Temperature overrides nutritional cues for optimal oviposition decision in a polyphagous invasive insect.

Polyphagous insects rely on multiple cues to choose oviposition sites, including substrate temperature and nutritional quality, which often do not coincide. We examined how females of the invasive fly Drosophila suzukii make oviposition decisions when temperature and nutrition mismatch, and whether infection with the symbiotic bacterium Wolbachia influences these choices. We first quantified female performance (egg number, offspring development time, survival and mass) on four fruit purees at three ambient temperatures. We then assessed oviposition preferences when either substrate temperature or fruit quality varied independently. Finally, we conducted multi-choice experiments combining thermal and nutritional cues to test which most strongly drives oviposition. Both temperature and fruit quality affected offspring performance. Although females did not always choose the most favourable fruit, they consistently prioritised thermally optimal sites, even when these were nutritionally suboptimal. This behaviour gave partial support to the preference-performance hypothesis, which mainly held for temperature - the factor with the strongest effect on offspring development and survival in no-choice tests. Wolbachia infection enhanced offspring survival and reduced development time. It also altered oviposition patterns, leading to a more even distribution of eggs across fruit, though females maintained their preference for thermally favourable sites. Our findings suggest that the invasive success of D. suzukii could partly result from its capacity to select oviposition sites that maximise offspring performance under variable conditions. More broadly, they highlight the need to study behavioural decisions under conflicting environmental constraints to understand how behavioural flexibility contributes to individual fitness and population persistence in changing environments.