Helicoverpa armigera Larval Growth Inhibition in Artificial Diet Containing Freeze-dried Pigeonpea Pod Powder

Helicoverpa armigera (Hu¨ bner) (Lepidoptera: Noctuidae) is a key insect pest of pigeonpea, Cajanus cajan (L.) Millspaugh, in the semi-arid tropics and one of the primary production constraints (Lateef & Pimbert 1991; Shanower et al. 1999). Cajanus scarabaeoides (L.) Thouars, a wild relative of pigeonpea, is reported to be resistant to H. armigera (Lateef et al. 1981; Saxena et al. 1990; Shanower et al. 1997). Larvae feeding on flowers and green pods of C. scarabaeoides grow slower, take longer to pupate, and form smaller pupae than those that feed on C. cajan (Lateef et al. 1981; Shanower et al. 1997). A high density of pod surface trichomes, relatively tough pod wall, and differences in the structure of pod tissue may contribute to the poorer growth of H. armigera and the lower level of pod damage in C. scarabaeoides compared with C. cajan (Lateef et al. 1981; Romeis et al. 1999). In addition to these physical factors, chemicals in or on the pods may also contribute to C. scarabaeoides resistance to H. armigera. Compounds that elicit feeding responses from H. armigera larvae have been extracted from Cajanus spp. pod surfaces. An acetone-soluble feeding stimulant was found on C. cajan pods and a water-soluble feeding deterrent was observed on C. scarabaeoides pods (Shanower et al. 1997). These feeding responses were detected using a 24 h filter paper feeding test (Blaney et al., 1987). The purpose of this study was to evaluate the impact of these compounds, in the absence of physical features, using longerduration feeding tests with semi-artificial diets containing freeze-dried powder of Cajanus spp. pods or their extracts.