A3328

orn rootworms CJ.L. WEDBERG Common names Scientific names slopes. Hilltops tend to be a less- Northern corn rootworm barberi favored egg-laying site. Western corn rootworm Diabrotica virgifera virgifera Eggs begin to develop when Southern corn rootworm Diabrotica undecimpunctata howardi spring soil temperatures reach 50° to 52°F. Larvae emerge and begin Appearance yellow to green in color and about invading corn roots by mid-June, Rootworms—fully grown the same size as the northern and with the largest number of larvae larvae—are slender, white worms, western corn rootworm . This usually found in early to mid-July. approximately 1⁄2 inch long with species does not overwinter in Corn rootworms go through three brown to black heads and a dark Wisconsin, and is not a threat to corn larval stages (instars) and can move plate on the top side of the rear produced in the state because it as much as 20 inches through the soil segment. arrives too late in the summer to during this time. Northern corn rootworm beetles cause damage. Newly hatched larvae (first instar) feed on the smaller, branching are pale yellow to tan when they first Life cycle emerge from the soil but soon darken corn roots. Later instars invade the Northern and western corn root- to light green. They are about 1⁄4 inch inner root tissues that transport worm beetles lay very small (less long. water and needed mineral elements than 0.1 mm long), white eggs that Western corn rootworm beetles to the plant. In most instances, larvae are shaped like a football. Eggs over- have alternating black and yellow migrate to feed on the newest root winter in the top 6 inches of soil, stripes running lengthwise on the growth. although some have been found 10 to wing covers; these black stripes con- 12 inches below verge to various degrees on the the surface. In males. The adult western corn root- hilly fields, the worm is about 1⁄4 inch long. highest concen- The southern corn rootworm trations of eggs beetle—also known as the 12-spotted are often found cucumber beetle—has 12 conspicuous on west-facing black spots on its wing covers. It is Western corn Northern corn rootworm beetle rootworm beetle

Corn rootworm larva Lodged corn plants resulting from corn rootworm damage. CORN ROOTWORMS

Northern and western corn root- Damage worm beetles first appear in Rootworms cause economic Wisconsin between July 16 and 24, losses mainly in the southern two- according to a 3-year study at the thirds of the state. The corn rootworm University of Wisconsin’s Arlington beetles prefer loam soils for egg- research farm. The population peak laying and usually are not a problem for western corn rootworm male in peat, muck, and non-irrigated beetles occurs before the female beetle sandy soils. peak. And the population peak for the As mentioned previously, root- western corn rootworm beetle occurs worms tunnel in corn roots. This before that of the northern corn root- damage can consist of brown, elon- These pruned corn silks are the worm beetle. gated scars on the root surface, tun- result of beetle feeding. Adults reach sexual maturity neling within the roots, or varying slowly. Female corn rootworm beetles degrees of root pruning. do not begin laying eggs until at least Plants with pruned roots usually Different-sized (instars) larvae 2 weeks after mating. Although some lodge after storms accompanied by can be found at any given time. This egg-laying occurs in late July and heavy rains and high winds. Slight to is probably due to eggs hatching over early August in Wisconsin, most of moderate lodging can result in a long period of time and larvae the egggs are laid after August 15. reduced ear weight—the affected growing at different rates. In fact, While a female corn rootworm beetle plants will have a goose-necked larvae, pupae, and adults can be found is capable of laying more than 1,000 appearance up to harvest. With severe simultaneously in July and August. eggs, she usually lays 300 to 500 eggs lodging the corn roots will pull com- After feeding for about 3 weeks, over a period of several weeks. Most pletely out of the soil, and stalks will the larva leaves the roots, forms a of these eggs are in a (‘‘rest’’) fall flat. In this case, the ear often fails small earthen cell, and transforms to condition and must go through winter to develop. Or, if severe lodging the pupal stage. Transformation to the chilling before the embryo will occurs, the ear may be difficult to pick adult takes 1 to 2 days. Pupae are develop in spring. up with harvest equipment. white, delicate, resemble the beetle Soil moisture influences both the Although lodging is one indicator and are often found next to the plant number of eggs laid and where the of rootworm damage, lodging usually base but have been recovered as far as eggs are laid. Corn rootworm beetles occurs because of other factors. Some 25 inches away from the plant and will lay more eggs in moist soil than corn varieties lodge just because of 9 inches deep in the soil. This suggests dry soil. And high soil moisture wind and rain—without any root- that some adults must move a great content may induce the female beetles worm activity at all. This seems to be distance before reaching the soil to lay their eggs near the soil surface. particularly true for very tall corn surface. Research suggests that eggs near the before the brace roots are totally Northern and western corn root- surface need moisture to survive the developed. worm beetles feed on corn pollen and winter or they will probably die. Besides feeding on corn roots, many other types of pollen, as well as However, low soil temperatures due rootworm beetles feed on green corn corn silks. Occasionally, this pollen- to little snow cover may also con- silks and thereby reduce pollination. foraging habit results in corn root- tribute to egg mortality. However, most field corn is pollinated worm problems in residential flower before peak beetle populations occur, and vegetable gardens. In addition to so beetles present around mid-August pollen, the western corn rootworm or later have little impact on that corn. beetle feeds on corn leaves. The western corn rootworm beetle also feeds on leaves, and this can slow plant growth and on rare occasions kill plants of some corn Predicting potential rootworm When you check the ear, grasp varieties. Late-planted field corn and damage the ear tip tightly, enclosing the silks sweet corn—such as corn planted Another reason to scout for corn in the palm of your hand and count after early peas or first-crop alfalfa— rootworm beetles is to predict the all other areas of the plant first. The are most likely to suffer from adult potential for rootworm damage the silks often have the most beetles on feeding activity. following year. During some years, the plant, so a tight hold on the ear tip If you see lodging in your fields soil are applied to keeps beetles from dropping. Examine during July or August, dig several 90 percent of the continuous corn the ear tip by cutting it off with a lodged and unlodged plants along acreage in the rootworm area of knife, cutting only the silks. Open with a 6-inch core of soil. Shake off Wisconsin, yet only 40 to 50 percent of your hand slowly and count the the soil, thoroughly wash the roots, these fields have rootworm popula- beetles that come out of the silks. and look for rootworm feeding—the tions that justify such applications. Record both the number of root tips may be brown, or appear While this proportion will vary western corn rootworm beetles per tunneled and pruned back to the from year to year depending on plant and the number of northern plant base. You may find larvae in the natural fluctuations in the corn root- corn rootworm beetles. plant crown. worm population, it still indicates that Control Do not wait until fall to check many farmers could save money with lodged plants, because normal root prudent use. By scouting Chemical control decay present in fall usually makes it cornfields during August and evalu- Although a soil insecticide will impossible to accurately assess root- ating corn rootworm beetle popula- not kill all corn rootworms in a field, worm damage. tion levels, you can determine if an it controls them sufficiently to prevent insecticide will be necessary if you economic damage. However, situa- Scouting suggestions plant corn in the same field next year. tions such as extremely high popula- Determining immediate adult Beginning in late July, make three tions of larvae, heavy rains, and rootworm damage scoutings at 7- to 10-day intervals improper calibration of insecticide Corn rootworm beetles can inter- through late August. Occasionally, application equipment can result in fere with pollination by pruning silks beetle numbers are so dense (such as poor control. during this critical period; this is one in late-planted fields) that they liter- Depending on their label, insecti- reason you should scout your fields. ally are ‘‘all over the place.’’ In these cides can be banded ahead of the Check for corn rootworms before instances, detailed counts aren’t nec- planter presswheel, broadcast before 70 percent of the plants are in the essary—the odds are good that the planting or at early postemergence process of silking. Waiting until later field will have a corn rootworm infes- (before mid-June), or applied side- may result in yield losses before you tation that can cause substantial corn dress during cultivation. The pre- have a chance to apply insecticides. losses next year. ferred method is to apply a 7-inch Beetles do not reduce yields once pol- For lesser populations count the band of granular insecticide ahead of lination is over and silks are brown. number of western and northern corn the planter presswheel at planting. First, count the number of beetles rootworm beetles on 50 plants each While it is possible to plant corn on 10 random plants in five separate time you sample. Examine 10 plants so late that most of the rootworm areas (a total of 50 plants). Do this for selected at random in each of five larvae starve before corn roots are each planting date and each variety in areas in the field. available, dent corn yields from such the field. Record the number of Make sure you move quietly as late plantings are too low to make this beetles per plant and the number of you approach a plant so you don’t practical. If there are enough root- 1 plants with silks pruned back to ⁄4 disturb the beetles. Count the beetles worm eggs present to cause substan- inch or less. Also record the number on the entire plant—including the tial corn losses in a field, use a corn of plants that haven’t silked, the tassel, ear tip, leaf surface, and behind rootworm insecticide whenever corn number with fresh silk, and the leaf axils. Pull the leaves away from is planted before mid-June. number with brown silk. In addition, the stalk to examine leaf axils. Extension publication Field Crops determine if pollen is still being shed. Pest Management in Wisconsin (A3646) CORN ROOTWORMS contains detailed information on Crop rotation is also a good man- Although there may be some selecting and using corn rootworm agement option because it reduces the damage to first-year corn planted insecticides and provides information possibility of corn rootworms devel- after soybeans, alfalfa, small grains on treatment thresholds. It is revised oping resistance to a particular insec- and other crops, it seldom is great annually because treatment thresh- ticide. Insecticide resistance is pos- enough to warrant insecticide treat- olds and insecticide suggestions may sible whenever an population is ment for corn rootworms. Thus, change each year. This publication is continually subjected to selection annual crop rotation is an excellent available from your county Extension pressure from insecticide use. method of corn rootworm control. agent. Corn can be planted after soy- Resistant varieties. There are Non-chemical control beans without using a soil insecticide no commercially available dent or Crop rotation. Growing corn in as long as there was not an abundance sweet corn varieties resistant to corn the same field two years in a row of volunteer corn or weeds present in rootworms. However, there are vari- gives rootworms a chance to greatly the soybean field. Weed or volunteer eties that are rootworm tolerant—they damage corn the second year, so a soil corn pollen may attract sufficient corn have the ability to outgrow rootworm insecticide is often needed at planting rootworm beetles and result in damage and regenerate roots better time where rootworms are a problem. enough eggs deposited to produce than other varieties. Planting these Late-planted corn fields are espe- substantial corn losses. Good weed varieties will not eliminate the need cially attractive to rootworm beetles. control will help eliminate this for soil insecticide use, but used with Their green silks and pollen can problem. an insecticide they will help prevent attract large numbers of beetles from Corn rootworms occasionally are serious economic damage to your more mature corn in surrounding a problem in fields planted to corn crop, as well as lodging. Contact your fields, resulting in massive numbers after alfalfa. Adults attracted by area seed dealers for additional infor- of eggs being deposited in the soil. alfalfa or weed blossoms during the mation about tolerant varieties. Since soil insecticides do not control summer egg-laying period may lay Natural control. Nature also 100 percent of the rootworm larvae, enough eggs to result in a problem helps control insect pests. While adult an unacceptable amount of root level of larval feeding the following and larval corn rootworms are essen- feeding could still occur in these summer. tially free of parasites, ground beetles fields. In these cases, crop rotation You can plant corn after most and predacious mites in the soil help appears to be the most appropriate vegetables, except lima beans, control corn rootworms to some control choice. without using an insecticide at extent by feeding on rootworm eggs, Crop rotation is an excellent planting time. Lima beans blossom larvae and pupae. method of controlling corn root- over a long period of time and may worms. Larvae perish soon after attract corn rootworm beetles. In fact, hatching if a crop other than corn is some canners report corn rootworm planted in the field they inhabit. damage when sweet corn follows lima beans in rotation.

Copyright © 1996 University of Wisconsin-System Board of Regents and University of Wisconsin-Extension, Cooperative Extension Author: J.L. Wedberg is professor of entomology, College of Agricultural and Life Sciences, University of Wisconsin-Madison and University of Wisconsin-Extension, Cooperative Extension. Produced by Cooperative Extension Publications, University of Wisconsin-Extension. University of Wisconsin-Extension, Cooperative Extension, in cooperation with the U.S. Department of Agriculture and Wisconsin counties, publishes this information to further the purpose of the May 8 and June 30, 1914 Acts of Congress; and provides equal opportunities and affirmative action in employment and programming. If you need this material in an alternative format, contact Cooperative Extension Publications at 608-262-8076 or the UWEX Affirmative Action Office. This publication is available from your Wisconsin county Extension office or from Cooperative Extension Publications. You can print or order online at cecommerce.uwex.edu, call 877-WIS-PUBS (947-7827), or send a fax to 414-902-2679. A3328 Corn Rootworms