Trichogramma egg parasitism of Helicoverpa armigera on pigeonpea and sorghum in southern India

Trichogramma spp. (Hymenoptera: Trichogrammatidae) only rarely parasitize e ggs of Helicoverpa ar armigera (Hubner) (Lepidoptera: Noctuidae) on pigeonp ea (Cajanus cajan (L.) Millspaugh), while on other plants including sorghum (Sorghum bicolor (L.)) high parasitism levels of this host are found. In t his study two strategies designed to increase the parasitoid activity on pi geonpea were tested in the field: inter-cropping pigeonpea with sorghum and mass-releasing T. chilonis Ishii. Neither strategy led to an increase in p arasitism. On pigeonpea, H. armigera oviposited >74.8% of its eggs on calyx es and pods. Parasitism levels in host eggs collected from different plant structures varied significantly with 3.6, 0.3, and 40.7% of eggs on calyxes , pods, and leaves parasitized. Earlier studies have shown that calyxes and pods possess long glandular and non-glandular trichomes, and are covered b y sticky trichome exudates which inhibit parasitoid searching behaviour. Pa rasitism levels between 27.9 and 100% were recorded from host eggs on the i ntercropped sorghum. Trichogramma chilonis was the dominant parasitoid spec ies. The mean clutch size was 2.03, but up to six parasitoids emerged per e gg. Progeny sex ratio (% females) decreased with clutch size, from 63.1% at a clutch size of one to 46.0% at a clutch size of five. Sticky trap catche s showed that while the parasitoid population in sorghum increased when H. armigera started ovipositing, the population within pigeonpea did not benef it from either a high parasitoid population in sorghum or a high host egg d ensity on pigeonpea. During one of five seasons studied, however, high para sitism levels (up to 73%) were recorded on pigeonpea. During this season, H . armigera oviposited on pigeonpea plants in the vegetative growth stage an d a high proportion of eggs were collected from leaves. Parasitism levels w ere positively correlated with the percentage of eggs collected from leaves . This study shows that the parasitization efficiency of Trichogramma spp. on pigeonpea depends mainly on the location of the host eggs. This explains why parasitism levels of H. armigera eggs on pigeonpea did not increase wh en intercropped with sorghum or after mass-releasing T. chilonis.