Photo by marykbaird/morguefile.com.
In September, the corn in a field of continuous corn in central New York was exhibiting classic symptoms of corn rootworm damage: Goosenecked and lodged stalks resulting from severely pruned roots. However, this field was planted to a corn hybrid with the corn rootworm trait - in this case, the common Cry3Bb1 trait, the same one involved in other cases of corn rootworm resistance in the Midwest.
The first observation of rootworm Bt trait failure in a commercial cornfield was in Minnesota in 2009, though this discovery wasn't made public until some time later. As recently as a year ago, university entomologists were saying that rapid resistance development in the northeastern U.S. was less likely because corn rootworm pressure in this region has never been as high as in the Corn Belt. (Oh well, it sounded good at the time.)
There were other reasons for optimism: Farmers in the Northeast often rotate with a forage crop instead of soybeans, and continuous corn - one of the surest ways to develop insect resistance - is less common in this region. Because of these practices, there has been less use of corn hybrids with the rootworm resistance trait in the Northeast. The central New York discovery, confirmed by Cornell University entomologist Elson Shields, is a game changer, one that should be a dose of reality for farmers in the Northeast who were hoping this problem would be a long time in arriving.
- If it ain't broke, don't fix it. Hopefully someone (you or a crop consultant) has been scouting your cornfields, so you know not only the relative number of corn rootworms that have been present in your cornfields (a lot versus a few), but which of the two species, western or northern, is predominant. This is important, because to date, all confirmed cases of rootworms resistant to the Cry3Bb1 trait have been western corn rootworms.
Shields reports that western corn rootworms have been the predominant species in the Northeast since the 1990s, but this may or may not be the case on your farm. Western corn rootworms have been on the increase in parts of the Northeast and are now the dominant rootworm species in areas where northern corn rootworms were more common five or 10 years ago. A western corn rootworm is about twice as damaging as its northern cousin, aside from any resistance issues.
Regardless of the population or species of rootworms on your farm, you'll need to take preventative steps, but the need to take immediate, drastic measures should be influenced by your individual situation. Don't assume that if you harvest your corn for silage you can ignore the rootworm problem. Corn rootworms can severely affect the corn plant whether it's harvested for grain or silage. Goosenecking and lodging are the most visible problems, but yield loss is also a big factor. In fact, some studies have found that the yield impact of rootworms is even greater on corn that's harvested for silage.
- Use the rootworm trait only if the situation warrants it. The development of resistance can be greatly delayed by not using the corn rootworm trait unless there's a good chance you'll need it. Don't use the trait on first-year corn, and in most cases not on second-year corn either. The more extensively the rootworm trait is used, the greater the chance there is for insect resistance. Low infestations of corn rootworms in second-year corn can be controlled either by the use of a high rate of seed treatment (Poncho, Cruiser Extreme, etc.) or a reduced rate of a soil insecticide.
- For continuous cornfields, use refuge-in-a-bag (RIB) seed. For fields in corn three years or more that have a rootworm problem, use RIB seed, which mixes a small percentage (usually 5 percent) of rootworm-susceptible (non-rootworm trait) seed in a bag of otherwise resistant seed corn. An alternative is to plant seed corn with the rootworm trait while carefully observing insect refuge requirements. Note the word "requirements," not "suggestions."
Some entomologists believe that the reason corn rootworm resistance developed as quickly as it did was because too many Corn Belt farmers were ignoring refuge requirements. The farmers probably assumed that their neighbor would plant non-traited seed that would serve as the refuge. The problem: What if the neighbor was assuming the same thing as the farmer? The result: Lots of rootworm-resistant corn hybrids with no refuge.
- Give seed treatments a chance. Almost all seed corn now comes treated with an insecticide. Most is treated at the low rate (250 milligrams) to control early-season insects such as seed corn maggot. However, the high rate (1,250 milligrams), aka "the rootworm rate," should do a good job of controlling moderate infestations of corn rootworms.
Some university entomologists suggest using this rate on second-year corn. Seldom, if ever, is the 1,250-milligram rate needed on first-year corn. In areas where corn rootworm pressure is high (such as in the Corn Belt), this rate hasn't been providing adequate control for continuous corn. However, in much of the Northeast we don't have this level of pressure, so the use of the 1,250-milligram rate may provide adequate control.
Work closely with your crops consultant or extension educator, who may have a good idea of rootworm pressure in your area.
- Don't junk your insecticide hoppers. Applying a granular soil insecticide isn't as convenient as using traited seed corn, but in the many decades that soil insecticides have been used for corn rootworm control, there has never been a confirmed case of rootworm resistance. That's probably because soil insecticides only protect part of the corn root system, since they're applied in a band or in-furrow. The roots a foot or more away from the plant aren't affected by the insecticide, functioning in much the same way as a refuge, so there are always some rootworms that complete their life cycle.
Before the incidence of rootworm resistance, I was of the opinion that the combination of traited hybrids and insecticide-treated seed would render granular insecticide hoppers obsolete. In fact, when Miner Institute bought a new corn planter about 10 years ago, we ordered it without hoppers. In retrospect, that was probably a mistake. An increasing number of farmers are using both a soil insecticide and a rootworm-resistant hybrid, applying an insecticide overlay in the same fields where they plant the traited corn.
Ev Thomas has worked as an agronomist in New York for 45 years, first with Cornell University Cooperative Extension, then with the William H. Miner Agricultural Research Institute in Chazy, N.Y., including managing its 680-acre crop operation. He continues to work part-time for Miner Institute and is now an agronomist at Oak Point Agronomics. He has written our Forages column for 16 years and has been an expert contributor on a number of other topics.