Rootworms are Unpredictable, if Nothing Else!

No problems notices in second-year corn…this time!

Published on: Nov 26, 2007

Purdue University's standard recommendation over the past many years is likely not that different from most Midwestern Universities. If you're planting corn after corn, apply a soil insecticide to protect against corn rootworm. Since the pest has been identified and understood, it's a known fact that it lays eggs in corn fields, and that those larvae hatch the following spring, usually in late May or early June, depending on temperature accumulation during the spring season. If corn roots are nearby, the larvae will devour them, unless it's transgenic corn. Then they would take tiny bites and die, at least in theory. So today's recommendation may be tempered to include: either apply a soil insecticide or plant a transgenic hybrid in corn after corn. The message is the same- control corn rootworms.

Today, with a variant appearing more than a decade ago in east-central Illinois and moving eastward, plus somewhat to the south, it's also necessary to use protection in first-year corn following soybeans in many areas, particularly the traditionally hard-hit areas of east-central Illinois and northwest Indiana. These western corn rootworm beetle variants can also lay eggs in soybeans fields, meaning first-year corn could be impacted by larvae. But that doesn't mean there won't be eggs laid in corn fields that go back to second year or continuous corn, either.

This years' Corn Illustrated Project, sponsored by Farm Progress Companies included a high-yield trail on irrigated ground near Edinburgh, Indiana. Actually, part of the plot was irrigated, a third was irrigated beginning one week after tasseling, and the final portion wasn't irrigated at all. The soil consists of a loam with less than 2% organic matter, that turns into gravel at three feet below the surface. The land is actually owned by a gravel company as a future source for gravel.

The high-yield trial was planted on ground that raised seed-corn a year ago. So from the very early planning stages, the intent was to apply a soil insecticide on the entire plot. One of the drawbacks to working with farmer trials is that sometimes in the heat of battle, thoughts turn to moving forward instead of making sue that the protocol is followed to the letter. It's a point the Crops Illustrated crew promises to work on next year.

This year, the plot was finished and it was time for lunch, when the farmer got a sheepish, almost ghost-white luck on his face. On no, he remembered, the insecticide was ordered and sitting in his toolshed. He simply forgot to bring it over and apply it. Since he's in an area where the variant hasn't caused much if any trouble in first year soybeans, he's not accustomed to applying a soil insecticide in corn.

Sure enough, the bags were still sitting in the shed, and the corn was planted. There really wasn't a feasible solution to still get it applied that they could devise. So the insecticide eventually went back to the dealer, and the farmer and plot organizers held their breath and crossed their fingers.

When the corn was pushing waist-high, in mid-June, long after rootworm larvae had been noted discovered in Indiana field, the farmer and project staff got brave enough to dig up several roots. They dug up root balls, checked for chewing, of which they saw extremely little. Once roots were shaken over black plastic, they did find one tiny larva in more than a haff-dozen roots that were dug.
Apparently, they lucked out. Rootworms don't visit this second-year, non-transgenic corn- but it's still just not a risk worth taking. Later yields and absence of lodging prove that in this case, insects skipped that field.

Will the farmer and project crew apply insecticide again if its corn after corn. You bet! They feel lucky that they made one mistake and lived to farm another day!