Temperature-dependent survival and aggregation behaviour of Hermetia illucens larvae in response to Beauveria bassiana infection

Abstract Larvae of the black soldier fly (BSF; Hermetia illucens ) can feed on a wide range of organic residual streams, including those of agricultural origin. This may result in exposure to entomopathogenic fungi such as Beauveria bassiana that are increasingly used in biological control. The natural behaviour of BSF larvae to aggregate may be a defence mechanism to increase body temperature to combat infection. To study the effect of temperature on B. bassiana (strain ARSEF-Bb8028), we used two different methods of infection of the larvae. BSF larvae were infected either by injection of blastospores into the haemolymph or via exposure to blastospores in the substrate. To identify if temperature had an effect on B. bassiana infection in BSF, survival assays were performed at three different temperatures. Larval mortality was highest at 27 °C, the optimal temperature for B. bassiana growth. Larval mortality was lower at 34 °C with both infection methods. We investigated the effects of exposure to B. bassiana blastospores on larval behaviour. Exposure to B. bassiana did not enhance aggregation behaviour in the bioassay employed. At later time points since injection with B. bassiana , BSF larvae were more isolated than control larvae. This may be a consequence of disease caused by fungal infection and may be an adaptation to prevent spread of the fungus after death to conspecifics. This study provides a first understanding of how survival depends on temperature and reveals behavioural responses of BSF larvae to fungal infection that may be used as indicators of larval health.