Now that most Minnesotans can see the lawns again after a record-breaking winter, some aren’t too happy with what the melt has revealed. In the metro especially, people are noticing big, juicy white grubs feeding on their lawns.
What are these grubs?
White grubs are the immature form of scarab beetles. Common scarab beetles in Minnesota include June beetles, rose chafer and, of course, Japanese beetles. Depending on where you are in the state, the grubs you are seeing are likely Japanese beetle grubs, but it is still important to identify the insect you are looking at.
While scarab beetles are related, how long they spend in the soil as grubs and how much damage they do varies a lot by species.
If you are thinking about putting major time or money into dealing with white grubs, it is worth it to confirm which species you are dealing with. This tool from Cornell University can be used to compare the size, lifecycle, and pattern of hairs between different grubs.
Why do white grubs matter?
Different species have different potential to do damage. The main grub that does significant damage in the metro and southeastern Minnesota is the Japanese beetle.
Japanese beetle grubs feed on the roots of grasses from June through the fall, then they ride out the winter a few inches under the soil. As soil temperatures warm, they return to the top few inches of soil, feed on roots, then pupate.
In mid-June adult beetles will emerge and feed on many species of plants. These adults lay eggs and the cycle starts all over again.
Is now the time to treat?
Despite how your lawn may look and the number of grubs you are seeing, spring is not a good time to use pesticides to manage Japanese beetles. There are a couple reasons for this:
- These grubs are big, which makes it harder to kill them with pesticides.
- Most of the feeding damage has already been done.
- Healthy turf can tolerate some level of Japanese beetle feeding (about 10 larvae per square foot).
- Treating larvae now does not mean you won’t see adults feeding on leaves or grubs feeding on your lawn later this summer or next spring.
Think about your yard's history with this pest — do you see grub damage every spring? If so, put a reminder in your calendar to treat in June or July. Soil applications at this time target freshly hatched grubs, or the grubs that will be in your lawn from the summer of 2023 to the spring of 2024. These young grubs are easier to treat.
If this damage is new to you, you could adapt a wait-and-see approach. Do you see dead patches in your lawn later in the summer? If so, you could use a product to get a little bit of control if the problem reveals itself.
Depending on what time of year you are treating for grubs, there are a wide variety of products available, from conventional insecticides to biological nematodes.
For more information on the types of products available and what else you can do, see our Japanese beetle page.