Insects and Composting

- Cold compost piles harbor a variety of insects -

Summary: A variety of insects and worms can be found in cold-composted compost piles. These insects aid the decomposition process and should be left alone to do their jobs.

Jack DeAngelis, PhD
OSU Ext. Entomologist (ret.)


Composting is a great way to get rid of yard and vegetable waste in an environmentally sound manner. Not only do you keep the waste out of the landfill but in the end you gain a valuable product to add back to garden soil. Composting can be as simple or as involved as you want to make it. The simplest compost piles contain only yard waste which slowly breaks down without any additional inputs. It may take a year or longer to complete the decomposition process.

compost bin

compost bin

Hot composting

You can speed up decomposition by first chopping or shedding the yard waste which exposes more of the material's surface to the decomposition process. Decomposition is even faster if the chopped yard waste is layered with soil, and confined to a compost bin (see photo). The soil adds necessary nutrients and microorganisms, and the bin holds in the heat of decomposition. Decomposition can be further accelerated by turning the pile to add oxygen. Layered piles that are occasionally aerated by turning can become quite hot because of very rapid internal microbial decomposition and are call hot-composted. Piles become so hot in fact that insects and worms may not be able to survive in them.

Cold composting

Vegetable waste from the kitchen or garden, on the other hand, contains more water and then yard waste. The extra moisture greatly slows decomposition because the piles never heat up to the same degree as dry piles. Because the piles don't heat up internally they are called cold-composted.

Wet, cold composting piles typically contain large numbers of redworms, insect larvae, and other microarthropods that could not survive hot composting. In fact, redworms are sometimes added to the cold compost piles to start the decomposition process. The rearing of redworms for worm composting is called vermiculture.

Insects in compost bins

There are a number of insects that normally inhabit cold-composted piles especially those that contain large amounts of wet kitchen scraps. The insects generally are no reason for concern, in fact they aid overall decomposition. When vegetable-based kitchen waste is first added you may get a "bloom" of vinegar flies (aka fruit flies) until the mass of vegetable matter starts to break down. Vinegar flies lay their eggs in fruit and vegetable skins and the maggots feed on bacteria and fungi of the rotting plant matter.

Black soldier fly maggots (larvae) [picture of black soldier fly maggot] can be found in some compost bins. These large maggots often startle those new to composting but they cause no harm. The maggots may indicate the presence of meat or fat in the compost, neither of which should ever be added to your bin.

A variety of small insects, and related arthropods, like collembola (springtails) and sowbugs will colonize cold piles much as they colonize any organic garden soil. These organisms feed on the bacteria and fungi which in turn are feasting on the compost. Never attempt to control these critters with insecticides as this will disrupt the compost pile's ecology and greatly slow decomposition. Also, remember, never put meat, fat, dead animals, including slugs, or oils into your compost bin. These will rot causing foul odors and may attract house flies, other manure flies, and scavenging rodents. See Related Articles below.

Related Articles

What are Fruit/Vinegar Flies?

What are Collembola?

What are Sowbugs?

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