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Issue No. 15, Article 2/July 7, 2006

Tassel High by the Fourth of July: Prepare for Silk Clipping by Western Corn Rootworms

Western corn rootworm adults have taken flight across the Illinois agricultural landscape. Throughout much of central Illinois, many cornfields have begun to tassel, and we've received some reports of fields already being treated to prevent excessive silk clipping. Producers should consider a rescue treatment to protect pollination if there are five or more beetles per plant, pollination is not complete, and silk clipping is observed. There are many insecticides listed in the 2006 Illinois Agricultural Pest Management Handbook that may be used against corn rootworm adults to prevent silk clipping. Many of the products listed for control of western corn rootworm adults during the pollination period are pyrethroids (see Table 1). Efficacy of these products may be compromised somewhat under very hot conditions.

Tassels are now common throughout much of central Illinois (Champaign County, July 5, 2006).

Western corn rootworm adults on corn tassels (Champaign County, July 5, 2006).

Why are western corn rootworm adult densities seemingly so high every year? Many reasons help explain why corn rootworm adult densities continue to be so impressive year after year. The primary factors are these: (1) very mild winters (which may contribute to better egg survival), (2) excellent soil conditions (soil not saturated) at the time of larval hatch, (3) the long-term trend toward earlier and earlier planting of corn, (4) few significant natural enemies, and (5) greater overall agricultural landmass (first-year corn and continuous corn), which serves as a corn rootworm "nursery." Before the mid-1990s, western corn rootworm adults survived only in continuous corn production systems. This has changed significantly in the last decade with the growing importance of the variant western corn rootworm.

Why do we see so many western corn rootworm adults at pollination even when we've used a soil insecticide or a transgenic corn rootworm hybrid? Research conducted at the University of Illinois in the late 1980s clearly documented that adult emergence is plentiful even when a soil insecticide has been used. Because soil insecticides applied at planting are placed in narrow (7 inch) bands or in-furrow, the soil outside of these treated areas (between rows) serves as a refuge for corn rootworm larvae. In this sense, producers have unwittingly utilized a sound refuge strategy for soil insecticides for decades. The traditional granular soil insecticide products are typically very water insoluble. Consequently, very little movement of these products occurs outside of the treated areas. Corn rootworm larvae thrive in the soil between corn rows as corn roots proliferate throughout these untreated zones. In general, the soil insecticides accomplish their mission; that is, they protect the plant from lodging by preventing excessive pruning of brace roots. Soil insecticides were never designed to be population management tools.

Even though you've planted a transgenic corn rootworm hybrid (YieldGard Rootworm, MON 863, Cry3Bb1, Herculex RW, DAS-59122-7, Cry34Ab1/Cry35Ab1), corn rootworm adults will be found in those fields during pollination as well. Unlike the Bt hybrids used to control European corn borers that are characterized as high-dose events, Bt hybrids designed to limit corn rootworm larval injury allow for greater survivorship to the adult stage of development. Bt seed also is treated with a low rate of a neonicotinoid insecticide (Ponchoclothianidin, Cruiserthiamethoxam). So western corn rootworm adults that emerge from a field planted with a transgenic corn root hybrid have been exposed simultaneously to a Bt protein and a neonicotinoid insecticide. Despite this dual assault, many still survive to the adult stage and threaten the pollination process. Bottom linedon't assume that just because you planted a Bt hybrid for corn rootworms you're safe over the next several weeks through pollination. This could be a mistake. Take some time to scout your fields and consider the treatment recommendations suggested previously.--Mike Gray

Mike Gray

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