The Billion-Dollar Battle: Using Integrated Pest Management to Combat Corn Rootworm
There are four corn rootworm species in North America: northern, western, Mexican and southern. Northern and western corn rootworm are the most economically important and are found throughout the Corn Belt.
Both larvae and adults can damage corn – larvae feed on roots, which can lead to lodging, and adults feed on silks, which can interfere with pollination. On top of that, they are highly adaptable and resilient to forms of control.
“Corn rootworm is tough,” said Aaron Vammer, Pioneer product agronomist. “It’s basically the Palmer amaranth of the insect world. The reason it’s so tough is because of its ability to adapt and overcome basically everything we throw at it from a management standpoint.”
Northern corn rootworm eggs have developed extended diapause to survive in the ground for years until ideal conditions emerge. Western corn rootworm beetles have adapted to lay eggs in soybean fields instead of just corn.
Integrated Pest Management (IPM) Tips for Improved Control
The best way to manage corn rootworm is with an integrated pest management system that includes multiple effective control tactics. Single strategy tactics, such as relying only on short-lived, soil-applied insecticides or corn rootworm Bt corn may decrease the length of time that tactic is effective and reduce available options for management.
The first step to an effective IPM strategy is scouting. There are three main scouting techniques for corn rootworms: root scouting, adult scouting and sticky traps.
In early to mid-June, dig up two plants with the soil 6 to 8 inches around the stalk from five locations across the field. Sift over a sheet of black plastic or soak the root ball, looking for 1/32- to 1/2-inch-long larvae or brown scarring on the roots. If the average length of larvae is more than 1/2 inch or pupae are found, a rescue treatment may be too late.
In early to mid-July, evaluate fields for silk clipping. If pollination is in progress and the beetles have chewed the silks where less than 1/2 inch of the silk is exposed beyond the husks, consider an insecticide treatment. Fields may become re-infested 2 to 3 weeks after an application; consequently, some fields may require two applications of insecticide to significantly reduce egg laying.
Sticky traps should be used after silking starting at the blister stage. Place six traps per field, arranged down one row and the length of the field at least 100 feet from the edge. Weekly counts from each trap can help estimate the corn rootworm population the following year.
If there are fewer than 21 beetles per trap per week, or 21 to 50 beetles per trap per week, there will be low to moderate populations, respectively, the following year. If there are more than 50 beetles per trap per week, expect a high corn rootworm population the following year and apply insecticides.
After the threat level is known, growers can plan rescue treatments for this year and how to counteract corn rootworm populations the following year.
Some of the effective control methods for corn rootworms are:
- Rotation to a non-host crop.
- Planting hybrids with multiple Bt traits for corn rootworm control.
- Applying in-furrow and lay-by insecticides.
- Applying foliar insecticide to control adult beetles prior to egg-laying.
- Controlling volunteer corn in non-host crops.
“We continue to recommend that growers use best management practices to keep CRW populations in check in their fields,” Vammer said. “This includes things like in-furrow insecticide applications, chemigation rescue treatments during the larval stage if feeding is too heavy on the roots, or a well-timed beetle bombing treatment to keep populations in check for next year.”
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