Corn Rootworm in Sweet Corn
When considering insect management in sweet corn at planting, my first question is: "Did you rotate?". If you did, you effectively avoid problems from two corn rootworm species. If not, ideally you scouted for rootworm adults in the silking corn last summer, and can base a decision about a soil insecticide according to thresholds. If you are not rotating and did not scout, you should be concerned about corn rootworms and consider a soil insecticide at planting.
There are two species of corn rootworms: the western corn rootworm (WCR) and the northern corn rootworm (NCR). Both species are native to North America, and were recorded only from the north central states in the early to mid 1800s. The northern corn rootworm reached Pennsylvania in the 1950s, and the western corn rootworm in the 1980s. The WCR looks similar to the striped cucumber beetles, but the stripes are not as straight, and the abdomen is yellow in the WCR, but it is black in the striped cucumber beetle. The NCR is pale green to tan or dull yellow in color. In sweet corn or field corn, the adult beetles feed primarily on pollen and on the silk, and they can also feed on the leaves. In cucurbits, such as pumpkins, melons, etc., the beetles are easily found in the flowers mixing in with the striped cucumber beetle, probably also feeding on pollen. Thus, the adult feeding occurred last summer and fall.
The adults of both rootworm species lay their eggs at the base of corn plants in late summer or early autumn. The eggs overwinter in the top 6 inches of soil, and emerge next spring. Egg hatch occurs mostly during June, but extends from late May until early July in Pennsylvania. Larval development requires 4 to 6 weeks. Adult emergence follows for 4 to 6 weeks, with up to 1000 eggs per female being deposited from mid-July, thoughout August, and into September.
Newly hatched larvae feed on root hairs and outer root tissue, and older larvae tunnel into roots and can even enter the plant crown. Root tips appear brown and chewed back. The root feeding affects plant uptake of water and nutrients, so the amount of plant damage is influenced by water availability. Vigorous plants can compensate under moist growing conditions by growing new roots faster than they are pruned off, but effective plant compensation may be better in field corn than sweet corn varieties. Although this larval feeding is typically the most significant damage by rootworms, later in the season you need to consider damage by adults feeding on silks.
The larvae that emerge in the spring will cause significant damage to the roots of corn unless one of two control measures are taken: (1) the field is rotated out of corn or cucurbits; or (2) a soil insecticide is applied at planting if corn or cucurbits is planted. The most effective option is to rotate. Only a single year rotation is required, and only a short distance of a few feet will be sufficient. That is because the newly hatched larvae must find corn roots to feed on within a few feet of where they hatch. When corn is rotated to legumes, the great majority of the larvae will starve within 1 year. Rotation to other grasses (wheat, rye, barley, oats or sorghum) is also effective because the larvae do not survive well on these other grasses. Even a 2-year rotation on larger fields is effective in most years, because it takes some time for the rootworm populations to immigrate in and build up within a continuous corn field. Large rates of immigration could compromise a 2-year rotation, so scouting those fields in the fall is recommended. If you are rotating annually, it is not necessary to scout for rootworm adults - you can safely assume that the rotation is effective. There is an interesting case of a population that has adapted to a corn-soybean rotation by adults from one crop adapting to the other, but this concern is confined to the midwest.
If rotation is not practiced, insecticide needs to be applied when the adult population exceeds threshold. The threshold is determined last fall, by estimating the number of adults per plant. On field corn, the threshold varies from one to three beetles per plant. Sweet corn is more susceptible to damage, because of the smaller root system, and therefore should use the lower threshold. Insecticides can be applied at planting or at cultivation. Moving the time of application close to the time of rootworm egg hatch is the most effective, which occurs from late May until early July. Several of the insecticides listed for wireworms or grubs in the Commercial Vegetable Guide are also labeled for corn rootworms in sweet corn. Others listed for wireworm or grub control may also have a rootworm label in sweet corn. Check the label and follow directions on the label. Insecticides in continuous corn do protect the roots, but are not as effective as rotation - you can get rootworm population growth in unprotected areas of the soil even when soil insecticides are applied.
In Pennsylvania, rootworms are typically the most significant pest that would require a soil insecticide at planting in sweet corn. However, wireworms, grubs, or cutworms sometimes also cause problems with stand establishment. Wireworms are larvae of click beetles, and grubs are larvae of scarab beetles (June beetle, Japanese beetle, etc.). Problems from these are usually associated with weedy fields or fields recently moved into production from sod, forage, or pasture. Cutworms are larvae of night-flying moths. Weedy or minimum till fields are especially attractive for egg-laying.
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