Selection of cold injury treatments to facilitate release of the parasitoid Anisopteromalus calandrae (Hymenoptera : Pteromalidae) reared on the riceweevil (Coleoptera : Curculionidae)

The use of cold treatments to kill the rice weevil, Sitophilus oryzae (L.) before eclosion but allow the parasitoid Anisopteromalus calandrae (Howard) to complete development and eclosion was examined. Cold tolerance, crystal lization temperature, and developmental stage of rice weevils were examined at 2-d intervals between 18 and 30 d after rice weevil oviposition. The me dian crystallization temperature dropped from -16 to -24 degrees C: over th is time, and cold tolerance was lowest between day 20 and 24. Based on thes e data, rice weevil immatures were exposed on days 18, 19, 21, 22, 23, and 24 after oviposition for 2 h to various subzero temperatures. Hosts were ex posed to parasitoids for oviposition on day 20. Before parasitization, rice weevils were either frozen internally by exposure to -25 degrees C, or wer e subjected to nonfreezing chilling injury by exposure to -12 degrees C. Ri ce weevils exposed to cold after parasitization were subjected to chilling injury by exposure to either -12 or -10 degrees C. Freezing rice weevil lar vae before parasitization resulted in poor production of parasitoids and a sex ratio favoring males. Exposure to -12 degrees C for 2 h, 2 or 3 d after parasitization resulted in production of parasitoids similar to unchilled controls, and a slightly higher female-to-male ratio than the controls. The production of A. calandrae was better after exposure to -10 degrees C, but at this temperature some rice weevils eclosed. The treatment of 2 h at -12 degrees C, 3 d after parasitization gave the best parasitoid survival and ensured rice weevil mortality.