Effect of plant structure on host finding capacity of lepidopterous pests of crucifers by two Trichogramma parasitoids

Generalist parasitoids are confronted by different plant structures and differences in host abundance and distribution within and among plants. These differences may affect host finding success. In this study, we investigated the effect of plant structure on host finding success of two Trichogramma species using three species of Lepidoptera and three structurally, but closely related crucifers (cabbage, broccoli, Brussels sprouts). We also determined the searching pattern of female parasitoids by characterizing the distribution pattern of parasitized eggs according to leaf surface and plant height. Neither species of parasitoid showed a preference for any of the three plant varieties. We found significant effects of plant variety, pest species, and parasitoid species on parasitism and only the parasitoid × plant structure interaction was significant. Mean percent parasitism generally tended to decrease with an increase of plant structural complexity and was greatest with cabbage, lowest with Brussels sprouts, and intermediate with broccoli. On simple plant structures, both wasp species performed well on the three varieties of plants. Females parasitized more eggs at the base of plants and on the inferior side of leaves. This study demonstrates that plant structure can constrain parasitoids while they are foraging for hosts.