Carrots Are Defoliated By Carrot Weevils
Carrot weevil larvae defoliate plants, by devouring their foliate from tip to crown. After an attack, only the ribs of leaves and stalks remain. The pests also excavate zigzag tunnels into the carrot root. The adult carrot weevil is 1/5 inch long, brown, and hard-shelled. Their larvae are plump, pale worms, with brown heads and no obvious legs. If these weevils were a problem last year, try planting carrots later in the season if you live in the South or Northeast.
Wherever you live, you can protect your plants from the weevils by covering them with garden fleece. You can also use predatory nematodes, live microscopic roundworms, packaged as a powder to be mixed in water and sprayed on the soil. They will parasitize the larvae by growing inside them until the larvae die. Spray any visible adult weevils with a pyrethrin/pyrethrum insecticide product as directed on the label.
For more information see file on Dealing with Weevils.