Treating corn rootworm hybrids with soil insecticides boosted yields an average of 12 bushels per acre in university trials across the Corn Belt in 2007.
The 17 yield comparisons — which were conducted by university researchers at the University of Illinois, Purdue University, The Ohio State University, the University of Nebraska, and the University of Wisconsin — compared YieldGard and Herculex trait corn alone and in combination with various soil insecticides. Insecticides included Counter 15G, Fortress 5G, Aztec 4.67G, and Force 3G applied with the SmartBox closed handling and application system.
"Based on these results, most farmers would realize a very positive return on investment," says Paul Vaculin, marketing manager for granular insecticides and closed delivery systems at AMVAC Chemical Corp., which sponsored the trials. "In this high-value market, it can be a sound business practice to make an extra investment to maximize yields."
AMVAC undertook sponsorship of the trials at the request of growers, who had noticed a yield benefit from applying insecticides to corn with in-plant corn rootworm protection. "We wanted to test this idea to see if it was valid," says Vaculin.
"The university trials were not designed to determine why the soil insecticides improved yields. However, a combination of factors could be responsible," says Rich Porter, AMVAC’s technical manager for granular insecticides.
"In addition to controlling corn rootworm larvae, all of the soil insecticides in the trials control secondary insect pests, such as wireworms and grubs," Porter adds. "Since Counter is also a nematicide, suppression of corn nematodes could be a factor in trials where Counter was applied. Any or all of these factors could be at play in individual trials. Variable environmental conditions, as well as differences in pest spectrum and infestation level, could affect the yield boost seen on individual grower fields."
"In a small number of trials, the rootworm trait was challenged," Porter says. "In those trials, the addition of the granular insecticide substantially reduced root feeding and held a higher percentage of roots below an economic threshold."
"Besides protecting yield, using a soil insecticide in conjunction with rootworm hybrids also may be a sound strategy for managing development of resistant insects," Porter continues. "If a soil insecticide is present, it has the opportunity to control rootworm larvae that are not affected by feeding on the Bt (Bacillus thuringiensis) toxin in corn roots. This has the potential of reducing proliferation of resistant rootworm biotypes."
In most of the 2007 trials, soil insecticides were applied at lower labeled rates — typically at three-quarters of the full rate recommended to control rootworms. For 2008, AMVAC is recommending an application of low label rates of Counter or Fortress in conjunction with rootworm hybrids. At a three-quarter rate, these insecticides typically cost about $15 per acre.