No. 14 Article 2/June 30, 2006

Still Too Early to Assess Western Corn Rootworm Impact

Compared with 2005, the numbers of reports of severe rootworm larval damage and large numbers of western corn rootworm adults are few this year. Some people are speculating that the often-discussed long-term cycle of western corn rootworms is trending downward. Maybe, but it's still too early to tell. We will have a better handle on densities in July when rootworm larval injury reaches its zenith and rootworm adults are present during corn pollination. In the meantime, keep sending us your observations.

Fortunately for our research efforts, the amount of larval injury in our rootworm control plots appears to be significant. Before the recent rainfall, the untreated check rows in our experiment near Urbana were showing obvious signs of stress because of a lack of soil moisture and rootworm injury to the roots. The plants were rolling their leaves tightly during the heat of the day. The same signs of stress were apparent in some plots in which rootworm control products had been applied, so the results from our rootworm injury evaluations in July should be interesting. Eric Adee, superintendent at the Northwestern Illinois Agricultural Research and Demonstration Center near Monmouth, sent us some photographs of lodged plants in untreated check rows in one of our experiments at the center. The plants lodged after some stormy conditions on June 22. However, the corn in plots with rootworm protection remained upright.


Lodged rows of corn (foreground) in untreated check, Northwestern Illinois Agricultural Research and Demonstration Center (photo courtesy of Eric Adee, University of Illinois).

We also are awaiting word on suspected corn rootworm larval injury in some fields of corn planted after soybean near I-70 in the southwestern part of the state. The potential for the variant western corn rootworm to extend its range farther south in Illinois is bona fide, although we do not anticipate widespread problems in 2006. Nonetheless, assessment of rootworm larval injury in fields of corn planted after soybean is a sure method of verifying the problem.

As rootworm larvae continue to munch on corn roots, adults continue to emerge. Of note is the observation of a male western corn rootworm that emerged from a field of corn planted after soybean in Madison County. Brent Rains, with Crop IMS in Collinsville, filed the report from a cornfield in southern Madison County. He found no injury to a few of the roots examined, and he found few beetles. However, the presence of a male western corn rootworm is significant. Upon emergence, male western corn rootworms remain in the field to wait for females to emerge. Therefore, it's probable that the male emerged from the field in which it was found. Since that time, Brent has found a few western corn rootworm females.

As we continue to watch for western corn rootworm adults in cornfields, it seems appropriate to provide a thumbnail review of adult emergence and behavior after emergence. Male western corn rootworms begin to emerge before female western corn rootworms and remain in the field where they emerged while they wait for females to emerge. Males mate with females almost immediately after the females emerge from the soil. After mating, the female usually remains in the field feeding on corn tissues for several days. The eggs mature during a pre-oviposition period of two to three weeks, after which an individual female lays an average of more than 1,000 eggs in an average of 13 clutches about every five days. For more information about western corn rootworm biology, refer to the corn rootworm fact sheet on our IPM Web site.

As cornfields begin to tassel, followed shortly thereafter by pollen shed, corn growers will be concerned about the potential for silk clipping by rootworm beetles to interfere with pollination. The economic threshold for rootworm adults clipping silks is five or more beetles per plant before pollination is complete. Table 1 shows insecticides labeled for control of corn rootworm adults in corn.

Another reminder: Because of the earlier-than-usual emergence of western corn rootworm adults, people monitoring for western corn rootworms in soybean fields should consider placing Pherocon AM sticky traps in fields earlier than typically recommended (i.e., the last week in July). Consider placing traps as early as mid-July, especially in southern counties, where we hope to determine whether the range of variant western corn rootworms has expanded. For more information about trapping for western corn rootworm adults, refer to "Western Corn Rootworm Adult Emergence Has Begun" in last week's issue (no. 13, June 23, 2006) of the Bulletin and to the western corn rootworm fact sheet on our IPM Web site.--Kevin Steffey

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