Carrot fly Chamaepsila rosae adult fly, and larval damage to carrot crop
Female crane flies dip the tip of their abdomens into the soil to deposit clusters of eggs, which hatch a few weeks later. In some species eggs are laid in early autumn, with the young larvae overwintering and completing their feeding in the following spring or summer. Those species that lay eggs in early summer may have completed their larval feeding by autumn and will overwinter as pupae. When the adult fly is ready to emerge, the pupal stage wriggles upwards until it protrudes out of the soil. Empty pupal cases are sometimes seen in large numbers on lawns in late summer-early autumn.
While crane flies generally have a single generation a year, those of carrot fly, onion fly and cabbage root fly have at least two and sometimes three. These flies detect their host plants by scent and deposit eggs in the soil close by. The larvae feed initially on fine root hairs but later eat the larger roots or tunnel inside the fleshy tap roots. When fully fed, the larvae pupate in the soil. Onion fly and cabbage root fly overwinter as pupae, but larvae of the last generation of carrot fly can carry on feeding well into the winter.
Role of root-eating flies in gardens
While some flies have larvae that are unwelcome in gardens, their abundance at certain times of year means that they are important prey items for predatory animals such as bats, spiders and some birds. This is particularly true for crane flies, which are often present in large numbers in late summer and early autumn.
Most crane flies do not cause garden problems, however those that do are mainly a problem in lawns. Damage typically occurs in years when wet weather coincides with the egg-laying period, which ensures better survival of the eggs and young larvae. Leatherjackets can move from lawns into adjacent flower beds or vegetable plots and may damage young plants growing there.
Flies with larvae that eat the roots of vegetables can make growing vulnerable vegetables difficult. Young brassica plant are often killed by cabbage root fly, as are onions and leeks by onion fly larvae. A large proportion of the carrot crop can be rendered inedible by carrot fly tunnelling. There are some carrot cultivars that are sold as being resistant to carrot fly but ‘less susceptible’ is a better description.
There are no pesticides currently available for garden use that will give effective control of these insects, so growing vulnerable vegetables under insect-proof netting is the best means of preventing losses.
Other sources of information
Coe, R.L, Freeman P. and Mattingley P.F. (1950) Handbooks for the Identification of British Insects. Diptera 2.Nematocera: families Tipulidae to Chironomidae. Tipulidae 1. Out of print but available here.
Stubbs, A E. (2021) British Craneflies British Entomological and Natural History Society
Page text drafted by Andrew Halstead, reviewed by Andrew Salisbury, compiled by Steve Head