Protection from below: Silicon supplementation and entomopathogenic nematodes to manage turfgrass insects

We explore the impacts of silicon (Si) supplementation and entomopathogenic nematodes (EPNs) on controlling three primary turfgrass insects: black cutworm (Agrotis ipsilon), white grub (Anomala orientalis) and annual bluegrass weevil (Listronotus maculicollis), with a focus on the efficacy of EPNs (Steinernema carpocapsae and Heterorhabditis bacteriophora) when applied in tandem with Si fertilization. EPNs are obligate parasites that release endosymbiotic bacteria into the insect hemocoel, causing the death of host insects due to bacterial toxemia and septicemia. Si is a beneficial nutrient for plants that can provide multiple benefits to turfgrasses, including enhanced disease resistance. However, the effects of Si supplementation on turfgrass insects are not well-documented. Grasses are high Si accumulators, and Si may account for up to 10% of their shoot dry weight. Si deposition makes plant tissues more abrasive and tougher, reducing their masticability and digestibility to insect herbivores. Insects feeding on Si-supplemented plants can experience impaired growth and development and exhibit compromised immunity. It is unknown, however, how such weakened immunity affects insect susceptibility to their biological control agents, such as EPNs. We conducted a series of laboratory and glasshouse experiments to examine the impacts of Si supply to different turfgrasses (i.e., annual bluegrass, creeping bentgrass and perennial ryegrass) in combination with the application of EPNs on controlling these major turfgrass pests. Our results suggest that Si fertilization could augment plant resistance to these insects, particularly against cutworms, and could enhance insect susceptibility to EPNs, which may facilitate the biological control of these insects.