This section of the web site features Management Updates written by the turf specialists of the UMass Extension Turf Program. The messages cover regional problems, are geared toward regional conditions, and are posted frequently during the growing season.
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White Grubs Causing Headaches
I have had several people call or e-mail within the last three weeks, asking about grub damage. It appears that many turf sites throughout New England have experienced grub damage this fall, including some areas that were treated with a neonicotinoid in June or July. Here's my best effort at reconstructing what happened this year. I believe the issues that are involved are:
- Delayed egg laying because of dry soil conditions
- Failure to water in products at the time of application
- Reduced effectiveness in thatchy conditions
Dry soil conditions
Many parts of New England were relatively dry throughout the summer, going three to five weeks (or longer) between significant rainfall events. In some cases, availability of water for irrigation was limited. That means the soils in unirrigated areas were on the dry side for much of the summer, including the time adult beetles should have been laying eggs. It turns out that adult females often avoid laying eggs when soil moisture is low, and instead hold on to their eggs until soil moisture levels improve.
My guess is that egg laying of Japanese beetles, oriental beetles, European chafers, and their cousins was delayed in many locations this summer because the soil moisture was low. Meanwhile every neonicotinoid label indicates that applications targeting white grubs in turf should be made "when adults are laying eggs". Most years that is between late June (European chafers in southern New England) through early to mid August (Japanese beetles in northern New England). But this year egg laying may have been delayed three to five weeks.
That means that any neonicotinoid application that was made during the "usual" time may have been three to five weeks too early this year. As a result the active ingredient may have been slightly less effective, because it had an extra three to five weeks to break down after the application but before the new grubs began to appear in the turf. (Note that the lawn care companies with which I have been in contact indicated that the "breakthrough" rate was less than 5%, so it is not like there was an industry-wide calamity!)
In any case, I urge any of you who are planning to use neonicotinoids next year to target grubs to revisit your timing, and avoid applying a neonicotinoid any earlier than mid June. July would be an even better option.
Post application irrigation
We have conducted some field trials over the last couple years looking at whether water is necessary after application. While many turf labels do not require immediate water, our studies indicate that the effectiveness of the application is greatly enhanced when you DO water in. In some cases, the difference is really dramatic. So aim for at least 0.1 inch of irrigation after a grub application (0.25 inch is even better), or try and take advantage of rain fall.
Over the years we have had reports of "breakthroughs" of grub products. When we are able to eliminate the possibility of a misapplication (wrong rate of application, improper calibration, wrong timing), the common denominator often is a very thatchy turf. So if your grub product did not perform as well as you had expected, consider managing the thatch so that it is not too thick or dense.
What can we do?
At this point it is too late to apply any curative grub insecticide to target grubs that are active right now. Keep in mind that several species will begin moving downward as soon as the soils begin to cool down. Given the cooler night temperatures that are in the forecast, that should be happening very soon!
Your best bet is to "manage the damage". Often that means finding a way to get the skunks or raccoons or other foragers to go somewhere else. Trapping may be an option for some, while Dr. Dave Shetlar (The Ohio State University) reports that use of Milorganite™ can deter skunks or raccoons in some turf settings. I do not know what the agronomic impact of a mid-fall fertilizer application would be, but if it can be successful at moving the skunks or raccoons, that would be an obvious benefit!
Keep the turf growing as well as possible - perhaps a little water in areas that are on the dry side right now, raise the height of cut to encourage deeper rooting.
Now is a great time to review your insect control program and identify what worked well and what didn't. Who knows what next summer will bring? But at least we can all learn from this past summer.
Submitted by: Dr. Pat Vittum
For additional information about integrated management of turf pests, refer to our Professional Guide for IPM in Turf.