Issue No. 10, Article 3/May 27, 2005
Stalk Borers Found Moving to Corn
In a recent report from northern Illinois, Ryan Stoffregan, Advanced Crop Care, has found stalk borers moving from giant ragweed into corn along fencerows. It's time to offer our annual heads-up for stalk borers. Larval movement from weedy hosts to corn has begun in some areas of the state.
Native to North America, the stalk borer can be found in the United States in areas east of the Rocky Mountains, from southern Canada to the Gulf of Mexico. Though mainly thought of as a pest of corn, the stalk borer attacks more than 100 species of plants, including fruits, vegetables, broadleaf weeds, grasses, and ornamentals.
The stalk borer completes one generation each year. Females deposit eggs in weeds and grassy patches, particularly along fencerows, ditch banks, and waterways the previous fall. Larvae begin hatching from eggs in late April and early May. Newly hatched larvae feed on and tunnel into above-ground stems of the host plants. As larvae outgrow the grasses, they crawl to larger-stemmed hosts, such as corn. In addition to corn, stalk borers will tunnel in most large-stemmed plants, including ragweed, cocklebur, burdock, cotton, and tomato, and even in some smaller-stemmed crops, such as alfalfa, rye, wheat, barley, pepper, pea, and bean. Dispersal will occur for several weeks, depending on nutritional quality and stem size of the host. Some larvae may change host plants several times. In late July to early September, larvae begin to pupate within host stems or to drop to the ground and pupate in the soil. Moths begin emerging in late August.
Stallk borer larva.
Degree-day accumulations can be used as a predictor of when larvae hatch from eggs and dispersal begins. Figure 1 shows degree-day accumulations (base 41°F), from January 1 through May 24. Larvae begin to hatch from eggs after the accumulation of 600 to 650 degree-days, and after 1,100 degree-days larvae begin moving to corn. Degree-days offer a predictor for when these biological events may occur, but insect growth may be slightly ahead of or behind these predictions.
Border rows of crops reveal the first telltale sign of injury. Young larvae tunnel into corn stems or climb into the whorl. As a result, the unfolding leaves show irregular holes and ragged edges where feeding occurred. Young plants (less than three-leaf stage) may wilt because of tunneling; older plants (four- to seven-leaf stages) usually discolor, wilt, and die, a symptom referred to as "dead heart." After plants reach the seven- to eight-leaf stages, corn usually tolerates feeding with no visible symptoms. Most loss occurs to border rows, as larvae do not move far from the grasses. However, the availability of preferred weedy hosts, such as giant ragweed, in or near the field can be linked directly to the abundance of stalk borer.
Stalk borer-infested corn plants.
Good management of stalk borers begins with sound weed-management practices. Destruction of weeds and grasses near field borders will reduce egg-laying sites in late summer. In the spring, postemergence insecticide treatments are effective when larvae are moving from weed hosts to corn plants. Decisions regarding the necessity of an insecticide treatment need to be made within the heat unit accumulation range of 1,400 to 1,700 (base 41°F). Some insecticides are labeled for use in tank mixes with herbicides. As larvae leave dying weeds, they come in contact with the insecticide and are killed. Insecticides labeled for control of stalk borer can be found in Table 1. Please read and follow all label instructions.
Growers are encouraged to begin scouting the border rows of their cornfields for stalk borer injury.--Kelly Cook