Plant–Herbivore–Trichogramma Interactions: Chemical Ecology, Host Quality, Diapause Regulation, and Implications for Biological Control

Diapause is an important seasonal adaptation that enables Trichogramma egg parasitoids to survive unfavorable environmental conditions and maintain synchrony with host availability. Although temperature and photoperiod are established regulators of diapause, plants can influence parasitoid ecology through changes in host availability, volatile emissions, defense responses, host egg quality, and local environmental conditions. This review synthesizes evidence on plant–herbivore–Trichogramma interactions and examines their potential relevance to diapause regulation and biological control. Studies demonstrate that plant species, host-plant phenology, herbivore-induced and oviposition-induced volatiles, and plant defense responses can affect parasitoid host searching and parasitism. Plant-mediated changes in host nutrition and plant architecture also influence parasitoid development, dispersal, and performance, although responses vary among species and environmental conditions. We further review evidence on the regulation of Trichogramma diapause by temperature, photoperiod, maternal effects, metabolic pathways, and population-specific responses, together with recent findings on lipid metabolism, carbohydrate transport, endocrine signaling, and symbiont-associated variation. Importantly, direct experimental evidence linking plant-derived cues to diapause induction remains limited. Climate change may further affect parasitoid–host synchrony by altering thermal conditions, plant development, host phenology, and extreme temperature exposure. Future research should integrate plant chemistry, host egg quality, maternal conditions, and physiological measurements with direct assessments of diapause under realistic environmental conditions. Such studies may improve our understanding of seasonal adaptation and support more reliable Trichogramma-based biological control.