# Can Eretmocerus eremicus Assess Oviposition Sites with Varying Host Densities and Predation Risks, and Make Decisions Based on Scent Cues?

## TL;DR

This study explores how the parasitoid wasp Eretmocerus eremicus uses scent to choose where to lay eggs, focusing on host presence and predation risk.

## Contribution

The study reveals that Eretmocerus eremicus responds more to strong scent contrasts than subtle differences in host density or predation risk.

## Key findings

- Eretmocerus eremicus can distinguish between host and non-host patches using scent cues.
- The wasps avoided patches with high predation risk but did not respond to smaller risk differences.
- They responded more to strong scent contrasts than to subtle scent differences in host density.

## Abstract

Parasitoid wasps can help control crop pests, but to do so effectively, they must detect where pests are and avoid places where predators could kill them. We tested whether the parasitoid wasp Eretmocerus eremicus relies on scent cues to select oviposition sites that optimize host encounters while minimizing predation risk. In laboratory choice tests conducted in a wind tunnel under a continuous airflow, we measured which patch the wasps selected, how long they took to decide, and their searching and oviposition behavior after arrival. The wasps clearly distinguished hosts from non-host patches using scent cues, but they did not reliably choose between patches with different host numbers under our assay conditions. They also avoided patches with very high predation risks more than they did safe patches, but they did not consistently respond to smaller differences in risk. Overall, under our experimental odor context, this parasitoid responded more clearly to strong scent contrasts than to subtle scent differences. Understanding how these wasps respond to host- and predator-related scent cues highlights the need for further research to identify which scent signals are most informative and how they could be used to improve parasitoid release planning for pest control.

Parasitoids use different signals to locate their hosts, and these signals can modulate their behavioral decisions. Thus, patch selection and foraging in patches with different characteristics depend on their ability to gather and use such information efficiently. In this study, we evaluated whether the parasitoid Eretmocerus eremicus (Hymenoptera: Aphelinidae), a natural enemy of Trialeurodes vaporariorum (Hemiptera: Aleyrodidae) on tomato plants (Solanum lycopersicum), uses scent cues to select and forage in patches that differ in host density and predation risk. Using choice bioassays in a wind tunnel under a continuous airflow, we recorded selection patch and selection time, as well as foraging parameters, including residence time, oviposition events, and attacks. Our results show that E. eremicus discriminated between sites with and without hosts using scent cues, but discrimination between patches with different host numbers was not detected under our assay conditions. It also distinguished between patches with maximum risk and those without risk, but not between subtle differences in risk. These findings suggest that E. eremicus, responded mainly to contrasting olfactory cues rather than to subtle odor differences. From an applied standpoint, our results motivate deeper investigation into how host- and predator-associated olfactory cues could fine-tune parasitoid deployment in biological control.