Deadly smells: Plant volatiles increase mortality of entomopathogenic nematodes during host infection

When plants are attacked by insects, they commonly mobilize various defense mechanisms including the induction of herbivore-induced plant volatiles (HIPVs). These compounds can serve as intraspecific and interspecific signaling compounds. Some HIPVs are well known to have negative impacts on herbivores, while others, especially in the case of specialized herbivores, can promote herbivory. Some HIPVs are also known to affect insects' defense against pathogens.  Entomopathogenic nematodes (EPNs) are bacterivores that during host infection release symbiotic bacteria that consume the infected insect. Since many EPNs are attracted to, and even rely on HIPVs for host finding, experiments were conducted to test if Galleria mellonella  exposed to HIPVs impacted EPN survival during infection. Compounds of importance to EPNs (such as pregeijerene, β-caryophyllene, and α-pinene) and compounds that have been determined to increase or decrease susceptibility of insects to pathogens (such as (Z)-3-hexenyl acetate, linalool, and β-ocimene) were selected. Mortality was measured by dissecting the insect 48hr after infection and counting of dead versus live nematodes. We found that exposure to pregeijerene, linalool, β-ocimene, and α-pinene significantly increased mortality. The highest average mortality (36%) was seen for α-pinene exposure, while exposure to Z-3-hexenyl acetate, and β-caryophyllene did not result in increased mortality. These results indicate that the presence of HIPVs can greatly impact the performance of EPNs, even in a highly susceptible host such as G. mellonella.