Manage Adult Corn Rootworm Populations and Protect Silks

Categories: GROWING, CORN
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A general depiction of the key growth stages in which CRW management tools are suggested for use.

When it comes to high pressure corn rootworm (CRW) fields, even under intense management with soil applied insecticides and/or CRW-traited corn hybrids there is a possibility that economically damaging populations may be encountered. If planning to plant corn again the following year, or if simply attempting to protect current yield potential by avoiding silk clipping, scouting to determine adult CRW beetle populations and knowing economic thresholds are the first steps in the decision-making process. Here are some insights to keep in mind when deciding if and when to apply a foliar insecticide.

Why Manage CRW Adults?
Scouting and potentially applying foliar insecticides for CRW beetles can be done for different reasons. These reasons vary from protecting silks for good pollination, to proactively managing CRW populations in fields that you intend to plant to corn in the following year.

Managing silk clipping: Clipped silks, especially in severe circumstances, can hinder pollination enough to negatively impact kernel development and, therefore, yield. If the population in a field reaches critical levels where the silks are clipped down to a half inch or less before 50% pollination, spraying insecticide directly after beetle emergence may be warranted.  

Managing next year’s populations: Early insecticide applications timed for silk clipping (or to enable tank mixing with fungicide applications) may not align with the best timing for controlling gravid females (abdomen visibly distended with eggs). Due to improper timing, insecticide applications may not help reduce CRW populations the following year. For best results reducing future CRW populations, it’s best to time applications just prior to female beetles starting to lay eggs. Male beetles will typically emerge 1-2 weeks prior to females. Upon female emergence, they will require 1-2 weeks of feeding on pollen and silks before they begin to lay eggs. 

Time insecticide applications after economic thresholds are exceeded (see below) and when 10% or more of the female beetles are gravid. Keep in mind the critical importance of the timing on this type of application. Reducing field trips by combining insecticide with fungicide applications may not yield the desired long-term management results. Additionally, consider the potential for spider mites being present. Most pyrethroid and organophosphate insecticides commonly used can also reduce the number of spider mite predators, leading to potential spider mite flare ups in some situations.

Adult Monitoring Methods and Thresholds

Adult beetle trapping: The most accurate way to determine if it is economically justifiably to spray adult beetles is determined by monitoring adult populations. Non-baited, pheromone sticky traps are the preferred choice for monitoring and can be purchased from various vendors. The optimal time to monitor is the start of corn silking and continues until reaching grain dent stage or reaching economic thresholds (generally mid-July through August, potentially into early-September).  

Attach the trap directly to the corn plant just below the ear (shown right) and monitor weekly. Replace traps as needed to ensure accurate numbers are being recorded. Determine average beetles/trap/day by dividing the total number of beetles caught from all traps by the number of traps placed, and then by the number of days traps were in the field.  University recommended economic thresholds range from 2+ beetles per trap per day in central Corn Belt growing areas and as high as 6+ beetles per trap per day in northern Corn Belt areas for deciding when to use insecticides to reduce adult beetle egg laying capacity.    

Example calculation:

On one of your weekly checks, you count a total of 74 beetles that were caught amongst 6 traps in a field. The traps were set out 7 days ago.  

74 beetles/6 traps/7 days = 1.76 beetles/trap/day

This average hasn’t quite reached the threshold to justify a spray yet, but is enough to heighten concern.  

For more information on trap placement, the quantity of traps to set up, and the new Golden Harvest CRW beetle monitoring initiative, check out our in-field video demonstration.  

Adult field scouting (July/August): Another option in determining a field’s CRW pressure is by performing routine scouting every 3 or so days from silking through dent (if beetles are present). Beetle activity is highest mid-morning and late afternoon, so scouting at these times is recommended. Do this by selecting 5-10 random areas of the designated field and counting the number of beetles on 10 corn plants (for a total of 50-100 plants). Pick corn plants that are not next to each other, but randomly spaced.  Check the tassel, silk, ear, and top and bottom of the leaves and count how many beetles you observe. Divide the total number of beetles counted in the field by 50 (plants) to find the plant average. University studies suggest the following thresholds in the included table (above). If both species are present, use the threshold for the one that is most predominant. Thresholds may be higher in northern growing areas where overwintering survival of eggs may decrease.

For more information on corn rootworm management or help with scouting and decision making, contact your local Golden Harvest Seed Advisor or agronomist. 

Photos are either the property of Syngenta or used under agreement.

Syngenta hereby disclaims liability for third-party websites.


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