Substrate Preference of Black Soldier Fly Larvae Is Influenced by Rearing History

ABSTRACT Black soldier fly larvae ( Hermetia illucens Linnaeus (Diptera: Stratiomyidae; BSF)) have a natural diet of diverse decomposing materials with variable nutrient and water contents. BSF larvae prefer substrate pre‐colonised by conspecifics over uncolonised substrates. The chemical cues left behind in the feed substrate by conspecifics may vary with rearing history. To investigate larval feed preference behaviour, BSF larvae were reared under six different conditions: low, medium or high larval density and three different feed water contents. Larval developmental stage and body weight of the six groups of larvae were recorded after eight days of growth. Medium feed water content, medium and low larval density conditions were most favourable for development and growth; high water content and high density were intermediate, while low feed water content was the least suitable condition. The six groups of larvae were offered a choice between uncolonised and pre‐colonised substrates. In addition, they were offered a choice between substrates colonised by conspecifics reared on feed with low water content and substrate colonised by conspecifics reared on feed with medium water content. Behavioural variables recorded were number of substrate contacts and entries, and number of larvae and time spent within the substrates. Our results show that BSF larvae reared under all six conditions prefer substrate colonised by conspecifics, with insects reared on high‐water feed showing the strongest preference. Larvae reared at high density showed the highest number of substrate contacts, entries, and spent most time within feed substrates. When given a choice between substrates colonised either by conspecifics reared on low‐water feed or on medium‐water feed, substrate colonised by conspecifics reared on medium‐water feed was preferred; however, only by larvae reared on either low‐ or high‐water feed. This indicates that water stress may cause changes in conspecific cues that are only recognised by individuals that experienced such stress.