Q: Yesterday while repairing grub damage to my lawn, I noticed a large number of holes in areas not affected by grubs. The holes are about 1 inch deep and 1 inch wide. It looks like the lawn had been aerated, but there was no loose soil evident. What do you think is causing this? I didn't treat for grubs this year. Next year, what is the best time to apply a grub preventer? Do I need to use it every year?
A: The holes sound like the work of birds mining for grubs. Grubs are a nutritious bird lunch, and this digging is helping you hold down next year's problem already.
Once grubs polish off a section of lawn, they move into live turf and feed on those roots until knocking off for winter deeper in the soil. The fact that you're seeing holes tells me grubs are still at work in what appears to be unaffected lawn -- for now. Given enough time, those sections would brown out, too.
Grub feeding will be winding down shortly, so I wouldn't bother putting down any kind of insecticide now.
The bird holes will fill in next spring. However, openings like that are also weed invitations, so you can either scatter some grass seed now to "re-grass" the holes (beating weeds to the punch) or worry about killing weeds next spring, if necessary. Either way, the birds are doing more good than harm.
Skunks also love to go grub-hunting, but they tear up a lawn far more by digging and scratching their way to lunch than birds do with their more surgical, beaky grub excisions.
I don't use grub treatments at all. Like most bug problems, damage runs in cycles. You might get bad damage one year, then virtually nothing in another year. In my opinion, it's wasteful and unnecessarily polluting to apply insecticides "just in case" every year.
When grubs kill off sections of my lawn, I invest the time, effort and money on reseeding instead. In grub off years, I get off the hook altogether. In bad years, even a 25-pound bag of quality grass seed isn't much more expensive than treating the whole lawn with a grub preventer. I don't mind the work either. I look at it as good exercise in the fresh air.
That said, grub preventers do what they're advertised to do. The key is getting them down and watered in before the new "crop" of grubs hatches. Early summer is the time to get the preventer down -- June or July is ideal.
Source: Holes in the Lawn