Organic Answers Column – March 13, 2024 – Flies

Flies – The Good and The Bad

Some flies, especially biting ones, are nasty pests and need to be controlled. Horse flies, flesh flies, deer flies, houseflies, etc. are annoying and offer painful bites.

Adult black soldier fly (left) and their maggots (larvae) in a kitchen waste bin.

But not all flies are pests. Most of them are rather wonderful beneficial insects, such as the wasp-looking black soldier fly. The adult soldier flies are good plant pollinators and the larvae are maggots that help break down wet compost, are good food for chickens and other birds, and can be added to the soil to help introduce beneficial nematodes.

Charismatic Tachinid flies have dramatic eyes and bristly butts. They lay eggs on the backs of pest caterpillars that in turn hatch and the larvae eat and kill the worms. There are thousands of species and they all help control pest insects.

Tachinid flies – there are thousands of varieties of them. They feature big eyes and bristly tails.

A bee-looking beneficial fly is the hover fly. Adults look like pretty bees or wasps and can be seen “hovering” around your roses and other perennials. They are excellent pollinators and larvae are little slugs that eat pest insects.

There is a pretty little iridescent Long-legged fly that is a predator of pest insects on plants and in the soil. They are usually blue or green.

Hover fly adult (left) and a predatory Long-Legged fly

Hover fly larvae eat pest insects.

Fly larvae are beneficial in breaking down kitchen waste for the compost. Illustrated below is a bin that the soldier flies lay eggs in and the maggots go to town.

Soldier fly larvae are beneficial and efficient breaking down food waste.

Controlling the pest flies.

Good screens are effective tools for keeping these and other pests out so make sure window screens have no holes or tears. Also check the seals around doors to make sure there are no cracks that would allow insect access.

Flies love stinky things and dirty places. Keep your inside garbage cans as clean as possible. Don’t leave food out and clean up any spills promptly.

Flies in the house plants? Control fungus gnats and fruit flies by cleaning sink, bathtub and shower drains at least once a month. Pour boiling water down to kill any fly eggs and larvae then cover the drain after you pour the water to trap the heat and steam. Adding two ounces of orange oil per gallon of water will also help.

Old fruit should be discarded to discourage fungus gnats and potted plants should be allowed to dry out between waterings. For infestations, Bti products can be applied to the soil. Pest flies seem to hate the smell of lavender oil and it also has a calming effect on people.

For controlling dog ear biting flies, pick up the dog poop every day. The blood-sucking flies need the droppings for their egg laying and life cycle.

Heavy duty plastic bags filled with clear water hung out on the porch is a solution that some homeowners, farmers, ranchers and restaurants have used to repel flies with great success. Something about the way water bulging in a clear plastic bag causes a prism effect and repels flies. The rounder and plumper the bags, the better the control. Dusty bags need to be cleaned or replaced to continue to work. Clean ones work surprising well.

For more detailed information on the good and bad flies, read the Texas Bug Book by Garrett and Beck.