Rootworms have proven they are one of the most adaptive pests known to corn production and that they are gifted at developing resistance to our best pest management techniques. The resistance story begins decades ago, it stretches through the mid-90s, and it continues today. Today’s part of the rootworm resistance story largely revolves around the subject of trait-resistance. It began just a few seasons ago in Iowa when university researchers confirmed the pest had developed “in-field” resistance to the first rootworm trait (YieldGard). The story was soon followed by confirmed resistance in Illinois. In 2013, rootworms made themselves well known in the northern portion of the Burrus footprint with fairly extensive rootworm injury (92% of growers attending our Fairbury, IL winter agronomic meeting encountered rootworm injury within their neighborhood).
The update on trait-resistant rootworms can best be summarized by saying that they have “expanded their territory and their feeding habits.” According to the University of Illinois - Sangamon, McDonough, and Mercer counties have now been added to the list of Illinois counties with confirmed trait-resistance. Researchers at Iowa State confirm that rootworms have now developed “in-field” resistance to the Agrisure rootworm trait as well.
A Reminder on Resistance Management
The list of traits commercially available to battle rootworms is limited to four products, which includes that introduction of Duracade, when it is released. What comes after that? The answer is “nothing for the foreseeable future.”
This led Burrus to the following statement in our 2013 Harvest Report:
“We need to take the fact that we encountered inconsistencies in rootworm trait performance as our signal that we must manage resistance.”
Minus scouting information, we have advised that insecticide be applied with trait to ward off significant rootworm injury within the red “At Risk” zone (in picture above). That recommendation is a “just in case approach.” With no information about a field, it is the best recommendation we can currently provide. However, that kind of blind approach is not ideal and its continuous use is unsettling.
The best approach is to use scouting information to determine a need for an insecticide with trait. There are two ways for growers to scout. The first is called the “root dig method” and the second is called the “rootworm beetle count method.”
Root Dig Method:
1) Record your gut feeling about the rootworm beetle population when walking the field in traited acres during July and August.
2) Dig roots in July and August to evaluate rootworm control in traited and refuge acres.
3) If the rootworm beetle population is high and root ratings of rootworm traited hybrids are 0.25 or higher use a granular insecticide the next year.
4) Keep annual records of your observations.
Rootworm Beetle Count Method:
1) Scout rootworm-traited acres at pollination.
2) Examine four consecutive plants in five areas of the field:
a. Start at the bottom of the plant.
b. Count beetles on the leaf surface, leaf axils, stalk, ear tip, and tassel.
3) Divide the total number of beetles by the number of plants examined to determine the average number of beetles per plant.
4) If the average rootworm beetle population is 0.75 beetles or greater per plant, consider using a soil insecticide with rootworm traits the next year.
5) Should the rootworm beetle population be less than 0.75 beetles per plant, wait two weeks and scout again.
6) When the beetle population remains low for six weeks after the scouting trip, no soil insecticide is necessary with rootworm traits the next year.