BSF Eggs and Larvae | A Close-up Viewby Terry Green on 08/08/14
Larvae of the Black Soldier Fly (BSF), a non-pest and non-disease spreading insect, efficiently feed on discarded wastes such as food scrap, accelerating its decomposition and the recycling process, and are of great interest because of their phenomenal growth rate and potential commercial value in recycling applications (see for example Black Soldier Fly Larvae | An Earth Friendly Feedstock?, BSF Metrics & Yields| Scale Up Production of Black Soldier Flies, and Recycling Biodegradable Wastes? | Take Your Cue from Mother Nature). This blog describes what to look for in decaying waste in identifying and becoming more familiar with the use of BSF in recycling waste starting from the egg stage up through the larval stage of their life-cycle.
Adult BSF live not more than about four to five days and do not feed on waste. Their main role is to start a new generation of young BSF on their way. Males hide out in bushes and trees near decaying waste in search of females. Males and females mate mid-air during flight, spiraling to the ground during the mating process, after which the female breaks away in search of a suitable place to deposit her fertilized eggs.
Females lay eggs in clutches of upwards of 400 or more eggs on surfaces near decaying waste, especially in nooks and crannies, where newly hatched larvae can easily gain access to decaying waste (Fig. 1a), or, alternatively, directly in the waste, itself (Fig. 1b). With day time temperatures in the mid-80’s F (26+ C), larvae hatch from egg clutches around the 4th day after deposition. They peak in size approximately three to four weeks after hatching from an egg clutch (Fig. 1d).
Fig. 1. Images of BSF eggs and larvae relative to the size of a US dime. (1a), eggs deposited directly on the surface of a plastic lid near decaying food scrap; (1b), eggs laid directly in decaying food scrap waste; (1c), newly hatched larvae emerging from the same egg clutch in 1a (four days later); (1d), image of a three week old larva grown off of food scrap waste. Copyright © 2014 Terry Green, All rights reserved.
Newly hatched larvae are very small, blend in with food waste, and cannot be easily seen with the naked eye mixed in with food scrap. Larvae hatched from egg clutches quickly put on weight however as they convert the nutrient value of food scrap while also consuming microbes processing the food scrap into insect biomass. They can be grown in dense concentrations on their own, or attended to in a BSF nursery, and are easily visible by the time they are about one week of old (Fig. 2).
Fig. 2. Image of BSF larvae about one week of age growing rapidly in a nursery in dense concentrations. Copyright © 2014 Terry Green, All rights reserved.
Young larvae breath through tiny spiracles which appear as bristles protruding from each segment of their soft exoskeleton (Fig. 3). This allows them to reside in pretty much any orientation while continuously feeding on waste and exchanging gases (oxygen and carbon dioxide) as they take in liquids, microbes and small pieces of waste passing through their mouth parts into their gut where nutrients get incorporated into insect biomass. That which cannot be incorporated on passing through the gut in one cycle is excreted and may yet be picked up again and passed once more through their gut by the peristaltic action of their gut as they burrow back and forth through the decaying waste on which they are feeding.
Fig. 3. Close-up view of a pair of young BSF larvae. Note the sharp mouth part at the front end of the larvae, the two eyes located laterally at the base of the first segment of their exoskeleton, and the breathing spiracles located symmetrically around each segment. Copyright © 2014 Terry Green, All rights reserved.
Newly hatched BSF larvae are about 1/50th of an inch (~0.5 mm) in length whereas full grown larvae ready to enter their prepupae stage of their life-cycle are about 5/8ths inches (~180 to 200 mm) in length (note size of larvae in Fig. 1c relative to that in Fig. 1d), a growth rate of nearly 400-fold over a three week interval in body mass! Their mouth is located at the more pointed front end of their body just ahead of a pair of eyes located laterally at the base of the first segment of their exoskeleton. Their movement is smooth and fluid-like which is quite distinct from the rapid, herky-jerky motion of housefly larva. The exoskeleton of housefly larvae is also generally a brighter white as opposed to the off-white, yellow-cream-like color of BSF larvae. Moreover, the overall appearance of BSF larvae is much closer to that of a slightly flattened cigar as opposed to that of the common housefly which is shaped more like a sharpened pencil (one end blunt and the other end tapered to a sharp point).
As BSF larvae reach the prepupae stage, their outer segmented creamy-yellow to light brown exoskeleton darkens to a deep brown, almost black color. At this stage they cease feeding on waste and climb free of the waste in seeking a dark dry hiding place to pupate. The outer shell tends to darken further, sometimes turning slightly ashen or dusty grey. Depending upon the temperature they are stored at during the pupation phase of their life cycle (see The Effect of Temperature on Emergence of Black Soldier Flies), new adults then emerge from their puparia in approximately 15 to 25 days ready to start the whole cycle over again with a new generation of offspring.
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