Got Black Soldier Flies? | Say Goodbye to Houseflies, Gnats & Fruit Flies : The Life and Times of BSF (Black Soldier Flies)
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Got Black Soldier Flies? | Say Goodbye to Houseflies, Gnats & Fruit Flies

by Terry Green on 11/13/13

Black Soldier fly (BSF) larvae grown off of food scrap and agricultural waste could one day replace much of fish meal as a high quality protein and lipid supplement in animal feedstock formulations (see, for example, Black Soldier Fly Larvae | An Earth Friendly Feedstock?, Black Soldier Fly Larvae for Waste Treatment – Prospects and Constraints, Alternative Biological Treatment of Manure, and Feedpedia). BSF also accelerate compost decomposition of food scrap waste (see Black Soldier Fly Processing of Biodegradable Wastes). Unlike common pesky house flies (Musca domestica Linnaeus), fungal gnats and fruit flies, BSF do not spread diseases, damage fruit crops or bore into and damage the root systems and stems of living plants. This blog highlights the observation that BSF on colonizing decaying waste rid the waste of house flies, fungal gnats and fruit flies.

Decaying food scrap, agricultural plant debris, even manure, uncolonized by BSF larvae, can quickly become infested with house flies, fungal gnats and fruit flies. A single female house fly can lay 75 to 150 eggs in successive batches over a three day interval in decaying waste. On hatching from the eggs, larvae (maggots) grow through three molting stages, pass into the pupa stage, and emerge from this stage ready to mate and further propagate the next generation within as little as seven to ten days. Adult flies live anywhere from 15 to 25 days, sometimes longer, depending upon environmental conditions and food availability.While feeding and flitting from manure to garbage bin to counter top to eating utensils, and to food left out in the open, adults easily pick up and spread disease carrying microbes, posing a health hazard (see House Fly: Musca domestica Linnaeus).

Fungal gnats and fruit flies are also prolific nuisance pests found in and about decaying waste. Given the right conditions, gnats lay eggs in strings in soil near plant stems which can get transferred into waste recycling bins. Fruit flies lay eggs in fruit, saps and decaying organic wastes. Unchecked, large numbers of larvae from these nuisance pests cause great damage to the agricultural and horticulture industries (see Fungus Gnats and The Fruit Flies’s Tale). BSF larvae however easily compete with house flies, fungal gnats and fruit flies in growing off of decaying organic wastes – so much so that there is good scientific evidence for their use as biological control agents (see, for example,  Hermetia illucen/ Control in Poultry Manure by Larviciding and Hermetia illucens (Linnaeus) as a Factor in the Natural Control of Musca domestica).

How BSF control and abolish house flies, gnats and fruit flies in decaying waste is not entirely clear. They are voracious predators and may simply consume house fly, gnat and fruit fly eggs as they encounter them while burrowing through and feeding on decaying waste. It appears from early published data that BSF larvae do not prey directly upon house fly larvae (see Hermetia illucens (Linnaeus) as a Factor in the Natural Control of Musca domestica Linnaeus).

BSF larvae, in combination with aerobic microbial activity accompanying larval processing of the waste, raise the temperature of the waste ~10 C (~18 F). It is possible that this temperature shift pushes the bin temperature too high for efficient survival and maturation of house flies, gnats and fruit flies relative to that tolerated and favorable for BSF larvae.

Shifts in environmental conditions can induce diapause, a biological strategy used by many insects for resetting and delaying maturation when environmental conditions become unsuitable pending restoration of normal conditions required for survival. Overcrowding, possibly by BSF larvae growing in the same environment, could also be another trigger inducing diapause in house flies, gnats and fruit flies forced together under these circumstances.

Figure 1 contrasts the appearance of common house fly pupae and a house fly larva with that of BSF prepupae. House fly larvae are pencil shaped, small off white in appearance, and exhibit rapid, irregular rather jerky movements as they navigate through waste. Their pupa shell is easily distinguishable from that of BSF as a bright, shiny red to orange, sometimes brownish, elongated bead about 1/10th the size of BSF pupae.

Fig. 1. Size and appearance of the common house fly (Musca domestica L.) larva and corresponding pupae relative to prepupae and pupae of the Black Soldier fly (Hermetia illucens). Fig. 1(a), house fly larva and corresponding pupae; Fig. 1(b) relative size of house fly pupa (red capsule, center) to that of BSF prepupae and pupae (dark brown – gray segmented objects surrounding house fly pupa). Copyright 2013  (c) Terry Green, All rights reserved.

In scaling up recycling operations in processing food scrap with BSF, house fly larvae and pupae may initially be in significant numbers before BSF fully colonize the waste. House fly larvae and pupae can be easily sorted out during harvest using screens that are porous to the house fly larvae and pupae while retaining BSF larvae and prepupae, and destroyed, if desired, and necessary. Alternatively, with patience the house fly population will on its own fall off as the BSF colony becomes established.

To further control and keep house flies, gnats and fruit flies in check, waste recycling operations can be done in an enclosed space by isolating processing bins from direct access from the outdoor environment. This can be done by running recycling operations inside a building, or outdoors by isolating the recycling bins inside mosquito or similar fine mesh netting.

In a BSF colonized bin we find during routine operations less than 1 house fly larva per 20,000 BSF larvae in our processing bins. Even though adult houseflies gain entrance to our recycling bins inevitably as we enter and exit our building containing our processing bins, they either fail to lay eggs in the decaying food scrap once it is transferred into our recycling bins, possibly because they detect pheromones given up by BSF larvae already present and feeding on the food scrap, or alternatively, the eggs they lay are for the most part consumed by grazing BSF larvae before they hatch.

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