Marion Paibomesai, OMAFRA Vegetable Crop Specialist, Guelph
Carrot weevil (Listronotus oregonensis (Le Conte)) is an important insect pest of carrots in Ontario. The adults are dark-brown, greyish beetles that are about 6mm long. They typically do not attack carrots until the first true leaf stage. When egg-laying occurs (mid-May to early June) the females chew holes in the petioles or crown of the carrot plant to place eggs in. After one to two weeks the larvae hatch and start to chew tunnels in the root of the carrot making it unmarketable or creating entry routes for secondary pathogens. When the larvae feed on young plants, the feeding damage may cause plants to wilt and/or die. The carrot rust fly larvae also cause similar damage to the roots. On mature carrots it is easier to distinguish the damage caused by carrot weevil versus carrot rust fly. The tunnels of the carrot weevil are darkened and partly open (Figure 1). Also, carrot weevil larvae tend to feed on the upper one-third of the root, whereas carrot rust fly larvae damage is mostly on the lower two-thirds of the root. Carrot weevil may also attack celery, parsnip, wild carrot and other Umbelliferous plants. In Ontario, there are typically 1 to 2 generations per year.
In terms of management, there are two products currently registered for carrot weevil control: Imidan 50 WP (phosmet) and Matador 120 EC (lambda-cyhalothrin). Please follow label for directions for use and safety protocols. Some use carrot weevil traps (i.e. wooden traps baited with carrots or mature carrot roots buried in the field) to determine when to apply products based on monitoring and thresholds (See: Integrated Pest Management of Onions, Carrots, Celery and Lettuce in Ontario for more information. A degree day model to predict egg-laying of carrot weevil has been developed. Egg-laying is expected to start at 147 growing degree days and 90% oviposition is expected at 455 growing degree days (I will start sending out weekly updates on degree day accumulations starting next week). As well, crop rotation and weed management surrounding the field may reduce overwintering populations of this pest.
Fig 1. a) Carrot weevil adult.; b) Carrot weevil larval feeding damage; c) carrot weevil larva