White Grub Damage in Perennial Grasses

Mercedes Moffett, Extension Educator, Claire LaCanne Extension Educator, Crops
Reviewed by Anthony Hansen Extension Educator, IPM, and Craig Sheaffer Extension forage specialist.

In Northeast Minnesota, we have been fortunate to receive some rainfall over the last few weeks. As temperatures begin to rise with the addition of some moisture, we have begun to see grasses greening. Even while things begin to change color, there have been some very noticeable yellow/brown patches remaining in grass stands in lawns, pastures, and hay fields alike. The predominant grasses that are being affected are timothy and bromegrass stands and the culprit is May or June beetles. We have not observed damage in legumes like alfalfa and clovers. 

White grub life cycle

The damage from these beetles is not caused by the adult flying stage, but instead the immature or larval stage, commonly referred to as white grubs (Photo 1). The name white grub sometimes gets used as an umbrella term to describe different grub species, but several species that we refer to as May or June beetles are the true white grubs causing issues in our neck of the woods.

These true white grubs have a three year life cycle. Adult females lay eggs in mid- to late summer of what is considered year one. That same year, grubs hatch from eggs and start feeding on existing roots. During this first year, the feeding injury is not observable. The grubs then overwinter in the soil. The grubs will continue to grow over the next two years passing through different stages called instars. Slight crop injury may be observed in the second year of a grub’s life. Once the grubs reach the spring of their third year, they are at their largest, and therefore, consuming the largest amount of plant roots leading to the greatest amount of damage. This is when feeding injury is particularly noticeable, which is what we seem to be experiencing this year. The grubs will pupate in this third year and then emerge as adults in the middle of the summer the following year. If infestations are severe enough, you can easily peel back sod in mats as if you were cutting up sod, due to loss of that root material.

Although we notice the damage most when the grubs are in the third year, the damage is fairly well done by then. It’s important to note that just because you see damage does not mean you have to treat right now to prevent more damage. In turfgrasses, you will want to determine thresholds, and for pastures, there are no labeled insecticides.

White grub identification

When it comes to determining if white grubs are the culprit of your losses, identification will be the first step. The large white grubs that you would find by digging down in the dirt are easily identifiable by their large size, white color, and by the brown hairs on the rear end in a distinct pattern of two straight lines that is almost zipper-like (Photo 2). The ones nearest the surface in the soil, will be the instars in the third year of the life cycle. These “year threes” can be as long as 1.5 inches. As you dig down further into the soil profile you will often find smaller ones that are younger. Typically year two instars will reside around 2-3 inches deep, and the first years are the deepest at 3-5 inches deep.

White grub treatment

If your concern is in turfgrass, visit Managing white grubs in turfgrass from Purdue. However, if you are seeing white grubs in your pastures or hay fields, you might consider stand renovations because no insecticides are labeled for use within either of these forage stands. Some grasses like smooth bromegrass and quackgrass that spread by underground stems called rhizomes may fill in and recover some, but others like timothy and orchardgrass cannot spread. Some management strategies include:

• Reseed heavily infested areas following tillage or by interseeding with a pasture mixture containing a diversity of grasses. Select grass species that differ from those on which you see the most feeding. Consider adding in a legume like red or white clover. This may help with overall stand viability because legumes can fix nitrogen that can be supplied to the grass and they are not primary feed sources for white grubs.
• Fertilize the existing stand. Nitrogen is especially beneficial for grass production and will help grass recovery and spreading.

Why are we noticing so much damage from white grubs this year?

We have had about three years of drought; specifically a dry summer and fall last year which was followed by an abnormally warm, dry winter. Our perennial grass stands have endured large amounts of stress, possibly making feeding from white grubs more apparent. Otherwise, healthy grass should be able to tolerate white grub feeding.