Assessing the suitability of flowering herbs as parasitoid food sources: flower attractiveness and nectar accessibility

Eleven insect-pollinated plant species were investigated with respect to their olfactory attractiveness and nectar accessibility for the parasitoid species Cotesia glomerata (Hymenoptera: Braconidae), Heterospilus prosopidis (Hymenoptera: Braconidae), and Pimpla turionellae (Hymenoptera: Ichneumonidae). Flowers differed considerably with respect to both their attractiveness and nectar accessibility. The results obtained from the three parasitoid species, on the other hand, showed a high level of congruency. Considering both parameters, flower attractiveness was not strictly correlated with nectar accessibility. Out of the 11 plant species tested, only two species (Aegopodium podagraria [Apiaceae] and Origanum vulgare [Lamiaceae]) were optimal as a parasitoid food source, as they combined olfactory attractiveness with accessible nectar. Two other species (Galium mollugo [Rubiaceae] and Leucanthemum vulgare [Asteraceae]) were attractive without providing an accessible food source. The remaining plant species either failed to attract the parasitoids (Daucus carota [Apiaceae], Erigeron annuus [Asteraceae], Medicago lupulina [Fabaceae], and Trifolium repens [Fabaceae]), or even repelled them (Achillea millefolium [Asteraceae], Trifolium pratense [Fabaceae], and Vicia sepium [Fabaceae]). These results show that the mere presence of flowering plants in an agroecosystem is not sufficient to guarantee nectar supply for parasitoids. It also underscores that mechanistic investigations can be a valuable tool in helping us tailor agroecosystems to the requirements of biological control agents.