The Carrot Rust Fly [CRF] is a weak flying insect pest that feeds its young on 107 different plants in the carrot family, including carrots, celery, parsnips, celeriac, parsley, and dill. The insect (Psila rosae) earned its common name due to the rusty coloured scarring left by its larvae as they feed on carrots, rendering them unmarketable.
The adult female CRF deposits her eggs in the soil near the base of the base of the carrot, and about one week later the larva hatches and begins to feed on the carrot root. Eventually the larva pupates (forms a cocoon) in the soil, emerges as an adult, and leaves the field to mate. Three generations of CRF may occur in a single year. In southern B.C. the first adult flies generally emerge at the end of April and are present until heavy frost.
Adults are about 6 to 8mm long, with shiny black bodies and reddish-brown head and yellow legs. Typically, the adults congregate at the edge of the field, not near the actual host plants. Yellow Sticky Traps just above carrot tops at each corner of the bed will indicate when the flies are about. More than one fly per trap per week is a problem.
Three organic methods can be used to eradicate the Carrot Rust Fly, or at least reduce damage to crops:
- Use crop rotation. Always plant carrots (and the other plants noted above) in a new area of the garden. CRF is a weak flier and will not infest fields from far away. If the adults emerge in the spring and find no suitable plants on which to plant their eggs, they will leave the area. For farms, plant new carrot plots over 1,000 meters from the previous year’s crop.
- Use lightweight row cover. On all new carrot beds, this cover prevents adults from accessing the plants in the first place. Carrot tops will push the cover up as the plants develop.
- Studies have shown that interplanting carrots with cover crops such as hairy vetch or crimson clover reduce CRF damage without affecting the yield. This has the added benefits of adding nitrogen to the soil, reducing water evaporation, and creating habitat for beneficial insects. Plus after harvest the cover crop can be tilled under.