Controlling Grubs

Controlling Grubs

It seems the major factor influencing development of damaging numbers of grubs is soil moisture and rainfall. In general, in years with normal or above normal rainfall, grub populations increase. Well maintained turf next to ornamental plants favored by the adults (such as lawns adjacent to rose bushes for Japanese Beetles) seems to be more commonly attacked. However, some grubs do not feed in the adult stage and these pests build up in well watered and maintained turf. Try to keep from over watering and most importantly try to maintain good drainage of lawns by keeping areas mowed and removing any thatch layer that forms. Aeration is important and can be easily achieved manually with something as simple as a pitchfork. The type of grass might be a factor, with the fine and tall fescues not as severely attacked as Kentucky bluegrass and perennial rye grass.

Water Management

Practically all white grub species require moist soil for their eggs to hatch. The young larvae are also very susceptible to desiccation (drying out without proper moisture - they are a soft bodied insect). 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. Conversely, moderate grub infestations can be out grown if adequate water and fertilizer is 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 or irrigation bans may occur due to low water tables/drought.

Parasites

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. Feeding grubs pick up this bacterium 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, a long-term fix, which requires patience while the bacteria multiply enough to eradicate the grubs.

Parasitic Nematodes

Insect parasitic 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 desiccation.

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 which seem to have measurable success. In trials, imidacloprid and isofenphos generally perform best when applied before mid-August, or when white grub 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 re-applications 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 making 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 to the success of any grub treatment, along with good drainage, aeration, and dethatching.