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.