Frank Evans, of Naseby, asks :-
When and where do carrot flies lay their eggs? I dislike using poisons but they are a pest in my garden so is it possible to attack them at an early stage?
Ruud Kleinpaste, the bugman of media fame, responded.
Carrot fly - more appropriately known as carrot rust fly - is an accidentally introduced insect, associated with the plant Family Apiacea (formerly known as Umbelliferae), to which carrot, parsnip, turnip, celery and hemlock belong. Indeed the insect can be a pest of the root systems of all these plants.
The adult female flies will fly at low altitude (i.e. near the ground) to find their host. They then lay their eggs on the "neck" of the developing carrot - that orangey bit that sticks just above the ground. The larvae (or maggots) that hatch will waste no time tunnelling into the carrot roots, which are then open to infection by all sorts of fungi and bacteria. In the warmer months the life cycle takes about 5 weeks.
Chemical control used to centre around dusting the seed with insecticide; these days Diazinon granules and other specific soil insecticides are employed to make the root zone an unhealthy place for carrot rust fly maggots. But there are other tricks we can use to minimize damage by this insect: The first thing is to practise hygiene around the carrots: Don't leave old carrots in the ground longer than necessary; remove crop debris and get rid of alternative host plants in the vicinity. Reduce shelter and weeds around the crop too!
Low flying adult flies can literally be kept out of the vegie garden when you construct a "fence" around the vegie patch; the fence could be made of shade cloth and should be, say, 600 mm. high. The silly flies never think to fly over the fence!
Finally - and this is quite interesting - the interplanting of carrots with onions has been scientifically shown to be beneficial, as it reduces rust fly damage on carrots and onion fly damage to onions! One of the few kosher and proven cases of "companion planting".