Spatial Associations of Key Lepidopteran Pests With Defoliation, NDVI, and Plant Height in Soybean

In soybean, Glycine max (L.) Merrill, production, losses to, and control costs for insect pests can be significant limiting factors. Although the heterogeneity of pests has typically been ignored in traditional field management practices, technological advancements have allowed for site-specific pest management systems to be developed for the precise control of pests within a field. In this study, we chose to determine how the in-field distributions of the larvae of three major lepidopteran pests [velvetbean caterpillar Anticarsia gemmatalis (Hübner) (Lepidoptera: Erebidae), soybean looper Chrysodeixis includens (Walker) (Lepidoptera: Noctuidae), and green cloverworm Hypena scabra (Lepidoptera: Erebidae) (Fabricius)] were spatially associated with defoliation, Normalized Difference Vegetation Index (NDVI), and plant height in soybean. Spatial analysis by distance indices (SADIE) of data from two South Carolina soybean fields in 2017 and 2018 revealed a limited number of spatial aggregations for insect datasets. However, 14% and 6% of paired plant–insect datasets were significantly associated or dissociated, respectively. NDVI was found to be more associated with pest distributions than soybean plant heights and defoliation estimates, and the majority of all plant–insect associations and dissociations occurred in the first 4 wk of sampling (late July–early August). If changes are to be implemented regarding how a pest is managed, critical factors explaining the spatial distribution of pests must be identified. Results from this study advocate for the relationship between early-season distributions of pests and important plant variables such as NDVI to be further investigated to better determine the strength of the correlations across years and sites.

Bacillus thuringiensis chimeric proteins Cry1A.2 and Cry1B.2 to control soybean lepidopteran pests: New domain combinations enhance insecticidal spectrum of activity and novel receptor contributions

Two new chimeric Bacillus thuringiensis (Bt) proteins, Cry1A.2 and Cry1B.2, were constructed using specific domains, which provide insecticidal activity against key lepidopteran soybean pests while minimizing receptor overlaps between themselves, current, and soon to be commercialized plant incorporated protectants (PIP’s) in soybean. Results from insect diet bioassays demonstrate that the recombinant Cry1A.2 and Cry1B.2 are toxic to soybean looper (SBL) Chrysodeixis includens Walker, velvetbean caterpillar (VBC) Anticarsia gemmatalis Hubner, southern armyworm (SAW) Spodoptera eridania, and black armyworm (BLAW) Spodoptera cosmioides with LC50 values < 3,448 ng/cm2. Cry1B.2 is of moderate activity with significant mortality and stunting at > 3,448 ng/cm2, while Cry1A.2 lacks toxicity against old-world bollworm (OWB) Helicoverpa armigera. Results from disabled insecticidal protein (DIP) bioassays suggest that receptor utilization of Cry1A.2 and Cry1B.2 proteins are distinct from each other and from current, and yet to be commercially available, Bt proteins in soy such as Cry1Ac, Cry1A.105, Cry1F.842, Cry2Ab2 and Vip3A. However, as Cry1A.2 contains a domain common to at least one commercial soybean Bt protein, resistance to this common domain in a current commercial soybean Bt protein could possibly confer at least partial cross resistance to Cry1A2. Therefore, Cry1A.2 and Cry1B.2 should provide two new tools for controlling many of the major soybean insect pests described above.

Soybean Leaf Age and Plant Stage Influence Expression of Resistance to Velvetbean Caterpillar and Fall Armyworm

Numerous species of herbivorous insects are associated with soybeans, including the specialist velvetbean caterpillar (VBC), Anticarsia gemmatalis, and the generalist fall armyworm (FAW), Spodoptera frugiperda. Expression of plant resistance is influenced by factors intrinsic to host plants, such as leaf age and plant stage, which can differentially affect specialist and generalist insects due to varying levels of plant defense and corresponding insect adaptation. In this study four experiments were carried out to test the hypotheses that levels of antibiosis-resistance to VBC and FAW in the resistant genotype PI 227687 and susceptible genotype IGRA RA 626 RR are related to leaf age and plant stage of soybean. Furthermore, the concentrations of nutrients and selected flavonoids were quantified to give insights on possible chemical mechanisms underlying the resistance. As results, development of VBC and FAW were negatively affected when larvae fed leaves of the resistant genotype, older leaves from the lower part of plants, or leaves from reproductive-stage soybeans. The effects were partly different for each insect species, and the generalist FAW was more affected by higher resistance levels in the older leaves of soybean than the specialist VBC. Distribution and concentrations of nutrients and flavonoids in soybean in function of leaf age and plant stage may explain the varying levels of antibiosis-resistance to VBC and FAW. These results can benefit developments of specific protocols for screening resistant soybean genotypes and pest management strategies focused in plant parts and growth stages that insect-resistance levels are lowest.

Integral Characterization of the 16S rRNA Gene of Non-sporulating Bacteria and Its Action Against Anticarsia gemmatalis Hübner (Lepidoptera: Erebidae)

Brazil is the world&rsquo;s largest producer of soybean (Glycine max), an extremely important legume due to its source of proteins and essential oils for humans and animals, besides to its applications in the various branches of industry. The velvetbean caterpillar [Anticarsia gemmatalis H&uuml;bner (Lepidoptera: Erebidae)] is a great pest that affects this crop and has been controlled by chemical and biological pesticides based on Bacillus thuringiensis. The objectives of this work were to prospect soil microorganisms, to characterize them using the 16S rRNA gene and to perform bioassays to analyze the lethality or subletality of these isolates against A. gemmatalis larvae. The DNA sequencing of the marker gene was complete, covering all conserved regions of it to determine the phylogenetic position of the isolates. Regarding to bioassays, subletality efficacy were low both for sporulant and for the non-sporulant bacterial strains tested. However, based on the signature by complete 16S rRNA analyses of the non-sporulating bacterial isolates, new characteristics worth of studying and prospecting biotechnologically became available.

Elucidating Efficacy of Ingested Positively Charged Zein Nanoparticles Against Noctuidae

A meridic diet overlay bioassay using empty, positively charged zein nanoparticles ((+)ZNP) was performed on soybean looper (Chrysodeixis includens (Walker)), tobacco budworm (Heliothis virescens (F.)), and velvetbean caterpillar (Anticarsia gemmatalis Hübner) (Lepidoptera: Noctuidae). Assessment of effects on mortality and development weights 7 d after ingestion of (+)ZNP were evaluated on larvae of each species. Treatments involved different concentrations, with H. virescens and A. gemmatalis offered 0 and 3,800 ppm (+)ZNP, whereas C. includens colonies were offered 0, 630, 1,260, and 2,520 ppm (+)ZNP. Mortality of A. gemmatalis and C. includens increased after ingestion of the highest (+)ZNP concentrations, while H. virescens neonate mortality was unaffected. Neonate and third-instar weights of A. gemmatalis and C. includens, and neonate H. virescens, decreased with high (+)ZNP concentrations. Following mortality results from A. gemmatalis neonates, a concentration response test was performed using a range of (+)ZNP concentrations. The LC50 for A. gemmatalis was 1,478 ppm. The potential of (+)ZNP as a pest management tactic is discussed.

Specific binding of Bacillus thuringiensis Cry1Ea toxin, and Cry1Ac and Cry1Fa competition analyses in Anticarsia gemmatalis and Chrysodeixis includens

Anticarsia gemmatalis (velvetbean caterpillar) and Chrysodeixis includens (soybean looper) are two important defoliation pests of soybeans. In the present study, we have investigated the susceptibility and brush border membrane-binding properties of both species to Bacillus thuringiensis Cry1Ea toxin. Bioassays performed in first-instar larvae demonstrated potent activity against both soybean pests in terms of mortality or practical mortality. Competition-binding studies carried out with 125Iodine-labelled Cry1Ea, demonstrated the presence of specific binding sites on the midgut brush border membrane vesicles (BBMV) of both insect species. Heterologous competition-binding experiments indicated that Cry1Ea does not share binding sites with Cry1Ac or Cry1Fa in either soybean pest. This study contributes to the knowledge of Cry1Ea toxicity and midgut binding sites in A. gemmatalis and C. includens and sheds light on the cross-resistance potential of Cry1Ea with other Bt proteins aimed at controlling lepidopteran pests in soybeans.