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Issue No. 14, Article 1/June 24, 2005

Corn Rootworm Larvae Abundant

(Originally published as an "Alert" on June 14, 2005)

Since writing the article "First Reports of Significant Rootworm Larval Feeding This Year" for issue No. 13 (June 10, 2005) of the Bulletin, I have received several reports of observations of large numbers of rootworm larvae. Apparently a lot of folks are poking around cornfields to get a feel for the rootworm pressure they may have to face this year, and they do not like what they are finding. Most reports have focused on numbers of larvae per root mass, rather than extent of root injury, and that’s a key point I will address later in this article. Nonetheless, the relative ease with which people are finding rootworm larvae suggests that we may have to deal with some difficult rootworm-control issues this summer.

Following is a synopsis of several reports I have received from June 15-–17, generally arranged from south to north:

  • As many as 12 larvae per root mass in Cumberland and Jasper counties.
  • Significant numbers of larvae in a continuous cornfield in Clay County.
  • Eight, 12, and 15 larvae per root mass in three fields of corn planted after soybeans; 8 larvae per root mass in a field of corn planted after wheat stubble; and 12 and 30 larvae per plant in two fields of corn planted after corn. All of these fields were in Menard County.
  • As many as 12 larvae per root mass and light rootworm larval injury in cornfields in Adams and Pike counties.
  • As many as 10 larvae per root mass with evident root injury in Logan County.
  • As many as 15 larvae per root mass in corn planted after soybeans, but only four larvae per plant in corn planted after corn in some fields in Marshall County, with some root injury apparent.
  • Beginning of rootworm larval feeding injury in Knox County.

Some of the individuals who provided information for this synopsis were Jeff Bunting (Growmark), Jim Dunseth (Ag-View FS), John Fulton (Logan County Extension), Shawn Jones (Pioneer Hi-Bred International, Inc.), and Matt Montgomery (Sangamon/Menard Extension).

In addition to these reports, Shawn Jones added that he observed several newly emerged western corn rootworm adult males in a field in Pike County. The appearance of adult corn rootworms in mid-June is awfully early, especially considering only about 50% larval hatch in a lot of central Illinois (assuming you trust degree-day accumulations). Nonetheless, Shawn’s identification seems solid, and the Mississippi River bottomlands are an ecosystem unto themselves. This report of early emergence of rootworm adults suggests that we will have to be on our guard when corn silking commences.

A word about observations of rootworm larvae in cornfields treated with either soil insecticides or seed-applied insecticides. Some of the fields from which the aforementioned reports were generated were treated at planting with either a granular soil insecticide or a seed-applied insecticide. There is genuine concern about finding rootworm larvae in treated fields. However, finding larvae does not necessarily mean that the product applied has not provided rootworm larval control. No rootworm-control product provides 100% control. In fact, many rootworm larvae may survive outside the treated area of soil beneath the applied band or furrow. Therefore, it’s more important to assess the level of rootworm injury to the roots than to focus solely on numbers of rootworm larvae.

Regardless, as I mentioned in the previous article, early planting (early to mid-April), dry soils, and large numbers of rootworm larvae can be a lethal combination. Chemical insecticides often do not perform very well when soil conditions are dry. Although no one has reported serious lodging of corn yet, even after the recent storms, it’s likely that the corn simply is not tall enough to lodge much yet.

A couple of questions asked deserve some response. "Will rain now activate the product that was applied at planting?" Although rain will benefit the corn plant and possibly root growth to compensate for rootworm injury, I doubt very much that it will "activate" soil insecticides or insecticidal seed treatments sufficiently to improve control significantly. "With all of the larvae we have found and the rootworm injury to the roots, should I consider a rescue treatment?" I really wish I could provide a positive response to this question, but I am not confident in rescue treatments for rootworm larvae. The only products registered for such an application (generally, at cultivation) are Force 3G, Furadan 4F, and Lorsban 4E and 15G. Of these products, only Furadan 4F is relatively soluble. Applying relatively insoluble products to control rootworms in dry soil would be a wasted effort. And the results with Furadan 4F have been inconsistent. Despite its greater solubility, Furadan 4F still needs moisture for optimum results. For any of these products, the only rational application method is to apply the product on either side of the corn row and to work it into the soil. If the corn is too tall for such an application, we discourage foliar applications, which typically do not provide much control.

For corn that has suffered so much already this year, this report does not come as good news. However, we feel obligated to let you know what is developing in the insect world, and we’ll continue to provide the most current information we have.--Kevin Steffey

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