Corn rootworm beetles have emerged and are thick in some Iowa cornfields. This is the time of year we get questions about managing adult rootworms for the next growing season. The short answer: targeting adults is challenging and can be expensive. We recommend focusing on larval management, but there are scouting guidelines for adults and some products that work better than others.
Understand rootworm biology
Timing foliar insecticides to kill adults is problematic due to corn rootworm biology. First, adults emerge from the soil over a period of 6-8 weeks starting in late June or early July, depending on temperature and other environmental factors. This usually coincides with silking (R1), when many folks are applying fungicides already and are concerned about silk-clipping insects affecting pollination. An insecticide at silking could knock down beetles that are active, but the vast majority of beetles won’t emerge until later in the season.
In most cases, we want to spray when females are laying eggs. Spraying at peak female activity a few weeks after pollination would be ideal. Remember, beetles are constantly emerging for ~2 months, and to further complicate things, adults are highly mobile and moving between fields after emergence. Adult corn rootworms are active in the landscape for several months, and we can find beetles up until the first frost in the fall. Although adult activity in fields is variable annually, we typically see peak beetle counts from late July through late August.
What is the “best” timing for targeting corn rootworm adults?
Much research has attempted to answer this question, but it is difficult to get the ideal timing. Because many factors influence the occurrence of corn rootworm populations in a field, research suggests that the most economical approach is to apply an insecticide when approximately 10% of the females are gravid (have eggs in their abdomen). This requires walking through fields, distinguishing between males and females, and using the “squish test” to determine if females are full of eggs.
Watch a video on how to do the squish test!
Depending on the insecticide used, more than one application would likely be needed to make a significant reduction in eggs deposited for the next growing season (2-3 applications spaced 7-10 days apart). However, a product with a longer residual could provide protection over several weeks and limit the need for multiple insecticide applications.
Recent research from Nebraska suggests that Steward EC (active ingredient: indoxacarb; IRAC Group 22A) applied at 8 oz/A provides quick knockdown, similar to pyrethroids (IRAC Group 3A) and chlorantraniliprole (IRAC Group 28). However, there seems to be evidence Steward may provide longer residual control of corn rootworm beetles, potentially up to 21 days. This is consistent with anecdotal claims made by agronomists and farmers that have used this product in Iowa. The application in the study was based on the activity of several insects in corn, but followed whole plant counts of beetles exceeding the threshold (0.75 beetles/plant).
Be vigilant about resistance management!
History has shown corn rootworm to be an adaptable pest. The rapid evolution of resistance to Bt proteins in Iowa should be a testament to the ability of corn rootworms to adapt to management strategies used annually in a field. We strongly recommend creating a diversified management plan for larvae, including crop rotation, transgenic hybrids, or soil-applied insecticides. Now would be a great time to assess larval root injury!
Although we are not aware of any corn rootworm populations resistant to indoxacarb or any insecticide-resistant corn rootworms in Iowa, be aware corn rootworms resistant to pyrethroids are common in Nebraska. Be prepared to rotate modes of action if multiple insecticide applications are made in a field (for any insects), and plan to rotate modes of action each year (e.g., if indoxacarb is used this year, plan to use a different mode of action next year).
Making an insecticide application decision, especially for a highly mobile pest like corn rootworm adults, should not be taken lightly. Any time an insecticide is applied, it is exposing the insects to that chemistry and giving them a chance to become resistant to it. We have relatively few options available, and regulatory action threatens future use of many active ingredients. Resistance development has the potential to be very costly, so use them sparingly!
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