Drought Stress Impairs Communication Between Solanum tuberosum (Solanales: Solanaceae) and Subterranean Biological Control Agents

The attraction of entomopathogenic nematodes (EPNs) to herbivore-injured plant roots has been documented recently to be a common tritrophic interaction. Belowground tritrophic interactions are especially subject to modulation by many abiotic factors including drought. In this study, complementary greenhouse experiments were conducted to understand how drought stress might affect a potato plant's impact on EPN behavior. In separate trials, the responses of the EPN Steinernema diaprepesi (Nguyen and Duncan) (Rhabditida: Steinernematidae) to root herbivory by larvae of the weevil Diaprepes abbreviatus (Linnaeus) (Coleoptera: Curculionidae), in well-watered and drought stressed potato plants, were measured using soil-matrix olfactometers with two arms. The drought treatments were initiated in 30-d old plants. Drought-stressed potato plants received water when potentiometers read approximately 20 kPa, while for well-watered plants, the number was 8 kPa. Four weeks after initiating the treatments, 400 ml water was added to all pots, immediately before starting the experiments. The experiments revealed that S. diaprepesi infective juveniles (IJs) did not migrate preferentially toward drought-stressed or well-watered plants when neither were subjected to herbivory [t₍₂₁₎ = 1.13, P = 0.269]. However, plants with roots damaged by herbivory attracted more S. diaprepesi IJs if they were well watered than if they were drought stressed [t₍₂₄₎ = 3.19, P = 0.004]. If both plants in the olfactometers were drought stressed, EPNs moved preferentially toward those with root herbivory than those with undamaged roots [t₍₂₃₎ = 3.19, P = 0.004]. No difference was detected in gas chromatography profiles between droughted and well-watered plant roots subjected to herbivory [F₍₂₄, ₃₃₆₎ = 0.68, P = 0.87]. GC analysis showed that three compounds, including 3-nonanone [t₍₆₎ = 4.83, P = 0.003], artemisyl ketone [t₍₇₎ = 6.21, P = 0.000], and benzoic acid, 4-ethoxy-, ethyl ester [t₍₇₎ = –4.22, P = 0.004] were significantly higher in drought stressed than control plant roots. These results indicate that potatoes, like other plants, can recruit EPNs in response to root herbivory, and that drought stress dampens this tritrophic interaction where choice is involved. Additional research that resolves the mechanisms of these interactions may provide insights to exploit EPNs for crop protection.