|The larva of the carrot rust fly is a maggot that tunnels into carrots. Photos by Eric Sideman.|
by Eric Sideman, Ph.D.
I have lived in the same house in Greene, Maine, for over 20 years, and the carrot rust fly has been a problem only one of those years. But it was awful that one year, and while I was discarding carrot after carrot from the root cellar that winter, I kept thinking that a commercial carrot grower would be down one customer if he or she sold such carrots to somebody.
This pest rears its ugly self from tunnels that it burrows through carrots. The tunnels can be seen on the surface of the carrot and are rusty in color due to the maggot’s excrement – hence the name of the pest. Cutting the carrot for your salad frees the wriggling maggot. Although the part of the carrot without tunnels is fine to eat, many people lose their carrot craving after seeing the maggot.
When you have this problem, it is a serious pest that needs to be dealt with, but since its occurrence is spotty, I hesitate to implement defenses. Actually, I haven’t done anything since my single infestation and have never had the problem again.
The light yellowish maggot is the larva of a blackish fly that has some greenish markings and an orange head and is about a quarter of an inch long. The spring generation of the fly lays its eggs on the ground at the base of the carrot plant in mid-May to early June. The young larvae burrow into the soil and feed on the small roots of the growing carrot. Then the older larvae enter the main root. When the larvae mature they leave the carrot and pupate in the soil. The second generation of adult flies emerges from the pupae from mid-August to mid-September and lays another batch of eggs that produce the maggots that develop in storage carrots. If carrots remain in the garden, these larvae mature, leave the carrot, pupate in the soil over winter and emerge in spring as flies. Wild and volunteer carrots, parsley, celery, coriander and parsnips are other hosts, and rust fly larvae from these crops mature and pupate in the soil, so crop rotation is unlikely to provide control.
Larvae feeding during the summer cause stunted plants that turn yellow. Larger larvae destroy the crop. To add insult to injury, soft rot bacteria may take hold in the tunnels, so that the carrots decompose into a soft, smelly mess. Larvae in fall carrots may be small when the crop is harvested for storage and may go unnoticed, but they continue to develop into large larvae during storage.
Controls for carrot rust fly are all cultural modifications. If feasible, rid the growing area of all hosts the year before growing carrots, and, in any fields that had hosts, plow deeply in the fall to bury overwintering pupae.
Planting later than the end of May will avoid the first generation of egg-laying flies. Harvesting an early planting by mid-June will get carrots out before the larvae enter the taproot or grow large enough to be noticed. Harvest early plantings in blocks and be sure to harvest the crop completely so that the area will not produce second-generation flies.
Sprinkling rock phosphate around the base of the plant is said to deter egg laying. Some folks say that beneficial nematodes help reduce populations, especially when crops that confuse the fly, such as onions, leeks and crimson clover, are grown with the carrots. Recent research in Washington State did not support interplanting with crimson clover as a control.
By far, covering the planting with floating row cover is the best control – especially if you have had repeated problems with the pest and know that it overwinters regularly in your area. Carrots that are relatively large in August, when the second generation of egg-laying flies is active, and that are intended for late fall harvest are most important to cover. Early carrots that are large when the first generation is laying eggs, from mid-May to June, may also be important to cover if you plan to harvest those carrots in late summer, since that would give the larvae time to grow.
About the author: Eric is MOFGA’s organic crops specialist. You can contact him with your questions at [email protected] or 568-4142.