The philosophy of live and let live has its limits. One of these comes when small flies attempt to share your wineglass with you. “Fruit fly,” you cry while the little beast does the backstroke in your Chilean Santa Rita Reserva.
Drosophila, or fruit flies to you, get blamed for every infestation. But the true culprit might be from other members of the great family of flies, or Diptera, technically known as the Sciaridae or, in common parlance, fungus gnats.
Common to moist environments, they find the growing medium supporting houseplants a delightful pied-à-terre. The larvae of some fungus gnat species feed on the root hairs of plants. This does nothing to enhance the health of said plants.
Their small size – one to seven millimetres long – makes firm identification fraught with difficulty. You have to be either a # or an enthusiastic entomologist to indulge in such endeavours. This might explain why the Sciaridae are described by expert sources as one of the least studied large families of Diptera. Apparently, there are some 1700 described species plus a further estimated perhaps 20,000 species waiting to be discovered, mainly in the tropics.
Adult Sciaridae feed only on liquids, living just long enough to mate and the female to lay eggs, both sexes dying after about five days. Each female fungus gnat is capable of laying about 200 transparent eggs. These hatch in a week with about 90 per cent of the larvae being female.
Cedar wood chip mulches will discourage fungus gnats from infesting houseplant growing medium. Another useful passive device is a “sticky stick” – a yellow card coated with adhesive affixed to a plastic shaft inserted into the plant container soil. These also control other pests of indoor plants such as whiteflies and aphids.
These same “sticky sticks” will also trap the Drosophila often arising in clouds from fruit and vegetables. Most often these are D. melanogaster but a nasty new species has recently arrived on the scene, the Asian or spotted wing fruit fly. First identified in North America 2008 it is known from British Columbia, Alberta, Ontario and Quebec, as well as the United States, Spain, Portugal, France, Italy and Russia. Wherever it has arrived, it has become a serious pest of soft fruit and berries. Unfortunately for environmentally conscious Canadians, it answers to the name of Drosophila suzukii.
Wes Porter is a horticultural consultant and writer based in Toronto. Wes has over 40 years of experience in both temperate and tropical horticulture from three continents.
