Are you continuing to scout your fields weekly to better understand the current populations of rootworm beetle? Your answer should be yes.
For insect management purposes, it is important to know how to distinguish between the three rootworm types:
- Northern Corn Rootworm beetle (NCR)
- Western Corn Rootworm beetle (WCR)
- Southern Corn Rootworm beetle (SCR)
Northern Corn Rootworm Beetle (NCR)
NCR beetles are typically ¼-inch long, and their coloration ranges from tan to light green. Unlike the WCR and SCR beetles, there is no color difference between males and females; however, females typically have longer, larger abdomens. NCR and WCR go through one generation a year and overwinter as eggs in the northern part of the country.
Western Corn Rootworm Beetle (WCR)
WCR beetles are typically yellow to light green, with three dark stripes running the length of the wings. Stripes vary from three distinct dark to black lines on the males to one large strip covering most of the wings on the females. Females have slightly larger abdomens than do the males, but both are around 5/16-inch long.
Southern Corn Rootworm Beetle (SCR)
SCR beetles are generally around ⅜-inch long. Their coloration ranges from yellow-green to green, with 12 black spots on their backs. SCR typically is not an economic concern in the Upper Midwest, but they can be detrimental in the South, as they can run through two generations in a year.
Understanding the Lifecycle of Each Rootworm
Depending on where you are in the country, rootworm larvae will have emerged as adult beetles by early July and can continue to be a pest until early October. Male beetles typically emerge ahead of females by as much as 10-14 days, which is a key thing to know when deciding if an insecticide application is needed.
Rootworm larvae feed on corn roots and organic matter; however, adult beetles feed above ground on silk, anthers and leaf tissue. If populations become high enough, silk feeding can cause a yield reduction due to the inability of the kernel to be pollinated. Likewise, excessive feeding in the tassel area can limit pollen production. The WCR may also feed on leaf tissue, sometimes totally stripping the leaves.
Beetles will also migrate to soybean, alfalfa and other crops where they feed on pollen, flowers and foliage, but typically they are not more than a nuisance outside of corn. Consideration needs to be made regarding beetle numbers in soybeans in the current year if you plan to plant corn next year. Keep in mind that late maturing fields are like magnets to these beetles—they leave mature fields looking for late, immature fields to continue feeding.
Scouting For and Managing Rootworm Beetles
Scouting practices and beetle population numbers vary from region to region, and your local university extension website can typically be a good resource for this type of local information.
If you find that you have exceeded the economic threshold, you will want to apply an insecticide labeled for rootworm beetle control. Approved products with bifenthrin, lambdacyhalothrin and several other active ingredients work well to suppress beetle populations, potentially increasing your ROI for the year and limiting potential for heavy populations in next season’s crop. Remember that many insecticides will suppress beneficial insects as well. Use proper integrated pest management methods, scout your fields and only apply insecticides if it can improve your bottom line.
Understanding which variety of rootworm beetle is in your field will help you understand how to achieve optimum control.
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R.L. Croissant, Bugwood.org, Diabrotica undecimpunctata howardi and D. barberi and D. virgifera virgifera, CC BY 3.0