Interaction of age and temperature on heat shock protein expression, sperm count, and sperm viability of the adult black soldier fly (Diptera: Stratiomyidae)

Abstract

Information regarding black soldier fly (Diptera: Stratiomyidae) adult biology is vital as this is the life stage that produces eggs and thus drives population size. The goal of this study was to determine key biological characteristics of adult black soldier flies as they age in relation to: (1) the thermal preferences (T sel) of males and females; (2) the impact of temperature on heat shock protein expression in males and females; as well as (3) the sperm count; and (4) the sperm viability in males. Aging significantly impacted male and female temperature preferences. Young males (<24-h-old) preferred warmer temperatures (median=24.3 °C, range=19.3-28.2 °C) compared to females of the same age (median=20.2 °C, range=15.4-26.2 °C). However, in older adults (i.e. 72-h-old males and 48-h-old females), temperature preferences converged between 21 and 24 °C. Temperatures tested did not impact hsp expression in males or females. However, aging males, but not females, had increased expression of the heat shock proteins (hsp) hsp70 and hsp90. Furthermore, age impacted sperm count but not sperm viability in males. In particular, 48-h-old males had the greatest sperm count (322.5/sample) and sperm viability (60-78%) compared to all other aged males. Thermal data in conjunction with sperm data potentially explain why early thermal segregation behaviour between males and females occurs. Once adult males and females reached 72-h-old and 48-h-old, respectively, they exhibited a common thermal preference, which coincided with the greatest number of viable sperm in males. Forcing adults into environments (i.e. cages) outside these selected preferences could result in premature or delayed mating or low fertilisation rates. Future research exploring cage design and conditions are needed to optimise black soldier fly colony maintenance and fertile egg production, and can leverage information such as the results described here.