I have received a few phone calls and e-mails this week from people asking about white grubs, which are starting to make their presence known in some parts of southern New England. Soil temperatures are still a little on the cool side, thanks to the cool and cloudy weather, so grub damage has not been showing up as quickly as it sometimes does. The steady rains have been masking grub damage in some locations, and eventually the skunks and raccoons will figure out where the grubs are!
The main question seems to be whether there is anything that can work at this point to control the grubs?
The only curative product we have that might have an impact on the grubs is trichlorfon - but it probably will not kill more than 40 to 50% of the grubs that are present right now. That is partly because the grubs are larger than they were in the fall, and partly because their physiology is already beginning to change in anticipation of pupation in a few weeks. Many people find that such a low mortality rate makes it difficult to justify the cost of the application.
Remember that trichlorfon cannot be used on school grounds in Massachusetts and has similar restrictions in several other northeastern states. It works very quickly (usually within three days) and breaks down very quickly. However, it is very soluble in water so it would be more likely to run-off or leach than other products. Note that trichlorfon is also very sensitive to high pH, so if the water in your sprayer is higher than 7.2, you need to use one of the additives to buffer the reaction or lower the pH.
Dr. Dave Shetlar, my colleague at Ohio State University, has claimed in the past that chlothianidin could provide some curative effect on white grubs in as little as seven days. I don't know whether he has tested chlothianidin in a spring application. But when we conducted a trial in September a few years ago with chlothianidin, we did not see a significant reduction in grub numbers 10 days after application. (We did see a reduction 30 days after application.) Dr. Shetlar's trials were looking at Japanese beetles and masked chafers, which are much more susceptible to insecticides than the European chafer and oriental beetle, which are our primary species. So personally I doubt that chlothianidin would provide any better relief than trichlorfon for a spring "rescue" treatment.
If you are experiencing major grub damage now or you begin to see activity in the next week or two, as soils finally begin to warm up, you might want to apply a preventive insecticide. One option would be chlorantraniliprole, which is outstanding against all the white grub species we see in New England and has no activity against bees, ants, or wasps. (It is also virtually non-toxic to vertebrates.) It should be applied any time from now until the first week of June. It will not affect the grubs that are present now, but it will control the grubs that emerge later in the summer.
The second option would be to apply a neonicotinoid (e.g., chlothianidin, imidacloprid, or thiamethoxam) between mid June and early August. These products can be very effective against European chafers, oriental beetles, masked chafers, and Japanese beetles, but have no effect on asiatic garden beetles. Remember that neonicotinoids are toxic to honey bees and other pollinators, and must not be used when bees are foraging (for comprehensive information on minimizing the impact of neonicotinoid insecticides on pollinators, see our Neonicotinoid Turf Insecticides and Pollinators fact sheet).
Whatever approach you use, remember that the product needs to be watered in to enhance its effectiveness. And remember that thick or dense thatch often makes it harder for the insecticide to get to the root zone, so thatch management might be the first step to take.
Submitted by: Dr. Pat Vittum