Laboratory Studies of the Interactions of Environmental Conditions on the Susceptibility of Green June Beetle (Coleoptera: Scarabaeidae) Grubs to Entomopathogenic Nematodes

In the laboratory, four nematode species differed significantly in the percentage mortality that each caused in second-instar green June beetles (grubs), Cotinis nitida (L.). Water suspensions of 1,000 infective juveniles (IJ)/one grub/Petri dish each of Heterorhabditis bacteriophora, Steinernema glaseri, and S. feltiae caused 34, 22 and 18% grub mortality, respectively. These values were significantly (P = 0.05) greater than the grub mortality caused by either S. carpocapsae (12%) or the water check (0%) that were similar. Several factors affected third- and fourth-instar susceptibility in the laboratory when exposed to 10,000 S. carpocapsae or H. bacteriophora IJ in 75 g of soil (133 IJ/cm³ soil)/plastic cup. Soil at 30% moisture by weight resulted in significantly more (P= 0.006) dead grubs (6.6%) than at 10% soil moisture (2.8%). Significantly more (P = 0.03) grubs died when at a grub density of 2 grubs/plastic cup (5.6%) than at 1 grub/plastic cup (3.8%). Significantly more (P = 0.01) grubs were killed by either S. carpocapsae and H. bacteriophora at a soil temperature of 25°C (6.5%) than at 12°C (2.8%). There was also a significant interaction of temperature and nematode species. Heterorhabditis bacteriophora killed significantly more (P= 0.02) grubs at 25°C (9.5%) than at 12°C (1.9%) and more than S. carpocapsae killed at either 12°C or 25°C (<3.9%). More virulent nematode strains or species than these four nematode species must be identified if green June beetles are to be controlled by nematodes.