Effects of temperature and humidity on the presence and prevalence of a common fungal parasite on an invasive ladybird

Abstract Invasive species can significantly alter ecosystems, often through competitive dominance and indirect ecological effects. The harlequin ladybird (Harmonia axyridis) has become a widespread invasive species that, after a period without natural enemies, became parasitized by different organisms in the invaded range, including the microfungus Hesperomyces harmoniae. These parasite–host interactions remain understudied. Here, we investigated how temperature and humidity influence parasitism of He. harmoniae on Ha. axyridis using controlled laboratory experiments. We tested different environmental conditions: low versus high relative humidity and low versus high temperature. We assessed parasite prevalence (proportion of infected ladybirds), rate of development (time to maturity of fungal thalli) and parasite load (average number of thalli per host). High humidity increased both parasite prevalence and rate of development. High temperature had a slight negative effect on both measures, whereas low humidity and low temperature had a strong negative effect. Hesperomyces harmoniae thrives best under moderately warm and humid conditions and its prevalence may be constrained in drier or cooler environments. Understanding the environmental factors influencing He. harmoniae dynamics provides insights into the role of abiotic conditions in shaping parasite–host interactions. Our findings have implications for the population ecology of Ha. axyridis in different climatic regions.