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Issue No. 17, Article 1/July 16, 2004

Time to Begin Scouting for Corn Rootworm Adults in Soybean and Corn

On July 13, I spent a very enjoyable morning visiting with producers at the Northern Illinois Research and Education Center located near Shabbona, Illinois. We discussed several entomological topics of interest this growing season; however, most of our conversations focused on corn rootworms and performance issues related to soil insecticides and insecticidal seed treatments.

Following our discussion, we examined root systems from our insecticide evaluation trial at this location. Many in attendance, including me, were surprised to see the level of root injury among the various treatments. A cursory look at the plot revealed seemingly uninjured plants. No plant lodging was evident. After a few plants were removed from each treatment and the soil was washed away from the root systems, significant root pruning became readily apparent.

On the way back to Champaign-Urbana, I encountered heavy rain, hail, and wind, particularly in LaSalle County. I suspect this storm may contribute to lodging in fields with moderate to heavy root pruning in this area. In the next few weeks, I won't be surprised to learn of increasing reports of corn rootworm problems and concerns related to performance issues of soil insecticide products.

Many producers indicated that corn rootworm adults are now common inhabitants of cornfields and soybean fields. This includes even those fields that received a soil insecticide application this spring. Frequent readers of the Bulletin may recall that previously we've discussed the fact that soil insecticides are not population suppression tools. Adult emergence between corn rows (areas outside of the insecticide band) occurs each season.

Monitoring Cornfields to Prevent Excessive Silk Clipping

Controlling corn rootworm adults to prevent egg laying and controlling adults to prevent silk clipping do not overlap in time. The great majority of corn rootworm eggs are laid after the time when treatments to prevent silk clipping would be necessary. Densities of at least five adults per plant typically are required to affect pollination in commercial cornfields.

Seed-production fields are likely to be at greater risk to economic losses caused by silk clipping. In addition to noting the number of adults observed on plants, pay close attention to the amount of silk tissue protruding from the tips of ears. When 1/2 to 1 inch of fresh silk remains and soil moisture is abundant, successful pollination is likely occurring. Many insecticides are labeled as rescue treatments to prevent excessive silk clipping (Table 1).

Excessive silk clipping by corn rootworm adults.

Control of Corn Rootworm Beetles to Prevent Egg Laying in Cornfields

Controlling corn rootworm adults to prevent them from laying eggs is recommended only for corn planted after corn, not for corn planted after soybean. Scout for corn rootworm adults in corn from mid-July through early September to determine the potential for rootworm larval damage in 2005. The requirements that must be met for a successful beetle-suppression program are complex. Scouts must identify both species of corn rootworm adults (western and northern), distinguish between the sexes, and also determine whether the females are gravid (have eggs present).

Frequent scouting trips in corn over a lengthy period of the hot summer are essential. Although one properly timed adult-suppression treatment should replace a soil insecticide the following season, some fields require two insecticidal sprays. Two adult treatments or a spray plus a soil insecticide the following spring may hasten the development of insecticidal resistance.

If you choose to use an adult-suppression program, apply an insecticide when the number of beetles reaches or exceeds 0.75 per plant and 10% of the females are gravid. Continue to monitor fields weekly after a treatment has been applied for the beetles. A second insecticide application may be necessary if the number of beetles reaches or exceeds 0.5 beetle per plant. Adult-suppression programs to prevent egg laying have worked very well for many years, particularly in some western states of the Corn Belt, such as Nebraska. However, these broadcast treatments have led to the development of insecticidal treatments.

Scouting Cornfields to Determine the Need for a Soil Insecticide Next Season

If you intend to grow corn after corn and if rootworm beetles averaged 0.75 or more per plant in continuous corn or 0.5 per plant in first-year corn (for any sampling date), consider the use of a soil insecticide during planting in 2005. Fields that will be planted to corn in 2005 should be scouted weekly between mid-July and early September. Examine two plants selected at random in each of 25 areas of a given field. Count all the western and northern corn rootworm adults each time. These scouting efforts require about 45 minutes in a 40-acre field. As you approach a given plant, avoid disturbing the beetles as much as possible. Count the beetles on the entire plant, including the ear tip, tassel, leaf surface, and behind the leaf axils.

Scouting for Western Corn Rootworm Adults in Soybean Fields

An adult-suppression program to prevent egg laying by the western corn rootworm variant in soybean fields is not recommended. Scouting protocols and thresholds in soybean fields are designed to aid producers in making more-informed decisions regarding the potential need for a soil insecticide in first-year cornfields the following season.

Yellow sticky traps (Pherocon AM) should be deployed in soybean fields during the last week of July and for each of the first three weeks of August. Old traps should be replaced with fresh traps on a weekly basis. Twelve traps should be evenly distributed across a given soybean field. The cost of traps for the season is approximately $50 (roughly $1 per trap). If trap averages for the 4-week period equal five beetles per trap per day, growers should anticipate the potential for economic injury the following season in first-year corn and may elect to use a soil insecticide. For more details on this management approach, please consult the following Web site. --Mike Gray

Mike Gray

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