Microbial volatile organic compounds from tempered and incubated grain mediate attraction by a primary but not secondary stored product insect pest in wheat

Authors: VAN WINKLE, TAYLOR - Michigan State University; PONCE, MARCO - Kansas State University; QUELLHORST, HANNAH - Kansas State University; Bruce, Alexander; ALBIN, CHLOE - Kansas State University; KIM, TANIA - Kansas State University; ZHU, KUN YAN - Kansas State University; Morrison, William - Rob

Submitted to: Journal of Chemical Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/22/2021
Publication Date: 9/20/2021
Citation: Van Winkle, T., Ponce, M., Quellhorst, H.E., Bruce, A.I., Albin, C.E., Kim, T., Zhu, K., Morrison Iii, W.R. 2021. Microbial volatile organic compounds from tempered and incubated grain mediate attraction by a primary but not secondary stored product insect pest in wheat. Journal of Chemical Ecology. 48:27-40. https://doi.org/10.1007/s10886-021-01312-8.
DOI: https://doi.org/10.1007/s10886-021-01312-8

Interpretive Summary: Preventing stored product insects from finding and infesting stored grain and other foods is critical in pest management. It is likely that insects use volatile cues or odors to find food sources, but the specific odors they use is unknown. If we could identify these odors, we could exploit them to manipulate where pest insects move and/or to draw insects away from food sources and into traps. Moldy grain is sometimes preferred by insects, so the odors given off from this type of grain might be worth further exploration. The use of these moldy odors offers an advantage relative to other odor sources, because they are typically absent from appropriately-stored grain at food facilities, and so would be unique, possibly preferred stimuli over grain odors that could then be incorporated into traps. Primary stored product insects (those that feed and reproduce directly on intact grain) and secondary stored product insects (those that can only feed and reproduce on damaged or broken grain) may respond differently to moldy odors as a function of their life history. Light colonization of intact grain by fungi may weaken the outer shell of the grain sufficiently to allow secondary pests to use otherwise inaccessible nutrients in intact kernels, for example. Contrary to expectations, we found that the primary pest (lesser grain borer) was attracted to microbial odors from grain tempered to 12–19% grain moisture and incubated for longer periods (18–27 d) in a wind tunnel, and a release-recapture assay using commercial traps. The movement and orientation of the secondary pest (red flour beetle) was unaffected by moldy odors. Increasing grain moisture resulted in elevated grain damage detected through near infrared spectroscopy, and resulted in small but significant differences in the blend of volatiles emitted by treatments in odor collections and analysis. In sequencing the microbial community on the grain, we found a diversity of fungi, suggesting that a diverse assemblage was responsible for emissions. We conclude that the lesser grain borer is attracted to a broader suite of microbial cues than red flour beetle, and believe some of the compounds identified in this study will be useful as novel stimuli for manipulating the movement of pest insects in the future.

Technical Abstract: There has been a dearth of research elucidating the behavioral effect of microbially-produced volatile organic compounds (MVOCs) on insects in postharvest agriculture. Demonstrating attraction to MVOCs by stored product insects would provide an additional source of unique behaviorally-relevant stimuli to protect postharvest commodities at food facilities. Here, we assessed the behavioral response of a primary (Rhyzopertha dominica) and secondary (Tribolium castaneum) grain pest to bouquets of volatiles produced by whole wheat that were untempered, or tempered to 12, 15, or 19% grain moisture and incubated for 9, 18, or 27 days. We hypothesized that MVOCs may be more important for the secondary feeder, because they signal that otherwise unusable, intact grains have become susceptible by weakening of the bran. However, contrary to our expectations, we found that the primary feeder, R. dominica, but not T. castaneum was attracted to MVOCs in a wind tunnel experiment, and in a release-recapture assay using commercial traps baited with grain treatments. Increasing grain moisture resulted in elevated grain damage detected by near-infrared spectroscopy, and resulted in small but significant differences in the blend of volatiles emitted by treatments detected by gas chromatography coupled with mass spectrometry (GC-MS). In sequencing the microbial community on the grain, we found a diversity of fungi, suggesting that an assemblage was responsible for emissions. We conclude that R. dominica is attracted to a broader suite of MVOCs than T. castaneum, and that our work highlights the importance of understanding insect-microbe interactions in the postharvest agricultural supply chain.