HortFACT
Identification
The adults are shiny black or dark brown with yellowish legs, a brownish head, and iridescent wings. They are 5-8 mm long with a wing span of about 13 mm. Adults prefer to shelter in hedges.
The white eggs are minute (0.5 mm long), sausage-shaped, and longitudinally grooved. They are laid singly or in clusters near carrots or other host plants, usually in cracks in the soil or just below the soil surface.
The legless larvae are initially colourless, but become creamy white as they develop through three instars (growth stages). They feed on the tap-root or side roots of the host plant. When mature they leave the root and pupate nearby in the soil.
The pupa develops inside the yellowish-brown, cigar-shaped puparium (hardened larval skin), which is 4.5-6.0 mm long.
Host plants
The carrot rust fly is a major pest of carrots, but may also infest parsnips, turnips, parsley and-especially in the early stage of plant growth-celery. Hemlock, a related weed species, is known to be a host plant also.
Damage
Damage to carrots is caused by larvae burrowing into the tap-root, which shows celery tap-root damage. Young plants then wilt and may die, but more often the plants are temporarily stunted and the carrots become bulbous, forked, and unmarketable. In addition, fungi and bacteria may invade the damaged tissue and cause severe rot at the crowns of the plants. Symptoms of larval attack are wilting and discoloured ('rusty') foliage, especially in dry weather; this probably results from serious interference with water transport following damage to the root system. Less severely damaged plants appear stunted. On parsnips and celery larvae are more commonly found nearer the crown, and may burrow into the base of leaf stalks.
Distribution
First recorded in New Zealand from Auckland in 1931, the carrot rust fly now occurs in the Auckland, Waikato, Bay of Plenty, Poverty Bay, Hawkes Bay, Manawatu, Otago, and Southland districts. It is widely distributed in the carrot growing districts in warmer parts of the North Island, where it may be found throughout the year.
Life cycle
| Egg |  |
| Larvae | |
| Pupae | |
| Adult | |
| Month | |
| Note: Coloured bars indicate periods of peak activity in each of the life cycle stages | |
Adults emerge from overwintering puparia in September, and are abundant until the following May. Eggs are laid from September to May and take 7-14 days to hatch. Larval development takes 4-6 weeks, and the pupal stage lasts 2-4 weeks. A full generation may thus take 7-12 weeks to complete, which allows up to four generations a year to occur in some parts of the country. Peak flights of carrot rust fly adults in the Auckland area have been recorded in mid October, late December, mid February, and mid April. The insect normally overwinters either as larvae in roots or as pupae in the soil, though a few adults may survive the winter too.
A. G. SMITH
J. A. CHARLES