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 entomopathogenic nematodes against the sugarcane rootstalk borer weevil Diaprepes abbreviatus (L.) were tested under laboratory conditions. Virulence and reproductive potential were compared among 3 nematode species: Steinernema riobrave (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-10th 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 H. indica was greater than H. bacteriophora in 50-d-old D. abbreviatus larvae at all temperatures, and greater than the other 2 nematode species in 20-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 (in 50-d-old larvae). Reproductive potential was greatest in H. indica followed by H. bacteriophora. The high level of reproduction in heterorhabditid species indicates a potential for nematode recycling in field applications. We conclude that temperature and host age should be considered critical factors in determining the time of nematode application.