Adult Flies themselves are rarely a major nuisance in vegetable and flower gardens. The larvae are the problems, maggots and leaf miners. The immature stages of flies that feed on roots, foliage, flowers, fruits and vegetables.
Carrot Rust Fly
Females lay their eggs at the crown of host plants. When the eggs hatch, larvae burrow into the soil and start feeding on the roots.
The adult is a common Garden fly, it is a slender, shiny, black fly, about 5 to 6 mm in length ( 1/5 inch), with a small reddish head and elongated yellow legs.
It feeds on several host plants including carrots, parsnips, celery, parsley, celeriac, fennel, dill, caraway, and coriander.
The larvae mine in the roots, causing holes that are subject to rot by secondary organisms. Heavy maggot feeding is indicated by drooping, discolored foliage.
The adult flies emerge in May and June. There is a pre- oviposition period of a few days before the females begin depositing eggs in the soil alongside host plants. The eggs hatch in 7 to 10 days.
The young larvae move downward feeding on the fibrous roots. As they grow, the larvae attack the taproot — girdling it about halfway down. Young plants wilt and may die.
There are several generations each season. Maggots from the Late Summer generation cause the greatest damage. The damage generally increases the longer the carrots are left in the ground.
Eliminate host-weeds such as wild Carrots, wild Parsnips and Hemlock in the area.
Neem acts as a deterrent against the adults, and is fatal to the larvae. It need to be frequently re applied as it degrades rapidly in nature. Garlic Spray also acts as a deterrent, it is not 100% effective but does help tominimize infestations.
Vegetation debris, in particular unharvested carrot roots, as well as other host crops should be removed at seasons end as there is a good possibility they are harboring larva awaiting next years crop.
Floating row covers prior to seedling emergence are advisable.
Sawfly
There are over 400 species of sawfly and they are found on a wide range of host plants including apples, pears, berries, and other soft fruit. The larvae look like a caterpillar, but they are in fact related to bees and wasps.
They are ferocious feeders, and will strip plants bear in a very short time.
Many sawfly species produce 2 to 3 generations a year. The adults lay the eggs on the underside of the leaves low down in the center of the plant, and the larvae are often not detected until the damage has been done.
Defoliated plants and bushes often give poor yields for several seasons, if they recover at all.
Although pyrethrin is a potent insecticide, it also functions as an insect repellent at lower concentrations.
Vinegar Flies and Fruit Flies
Vinegar flies are small, slow-flying insects usually found in the vicinity of over-ripened fruit and vegetables, and are generally confused with fruit flies, which is not much of a problem as the prevention of both is identical.
These insects are most abundant in the late summer months when fruits ripen and begin to ferment.
They are common nuisance pests anywhere there is fruit or vegetables be it Home, Markets, Restaurants.
Although they are an unsightly nuisance, they do not do much damage to small scale crops such as vegetable garden, they can be problematic on Commerical operations.
Cabbage Whitefly
The Cabbage whitefly is another garden fly which is found on many brassica such as Cabbage, Brussels sprouts, broccoli.
They are small white winged insects that live on the underside of brassica leaves. When disturbed they fly up in hoards . Adult whiteflies are structurally similar in appearance to aphids, but they are covered in sooty grey hairs and have white wings. When flying in clusters they resemble a cloud of gnats.
The flies themselves do not often cause severe damage, but the “honey dew” excretions they produce can disfigure the plants.
This is not so much to do with the honeydew itself but the sooty or black molds which grow on the honeydew. The sooty molds will prevent leaves from photosynthesizing, and leave remaining plants unpalatable.
This pest is tolerable unless infestation become severe.
Adult whiteflies can be caught on Yellow Sticky Traps. Spent adult carcasses should be vacuumed from leaves , floors in indoor environments. Remove any infested foliage.
Reflective mulches [Aluminum] in outdoor environments are advisable if whitefly is a severe problem. Studies have proven that they reduce whitefly populations. See: Combining reflective mulch and host plant resistance
Parasitic wasps will help control whiteflies in green houses, outdoors you might want to attract native parasitic wasps and Ladybugs.
Diverse gardens that include a symbiotic mingling of flowers and vegetables are a poor environment for whitefly and other pests as they also attract many natural predators, lacewings and ladybugs in particular