Lawn Grub Control and Treatments

Grubs are the larvae of Japanese beetles that feed on the roots of grass. They hide under the surface of your lawn and usually go unnoticed until the tell-tale unsightly brown spots appear. Unlike other lawn diseases, grub damage will cause the sod to lift in sheets. The key to getting rid of grubs is to manage the problem before it arises.

Factors Contributing to the Appearance of Grubs
The major factors that encourage damaging numbers of grubs are soil moisture and rainfall. Grubs increase in years with normal or above-normal rainfall. Well-maintained turf next to ornamental plants favored by the adults, such as lawns adjacent to rose bushes, tend to be attacked. Try to keep from over-watering and maintain good drainage of lawns by keeping areas mowed and removing any thatch that forms. Aeration can be easily achieved with something as simple as a pitchfork. The type of grass might also be a factor; fine and tall fescues aren't as severely attacked as Kentucky bluegrass and perennial ryegrass.

Water Management
Practically all white grub species require moist soil for their eggs to hatch. The young larvae are also very susceptible to desiccation, or drying out without proper moisture. In areas where turf can stand some moisture stress, do not water in July and early August, when white grub eggs and young larvae are present.

Moderate grub infestations can be outgrown if adequate water and fertilizer are applied in August through September and again in May, when the grubs are feeding. This last strategy is not preferred because mammals may dig up the turf to get at the grubs, or irrigation bans may occur due to low water tables or drought.

Several parasitic wasps, Tiphia spp. and scoliids, attack white grubs and may effectively reduce populations in certain areas. However, these parasitic wasps may take two to three years to build up effective populations, during which time turf damage may occur. This is a long-term commitment rather than a quick fix.

Milky Diseases
Several strains of the bacterium Bacillus popilliae have been found that attack white grubs. The commercial preparations of this bacterium are extracted from Japanese beetle grubs and therefore are most active against this species. This bacterium is picked up by feeding grubs, and it causes the body fluids to turn a milky white before grub death. Fresh bacterial preparations should be used, and three to five years are needed to provide lasting controls. Again, this is a long-term fix that requires patience while the bacteria multiply enough to eradicate the grubs.

Parasitic Nematodes
Nematodes in the genera Steinernema and Heterorhabtitis have been shown to be effective against white grubs. Since they are living (as are parasites and milky disease), extreme care and attention to directions for application are essential to keep them from dessication.

Preventive Pesticide Applications
Since white grub occurrence is rather sporadic, applying pesticides for control of anticipated grub populations is not recommended. However, in areas where adult activity has been observed or perennial infestations have occurred, preventive applications may be warranted. Currently, imidacloprid, isofenphos and isazophos are the only registered products that seem to have measurable success. In trials, imidacloprid and isofenphos generally perform best when applied before mid-August, or when egg laying is underway.

Early Reactive Pesticide Applications
Most of the modern soil insecticides have short active residual periods (three weeks or less) and must be used when the grubs are actively feeding. No insecticide is 100% effective; they usually kill 75% to 90% of the grubs present in any given area. This is why reapplication may be necessary when grub populations get very high. Timing of treatments is critical for success. You should apply the pesticide when the grubs are small and actively feeding, yet late enough to catch all of the population. In general, reducing thatch and using good irrigation after a pesticide application will increase control.

Chemical control of large grubs is difficult at best. The highest success rate will be achieved when grubs are small, in the first few instar stages. Always be sure to irrigate well after any application in order to keep the grubs near the soil/thatch interface and to wash in the pesticide. Adequate watering in is critical.

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