Conventional soil management may promote nutrients that lure an insect pest to a toxic crop

Authors: Schmidt, Rebecca; MORETTI, ERICA - US Department Of Agriculture (USDA); WICKINGS, KYLE - Cornell University; WOLFIN, MICHAEL - Pennsylvania State University; NORTHFIELD, TOBIN - Washington State University; LINN, CHARLIE - Cornell University; NAULT, BRIAN - Cornell University

Submitted to: Environmental Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/15/2020
Publication Date: 12/30/2020
Citation: Schmidt-Jeffris, R.A., Moretti, E.A., Wickings, K., Wolfin, M.S., Northfield, T.D., Linn, C.E., Nault, B.A. 2020. Conventional soil management may promote nutrients that lure an insect pest to a toxic crop. Environmental Entomology. 50:433-443. https://doi.org/10.1093/ee/nvaa167.
DOI: https://doi.org/10.1093/ee/nvaa167

Interpretive Summary: In some cases, insect herbivores are more attracted to crops grown in soil managed conventionally versus organically. This is thought to be due to improved health of organically grown plants, which occurs because organic fertilizers like manure provide a slower and more consistent release of nutrients. Insect herbivores are able to detect less healthy, less defended plants and will preferentially eat or lay eggs on these plants. Because some conventional crops, like corn, are planted with GMO varieties that are toxic to pests, it is possible that this boosts the efficacy of those varieties because insects are unintentionally attracted to the toxic crop. Researchers at the USDA-ARS in Wapato, in collaboration with Cornell University, Pennsylvania State University, and Washington State University, conducted a series of laboratory assays with soil collected from conventional and organic corn fields. These assays examined the health of the soil, the nutritional content of corn plants grown in the soil, and attraction to the corn plants by European corn borer, a key pest of corn. They also monitored European corn borer activity in adjacent conventional and organic fields. They determined that European corn borer was more attracted to the plants grown in conventional soil but could not discriminate between corn with and without the GMO trait. Their results indicated that the plants that were more attractive to European corn borer had lower magnesium content, which was linked to a lower ratio of magnesium to potassium in the conventional soil. This research increases understanding of how soil management practices impact pest insect preferences and indicates that conventional soil management may unintentionally attract pests to toxic crops, benefitting farmers

Technical Abstract: 1. Slow and consistent nutrient release by organic fertilizers can improve plant nutrient balance and defenses, leading to herbivore avoidance of organically managed crops in favor of plants with weaker defenses. We propose that this relative attraction to conventional plants, coupled with the use of genetically-modified, insecticidal crops (Bt) has created an unintentional attract-and-kill system. 2. We sought to determine if Bt and non-Bt corn plants grown in soil collected from five paired organic and conventional fields differed in attractiveness to European corn borer (ECB) moths, by conducting ovipositional choice and flight tunnel assays. Then, we examined the mechanisms driving the observed differences in attraction by comparing soil nutrient profiles, soil microbial activity, and plant nutrition. We also investigated the volatile production of the corn grown in the soil treatments. Finally, we assessed whether ECB abundance near corn fields differed based on soil management. 3. ECB preferred plants grown in conventional soil but did not discriminate between Bt and non-Bt corn. Individual nutrients (calcium, magnesium, potassium, phosphorus) within plants, as opposed to a nutrient balance index, differed between the soil management types. In a structural equation model analysis, organic management and more alkaline soil was associated with an increased soil magnesium:potassium ratio, which increased plant magnesium, and was linked to reduced ECB oviposition. 4. The ratio of the volatiles 2-hexenal:4-hydroxy-4-methyl-2-pentanone did not significantly differ between plants grown in the two soil management types. However, the ratio was correlated with decreased flight orientation. 5. There was an inconsistent trend for higher ECB moth activity near conventional corn fields. 6. Synthesis and applications. Our results extend the plant nutrient balance hypothesis describing conventional plant preference by showing that it can also improve attraction to plants with genetically inserted toxins. Further, we identified individual plant nutrients that impacted herbivore choice. Unintentional attract (to conventional) and (Bt) kill is a plausible scenario for pest declines in response to Bt corn adoption, but this effect may be obscured by variation in other management practices and landscape characteristics