Susceptibility of Brazilian Populations of Helicoverpa armigera and Helicoverpa zea (Lepidoptera: Noctuidae) to Vip3Aa20.

Transgenic maize expressing the insecticidal protein Vip3Aa20 is increasingly being adopted in Brazil. In this study, we determined the baseline susceptibility of Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) and Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae) to Vip3Aa20, as part of an Insect Resistance Management (IRM) program. Diet-overlay bioassays were conducted with neonates exposed to Vip3Aa20 for 7 d. The baseline susceptibility data were obtained for seven field populations of H. armigera and six of H. zea collected from major soybean-, cotton-, and maize-producing areas in Brazil. To validate the diagnostic concentration, 11 field populations of H. zea were tested from 2014 to 2015. The LC50 for H. armigera populations ranged from 2.97 to 8.41 µg Vip3Aa20/cm2 (threefold variation), and for H. zea populations from 0.04 to 0.21 µg Vip3Aa20/cm2 (fivefold variation). The EC50 for H. armigera ranged from 0.099 to 0.455 µg Vip3Aa20/cm2 (fivefold variation), and for H. zea from 0.004 to 0.020 µg Vip3Aa20/cm2 (fivefold variation). H. armigera was more tolerant to Vip3Aa20 protein than was H. zea (≈40- to 75-fold, based on LC50). Based on the LC99 value, the concentration of 6.4 µg Vip3Aa20/cm2 was defined as a diagnostic concentration for susceptibility monitoring in H. zea, and >44 µg Vip3Aa20/cm2 for H. armigera. Our baseline susceptibility data for Vip3Aa20 in H. armigera and H. zea populations will be useful in IRM programs in Brazil.

Pan-American Similarities in Genetic Structures of Helicoverpa armigera and Helicoverpa zea (Lepidoptera: Noctuidae) With Implications for Hybridization.

The genus Helicoverpa (Lepidoptera: Noctuidae) includes phytophagous and polyphagous agricultural insect pests. In the Americas, a native pest, Helicoverpa zea (Boddie), and an invasive pest, Helicoverpa armigera (Hübner), are causing severe damage in vegetable and agronomic crops. The population structure of both species in South America is poorly understood, and the phylogenetic relatedness of H. armigera and H. zea suggests natural interspecific gene flow between these species. Using microsatellite loci, we investigated: 1) the genetic diversity and gene flow of H. armigera specimens from Brazil; 2) the genetic diversity and gene flow between H. zea specimens from Brazil and the United States; and 3) the possibility of interspecific gene flow and the frequency of putative hybrids in Brazil. We detected high intraspecific gene flow among populations collected in the same country. However, there is a geographic limit to gene flow among H. zea individuals from South and North America. Pairwise Fst and private alleles showed that H. armigera is more similar to H. zea from Brazil than H. zea from the United States. A comparative STRUCTURE analysis suggests natural hybridization between H. armigera and H. zea in Brazil. High gene flow and natural hybridization are key traits to population adaptation in new and disturbed environments, which can influence the management of these pests in the American continent.

Cross-species amplification and polymorphism of microsatellite loci in Helicoverpa armigera and Helicoverpa zea (Lepidoptera: Noctuidae) in Brazilian cropping systems.

The Old World bollworm Helicoverpa armigera (Hübner) was recently discovered in Brazil. This species is closely related to the New World bollworm H. zea (Boddie), and mating between these species has already been reported under laboratory conditions. Here, we tested the cross-species amplification of 20 microsatellite (SSR) loci in field populations of H. armigera and H. zea collected from Brazilian cropping systems. Seven SSR loci were successfully amplified and polymorphic in both species except for the locus HaC14, which was monomorphic for H. zea. All SSR loci were in linkage equilibrium, and deviations from Hardy- Weinberg equilibrium were only observed for the locus HarSSR1 in the HaRS-2 population, where null alleles were present. A moderate level of polymorphism was detected in H. armigera and H. zea populations with a mean allele number of 4.14, and 2.24, respectively. Interestingly, most of the populations of the recent invader H. armigera showed higher genetic diversity and inbreeding coefficients than H. zea populations. The genetic identity of each species was recovered using a STRUCTURE analysis, where the populations formed two clusters (K = 2) according to their species. STRUCTURE also suggested the occurrence of potential hybrid offspring between H. armigera and H. zea individuals in natural conditions. These SSR loci will be valuable in characterizing population differentiation, invasion routes, adaptation, reproductive behavior, and intra- and interspecific gene flow in H. armigera and H. zea populations in Brazil, the USA, and other areas where these two pests occur.