The effects of biotic and abiotic factors on insect life histories have been extensively studied. However, the impact of some crucial aspects, such as larval density (crowding) and the effects of environmental interactions, have often been overlooked. This study aims to determine the effects of larval density and temperature on life-history traits in the black soldier fly (BSF). Our results showed an increase in prepupal mass, pupal mass, prepupal-to-pupal mass loss, survival, prepupal fat content, adult mass, adult longevity and a reduction in larval and pupal development time at low densities. Larval density was maintained throughout the entire larval period and larval survival was determined at the 4th instar and at prepupation. Larvae were reared at three different larval densities (1, 5 and 10 larvae/cm2), at three temperature treatments (23, 27 and 30 °C) and food was provided ad libitum. High densities, on the contrary, resulted in an increase in development time, mortality and a decrease in prepupal mass loss. Temperature significantly affected all studied traits except survival, prepupal fat content and adult longevity, and notably modified the larval density effects on prepupal mass, pupal mass, adult mass, prepupal-to-pupal mass loss, prepupal fat content, duration of larval period, and adult longevity. Males and females differed in all studied life-history traits except adult mass. We conclude that density and temperature and their interaction-related effects during larval development considerably affect BSF larval life-history traits. Therefore, these effects should be carefully considered when planning for insect mass rearing.

Organisms are expected to invest more in their immune function when the risk of disease infection is high. However, induction of a robust immune response is costly and may not be achievable in suboptimal environments. High conspecific density could simultaneously imply high infection risk and a suboptimal environment for many insect species. We focus on the economically important dipteran species (black soldier fly, BSF) that represents the insect order that has been ignored in previous research on density effects on immunity. The experimental part of the study was carried out to evaluate the effect of larval density (three density treatments: 1, 5 and 10 larvae/cm2) and temperature (three thermal treatments: 23, 27 and 30 °C) on the immune function of BSF larvae. The larvae that were reared at high compared to low larval densities and at higher than lower temperatures had significantly higher activity of phenoloxidase, an enzyme that plays an essential role in insect immune function. Sex did not have a significant effect on phenoloxidase activity and prepupal mass, pupal mass and adult mass were not affected by the levels of phenoloxidase activity of fifth instar larvae. In addition, we give an overview of larval density effects on insect immunity and show that density-dependent prophylaxis (stronger immune response in high larval density environments) is indeed common in the results of published case studies. However, cases with no correlation between density and immunity traits were as frequent. Moreover, in more than half of the studies, qualitatively different within-species patterns in different immunity traits were observed. We conclude that BSF larvae exhibit density-dependent prophylaxis, and larvae invest more into their immune system at high larval densities and temperatures than they do at low larval densities and temperatures.