Dose–response modelling uncovers compound‐specific inhibitory effects of host defence volatiles on mountain pine beetle fungal symbionts

BACKGROUND: The mountain pine beetle relies on symbiotic fungi to overcome host tree defences, yet these fungi must tolerate the toxic volatile compounds produced by their hosts. However, how individual volatiles affect fungal symbionts and whether these effects constrain fungal performance when exposed to such toxic compounds remain poorly understood. Quantifying the responses of mountain pine beetle fungal symbionts to host defence volatiles can help explain growth constraints within beetle-fungus symbioses in host trees. RESULTS: We evaluated the toxicity of eight pine defence volatiles, seven monoterpenes and one phenylpropene, 4-allylanisole, on the three primary fungal symbionts, Grosmannia clavigera, Leptographium longiclavatum and Ophiostoma montium, of the mountain pine beetle in vitro. Fungal cultures were exposed to a range of concentrations of each volatile, and individual growth responses were quantified to determine dose-response relationships. All volatiles reduced fungal growth, except for (-)-α-pinene against L. longiclavatum, although sensitivity varied substantially among fungal species and volatiles. Grosmannia clavigera was overall the most strongly inhibited. We found that toxicity differed between enantiomers. Among the volatiles tested, 4-allylanisole and terpinolene were highly toxic to fungi, whereas (-)-α-pinene was the least inhibitory overall. Even with comparable concentrations at intermediate inhibitory potency, some volatiles (e.g. 3-carene) required much higher concentrations to achieve near-complete growth inhibition. CONCLUSION: These findings show that pine defence volatiles exert differential, compound-specific impacts on the fungal symbionts of mountain pine beetles, suggesting that chemical diversity in tree defences may enhance their defences by simultaneously targeting multiple components of the beetle-fungus symbioses. © 2026 The Author(s). Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.