Submitted to: Entomologia Experimentalis et Applicata
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/3/1998
Publication Date: N/A
Citation: Romeis, J., Shanower, T.G., Zebitz, C.P. 1999. Trichogramma egg parasitism of Helicoverpa armigera on pigeonpea and sorghum in southern India. Entomologia Experimentalis et Applicata. 90(1): 69-81. DOI: 10.1046/j.1570-7458.1999.00424.x. Interpretive Summary: The pod borer Helicoverpa armigera attacks pigeonpea, sorghum and many othe crops in India. Trichogramma spp. egg parasites are important natural enemies of the pod borer but are relatively ineffective on pigeonpea as compared to sorghum. The purpose of this study was evaluate methods for enhancing the activity of the parasitoid on pigeonpea. A secondary objectiv vwas to understand why the parasitoid is ineffective on pigeonpea. Two methods were evaluated: intercropping pigeonpea and sorghum, and mass-releasing the parasite in pigeonpea fields. Neither strategy lead to increased levels of parasitism on pigeonpea. On pigeonpea the podborer lays most of its eggs on the pods. Parasitism levels varied widely among eggs collected from different plant structures, with the highest levels recorded from eggs on leaves and the lowest levels from eggs on pods. This study demonstrated that the parasite's efficiency on pigeonpea depends primarily on the location of the host egg. This explains way parasitism levels did n increase when pigeonpea was intercropped with sorghum or after mass-releasing the parasite.
Technical Abstract: Trichogramma spp. (Hym.: Trichogrammatidae) only rarely parasitize eggs of Helicoverpa armigera (Lep.: Noctuidae) on pigeonpea, while on other plants including sorghum, high parasitism levels of this host are found. In this study two strategies designed to increase the parasitoid activity on pigeonpea were tested in the field: intercropping pigeonpea with sorghum an nmass-releasing T. chilonis. Neither strategy led to an increase in parasitism. On pigeopea, H. armigera oviposited > 74% of its eggs on calyxes 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 ar covered by sticky trichome exudates which inhibit parasitoid searching behavior. Parasitism levels between 28 and 100% were recorded from host eggs on the intercropped sorghum. Trichogramma chilonis was the dominant parasitoid species. The mean clutch size was 2.03, but up to six parasitoids emerged per egg. The percentage of females decreased from 63% at a clutch size of one to 46% at a clutch size of five. Sticky trap catches showed that the parasitoid population in sorghum increased when H. armigera started ovipositing, but pigeonpea did not benefit from either a high parasitoid population in sorghum or a high host egg density on pigeonpea. This study shows that the parasitizaiton 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 when intercropped with sorghum or after mass-releasing T. chilonis.