Fruit Flies as Creatures of the Compost
Written by Angus Stewart
The name fruit fly is a rather tricky one as there are literally hundreds of species that are given this title, with the trouble being that some species such as the Queensland fruit fly (Bactrocera tryoni) and the Mediterranean fruit fly (Ceratitis capitata) are some of the worst horticultural pest species in the world, as their adults lay eggs in ripening fruit that hatch into maggots that in turn ruin the fruit for human consumption.
Another insect known as fruit fly, is also known more commonly as vinegar fly (Drosophila melanogaster), which proliferates not only in rotting fruit, but also in other types of rotting organic matter other than fruit, and it is this species that is most commonly seen in composting environments. Generally speaking, this species does not cause a problem in ripening fruit still on trees, and so is not so much of a practical problem in the garden. The vinegar fly can be distinguished by its red eyes, something the more harmful fruit flies don’t have. But all the different ‘fruit flies’ can be identified by having two wings (as do all insects generally called flies)
The more harmful species of fruit flies can end up in compost if previously infected fruit goes into the food scrap mix and then into the compost. Vinegar flies, on the other hand, tend to be more ubiquitous in urban environments and will naturally start to breed wherever there is rotting food waste.
Because it may not be easy for you to identify exactly which type of fruit fly you have it is probably best to assume that they are unwanted in your worm farm or compost bin. Indeed, even the more benign Vinegar fly can breed up quickly to big numbers that swarm around whenever you are tending your compost and as such it is best to try and eliminate them, especially since there may be a risk that the more harmful fruit flies may be present as well as vinegar flies
Burying your kitchen scraps deep within your compost bin or worm farm will help to keep fruit flies from colonising and breeding, as will a worm blanket or covering over your compost bin, as will adding plenty of shredded paper and cardboard plus lime or worm farm conditioner, both of which will help minimise the attractive smell.
Keeping your kitchen scraps in a container with a lid that can seals tightly will help to prevent vinegar flies invading, as will sticky traps hung up near your composting area. You can make your own sticky traps or purchase ready made ones, and these have the advantage of giving you advanced warning of a developing problem, as well as giving you a sample that experts can use to identify the fruit fly exactly. This can be particularly important in states that are free of the pest species of fruit fly as it can help government departments of agriculture identify outbreaks and eliminate them in states that are fruit fly free.
In ground worm farms and the Gedye compost bin are designed to have as few entry holes as possible for pests, so these are an alternative to above ground worm farms. Being deep within the soil, they also will emit less of the attracting odours. Be sure to cover any additions with a nice layer of neutral paper or cardboard.
In summary, if you see little flies hanging around your worm farm or compost bin it is best to assume they are harmful and take our recommended measures to control them.