Non-consumptive effects of parasitoids and predators in stored products: The case of Theocolax elegans and other field-collected predators on the foraging of lesser grain borer and rice weevil

Authors: Hetherington, Matthew; SAKKA, MARIA - University Of Thessaly; MAILLE, JACQUELINE - Kansas State University; STOLL, IAN - University Of Missouri; ATHANASSIOU, CHRISTOS - University Of Thessaly; Scully, Erin; Gerken, Alison; Morrison Iii, William

Submitted to: Pest Management Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/2/2025
Publication Date: 10/21/2025
Citation: Hetherington, M.C., Sakka, M., Maille, J.M., Stoll, I., Athanassiou, C.G., Scully, E.D., Gerken, A.R., Morrison III, W.R. 2025. Non-consumptive effects of parasitoids and predators in stored products: The case of Theocolax elegans and other field-collected predators on the foraging of lesser grain borer and rice weevil. Pest Management Science. 2025; 81: 7529–7541. https://doi.org/10.1002/ps.70230.
DOI: https://doi.org/10.1002/ps.70230

Interpretive Summary: Parasitoids are arthropods that predate and feed on live insects. Thus, they are often deliberately released to reduce populations of insect pests, including stored product insects. Prior research on the effects of parasitoids on the biological control of stored product insects has focused exclusively on the effects of direct predation on stored product insect populations; however, in other insect species, the mere presence of a parasitoid can be sufficient to induce fear, which can change the behavior of the insect in a way that reduces feeding and reproduction, thus also reducing populations. In this study, we evaluated the behavioral responses of two common stored product insects (lesser grain borer and rice weevil) to a parasitoid (Theocolax elegans), other insect predators found around food facilities, and odors emitted by these insects. Movement by lesser grain borer and rice weevil was significantly reduced in the presence of parasitoids. The presence of parasitoid odors led to a 48–58% reduction in distance moved and 40–56% reduction in speed of movement by rice weevil and lesser grain borer. Lesser grain borer showed a 73% reduced frequency of entering zones filled with food when parasitoids were present, indicating a reduction in foraging behavior. We conclude that fear from nearby parasitoids and predators may result in alterations in behavior by stored product insects, which could be beneficial for reducing populations.

Technical Abstract: BACKGROUND: To-date, prior research on the biological control of stored products has focused on the direct consumptive effects of parasitoids and predators. However, in other systems, trait-mediated indirect interactions or non-consumptive effects have been shown to be as or more important in maintaining the suppression of pest populations. Thus, in this study, our aim was to evaluate the effect of the parasitoid, Theocolax elegans, the predator community around food facilities, and their associated chemical cues on the foraging behavior of Sitophilus oryzae and Rhyzopertha dominica. RESULTS: We evaluated consumptive predation of a variety of insect and arachnid predators on S. oryzae as well as effects on the foraging behavior of S. oryzae and R. dominica in a series of movement assays after exposure to adult T. elegans, T. elegans or gryllid headspace extracts, and a facsimile gryllid model to determine potential non-consumptive effects. We found the headspace from T. elegans colonies were distinct from other treatments. The strength of non-consumptive effects appeared to be greater after exposure to T. elegans adults compared to other predators or extracts. The presence of T. elegans in a block led to a 48–58% reduction in distance moved and 40–56% reduction in instantaneous velocity for the control and wheat treatments by S. oryzae and R. dominica compared to when wasps were absent. Rhyzopertha dominica exhibited 73% reduced frequency of entering the zone with wheat when adult T. elegans was present. CONCLUSIONS: We conclude that non-consumptive effects may result in alterations in behavior by stored product insects, but these effects may vary by the coevolutionary relationship between the natural enemy and pest.