Answer: Your crop has carrot rust fly, says Dan Mayer, Extension entomologist at Washington State University in Prosser. The adult fly emerges from the soil in June and lays tiny white eggs around the base of carrot plants. In a few days, the eggs hatch and the maggots tunnel into the soil and feed on the developing root. After three to four weeks the maggots pupate in the soil. In Seattle you could have two or three generations a year, says Mayer. The most destructive are the last two generations, as there is more root surface to feed on later in the season. The best protection would be to plant carrots early, in April or May in your area, under a floating row cover to keep the adult fly from laying its eggs, he continues. After you've removed all carrots from the ground, till the soil to help reduce the overwintering population. Rotate your carrot crop every year, and since the fly also attacks celery, parsnips, parsley and other Umbelliferae plants such as dill and Queen Anne's lace, don't plant these crops in infested areas.
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