Distribution. Carrot beetle is found throughout the United States and southern Canada, and in north ern Mexico. This native insect is not usually considered to be a pest in northern states and in Canada, but can be injurious in the southern Great Plains.
Host Plants. This insect feeds on a fairly broad range of plants, and carrot is not one of the crops most frequently damaged. Chittenden (1902) argued that it was the most serious pests of carrot in the past century, and his opinion apparently prevailed. Carrot weevil, Listronotus oregonensis (LeConte), and carrot rust fly, Psila rosae (Fabricius), have since become much more serious pests of carrot, but the name of this beetle, though inappropriate, is unlikely to change. Hayes (1917) registered his dislike with the name "carrot beetle," but failed to suggest alternatives. The name "sunflower root beetle" might be a more accurate reflection of this insect's ecology.
Vegetable crops damaged by the carrot beetle include cantaloupe, carrot, celery, corn, parsnip, pepper, potato, sweet potato, tomato, and watermelon. Other crops injured are cotton, sugarbeet, sunflower, and perhaps oats and wheat. Weed hosts are quite numerous, and include prairie sunflower, Helianthus petiolaris; carelessweed, Amaranthus palmeri; horse-weed, Conyza canadensis; sawleaf daisy, Prionopsis ciliata; silverleaf nightshade, Solanum elaeagnifolium; and white rosinweed, Silphium albiforum. Most plant feeding is accomplished by adults. Larvae develop principally on decaying organic matter, and are often associated with animal manure. Occasionally, they feed on roots, however, and also are cannibalistic.
Natural Enemies. Several flesh flies (Diptera: Sarcophagidae) are parasitic on this beetle, including Gymnoprosopa argentifrons Townsend, Sarcophaga sar-racenioides Aldrich, S. cimbices Townsend, S. helicis Townsend, and S. rudis Aldrich (Hayes, 1917; Rogers, 1974). Additionally, undetermined fungi and bacteria are associated with eggs and larvae. A bulb mite, Rhi-zoglyphus echinopus (Fumouze and Robin) (Acari: Acar-idae) builds up to high densities on the overwintering adults. The importance of these natural enemies is unknown.
Life Cycle and Description. There is a single generation annually, with adults serving as the overwintering stage. Overwintering adults emerge in April-June, feed and oviposit. Larvae develop during the summer months, and produce a new brood of adults during July-October that overwinter and emerge the following spring.
The most complete biology of carrot beetle was given by Hayes (1917), but more recent studies by Bottrell et al. (1973) and Rogers (1974) are significant contributions.
Damage occurs on a wide variety of plants, but has been rigorously studied only on sunflower. In such a crop, where the seedlings do not emerge until mid-
summer, it is the summer rather than the overwintering generation of beetles that causes injury. Damage to early season crops does occur, but it is caused by overwintering adults. Larval damage usually is insignificant. Adults feed at an average depth of 3.5 cm (range 0.6-12.9 cm), and a single adult can destroy virtually all the lateral roots of a small plant during the average feeding bout of 3.6 days duration. The primary, or tap root, is often only damaged at the surface. Sunflower varieties differ in susceptibility to injury, with perennial species less damaged (Rogers, 1980).
Carrot beetles have proven to be difficult to control unless highly residual insecticides are applied to the soil. Granular and liquid formulations may be applied, but success is not always guaranteed (Bottrell et al., 1970; Rogers and Howell, 1973). Fortunately, they are usually associated with wild plants rather than crops. The principal exception is damage to sunflower, which can be a problem when this crop is grown in the southern Great Plains. Rogers et al. (1980) reported on evaluation of resistant sunflower varieties.
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