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Study finds possible problems with control measures for corn rootworm

(Wednesday, Sept. 29, 2004 -- CropChoice news) -- Purdue University via Farm Industry News:
Corn rootworms may keep on munching, even if you applied insecticide, used treated seed, or planted CRW Bt hybrids. Areas of lodged corn aren't necessarily a sign that your control product failed, say John Obermeyer and Larry Bledsoe at Purdue University. It may be that the rootworm pressure in affected areas was very high. Lodged corn in treated fields is often the result of a concentration of egg laying the previous season, they say.

Rootworm control products are challenged to protect roots where numerous eggs/larvae exist. Efficacy data from two different plots show the difference between "normal" and "serious" rootworm pressure. Even Bt rootworm corn appears inundated with rootworm pressure at one location in Indiana.

Rootworm larval pressure and product performance varies within the field and from field to field. We, and other universities, evaluate efficacy of rootworm control products following very late-planted corn or "trap-crops." Trap-crops attempt to provide uniform larval feeding pressure the following season by attracting massive numbers of rootworm beetles that feed on the pollen/silks leading to mating and egg laying within that field. We have been doing this for years to test experimental and labeled rootworm products.

However, rootworms often don't cooperate in providing us the severe, and certainly not uniform, pressure desired. That's why we conduct these tests at multiple locations in Indiana and randomly replicate treatments within the plots. Producers often unwillingly create similar scenarios by replanting drowned out areas, allowing weed escapes in areas of a field, or not monitoring for variant western corn rootworm beetles in soybean. When producers purchase and apply rootworm materials they expect CONTROL. Still it is not unusual for producers to notice areas of cornfields that have been lodged while out combining. These lodged areas may be a result of serious root feeding from a concentration of eggs laid the previous season. The numbers of eggs, or "egg load," deposited in a given area or whole field is the most important of all variables determining the following year's risk to rootworm feeding.

High egg loads, coupled with optimal soil conditions for larval survival in the spring, will likely cause significant rootworm damage to corn causing plant lodging and possibly death. This will occur even when control products are used properly (time of application, placement, etc.) and at the labeled rate. In other words, control products can be "overwhelmed" with rootworm numbers. [Our data shot that] we got serious damage at the Pinney Purdue Agricultural Center (PinPAC) at Wanatah in northwestern Indiana. The PinPAC site was moderately dry throughout the summer, but had well-timed rains that resulted in excellent growth of the corn.

Although the results at the Northeast Purdue Agricultural Center (NEPAC) at Columbia City represent average damage from rootworm larvae for that region, this damage was highly variable. That site received an atypical amount of rain (about 20 inches) from May through July with more than 7 inches in 72 hours in mid June and probably accounts for the increased variability and overall low rootworm damage. Two other Indiana test sites were abandoned because of early-season saturated soils that resulted in near total mortality of rootworm larvae. The biggest surprise when comparing 2004 data is the reduced efficacy of YieldGard Rootworm at PinPAC. Granted this is only one site in Indiana that we are reporting, but it is the first time that we have measured YGRW trait performance that is much lower than anticipated.

Apparently rootworm Bt corn is also showing some surprisingly poor ratings in University of Illinois plots where rootworm pressure was excessive. No one at this time knows for certain why this new technology for rootworm control has shown a "kink in the armor." You can be certain that many rumors concerning Bt-corn performance will spread this fall and coming winter. University and industry entomologists will be closely studying this situation. It simply may be a matter of the rootworm larval numbers overwhelming the Bt-toxin expressed in the roots. We know that larvae must feed on the roots to ingest the Bt and then either die or become dramatically delayed in growth and development. Enough "sick" larvae may still do significant damage when larval numbers are extremely high.

Unlike Bt corn for above-ground lepidopteron insects (e.g., corn borer) that has given near 100% control, YieldGard Rootworm expresses a lower dose of the toxin in the roots which allows for a small percentage of surviving larvae. To keep this in perspective, other than this one test plot, we are not aware of any suspected poor performance of rootworm Bt corn in Indiana at this time. In the coming days of harvest, farmer's observations made through their combine windshields and by their yield monitors will add to our understanding of how this product protected corn in many environments and how that protection was associated with productivity. Corn rootworm, especially as it pertains to the western corn rootworm variant, and additional product performances will be featured in the November issue of the Pest&Crop. Visit the Purdue agronomy Web site at http://www.entm.purdue.edu/Entomology/ext/targets/newslett.htm.