Metabolomics Differences of Glycine max QTLs Resistant to Soybean Looper

Quantitative trait loci (QTLs) E and M are major soybean alleles that confer resistance to leaf-chewing insects, and are particularly effective in combination. Flavonoids and/or isoflavonoids are classes of plant secondary metabolites that previous studies agree are the causative agents of resistance of these QTLs. However, all previous studies have compared soybean genotypes that are of dissimilar genetic backgrounds, leaving it questionable what metabolites are a result of the QTL rather than the genetic background. Here, we conducted a non-targeted mass spectrometry approach without liquid chromatography to identify differences in metabolite levels among QTLs E, M, and both (EM) that were introgressed into the background of the susceptible variety Benning. Our results found that E and M mainly confer low-level, global differences in distinct sets of metabolites. The isoflavonoid daidzein was the only metabolite that demonstrated major increases, specifically in insect-treated M and EM. Interestingly, M confers increased daidzein levels in response to insect, whereas E restores M’s depleted daidzein levels in the absence of insect. Since daidzein levels do not parallel levels of resistance, our data suggest a novel mechanism that the QTLs confer resistance to insects by mediating changes in hundreds of metabolites, which would be difficult for the insect to evolve tolerance. Collective global metabolite differences conferred by E and M might explain the increased resistance of EM.

Mixture compatibility of ChinNPV baculovirus with herbicides and fungicides used in soybean

Chrysodeixis includens Walker, 1858 (Lepidoptera: Noctuidae), commonly known as soybean looper, is one of the main pests that reduces soybean yield due to its defoliation capacity. Population outbreaks of this pest can occur in the vegetative stage of the crop, together with the occurrence of weeds or in the reproductive stage, at the same time as that of diseases. This often requires the use of pesticides against pests and weeds, or fungi at the same time. Thus, the objective of this study was to evaluate the compatibility of baculovirus ChinNPV with different synthetic chemical herbicides and fungicides used in soybean. Four bioassays were carried out, with or without the addition of ChinNPV to different herbicides and fungicides. The artificial diets were immersed in the solutions of the pesticides and their mixture and supplied to the caterpillars of C. includens, immediately and after one and two hours of mixing. The evaluation was performed by quantifying the number of dead caterpillars. The results showed that the ChinNPV baculovirus is compatible for mixing with all studied herbicides and fungicides, even after two hours of mixing. In all the studied scenarios, baculovirus mixed with chemical pesticides triggered the same mortality as sprayed with baculovirus alone, and values of over 80% mortality of C. includens have always been recorded.

Evaluation of Reference Genes and Expression Level of Genes Potentially Involved in the Mode of Action of Cry1Ac and Cry1F in a Susceptible Reference Strain of Chrysodeixis includens

Soybean looper (SBL), Chrysodeixis includens (Walker), is one of the major lepidopteran pests of soybean in the American continent. SBL control relies mostly on the use of insecticides and genetically modified crops expressing Bacillus thuringiensis (Bt) insecticidal Cry proteins. Due to the high selection pressure exerted by these control measures, resistance has developed to different insecticides and Bt proteins. Nevertheless, studies on the mechanistic background are still scarce. Here, the susceptibility of the laboratory SBL-Benzon strain to the Bt proteins Cry1Ac and Cry1F was determined in diet overlay assays and revealed a greater activity of Cry1Ac than Cry1F, thus confirming results obtained for other sensitive SBL strains. A reference gene study across larval stages with four candidate genes revealed that RPL10 and EF1 were the most stable genes for normalization of gene expression data obtained by RT-qPCR. Finally, the basal expression levels of eight potential Bt protein receptor genes in six larval instars were analyzed, including ATP-binding cassette (ABC) transporters, alkaline phosphatase, aminopeptidases, and cadherin. The results presented here provide fundamental knowledge to support future SBL resistance studies.

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.

Effects of soil management and plant breeding on insect herbivore growth and consumption

[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT REQUEST OF AUTHOR.] The benefits of sustainable soil management practices on soil health have been well documented for over a century. In 2017, 42,087,306 ha of farm land were cultivated with no-till practices, which reduce soil erosion. Crop rotation can also improve soil health by increasing soil nitrogen content. As nitrogen is a limiting factor to plant and insect growth, changes in soil nitrogen will likely affect plant-insect interactions. Furthermore, far fewer studies have examined long-term effects of soil management on insect communities. We examine the effects of longterm soil management practices (27 years) on soil nitrogen availability, foliar nitrogen content and secondary metabolites, and insect herbivory. We compare leaf-chewing insect growth and consumption rates feeding on tilled continuous, no-till continuous, tilled rotated, and no-till rotated plots containing soybean (Glycine max). While piercing-sucking insects were fed corn (Zea mays) kernels from rotated no-till or rotated tilled plots or soybean pods from rotated no-till or continuous till plots. Piercing-sucking insects experienced higher growth on continuous tilled corn in comparison to the soybean treatments, likely due to the senescence of the soybean pods. In 2018, soybean looper (Chrysodeixis includens Walker) feeding on continuously planted, tilled soybean had greater relative growth rates than those feeding on rotated, no-till soybean. However, in 2019, soybean looper consumption was higher on the rotated, no-till treatment than on continuous till soybean. Fall armyworm (Spodoptera frugiperda J.E. Smith) consumption and growth did not differ among treatments in both years. While percent soil nitrogen levels differed, our bioassay results led us to conclude that management practices that promote soil health such as no-tillage and crop rotation do not greatly influence insect herbivory. Additionally, for thousands of years, humans have artificially selected for yield in food crops. In recent years, this has become more critical as arable land has decreased while human populations have increased. Increase in yield influences plant physiology, photosynthetic rates, and nitrogen uptake. As plant growth has been selected for, plant defenses may have decreased, leading to potential increases in insect herbivore growth, consumption, efficiency of conversion of digested food, and efficiency of conversion of ingested food. We conducted bioassays feeding soybean looper (Chrysodeixis includens Walker) larvae leaves from 19 soybean cultivars released over the last 86 years. Larval growth was not affected by soybean release year, but consumption decreased on recent varieties. Efficiency of conversion of digested food and efficiency of conversion of ingested food also increased on more recent varieties, perhaps, because of differences in foliar nitrogen or terpenoids. This is important as selection for yield has not necessarily resulted in selection for higher insecticide application.