The fungistatic and fungicidal effects of volatiles from metathoracic glands of soybean-attacking stink bugs (Heteroptera: Pentatomidae) on the entomopathogen Beauveria bassiana

- Volatiles from metathoracic glands of pentatomids affect fungal germination.
- Effect on Beauveria bassiana is fungistatic (inhibitory) and/or fungicidal.
- Volatiles of majority of tested soybean-attacking pentatomids are only fungistatic.
- The fungistatic effect is less pronounced for B. bassiana-susceptible species.
- Volatiles of Nezara viridula have also a fungicidal effect.

This study was initially designed to evaluate the differential susceptibility of three soybean-attacking pentatomids to the entomopathogenic fungus Beauveria bassiana in standardized bioassays. Euschistus heros (Eh) was shown to be significantly less susceptible than Chinavia ubica (Cu), whereas Dichelops melacanthus (Dm) adults were highly susceptible to fungal infections. A deeper look at the mechanisms involved in the possible role of volatiles from metathoracic glands on fungal infections was undertaken, and gland extracts from Nezara viridula (Nv), a species known for its resilience to fungal infections, were also included in the assays. Atmospheres with volatiles from pentatomids with very low-susceptibility to B. bassiana infections (Eh and Nv) had a significant effect on speed of germination as shown in counts performed up to 22 h post-inoculation, by which time 0.1 (control), 0.6 (Dm), 17.9 (Cu), 32.6 (Eh), and 43.4% (Nv) of conidia had not germinated. The fungistatic (inhibitory) and fungicidal (lethal) effects of Eh and Nv volatile-rich atmospheres were subsequently quantified in Petri dishes with either PDA or PDA medium amended with carbendazim, which allowed germination rates to be determined at 18 and 48 h post-inoculation, respectively. As opposed to control, Eh volatile-rich atmosphere had a clear fungistatic effect, since germination rate was only 27.4% within 18 h, but reached 99.4% at 48 h post-inoculation. For Nv volatile-rich atmospheres, only 15.1% of conidia germinated within 18 h, and by 48 h post-inoculation, approx. 18% of conidia were unviable (neither germ tubes nor intumescence), whereas in the control treatment rates were >99% at both reading times. Therefore, the gaseous phase of defensive secretions from fungus-resilient pentatomids possess a strong inhibitory effect and may display a less pronounced lethal effect on fungal germination, as was the case for Nv.