Figure 1. True white grub. Credit: A. Varenhorst.
In South Dakota, true white grubs (Figure 1) have steadily become a significant issue for rangeland and pasture. During the second and third year of their lifecycle, the white grub larvae feed on a large amount of root tissue and leave behind barren or brown circular patches in rangelands. The adults of the true white grub are commonly referred to as June beetles (Figure 2). Larvae of the true white grubs can be distinguished from other grubs by the “zipper” pattern of hairs or raster at the end of their bodies (Figure 3). Last spring, we received reports of large June beetle populations throughout much of Central South Dakota, which indicated the start of a new three-year lifecycle for the true white grubs. However, not all June beetles are on the same cycle so adult emergences during 2017 will not be an uncommon occurrence.
|Fig. 2. June beetle.
Credit: A. Varenhorst.
|Fig. 3. “Zipper” like raster used to identify true white grubs.
Credit: A. Varenhorst.
Behavior & Lifecycle
During the summer that the adults emerge, eggs are laid and true white grubs feed on grass (2016). During this time, the true white grubs are fairly small and feeding injury to the roots is not readily apparent. After the growing season ends, grubs overwinter in the soil. During the second year (2017), the grubs are larger and feeding injury will be more apparent, especially in areas experiencing drought stress. These patches will initially appear as a small area of yellowing or brown grass, but will spread outwards from the initial point. The grubs will then overwinter for an additional year. During the third year (2018), the true white grubs are much larger and feeding injury to grass will be readily apparent. The growth of the patches is likely dependent on drought stress as well as the grub density. In July, the grubs pupate and adults will be present in the soil where they will overwinter. During the following spring (2019), adults will emerge and start the cycle all over again.
Based on the proposed timeline, we are entering the second year of the three-year grub lifecycle. This indicates that areas of South Dakota that have had issues with grub injury in recent years should monitor grass health through the summer to determine if issues are worsening. There are no insecticides labeled for white grub management for pastures and rangeland. Currently, the recommendation for dealing with grub injury is to avoid stressing grass, especially during drought conditions. Reseeding rangeland and pasture may be an option in certain circumstances where significant damage occurred.
While no formal range recovery protocols exist in South Dakota for post-grub damage, it is recommended that landowners keep a few principals in mind when considering active range seeding/recovery.
Native Range & Pasture
For native range and pasture (virgin sod), it is likely most practical and economical to allow the range to recover on its own. If reseeding is pursued, it is recommended that producers work with a local rangeland resource person and reseed only native species for their given area. Since grub infestations are often associated with exotic cool season species such as Kentucky bluegrass and various brome grasses, selecting locally adapted native species will likely provide the best opportunity for reestablishment. However, little work has been done in South Dakota to prove or disprove this assumption. Breaking sod in an attempt to control white grub infestations is not recommended and is highly discouraged by range management professionals.
For planted hayfields (not virgin sod) dominated by exotic grasses, replanting can be an option. It is recommended that landowners pay particular attention to what stage of the life cycle the white grubs may be in, as it is likely futile to attempt a replant while grubs are still present in the soil as these may feed immediately on the tender roots of newly planted grasses. While not recommended, in extreme circumstances some South Dakota producers have decided to till hayfields infested with grubs in an attempt to kill/expose the grubs. These breakings are followed with a traditional row crop with the intent of returning the field to grass/hay in the future. No specific data on this method is available at this time.
S.D. Infested Areas
Figure 4 is a map highlighting areas of South Dakota that were greatly affected by true white grub populations during the previous cycle. If you are in the areas listed as high infestations during the previous cycle please pay careful attention to your range. If you have damaged range that does not fall into a previously reported county please contact us so that the map may be further updated.
Figure 4. South Dakota map of reported true white grub infestations and associated damage. Credit: P. Bauman.