Substrate composition effect on growth of Cotinis mutabilis (Scarabaeidae) larvae: a case for detritivore scarabs in the insect agriculture industry

Insect farming has potential to help alleviate food insecurities, which are increasing globally. Insect farming can also help reduce landfill loading with food loss and waste, which is nearly 33% of all food produced globally. Farming of detritivore insect species, those that can consume decaying organic material including manures, increases the economic and environmental sustainability of the insect agriculture industry. The green fig beetle (Cotinis mutabilis, Scarabaeidae) is a native to the Sonoran Desert in the United States and Mexico. This study explored the effects of ten organic waste diets differing in the ratios of four waste types, dairy cow manure, mushroom spent substrate, spent brewer’s grain, and leaves, on the growth and survivorship of the larvae. Results show that this species yielded highest grub peak weights when fed a diet containing spent beer grains. However, these grubs also had low survivorship. Highest food efficiency conversions were found in grubs that were fed a diet dominated by spent mushroom media. Manure-based compost yielded poor growth and survivorship in these grubs, despite observations of wild beetles laying eggs in compost. A correlation between diet density and peak weight of the grubs suggests that a low-density diet is a more suitable substrate.