When you think about it, the premise behind corn rootworm insecticides is pretty simple. After farmers apply them at planting, they kill rootworm larvae that munch on corn roots.
Or do they?
Numerous studies show that the number of adult beetles that emerge in fields infested with larvae can actually be greater in some fields treated with soil insecticides than in untreated fields, says Mike Gray, University of Illinois extension entomologist.
Soil insecticides kill larvae in the band to which they are applied, Gray says. However, many larvae survive and thrive on roots outside the treated band. These larvae then emerge as adult beetles, which lay eggs that become the next generation of larvae.
Soil-applied insecticides are designed to protect roots but do little to manage a corn rootworm population over the long run, Gray explains.
Investigating an alternative. USDA-ARS entomologists have evaluated an adult corn rootworm control project to better manage rootworm populations with minimal environmental concerns.
Illinois, Indiana, Iowa, Kansas, South Dakota and Texas were included in this ARS-funded, multi-year effort. In each trial, researchers blocked off 16 square miles of corn (as well as soybeans in Indiana and Illinois) and treated the block with aerially applied, insecticide-laced baits.
To determine if the field needs to be sprayed, we monitor the adult corn rootworm beetle population, says Corey Gerber, areawide IPM specialist for Purdue University. Once we hit a specific threshold, we spray the area to knock down the beetle population. The idea is to kill enough female beetles before they can lay their eggs.
Over time, researchers hope to reduce the rootworm population. Farmers have used this strategy for decades in western Corn Belt states such as Nebraska. However, they treated these fields with broadcast treatments of carbaryl or methyl-parathion. Although effective, these treatments kill beneficial insects and spur insecticide resistance, Gray says.
How it works. In the ARS program, an insecticide-laced product in droplet form called cucurbitacin acts as a feeding stimulant for adult corn rootworm beetles. Because the beetles compulsively eat the material, insecticide rates are 90 to 95% less than a broadcast application.
You don't have to hit them with contact because they ingest the product, says Clint Hoffmann, an ARS research agricultural engineer based in College Station, TX. Because it's not a contact material, you also aren't killing the beneficial insects.
The program's goal is to break the corn rootworm cycle by keeping populations below economic threshold levels. Soil insecticides never really break the cycle, Hoffmann says.
The researchers have tested three insecticide-laced baits: Slam, CideTrak and Invite. All three use cucurbitacin as the attractant. Slam contains carbaryl, but Invite and CideTrak can use any insecticide labeled for corn at approximately one-tenth of the labeled rate.
Right timing, cooperation necessary. The products have a control rate of 90 to 98% for adult corn rootworm beetles, Hoffman says. But correct timing is crucial to make these products work. Female beetles must be treated before they lay eggs in the ground.
Through scouting, you can keep track of populations and treat them, Hoffman says. It's not eradication, but population management. In a lot of the sites, we can get overall reduction of populations, but it usually takes two to three years. We hit them hard the first year and do a cleanup treatment the second and third years.
For example, one Texas study treated 2,000 acres during the first year, 800 in the second year and a final 80 acres in the third year.
If researchers achieve satisfactory control with just one application, the $9 to $10/acre cost is superior to the $12 to $15/acre soil insecticide cost. But there's a hitch: If two applications are necessary, adult treatment costs exceed those of soil insecticides.
The main concern we've had is if there's enough of an economic advantage for growers, who are fairly independent people, to block their acres together, Wilde says. Treating fields on a piecemeal basis won't work, because beetles can easily migrate from one field to another.
Researchers also recommend using 16-square-mile blocks. You also have to be scouting, Wilde says. If you spray again, it's another cost.
Hoffman adds that farmers with whom he has worked still apply soil insecticides. In areas where adult populations were reduced, we went back in the next year with soil insecticides with a one-third to one-half rate, Hoffmann says. It doesn't eliminate their use, but it does reduce it. And if you can achieve population control of adult beetles over time, you eventually won't need the soil insecticide.
Promising control. So far, the ARS program shows promising rootworm control. When beetle population densities warrant it, there seems to be an economic advantage if all the growers in an area spray for beetles, Wilde says. In Kansas, just 30 to 40% of treated acreage has had beetle numbers above threshold levels, compared with 70 to 80% of the acreage where farmers used soil insecticide treatments.
The program also shows good potential in the eastern Corn Belt. In the Indiana/Illinois trials, researchers analyzed treatment effectiveness by digging up roots from treated fields and comparing them with roots from untreated areas. They also examined differences between the two areas by using emergence cages that track beetle numbers in early summer.
So far, the damage to corn roots is much less in fields inside our managed site than in fields outside of this site, Gerber says. We are impacting the beetle population in our study site.
But for the program to fly on its own, scouting and cooperation among neighbors will be paramount. Now scouting may be easier because farmers can use new devices such as a kairomone trap to monitor beetle populations without having to do a hand count. We're hoping we'll make scouting more efficient and cost-effective, Hoffmann says.
The baiting strategy may someday grow in popularity if manufacturers discontinue some soil-applied insecticides or if the federal government phases them out, Hoffman says. Although companies in the future may market genetically modified (GM) hybrids with rootworm resistance, consumer hesitance about GM foods compounded by the recent StarLink episode may delay the marketing of these goods. If no one will buy it, no one will grow it, Hoffmann states.
For more information about CideTrak, contact Trece Inc., Customer Service, Dept. FIN, Box 6278, Salinas, CA 93912, 831/758-0204, www.trece.com or circle 200.
For more information about Invite, contact Florida Food Products, Dept. FIN, 2331 W. Hwy. 44, Eustis, FL 32727, 352/357-4141, www.floridafood.com or circle 201.
For more information about Slam, contact the Micro Flo Co., Dept. FIN, Box 772099, Memphis, TN 38817, 800/451-8461, www.microflocompany.com or circle 202.