This is what you have, info from Canadian Agric. Dept
Carrot Rust Fly
Characteristics: The adult is shiny black, with pale yellow head and legs. It is less than 5 mm in length. Damage is caused in feeding by yellowish-white, legless maggots which are 8 mm long when fully grown. Attacks result in stunting of carrot and parsnip plants. Roots may be reduced in size or distorted, scarred and riddled with rust-red burrows of the larvae. Seedlings may be killed if the growing tips are severely injured. Adult flies appear twice, in early summer and mid August, and lays eggs, which hatch 6 to 12 days later into larvae. Adults start at field margins increasing risk of damage in these areas. Small sheltered fields and poor rotational practices increase the chances of rust fly injury. This insect is a weak flier and does not do well in a windswept field.
Control: Follow a good crop rotation. Do not plant in fields adjacent to last years carrot crop. Where a known population exists chemical controls should be applied at planting. Planting may also be delayed until after mid-June to avoid the first generation of flies. Carrots may be harvested early (in late August or early September) before the maggot leaves the hair roots and enters the main tap root. Individual fields should be monitored using orange sticky traps to determine if control measures are necessary. Place traps in sheltered areas of the field, along the field edge. Apply spray when adult flies are first caught in the traps. Polyester or other fabric covers that exclude this fly could be applied on small fields. Do not allow holes to develop in the material or control will not be obtained. Row covers have not been researched in this area and may aggravate foliar and root diseases.