Fungal infection dynamics in response to temperature in the lepidopteran insect Galleria mellonella

This is an investigation of how temperature modulates the dynamics of fungus – insect interactions. The study focusses on the ability of the entomopathogenic fungus Metarhizium robertsii to infect the wax moth, Galleria mellonella, which is a well-studied and important model insect. Wax moth larvae in the wild develop optimally at around 34°C in beehives. However, surprisingly little research on wax moths has been conducted at temperatures relevant to the wild. Metarhizium robertsii was considerably more pathogenic at a constant temperature of 24°C compared with 34°C. The cooler temperature allowed greater condial adhesion to the cuticle, germination and haemocoel invasion. The wax moth larvae immune responses also altered with the temperature, and with the infective dose of the fungus. Enzyme-based immune defenses (lysozyme and phenoloxidase) exhibited enhanced activity at the warmer temperature. The predominant insect epicuticular fatty acids (palmetic, oleic and linoleic acids) were synthesised in greater quantities in larvae maintained at 24°C compared with those at 34°C, but these failed to exert fungistatic effects on topically applied fungus. It is clearly important to choose environmental conditions that are relevant to the habitat of the insect host when investigating the dynamics and outcome of insect / fungus interactions. The study has particular significance for the application of entomopathogens as biocontrol agents