Do It by Yourself: Larval Locomotion in the Black Soldier Fly Hermetia illucens, with a Novel "Self-Harvesting" Method to Separate Prepupae

Abstract

The neotropical insect Hermetia illucens has become a cosmopolite species, and it is considered a highly promising insect in circular and sustainable economic processes. Being able to feed on a wide variety of organic substrates, it represents a source of lipids and proteins for many uses and produces recyclable waste. We investigated the characteristics and differences in the poorly-known locomotory behaviour of larvae of different instars, paying particular attention to the unique characteristics of the prepupal stage, key to farming and industrial processes. Moreover, we attempted to develop a "self-harvesting" system relying on the behavioural traits of prepupae to obtain their separation from younger larvae under rearing condition with minimum effort. Prepupae differ from younger larvae in their response to physical disturbance in the form of tonic immobility and significantly differ in their locomotory movements. Both prepupae and younger larvae reacted similarly to heat or light-induced stress, but low light and high moisture induced only prepupae to migrate away, which resulted in the development of a highly efficient separation methodology. The new data on the behaviour of H. illucens not only shed new light on some unexplored aspects of its biology, but also led to develop an inexpensive self-harvesting system that can be implemented in small-scale and industrial farming.

Keywords: Hermetia illucens; black soldier fly; circular economy; insect farming; insects behaviour; locomotor activity; self-harvesting.

Figure 1

Larval instar classification; on the above the larval instar 6 and below prepupae. Scale bar: 1 mm.

Figure 2

Experimental apparatus to separate prepupae from younger instars. (a) View from above. (b) View of the underside, the red arrow show the passage for larve (c) lateral section view, as shown above the red arrow indicates the 1 cm-wide passage at the base of the barrier through which the larvae can reach the collecting arena.

Figure 3

Locomotory patterns of 6th instar larvae (a–f) and prepupae (g–n) as described by the trajectory-tracking function of the software Kinovea. The X-axis and Y-axis indicate the horizontal and vertical components of the registered respectively movements.

Figure 4

Movements in the locomotory pattern of 6th instar larvae (a,d,g; n = 10) and prepupae (b,e,h; n = 10) and analysis of their differences through measurements (c,f,i). Each row corresponds to one of the three movements: i = when the larva stretched its head forward (a,b), ii = when the head then touched the surface before being used as a pin to crawl forward (d,e); iii = when the head is used as a pin to crawl forward (g,h). In pictures (a–e) the red line and green lines indicate the measured characters (the length of the red line is divided for the length of the green one to compute the lifting index), in pictures (g,h) the measured angle is indicated in red.

Figure 5

Locomotory speed of 6th instar larvae (above: a,b) and prepupae (below: c,d) under different stress conditions. Different letters highlight statistically different groups according to pairwise comparisons. Violins show the probability density of the data at different values.

Figure 6

Scatter plots describing the number of prepupae (y axis) and younger larvae (x axis) found in in the collecting arena following separation experiments conducted at different moisture conditions and under different light treatments. A higher efficiency is reached near the upper-left corner of the graph, indicating a high number of prepupae but also a low number of younger instars at the same time.