Effects of temperature and host age on suppression of Diaprepes abbreviatus (Coleoptera : Curculionidae) by entomopathogenic nematodes

Effects of temperature and host age on the biocontrol potential of entomopa thogenic nematodes against the sugarcane rootstalk borer weevil Diaprepes a bbreviatus (L.) were tested under laboratory conditions. Virulence and repr oductive potential were compared among 3 nematode species: Steinernema riob rave (Cabanillas, Poinar & Raulston), Heterorhabditis bacteriophora Poinar, and H. indica Poinar, Karunakar & David. Assays were conducted in plastic cups filled with moist sand. Three soil temperature regimes (21, 24, and 27 degrees C) and 5 larval ages (20, 30, 40, 50, and 100 d old) were combined in various treatments. The larval ages were estimated to represent 4th-10t h instar. Older larvae (i.e., 100 d old) were less susceptible to nematode infection than younger larvae. Nematodes were less virulent at 21 degrees C than at 24 or 27 degrees C. The virulence of II. indica was greater than H . bacteriophora in 50-d-old D. abbreviatus larvae at ail temperatures, and greater than the other 2 nematode species in 90-d-old larvae at 24 degrees C. Heterorhabditis bacteriophora was more virulent than S, riobrave in 20-d -old larvae (at 24 degrees C), whereas S. riobrave was more virulent than H . bacteriophora at 21 degrees C (tin 50-d-old larvae). Reproductive potenti al was greatest in H. indica followed by H. bacteriophora. The high level o f reproduction in heterorhabditid species indicates a potential for nematod e recycling in field applications. We conclude that temperature;md host age should be considered critical factors in determining the time of nematode application.