Tomato chemical defenses intensify corn earworm (Helicoverpa zea) mortality from opportunistic bacterial pathogens

Authors: Mason, Charles; PEIFFER, MICHELLE - Pennsylvania State University; HOOVER, KELLI - Pennsylvania State University; FELTON, GARY - Pennsylvania State University

Submitted to: Journal of Chemical Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/13/2023
Publication Date: 3/25/2023
Citation: Mason, C.J., Peiffer, M., Hoover, K., Felton, G. 2023. Tomato chemical defenses intensify corn earworm (Helicoverpa zea) mortality from opportunistic bacterial pathogens. Journal of Chemical Ecology. https://doi.org/10.1007/s10886-023-01420-7.
DOI: https://doi.org/10.1007/s10886-023-01420-7

Interpretive Summary: Insects can face multiple, simultaneous challenges that threaten their ability to reach the reproductive life stages. The goal of this study was to determine how plant resistance interacts with opportunistic pathogens in the pest lepidopteran, corn earworm (Helicoverpa zea). This study employed multiple tomato genotypes, along with application by an elicitor, methyl jasmonate, to determine how variation in plant chemistry impacts herbivore survival from oral Serratia marcescens inoculation. Our results revealed that there are compounding effects by both plant and bacterial agents on larval mortality. Insect mortality was low when they fed on plants with low defenses and were not administered bacteria. Inoculation of the bacteria killed ~55% of the insects within five days of inoculation on plants with low levels of defense, and 95% on plants with higher levels of defenses. High levels of defenses increased systemic infection of the larval hosts by Serratia marcescens compared to plants which did not have their defenses induced. Both plant defenses and the microbe damage the gut of the larvae, and we discuss the ramifications of these combined assaults on the host. Plant defenses likely have wide-ranging interactions with opportunistic microorganisms naturally occupying the digestive system.

Technical Abstract: Insect herbivores face multiple challenges to their ability to grow and reproduce. Plants can produce series of defenses that disrupt and damage the digestive system, that are only heightened upon injury. Additionally, insects face threats from virulent microorganisms that can incur their own set of potential costs to hosts. Microorganisms that invade through the digestive system may function in concert defenses generated by plants, creating combined assailments on a host insect. In our study, we evaluated how tomato defenses interacts with an enteric bacterial isolate, Serratia marcescens in the corn earworm (Helicoverpa zea). We performed bioassays using different tomato cultivars that were induced by methyl jasmonate and larvae orally inoculated with a S. marcescens isolate. Untreated corn earworm larval mortality was low on constitutive tomato, while larvae inoculated with S. marcescens exhibited >50% mortality within 5 days. Induction treatments elevated both control mortality (~45%) and when combined with S. marcescens (>95%). Larvae also died faster when encountering induced defenses and Serratia. Using a tomato mutant, foliar polyphenol oxidase activity likely had stronger impact on S. marcescens-mediated larval mortality. Induction treatments also elevated the number of bacterial colony forming units in the hemolymph of larvae inoculated with Serratia. Larval mortality by S. marcescens was low (<10%) on artificial diets. Our results demonstrate that plant chemical defenses enhance larval mortality from an opportunistic gut microbe. We propose that the combined damage from both the plant and microbial agent overwhelm the herbivore to increase mortality rates and expedite host death.