Red turnip beetles in home gardens
Red turnip beetles, Entomoscelis americana, are typically associated with plants from the mustard family (also known as crucifers or Brassicaceae). They are more prevalent in areas with sandy soils. Red turnip beetles may migrate into home gardens from nearby fields where plants from the mustard family are currently growing or were grown in previous years.
Red turnip beetle adults are about ½ inch long. They have bright red wing covers with three black stripes that run lengthwise down its back. The head is red with a black spot on it and the prothorax (the area behind the head) is red with one large black patch and a small black spot on each side. Eggs are oblong and reddish-brown and typically laid on or near the soil. Mature larvae are black, about ½ inches long with a rough skin and short hairs covering a segmented body. Pupae are bright orange.
Red turnip beetles overwinter in the soil in the egg stage. Larvae hatch from the eggs starting in March and continue hatching until early May. The larvae feed on vegetables and weeds from the mustard family such as cabbage, turnip, flaxweed, shepherd’s-purse, and wild mustard. The larvae feed primarily at night although they have been observed during the day. If disturbed, they will drop to the ground and readily blend in with the soil. After feeding for about three weeks, the larvae will pupate within the top inch of the soil. Pupation takes about 2-3 weeks.
Adults emerge and appear in gardens from early June until early July. The adults feed for 2-3 weeks. Adults also feed on plants from the mustard family such as turnips, cabbage, radish, kohlrabi, hoary alyssum, sweet alyssum, and wild mustard. They have also been reported on strawberry, dandelion, bean, and potato which are from other plant families. From late June to mid-July, the adults burrow into the soil to rest for about one month. They re-emerge in late July or early August. The adults feed, mate, and lay eggs until late October. The eggs are laid singly or in clusters in shallow depressions in the soil or underneath leaves or other debris left on the soil. Each female produces about 300 to 400 eggs. There is one generation per year.
Figure 3. Red turnip beetle damage
In spring, both the larvae and adults feed on leaves, stems, and flowers. The extent of the damage depends on population levels. The damage can consist of light feeding to defoliation of the entire plant which can stunt or even kill it. Young seedlings and transplants are the most susceptible to injury from heavy feeding. Older plants can tolerate more severe defoliation. Later in the summer when only adults are present, the densities are usually not high enough to cause significant damage. Red turnip beetles are usually not an annual problem in home gardens and high numbers occur sporadically over the years.
Rototill or rake the soil in fall or early spring to bury the eggs. When the eggs hatch in spring, the larvae are unable to climb out of the soil to reach host plants. Cultivation from mid-May through June exposes the pupae to desiccation (drying out) and predation.
Plants from the mustard family are a primary food source for the emerging larvae. Therefore, eliminating weeds from the mustard family may help to reduce the number of red turnip beetles.
If only a few plants are affected, handpick and kill the larvae and adults (e.g., drop them into a pail of soapy water).
When possible choose a low impact insecticide that is less toxic, and “easy” on natural enemies and pollinators.
There are several options when considering a low impact insecticide. Neem is a plant based insecticide and acts primarily as an anti-feedant. Although insects are not killed quickly, it causes them to stop feeding and they eventually die. Spinosad is derived from a naturally occurring soil-dwelling microorganism, provides excellent control, but does not harm natural enemies.
Conventional, or broad-spectrum insecticides, are generally longer lasting but kills a variety of insects, including many natural enemies. Common examples of broad spectrum insecticides include permethrin, carbaryl, bifenthrin, and lambda-cyhalothrin.
*Long residual can persist as long as four weeks. Medium-long residual can persist as long as 10-14 days.
CAUTION: Read all insecticide label directions very carefully before buying and again before using to ensure proper application. Be sure that the label specifies it can be used on the specific cole crop you wish to treat, or cole crops in general. The label is the final authority on how you may legally use any pesticide. Whenever using any pesticide, including low impact, natural or organic insecticides, follow all label directions and use standard pesticide safety practices for transporting, storing, mixing, applying and disposing of pesticides to protect yourself, your neighbors and the environment.