Signals across trophic levels: Can different species of entomopathogenic nematodes protect plants from aboveground insect herbivores?

Plants respond to different threats in their environment by activating various defense pathways. Often, plants can detect damage in roots that will send signals to the shoots, alerting the plant to protect the aboveground tissue. Entomopathogenic nematodes (EPNs) are used for biological control of soil-dwelling insect pests such as the cucumber beetle (Acalymma vittatum (F.)) in cucurbit crops. A few studies have reported that soil inoculations of EPNs increase plant resistance to above ground insects. This included lower insect performance, higher expression of plant defense genes, and induced plant defense pathways associated with herbivory. An outstanding question is whether different species of EPNs induce plant defenses similarly. The objective of this study was to evaluate the influence of different species of EPNs on cucumber plant defenses and resistance to cucumber beetles. We exposed cucumber plants (Cucumis sativus) to one of three species of EPNs (Heterorhabditis bacteriophora, Steinernema carpocapsae, or Steinernema riobrave) or a water control and evaluated plant defense gene expression and defense hormone levels, as well as plant resistance to striped cucumber beetles (Acalymma vittatum). We predicted that cucumber plants exposed to EPNs will be more resistant to striped cucumber beetle herbivory and that increased expression of defense genes and phytohormones will mediate this interaction.