About this time, Japanese beetles are starting to come out. The soil is damp from the rain, turf has been fertilized, and now the nice warm weather is enabling rapid growth! This is prime "fattening up" time for most beetles. It's also prime time to control them.
Image courtesy of University of Illinios.
What people don't realize is that grubs, the larval form of beetles, can do just as much damage to grass and roots as the beetles do to plants. According to the U.S. Department of Agriculture, Japanese beetle grubs cause an estimated $234 million in damage each year making them the most damaging turf insect.
What are grubs?
Grubs are simply the infant form of beetles. The most prevalent are Japanese beetles and masked chafer beetles. Both start in the form of grubs. Let's look at the life cycle, and then we'll talk about prevention.
Late spring - larvae are moving up through the soil and will feed on plant and grass roots for a short time, and then hatch out of the soil as beetles.
Through the summer, June and July - Beetles will emerge to look for a mate and feed on plants and leaves, practically skeletonizing them. They particularly like roses and grapes. Periodically, they will burrow down into the soil to lay a few eggs. They will then return to the surface to feed. Beetles will repeat this cycle until they have laid 40-60 eggs a piece!
Late summer, late July into August - After eggs hatch, grubs begin to feed on grass roots. At this first stage, called first-instar, grubs feed close to the surface and are more vulnerable to control methods.
August through October - Grubs molt into second and third stages and consume more and more roots! Damaged turf will appear now.
Winter - Grubs move down in the soil as temperatures drop in late autumn and overwinter as third-instar grubs below the frost line.
Spring - Grubs move up in the soil and feed on roots for a short period of time. NOTE: Most of the damage seen in the spring was caused the previous fall.
Late spring - This is the time when grubs turn into pupae, which are resistant to insecticides. Completing the cycle, they emerge in June and July and start destroying!
The moral of the story is that grubs can do damage at two stages, in the soil as they consume grass roots and once they emerge and begin to feed on plants above ground. If you want to prevent and get rid of them, you need to know how to treat them in both stages.
How to prevent grubs?
Beetle stage - Are you seeing adult beetles now? There are a few ways to treat them that we've outlined in a previous post here. They are the most susceptible when they first emerge.
Grub stage - The larvae are very susceptible to treatment right after hatching in June and July. Treating at this time prevents the massive feeding frenzy that will go into August. With the development of new grub control chemistry like imidacloprid [Merit] and halofenozide [MACH2], applications in June and July will sufficiently protect lawns from new grubs into August.
If you wait until you can see damage in late fall, it will be too late for complete treatment, but there are products like trichlorfon and carbaryl available for rescue-type treatments.
We hope you don't have beetles consuming your plants and consequently grubs destroying roots later in the summer. But if you do, remember this blog post and call us if you need more hands on advice.