Symptoms
The larvae emerging from newly laid eggs feed on the root hairs, rootlets and tap roots of the host plant. Early symptoms are wilting and a reddish / yellow discolouration of the outer leaves.Early attacks can result in plant loss but the more common outcome is that surviving plants are distinctly stunted. As the plant roots develop and enlarge the larvae mine into the roots leaving unsightly characteristic reddish / brown tunnels below the skin surface.
Life-cycle
First generation adult flies are often on the wing when cow parsley is in full flower at the end of April. They migrate into crops from nearby sheltered areas such as hedgerows. The adults are very weak fliers and rarely rise above a height of 50 cms. Eggs are laid into soil crevices around the base of host plants. Depending on temperature the larvae usually hatch in about one week and feed on the plant roots.
Further damage can be caused by the larvae moving from plant to plant. After completing three growth stages (moults) the larvae pupate in the soil. The transition from egg to adult can be completed in 3 months.
Carrot flies can survive the winter in a variety of different ways. The adults can survive by sheltering in warm protected environments, the pupae can overwinter in the soil or the larvae can survive in the roots of host plants, especially in crops which have been covered with straw for protection from cold weather.
There are usually two generations per year but a third generation is possible especially if temperatures remain high into the autumn. The first generation arises in late April / early May and the second is on the wing in late July. It is the first two generations which are responsible for economic crop damage.