Abiotic and biotic influences on entomopathogenic nematode efficacy

Entomopathogenic nematodes (EPNs), microscopic roundworms that infect and kill insects, have gained importance as biological control agents for the safe, environmentally friendly control of insect pests. However, their adoption by managers is limited by variable field performance, to which variability in abiotic and biotic soil properties is a likely contributor. Abiotic soil properties such as texture, moisture, and porosity can affect the ability of EPNs to survive, move, and locate hosts. Also, many soil organisms can interact with EPNs in helpful or harmful ways, though their effects on EPN infectivity remain poorly quantified. Increased knowledge of the extent and relevance of abiotic and biotic soil properties’ effects on EPN performance will help managers better utilize EPNs against pests, both through increased ability to predict whether or not EPNs will be effective in a given soil and through soil management practices to increase their suitability for EPNs. My research is focused on examining the effects of biotic interactions on the ability of EPNs to infect hosts, and also to relate biotic and abiotic soil properties to EPN control of a pest insect in field soils. In laboratory trials, I evaluated the effect of soil microarthropod communities on EPN infectivity against the model host Galleria mellonella. Individuals of the EPN Heterorhabditis bacteriophora were applied into loam soil mesocosms containing or lacking a microarthropod community. Five G. mellonella larvae were introduced to each mesocosm sixteen days later and removed after an additional five days. The cadavers were dissected and average number of invading EPNs determined for each replicate, as well as the overall percentage of infection. Percent EPN infection of G. mellonella and average EPN establishment was significantly lower in soils with fauna than in soils without fauna. In addition, linear regression analysis suggested a significant negative correlation between the abundance of predatory mesostigmatid mites and EPN establishment within hosts. These results indicate negative effects of soil animals on EPN infectivity, likely due to predation. In the field, the relationship between EPN efficacy and biotic as well as abiotic soil properties was tested in turfgrass athletic fields. A series of field bioassays with the EPNs H. bacteriophora and Steinernema feltiae were conducted against the root-feeding turfgrass pest Popillia japonica. Sentinel P. japonica grubs were buried in replicated plots in two school soccer fields, in both the high-traffic areas in front of the goals and the low-traffic areas in the corners of the fields. EPNs were applied to the plots and the EPN infection of the grubs recorded. Additional measurements of abiotic soil properties and the abundances of four microarthropod groups were recorded. EPNs achieved modest control of P. japonica grubs at one of the two sites. Efficacy of S. feltiae was much higher in low-traffic soils than in high-traffic soils at one of the two tested fields in 2016 but not 2017. Sand content was the only soil property tracking EPN efficacy along a non-metric multidimensional scaling matrix, indicating abiotic properties take precedence over biotic properties in determining EPN efficacy in turfgrass field soils. While biotic interactions may reduce EPN infectivity under laboratory conditions, in the field setting of turfgrass soccer fields, abiotic soil properties are more important influences of EPN efficacy