True white grubs, the larvae of June beetles, have the potential to be significant pests of pasture and rangeland in South Dakota (Figure 1). The grubs feed on grass roots, leaving behind large circular patches of dead grass and bare ground (Figure 2). For most species of June beetles, they have a three-year life cycle, during which three years of their development time is spent underground as grubs. It is not until the end of their third year that they pupate and emerge as adults the following spring.
Figure 2. White grub damage. Photo by Amanda Bachmann.
In the spring of 2016, we received reports of large populations of June beetles in Central South Dakota, which indicates the start of a new cycle for the grubs. Based on these reports, it is estimated that 2018 will be the third year of the grubs’ life cycle. It is the final year of development before adult emergence next spring (2019). The main concern for 2018 is the potential for severe damage to occur in pasture and rangeland due to the large size of the third instar grubs. Last year, areas of feeding may have been observed, but these areas will likely increase this year. Areas that experienced drought stress in 2017 may have larger feeding areas due to the grubs.
June beetle adults can be ½ to 1 inch long and are light to dark red-brown in color (Figure 3). They typically emerge in May and June and are frequently seen near lights at night. The June beetle larvae, or true white grubs, are white to cream in color and almost always C-shaped. True white grubs can be distinguished from other grubs by the “zipper” pattern of hairs or raster at the end of their bodies (Figure 4).
Figure 3. Adult June beetle. Photo by Adam Varenhorst.
Figure 4. Zipper pattern of hairs on tip of abdomen used to identify true white grubs. Photo by Adam Varenhorst.
Lifecycle & Behavior
In the year that adult June beetles emerge, eggs are laid and the true white grubs hatch and begin feeding. During this time, the grubs are fairly small and feeding injury to the roots is not readily apparent (Year 1: 2016). After the growing season ends, grubs overwinter in the soil. During the second year, the grubs are larger and feeding injury will be more apparent, especially in areas experiencing drought stress (Year 2: 2017). 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, 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 (Year 3: 2018). During the following spring, adults will emerge and start the cycle all over again (2019). It is important to note that not all June beetles are on the same cycle, so adult emergences in other years is not uncommon.
As mentioned previously, we are now entering the third year of the three-year white grub life cycle (based on the proposed timeline). Areas that had signs of a white grub infestation last year will likely see considerably more feeding injury in 2018. Furthermore, damage could potentially be more severe in infested areas that are also impacted by drought.
There are no insecticides labeled for white grub management in pastures and rangeland. Doing so is considered an illegal activity and will cause more harm than good to the insect community that is present. Currently, the recommendation for dealing with grub injury is to maintain a healthy grassland system and to avoid stressing grass, especially during drought conditions. Reseeding may be an option in circumstances where significant damage has already occurred. When reseeding, be sure to choose plants that are less attractive to June beetles, such as native grasses and forbs. Non-native, cool season grasses can actually promote an infestation and should not be used.