Studies on olfactory systems of two parasitoid wasps: Diadegma semiclausum (Hellén) (Hymenoptera: Ichneumonidae) and Trichogramma pretiosum Riley (Hymenoptera: Trichogrammatidae) of the Diamondback moth Plutella xylostella (L.) (Lepidoptera: Plutellidae)

Abstract

The diamondback moth (DBM), Plutella xylostella (L.) (Lepidoptera: Plutellidae), is one of the most destructive insect pests of brassicaceous crops, which has shown problematic resistance to almost every common insecticide. In certain parts of the world, the economic production of crucifers has become nearly impossible due to the failure in controlling DBM. Consequently, increased efforts worldwide have been undertaken to implement biological control programs that are principally employing its natural enemies such as Diadegma semiclausum and Trichogramma pretiosum. D. semiclausum is a specialist DBM larvae parasitoid and one of the most common parasitoids in Australia. T. pretiosum is a generalist egg parasitoid, and it inserts its eggs into the host eggs, including DBM. However, limited studies have been conducted on these two parasitoid wasps regarding how they detect DBM, how their olfactory systems guide them to localize the DBM and what olfactory genes are involved in these DBM-seeking and oviposition behaviours. Without this knowledge, it is difficult to utilize them at maximum efficiency and effectiveness in DBM management programs. In this study, analytical chemistry, electrophysiology, scanning electron microscopy, genomics, transcriptomics, bioinformatics and molecular biology approaches were applied to investigate the olfactory systems of D. semiclausum and T. pretiosum. Eight candidate attractants were identified from DBM-infested canola Brassica napus, while some of them were found to be able to initiate significant antennal responses from the parasitoid wasp D. semiclausum. Male and female D. semiclausum exhibited different antennal responses to various tested volatile compounds. Candidate olfactory genes, including 17 odorant-binding proteins (OBPs) and 67 odorant receptors (ORs), were identified and characterized from D. semiclausum. Similarly, a total of 22 OBPs and 121 ORs were identified and characterized from T. pretiosum. These genes may play pivotal roles in the host-seeking and oviposition behaviours. This study improves our understanding of the olfactory systems of these two wasps and their host-seeking behaviours, which will assist in developing more efficient and environmentally friendly biological control strategies to manage DBM