BIOASSAY STUDY OF RESPONSE BY A PARASITOID TO FRASS AND FEEDING SUBSTRATES OF ITS HOST, THE STALK BORER ELDANA-SACCHARINA

In Petri dish and olfactometer tests Goniozus natalensis Gordh (Hymeno ptera: Bethylidae) exhibited a host-searching response upon contact wi th, and at a short distance from, frass of its natural host, Eldana sa ccharina Walker (Lepidoptera: Pyralidae), whichever the substrate on w hich E. saccharina had been feeding. The substrates were two host plan ts, sugarcane or Cyperus papyrus L.; diet media based respectively on sugarcane, papyrus, or cellulose; or a synthetic diet medium, containi ng no plant material. Host-searching behaviour increased in intensity with age and after mating of G. natalensis females. Parasitoids respon ded similarly to frass produced by male and female E. saccharina larva e. The substrates themselves elicited little response in G. natalensis . Frass of another borer, Sesamia calamistis Hamps. (Lepidoptera: Noct uidae), fed on sugarcane-based medium, also elicited little response i n G. natalensis. The results suggest that frass of E. saccharina larva e contains a material, evidently volatile, to which their parasitoid G . natalensis responds, irrespective of the substrate on which the E. s accharina subsist.