Insecticidal Traits of Variants in a Genotypically Diverse Natural Isolate of Anticarsia Gemmatalis Multiple Nucleopolyhedrovirus (AgMNPV)

Abstract

Outbreaks of Anticarsia gemmatalis (Hübner, 1818) (Lepidoptera: Erebidae), a major pest of soybean, can be controlled below economic thresholds with methods that do not involve the application of synthetic insecticides. Formulations based on natural isolates of the Anticarsia gemmatalis multiple nucleopolyhedrovirus (AgMNPV) (Baculoviridae: Alphabaculovirus) played a significant role in integrated pest management programs in the early 2000s, but a new generation of chemical insecticides and transgenic soybean have displaced AgMNPV-based products over the past decade. However, the marked genotypic variability present among and within alphabaculovirus isolates suggests that highly insecticidal genotypic variants can be isolated and used to reduce virus production costs or overcome isolate-dependent host resistance. This study aimed to select novel variants of AgMNPV with suitable insecticidal traits that could complement the existing AgMNPV active ingredients. Three distinct AgMNPV isolates were compared using their restriction endonuclease profile and in terms of their occlusion body (OB) pathogenicity. One isolate was selected (AgABB51) from which eighteen genotypic variants were plaque purified and characterized in terms of their insecticidal properties. The five most pathogenic variants varied in OB pathogenicity, although none of them was faster-killing or had higher OB production characteristics than the wild-type isolate. We conclude that the AgABB51 wild-type isolates appear to be genotypically structured for fast speed of kill and high OB production, both of which would favor horizontal transmission. Interactions among the component variants are likely to influence this insecticidal phenotype.

Keywords: Baculoviridae; bioinsecticide; genotypic variant; occlusion body production; pathogenicity; virulence.

Figure 1

Restriction endonuclease profiles of the genomic DNA of three different AgMNPV isolates following treatment with HindIII. m denotes the molecular marker. Fragment size in kilobases (Kb) is shown on the left. Red arrowheads indicate restriction fragment length polymorphisms (RFLPs) and asterisks on the left of each lane indicate the presence of submolar bands.

Figure 2

HindIII restriction endonuclease profiles of the genomic DNA of each of the 18 different AgABB51 genotypic variants (A to R). m denotes the molecular marker. Fragment size in kilobases (Kb) is shown on the left. The profile of genotype A is used as a reference to identify the presence (red arrowheads) or absence (green arrowheads) of characteristic restriction fragments in the other genotypic variants. Genotype A was selected for this purpose because of its similarity to the wild-type isolate (AgABB51) (shown in Figure 1).

Figure 3

Frequency of AgABB51 genotypic variants A through R. The n-value above each column indicates the number of clones exhibiting each variant’s restriction profile out of a total of 128 clones.

Figure 4

Mean percentage of mortality caused by AgABB51 wild-type isolate and each of the genotypic variants A–R on A. gemmatalis second instars inoculated with 1.1 × 10⁴ OBs/mL. Error bars indicate the standard error. Different lowercase letters indicate significant differences between variants (ANOVA, Tukey HSD; p < 0.05).

Figure 5

(a) Median time to death (MTD) values for the AgABB51 isolate and selected genotypic variants in A. gemmatalis second instars. Error bars indicate standard error and different lowercase letters indicate significant differences between variants (Bonferroni-adjusted t-test; p < 0.05). (b) OB production values obtained for AgABB51 and the selected genotypic variants in A. gemmatalis fifth instars. Error bars indicate standard error and different lowercase letters indicate significant differences between variants (Tukey HSD test; p < 0.05).