Grubs can be a recurring problem in South Dakota pastures, and recently SDSU Extension staff have fielded many inquiries. While damage from grubs can occur in any region, the majority of landowner concerns have generally originated from the south central portion of the state.
Life History & Life Cycle
Grubs are the larvae of beetles in the Scarabaeidae family, and are commonly called white grubs. There are three beetles that can cause damage in pastures as grubs: May/June beetle (native), Green June beetle (native), and European Chafer (introduced). A fourth species present in South Dakota is the Japanese beetle which is known to damage turf but is not thought to contribute to pasture damage as are the other three. Identifying the species damaging your pasture is important as understanding their lifecycle is key to understanding their damage potential and management strategies.
The white grub species are distinguished by the hair patterns on the rear end of the grub. Recent samples received from Kimball, SD were identified as grubs of the May/June beetle. It is assumed that most pasture damage that occurs in South Dakota is the result of the foraging activities of the May/June beetle grub. The larvae of June beetles are also known as true white grubs (Figure 1). The grubs are curled in a ‘C’ shape with a “zipper” pattern of hairs (two parallel rows) on the underside of their posterior end (Figure 2). Adults are shiny reddish brown in color and often seen attracted to lights. The May/June beetle has up to a three-year life cycle and damages grasses by feeding directly on their roots. In contrast the Green June beetle has a one-year life cycle and forages on decaying organic matter and damages grasses due to their surface burrowing. European chafers also feed directly on roots and have a one-year life cycle.
|Fig. 1. June beetle larva (true white grub).
|Fig. 2. Characteristic “zipper” hair pattern on rear end of the grub.
As stated, the June beetles have a three-year life cycle which makes it a more difficult species to control. They spend two years as grubs and emerge in the third year as an adult beetle.
- Year 1 - The adult beetles emerge in May and June and lay their eggs in the soil. The hatched grubs feed until late fall and overwinter in the deeper layers of the soil.
- Year 2 - In the spring, the grubs move close to the soil surface and continue feeding on the roots all season. By the end of fall, the grubs reach maturity and burrow deep into the soil to overwinter.
- Year 3 - In the spring, the fully grown grubs move back to the surface to feed on the roots until July and then undergo pupation. Adults emerge in few weeks but remain in the soil for winter and emerge in the following spring (Figure 3).
Anitha Chirumamilla, SDSU Extension
Assessing Grub Infestations
Grubs cause damage by feeding on the grass roots, resulting in brown dead patches in pastures and other grasslands (Figure 4). The presence of grubs in damaged spots can be verified by digging the soil about 1 foot deep in a square foot area and sifting the soil. No specific density has been established to assess infestation levels in South Dakota, but generally several (5 or more) grubs per square foot appear to be a fairly dense population.
Grubs generally infest areas of pastures where shallow-rooted non-native cool season grasses such as smooth bromegrass, Kentucky bluegrass, downy brome, or cheatgrasses have established. These areas also tend to be moist and are not at the extremes of being too wet or too dry. Most producers report infestations in moist but well drained lowlands or on the sides of slopes where moisture conditions are not at the extremes. Rarely do grubs significantly impact areas of healthy native vegetation with deeper more robust root systems. Some have reported infestations are heavier within ¼ to ½ mile of trees or shelterbelts where adult beetles generally congregate for breeding.
Jim Ristau, Pheasants Forever
White grub management in pastures is difficult as there are no proven effective insecticides that can kill the grubs in the soil. Further, since insecticides are often broad spectrum, any application of insecticide in a pasture could lead to a great deal of non-target impacts to beneficial insects like pollinators and dung beetle larvae. Grazing restrictions are also an important factor in the use of chemical insecticides.
Overall, prevention is the best strategy in managing grub issues in pastures. However once damage to the grassland is apparent, it is often too late for preventative measures as the damage is essentially ‘done’. In native rangelands, cultural practices such as maintaining a healthy pasture with a good balance of native grasses and native broadleaf plants and controlling non-native cool season grasses through grazing rotations are logical steps toward reducing potential infestations. In areas where non-native cool season grasses persist that have shown a tendency to attract grubs, increased grazing intensity in the fall may be beneficial as it will decrease the potential for a heavy residual layer of grass duff to insulate the soil, thus allowing freezing to penetrate deeper into the soil profile. Several producers have observed an increase in grub activity where grasses were allowed to form a matt and where freezing depths were shallow. In grass plantings or hayfields, mixing perennial legumes such as alfalfa with grasses are common suggestions.
Generally, the best advice is to allow the grubs to cycle. Most producers express concern regarding the perceived damage that animals such as raccoons, skunks, or badgers may be causing in the pasture as they forage for grubs. However, the grubs are the true source of the damage and the impact of the foraging animals is often only ‘skin deep’ as they search for grubs under the surface of the already dead sod (Figure 5). Natural predators such as birds, rodents and other small mammals also help by digging out the grubs. Allowing these animals to forage can help manage grub populations at reasonable levels.
NRCS Chamberlain, SD field office
Range management staff generally agree that infested sites will recover, and many actually show promise of recovering with an improved suite of native species. The wet conditions we are experiencing this summer might alleviate the situation by drowning the grubs or by increasing their susceptibility to diseases. Moreover, the excess moisture also helps the grasses to recover and fill in the patches. However, several producers have inquired about active replanting of infested areas. While not likely necessary in most cases, one NRCS range manager reported that producers who had tried to replant often had trouble with the loose sod layer resulting from the grub activity. If replanting is a strategy, removing this old sod would be advised.
There is very little available data that would indicate expected levels of replanting success, but there are a few basic grassland management principals to take into account if active planting is undertaken.
- Use native warm or cool season species and avoid exotic cool season grasses
- Plant a variety of native grasses and forbs
- Avoid intensive management of establishment areas that could drive the system back to a brome/bluegrass dominated plant community
- Avoid non-native legumes in native pasture settings
- Ensure good seed to soil contact in a firm seed bed without planting too deep. For planted grasslands or hayfields, establishing additional diversity of native plants or use of legumes could help curb a continual grub problem.
One Beadle county producer with a heavy infestation of grubs in a non-native pasture dominated by Kentucky bluegrass and smooth bromegrass suggested he may contain his herd of cattle to the 10 acre infestation site, essentially allowing them to trample the area with heavy hoof traffic with the intent of killing the grubs before they fully emerge as adults. While an interesting approach, it would be important in this situation to have a clear vegetation recovery plan that will take into account potential invasion of weeds into the disturbed site.
Regardless of the recovery plan, it is not recommended that landowners till native pasture soils in an attempt to control pasture grubs as the long-term impacts of this land conversion may be much more damaging than the short-term impacts of the grubs.