Developing a strategy for using entomopathogenic nematodes to control the African black beetle (Heteronychus arator) in New Zealand pastures and investigating temperature constraints

The African black beetle (Heteronychus arator F.) is a pest of pastures in the North Island of New Zealand. Except during dispersal activity, adult beetles live in the soil and feed at the base of grass stems during spring and fall and to a lesser amount over-winter. Larvae develop over summer months feeding on the grass roots. We tested three life-stages of H. arator (first and third instar larvae and adults) for susceptibility to the four entomopathogenic nematode (EPN) species present in New Zealand: Steinernema carpocapsae, S. feltiae, Heterorhabditis bacteriophora and H. zealandica. All H. arator stages were found to be susceptible to EPN, but different life-stages varied in susceptibility to EPN species. Based on our laboratory assays, we chose to target adult beetles using S. carposcapsae. We ran three field experiments, one in spring and two in fall but the EPN did not reduce beetle numbers in any experiments. Field temperatures were cool, but within the stated thermal niche breadth of S. carpocapsae for killing hosts. Sampling on nematode- treated plots revealed several susceptible lepidopteran larvae clearly infected with S. carpocapsae but no infected H. arator. We tested the hypothesis that the effective niche breadth for killing hosts may vary with host-susceptibility. We studied the temperature response of S. carpocapsae against Galleria mellonella and H. arator at 12.5, 15, 18, 20 & 25 °C. While S. carpocapsae caused significant mortality of G. mellonella at all temperatures, no H. arator mortality was seen at 12.5% or 15 °C despite using a high nematode rate and incubating for two weeks. Our data highlight the importance of testing susceptibility of different insect life stages at the temperatures likely to be encountered in the field following nematode application.