, we confirmed and investigated the metabolic mechanisms of pyrethroid resistance in H. armigera populations from Brazil. Mortality of H. armigera populations was lower than 50% at the highest dose (10µg a.i./3rd instar larva) of the pyrethroids deltamethrin and fenvalerate in dose-response bioassays. Very low mortality (10 to 40%) was obtained at a diagnostic dose of 10µg a.i./larva for each pyrethroid in H. armigera populations collected from different agricultural regions in Brazil, from 2013 to 2016. In synergist bioassays, when larvae were treated with PBO synergist, the mortality of all populations tested was 100%. The frequency of the cytochrome P450 CYP337B3 gene was above 0.95 in all populations of H. armigera. We found only fourteen heterozygous H. armigera out of 497 individuals tested for this gene subfamily. Our results indicated that H. armigera populations from Brazil have different degrees of susceptibility to deltamethrin and fenvalerate, but all populations can be considered tolerant to pyrethroid insecticides. The chimeric P450 CYP337B3 enzyme is one of the main mechanisms of pyrethroid resistance in Brazilian H. armigera populations.

High Susceptibility to Cry1Ac and Low Resistance Allele Frequency Reduce the Risk of Resistance of Helicoverpa armigers to Bt Soybean in Brazil.

The Old World bollworm, Helicoverpa armigera (Hübner), was recently introduced into Brazil, where it has caused extensive damage to cotton and soybean crops. MON 87701 × MON 89788 soybean, which expresses the Bt protein Cry1Ac, was recently deployed in Brazil, providing high levels of control against H. armigera. To assess the risk of resistance to the Cry1Ac protein expressed by MON 87701 × MON 89788 soybean in Brazil, we conducted studies to evaluate the baseline susceptibility of H. armigera to Cry1Ac, in planta efficacy including the assessment of the high-dose criterion, and the initial resistance allele frequency based on an F2 screen. The mean Cry1Ac lethal concentration (LC50) ranged from 0.11 to 1.82 µg·mL-1 of diet among all H. armigera field populations collected from crop seasons 2013/14 to 2014/15, which indicated about 16.5-fold variation. MON 87701 × MON 89788 soybean exhibited a high level of efficacy against H. armigera and most likely met the high dose criterion against this target species in leaf tissue dilution bioassays up to 50 times. A total of 212 F2 family lines of H. armigera were established from field collections sampled from seven locations across Brazil and were screened for the presence of MON 87701 × MON 89788 soybean resistance alleles. None of the 212 families survived on MON 87701 × MON 89788 soybean leaf tissue (estimated allele frequency = 0.0011). The responses of H. armigera to Cry1Ac protein, high susceptibility to MON 87701 × MON 89788 soybean, and low frequency of resistance alleles across the main soybean-producing regions support the assumptions of a high-dose/refuge strategy. However, maintenance of reasonable compliance with the refuge recommendation will be essential to delay the evolution of resistance in H. armigera to MON 87701 × MON 89788 soybean in Brazil.

Resistance Risk Assessment of Spodoptera frugiperda (Lepidoptera: Noctuidae) and Diatraea saccharalis (Lepidoptera: Crambidae) to Vip3Aa20 Insecticidal Protein Expressed in Corn.

Transgenic Agrisure Viptera 3 corn that expresses Cry1Ab, Vip3Aa20, and EPSPS proteins and Agrisure Viptera expressing Vip3Aa20 are used for control of Spodoptera frugiperda (J.E. Smith) and Diatraea saccharalis (F.) in Brazil. To support a resistance management program, resistance risk assessment studies were conducted to characterize the dose expression of Vip3Aa20 protein and level of control against these species. The Vip3Aa20 expression in Agrisure Viptera 3 and Agrisure Viptera decreased from V6 to V10 stage of growth. However, Vip3Aa20 expression in Agrisure Viptera 3 at V6 and V10 stages was 13- and 16-fold greater than Cry1Ab, respectively. The Vip3Aa20 expression in lyophilized tissue of Agrisure Viptera 3 and Agrisure Viptera diluted 25-fold in an artificial diet caused complete larval mortality of S. frugiperda and D. saccharalis. In contrast, lyophilized tissue of Bt11 at the same dilution does not provide complete mortality of these species. Agrisure Viptera 3 and Agrisure Viptera also caused a high level of mortality against S. frugiperda and D. saccharalis. Moreover, 100% mortality was observed for S. frugiperda larvae (neonates through fifth-instar larvae) when fed in corn with the Vip trait technology. Viptera corn achieves a high level of control against S. frugiperda and D. saccharalis providing a high dose, which is an important determination to support the refuge strategy for an effective resistance management program.

Baseline susceptibility and monitoring of Brazilian populations of Spodoptera frugiperda (Lepidoptera: Noctuidae) and Diatraea saccharalis (Lepidoptera: Crambidae) to Vip3Aa20 insecticidal protein.

The genetically modified maize expressing Vip3Aa20 insecticidal protein from Bacillus thuringiensis Berliner is abiotechnological option for the control of Spodoptera frugiperda (J.E. Smith) and Diatraea saccharalis (F.) in Brazil. To support an Insect Resistance Management program, we conducted studies of baseline susceptibility and monitoring of Brazilian populations of S. frugiperda and D. saccharalis to the Vip3Aa20 insecticidal protein. Neonates were exposed to Vip3Aa20 applied on artificial diet surface. Mortality and growth inhibition were assessed after 7 d. All populations were susceptible to Vip3Aa20. The LC50 ranged from 92.38 to 611.65 ng Vip3Aa20/cm2 for 16 populations of S. frugiperda (6.6-fold variation), and between 61.18 and 367.86 ng Vip3Aa20/cm2 for 6 populations of D. saccharalis (sixfold variation). The EC50 ranged from 21.76 to 70.09 and 48.65 to 163.60 ng Vip3Aa20/cm2 for S. frugiperda and D. saccharalis, respectively. There was a low interpopulation variation in susceptibility to Vip3Aa20, which represents the natural geographic variation in the response, and not the variation caused by previous exposure to selection pressure. For these two pests, the diagnostic concentrations of 2,000 and 3,600 ng of Vip3Aa20/cm2 caused high mortality. These diagnostic concentrations will be used in resistance monitoring programs in Brazil.