Lawns in the Midwest often are subject to severe injury by the larval stages (grubs) of various species of scarab beetles. Japanese beetles and May/June beetles are the predominant damaging white grub species found within home lawns. Several other white grub species including: European chafer, Asiatic garden beetle, green June beetle, masked chafer grubs, and Oriental beetle are sporadically found in lawns and may cause some damage.
Many white grubs look similar to each other but vary in size. Mature grubs range in size from 3/8” inch – 2″ inches. Grubs are C-shaped and have three pair of thoracic legs (ALIENS!!!). The head is dark, but the body is usually creamy white in color. White grub species identification is not necessary because the cultural control practices are similar. The arrangement of hairs and spines on the posterior end of the grub, called the raster, is a distinguishing feature between species, if identification is warranted.
Grubs chew off grass roots and reduce the ability of the lawn to take up water. During the hot, dry weather of late summer, large dead patches of lawn will begin to develop. Irrigated lawns may not show the damage as quickly, because the lawn is being watered regularly. Sometimes the damage can get farther along before it is noticed in an irrigated lawn compared to a non-irrigated lawn. The sod in those dead patches can be quite easily rolled up like carpet to reveal the grubs beneath, because the grubs have chewed through all the roots. This is also the time when skunks, starlings, moles, shrews, voles and other furries start to forage for their favorite, plump snacks, which causes digging in the lawn.
I’ve spoken to my spray technician about what to expect this year for grub damage. She feels that the severe cold that we experienced will not make much impact on the populations of beetles this year. The grubs here can generally be put into two categories, the May/June Beetles (#1) and the Japanese beetle (#2) grubs.
The #1 grubs are generally bigger and closer to the surface. These grubs are also mostly on a 3 year cycle, living 2 years underground. Many of these beetles may not have made it, but they are also not the ones that cause a bunch of damage to the lawn as they emerge sooner, so less feeding during the summer and the lawn has had time to recover. Although, these being closer to the surface and larger makes them attractive to wildlife, who will dig feverously to get to the squishy snacks. The related thought to this, is that with the harsh winter we had, many of the furries most likely did not make it through the winter.
Regarding the severity of our winter. Yes, we did see temperatures of -16F here, but that was aboveground, air temperature. We also had a bunch of snow that does act as an insulator. Therefore, although the freeze line may have been deeper, it is still just a freeze line, no colder than freezing, just deeper.
The Japanese beetle grubs (#2) will go as deep as necessary to avoid the freeze. These emerge later in the season, thus will cause more damage as the feeding is continuing into the drier, summer months and the grass cannot keep up with lack of roots and it’s water needs.
- Allowing your lawn to go dormant during the dry summer months can help by not moving the eggs of the beetles into the lawn and they will dry-out on top of the lawn where they were laid.
- The two nematodes that are most effective against Japanese beetle grubs are Steinernema glaseri and Heterorhabditis bacteriophora. The latter is commercially available.
- Apply Milky Spore to your lawn area only if you’ve seen grub activity in your lawn during the spring. Many experts do question it’s effectiveness, though.
- Make your yard attractive to birds that might eat them. Starlings and robins love to get them when they are freshly hatched.
- Attract the solitary fly (Istocheta aldrichii) and the parasitic wasp (Tiphia vernalis) that lays its eggs inside the adult beetles (fly) or the grubs (wasp). Adult wasps feed almost exclusively on the honeydew of aphids associated with the leaves of maple, cherry, and elm trees and peonies. (Hmmm, so aphids or grubs… which pest is worse!!)
- Unfortunately, if your lawn has been severely attacked, pesticides may be your only recourse. Responsible IPM methods can be employed to reduce the chemical impacts to the environment.
Prevention: An ounce of it…
Products containing imidacloprid, thiamethoxam or chlorantraniloprole, are preventive insecticides that work well on newly hatched grubs present in July, but do not for large grubs found from September to May. Remember, this will prevent the next generation of grubs from infecting your lawn; it has no effect on the ones that are currently maturing. There are different recommended timings for application depending on the active ingredient. Although the bag often states to apply anytime from May to Aug 15, it is highly recommended that products containing imidacloprid or thiamethoxam be applied and irrigated into the soil in June. Best to apply before a storm as it works best when watered in. Preventive products containing imidacloprid or thiamethoxam will consistently give 75%-100% reduction of grubs if they are applied in June or July.
There are two insecticides, carbaryl and trichlorfon, that are considered curative treatments. These kill all life stages of the of #2 type grubs, but do nothing to #1 type grubs. These two insecticides are the only choices available if high numbers of grubs are found in the fall after the middle of September and in the spring before early-May. They are not as effective as the preventive compounds in reducing grub numbers because they have a less active time in the soil and timing of the application is critical. Consider carefully whether it would be best to wait and apply a preventive next spring. If the need should arise to use a curative compound, make sure to keep the infested lawn watered regularly and fertilized. It is recommended to treat the area again with a preventive application the next summer or grubs will likely reoccur.
© The Naturarian