University of Illinois

No. 17/July 16, 1998

Controlling Rootworm Adults to Prevent Egg Laying

Adult corn rootworms have been out and about in Illinois for 2 to 3 weeks now, and more will continue to emerge from the soil in July. Because of the focus on the western corn rootworm "strain" that lays eggs in soybeans, people are more aware of rootworm beetles than ever before. Consequently, a lot of folks are scouting both corn and soybean fields to assess the presence or absence or densities of rootworm adults. In last week's issue (no. 16, July 10, 1998) of the Bulletin, we discussed rootworm adults and their propensity to clip silks. Now it's time to address the issue of controlling rootworm adults to preventthem from laying eggs, thereby eliminating the need for a soil insecticide at corn-planting time next spring. Although we have recommended this alternative for rootworm management for years, we have always emphasized that it's more complicated than applying soil insecticides and requires more management input. Most growers should consider hiring an experienced consultant to help implement a rootworm adult management program.

The idea of controlling adult corn rootworms to prevent egg laying originated with Union Carbide in the 1970s. They promoted the use of Sevin 4-Oil to suppress rootworm beetle populations for at least 3 weeks. To be effective in preventing egg laying, Sevin 4-Oil had to be sprayed when very specific thresholds were reached. Accurate and frequent scouting updates were required. Sevin XLR Plus replaced Sevin 4-Oil as the product of choice. The program worked effectively if farmers and their consultants were committed to a regimen of intensive and accurate scouting. Ideally, Sevin XLR needed to be sprayed only once at the appropriate time; if counts of adult corn rootworms did not return to critical levels in the field, egg laying was prevented and a soil insecticide was not necessary the following spring.

Prevention of egg laying by rootworm beetles has received renewed emphasis in Illinois over the past 5 to 10 years because other companies, such as Elf Atochem, have promoted their products (Penncap-M, for example) for the same use. Although the program offers corn producers a viable alternative for corn rootworm management, we maintain that the program works only if specific guidelines are followed by experienced scouts and consultants.

Following are some questions and our answers related to controlling rootworm adults to prevent egg laying.

If I treat a field to prevent excessive silk clipping, why can't I count on this application to eliminate the egg-laying threat of corn rootworm beetles?The threshold to protect pollination if rootworm adults are feeding on silks is five or more beetles per plant if pollination is not complete and silk clipping is evident. The thresholds associated with suppressing rootworm beetle numbers to prevent egg laying are much lower. In addition, treatments to prevent silk clipping generally occur too early in the season to significantly reduce egg laying by beetles later in the summer.

When should I start scouting for rootworm beetles, and what monitoring techniques and thresholds should be used in a beetle-suppression program to prevent significant egg laying? From mid-July through early September, you should be committed to scouting for rootworm beetles at least once each week. Determine the average number of beetles per plant by counting beetles on two plants selected at random in each of 25 areas of the field. Count all the western and northern corn rootworm beetles each time. The counts take about 45minutes in a 40-acre field. As you approach a plant, move quietly to avoid disturbing the beetles. Count the beetles on the entire plant, including the ear tip, tassel, leaf surface, and behind the leaf axils. Record the number of beetles you find per plant. If the average is more than 0.75 beetle per plant in corn after corn or 0.5 beetle per plant in first-year corn for any sampling date, plan to rotate away from corn, apply a rootworm soil insecticide to corn in 1999, or initiate a program for preventing egg laying. If densities of rootworm adults do not exceed these thresholds for any sampling date, rootworm larvae should cause no damage if corn is planted next spring.

In programs designed to prevent egg laying by rootworm adults, an additional threshold is required. An insecticide application is warranted if the beetle threshold (0.75 beetle per plant) is reached or exceeded and 10 percent of the females are gravid (with eggs). If more than 10 percent of the adult females within a field are gravid, significant egg laying probably has occurred, so the rootworm adult-suppression program probably will not be effective in preventing larval damage next year.

Why are the thresholds different for first-year corn and continuous corn? More females typically are found in first-year corn than in continuous corn. Therefore, there is a greater threat of more eggs being laid in first-year corn and, consequently, greater root damage occurring the following year.

Is it difficult to distinguish between male and female rootworm beetles and gravid and nongravid females? Distinguishing between male and female beetles is not difficult, particularly for western corn rootworms. Western corn rootworm males emerge first, followed by northern corn rootworm males, western corn rootworm females, and finally northern corn rootworm females. Northern corn rootworms, both male and female, are about 1/4-inch long and are uniformly green with no distinguishing stripes. The wing covers of male western corn rootworm beetles are mostly black, with a small tinge of yellow at the tips. Adult female western corn rootworms have three widely separated black stripes on yellow wing covers. Gravid females of both species usually have swollen abdomens filled with eggs. If you squeeze a female rootworm's abdomen and clear fluid is expressed, she probably is not gravid. If the fluid expressed looks and feels like tapioca, you probably have found a gravid female.

Will a single insecticide application sufficiently suppress egg laying by rootworm adults? Even if a field is treated in July, weekly scouting throughout August and into September is vital. Fields may become reinfested 2 to 3 weeks after an insecticide application. Consequently, some fields may require two applications of insecticide to prevent significant egg laying. However, treating a field twice to prevent egg laying may not be economically or ecologically viable. Refer to the preceding article regarding the development of rootworms resistant to insecticides in Nebraska.

Does planting date have any influence over which of my fields are most suited for an adult-suppression program? Late-planted fields, especially replanted fields, generally are not good candidates for an adult-suppression program. Large numbers of rootworm adults will be attracted to the fresh silks and pollen in late-planted fields and will move into these fields late in the summer.

I'm not real comfortable with the accuracy of my beetle counts because these insects always seem to be moving. Are there any other monitoring techniques besides counting beetles on plants? During the mid-1980s, research conducted at Iowa State University suggested that yellow sticky traps, specifically unbaited Pherocon-AM traps, could be used successfully to predict an economic infestation of rootworms the following season. To accomplish this task, 12 traps should be placed within a field throughout the egg-laying period of rootworm adults. The traps should be changed once per week. If six beetles per trap per day are captured, producers should consider rotating to soybeans or applying a soil insecticide at planting in 1999. Our on-farm research efforts in Illinois from 1991 through 1993 produced mixed results regarding the accuracy of these traps. However, Pherocon-AM traps are the scouting tool of choice for people who want to assess the status of western corn rootworm adults in soybeans. Refer to issue no. 15 (July 2, 1998) of this Bulletin and the fact sheet that was included with that issue.

Does the plant population influence thresholds for rootworm adults in corn? Yes, the corn plant population affects thresholds as it relates to the number of beetles per acre. Table 3 shows the thresholds for different plant populations and cropping sequences. If the density of rootworm beetles exceeds these thresholds, management of the 1999 larval population through crop rotation or an insecticide application is suggested.

Table 3. Thresholds for adult corn rootworms in corn.

Average number
of beetles per plant
Average number
of plants per acre
Continuous cornFirst-year corn

Kevin Steffey ( and Mike Gray (, Extension Entomology, (217)333-6652