Watch for High Corn Rootworm Populations and Their Damage

With rootworm beetles emerging from underground, this is the time of year to notice goosenecking corn (Figure 1) and/or high numbers of rootworm beetles in fields (Figure 2), both of which can be indicators of thriving populations of rootworm beetles. If lots of beetles are emerging from fields planted with hybrids containing Bt traits against rootworm, that is likely a good indication that beetle populations are evolving resistance to those transgenic traits.

Western corn rootworm is the most damaging pest of corn production in the U.S. The larvae of this beetle species feed on corn roots and when their damage is severe, corn plants can struggle to take up water and nutrients and often lodge, or fall over, for lack of a root system (Figure 3). In Pennsylvania and surrounding states, western corn rootworm is easily controlled with rotation. Rootworm larvae cannot survive on roots of soybeans, alfalfa, sorghum, and sorghum-sudangrass, among other crop species; therefore, rotating from corn to these crop species can effectively eliminate populations of rootworm in fields.

For dairies and other farming operations that grow continuous corn, however, western corn rootworm presents a challenge, and farmers typically rely on Bt corn hybrids with belowground protection provided by insecticidal traits, including the Yieldgard trait (containing the protein Cry3Bb1), the Herculex trait (protein Cry34/35Ab1), two Agrisure traits (proteins mCry3A and eCry3.1Ab), and the new dvSnf7 trait, which provides control via RNAi or interference RNA. This latter trait is only being offered in combination with one or two other below-ground traits. Continuous, annual use of any one of these proteins, however, for three years or more can lead to the development of local rootworm populations that are resistant to the trait. Scientists have generated resistance to all of these traits in lab settings and widespread field-evolved resistance has been discovered, mainly in the Midwest, to the Yieldgard and Herculex traits, as well as at least one of the Agisure traits (mCry3A). We have also had reports of resistance to these traits in Pennsylvania, but companies are not proud of this situation and tend to keep it quiet.

If you encounter high beetle populations or goosenecked corn at this time of the year, examining the root systems will indicate whether rootworms have been feeding heavily. Keep in mind, however, that there is little to do now to manage the problem; treating populations of adult beetles is an ineffective way of trying to limit next year's populations, so solutions will have to be implemented next spring. (If, however, rootworm beetles are numerous enough to clip silks and threaten corn pollination, it is prudent to control them.) The easiest way to fix this problem, or to prevent it from developing, is to regularly rotate fields from corn to soybeans, alfalfa, sorghum, etc., for the next growing season. If rotation out of corn is not possible, growers need to know which proteins they have been using and should switch to hybrids with two other traits that are active against rootworms, but the number of options and combinations is limited. This resource from Michigan State and Texas A&M can help cut through the alphabet soup of traits and protein names. Two options would be to remove Bt traits from the field entirely and 1) use soil-applied insecticides to control rootworms or 2) plant in late May or early June after most rootworms have hatched from their eggs and starved to death--this can be accomplished by tracking growing-degree days (contact me if you want details).

If you do encounter unexpected damage from rootworms, report the problem to the seed company; they have an obligation to investigate the situation. I also recommend contacting your local Extension educator or me because we can get involved and advocate on your behalf. The bottom line is that relying on one tactic for too long is a prescription for trouble, because resistance will develop—just as we have seen with glyphosate-resistant weeds. Rootworm beetles are skilled at adapting to consistent management tactics. Change up your approach to stay one step ahead of this important pest. To read more about this topic, please see our factsheet on managing potential rootworm resistance to Bt.