Reports in Iowa and from neighboring states indicate corn rootworm larval hatch for 2009 has begun. June 6 has been considered an average hatch date in Iowa in recent years, so rootworms that have now reached their first and second instar growth stage should be actively feeding--if rootworms are present in your cornfield. This is the time to apply a rescue treatment for corn rootworm if needed, and scouting your field will help determine if such action is necessary.
The other reason you should scout corn, dig some roots and check for rootworm feeding damage is so you get an idea of what kind of control you are getting from either the insecticide treatment you may have applied at planting--or to evaluate the control from Bt corn that has the corn rootworm resistance trait, if you planted that type of corn hybrid this year.
Iowa State University Extension entomologists Erin Hodgson and Jon Tollefson make the following observations and suggestions to help farmers and others scout cornfields and evaluate the corn rootworm control situation.
Evaluate your rootworm-resistant corn hybrids
There have been reported failures of Bt corn in Iowa the past several years, so Hodgson and Tollefson recommend scouting every field every year to ensure that you know you are getting root protection. There is a good discussion of this topic in the November 13, 2006 issue of ISU's Integrated Crop Management Newsletter, which is available online on the ISU Website. There is an archive of past issues of the newsletter which you can find on the site.
Of course, Bt fields require a refuge to be planted nearby, a refuge of non-Bt corn and that is where rootworm larvae are more likely to be feeding.
If an average of one or more larvae per plant is found, application of a rescue insecticide should be considered, say the ISU entomologists. Here are Hodgson and Tollefson's recommendations:
There are a limited number of products registered in Iowa for treatment of corn rootworm larvae after corn emergence (see Table 1). Be sure that you follow label directions and pay attention to the application guidelines. Post-emergent insecticides applied after the end of June will not be as effective because most of the root injury to the corn will have occurred already.
Table 1. Labeled post-emergent insecticides for corn rootworm larvae.
* Restricted use pesticide.
? Please read the article "Cancellation of Furadan for Crop Use" published online in the ICM Newsletter dated June 2, 2009. It gives recommendations regarding residues for 2010 and beyond.
Three additional resources that may be of interest for you to read regarding corn rootworm management are:
The Corn Rootworm Home Page
Interactive Node-Injury Scale
Evaluation of corn rootworm hybrids