That maggot could be the larval stage of the Carrot Rust Fly, which is a fairly common pest with carrots. Control lies with knowing its life cycle and making cultural changes to discourage its growth next year. For example, planting later than the end of May will avoid the first generation of egg-laying flies. Also, harvest early and completely (leave no plant behind). A sprinkling of rock phosphate around the base of the plant might inhibit egg laying. Covering the plant with a floating row cover is highly recommended.
That maggot could be the larval stage of the Carrot Rust Fly, which is a fairly common pest with carrots. Control lies with knowing its life cycle and making cultural changes to discourage its growth next year. For example, planting later than the end of May will avoid the first generation of egg-laying flies. Also, harvest early and completely (leave no plant behind). A sprinkling of rock phosphate around the base of the plant might inhibit egg laying. Covering the plant with a floating row cover is highly recommended.