Help! Grubs are killing my lawn!
Know your enemy : Identifying grub damage
Symptoms of white grub infestation include wilting of grass in patches, eventually turning brown or dying, sod that pulls up easily, in one piece, with the white grubs then visible underneath, feeding on the roots. Increased feeding activity of birds, moles, raccoons,etc. are another sign, you may notice holes in the sod where critters have been digging. The damage is most noticeable in Fall or in dry spells, though serious infestations may be visible in Spring.
Dead brown areas can have other causes, such as pet urine, improper use of fertilizer or insecticide, turf diseases or improper mowing, so be sure to identify the problem before trying to treat it! In the case of grubs, if the insects are not easily visible just under the turf, there is no point in treating for them.
The usual suspects....
The white grubs you will find feasting on grass roots are the larvae of several members of the beetle family, having an idea WHO you're hosting helps in controlling them.
All are fat white C-shaped grubs, with brown heads and six legs, but the larvae of the May or June beetle can easily be 2" in length. This species can remain underground up to four years before becoming a winged adult, so timing of chemical controls is not as crucial, though they must be applied when the ground is warm enough for the grubs to be actively feeding.
Japanese beetle larvae are smaller, about 1/2 inch, and have a noticeable V-shaped pattern on the 'raster', the dark bristly area at the end of the abdomen. Japanese beetles complete their life cycle in one year, laying eggs in mid-July which hatch in early August, chemical controls should be applied at that time while larvae are most susceptible.
Masked chafer grubs are similar to Japanese beetle larvae in size, but lack the V-pattern on the raster.Unlike Japanese beetles, the adults of this species are short-lived, and do not eat while in the winged state. Their life cycle is similar, with August being the time to apply chemical controls.
Dates given above for applications are most applicable for Zone 5, your local County Cooperative Extension Service will be happy to help you with timing treatments in your specific area. Any pesticide you choose MUST be labeled for use on white grubs, and label directions MUST be followed exactly.
Preventing grub infestations: biological controls
There are a few natural controls available to treat your lawn BEFORE grubs become a problem.
Milky spore disease, which is most effective against Japanese beetles, and to a lesser extent against May/June beetles. The bacteria that cause the disease affect only white grubs, and after proper 'inoculation' into a lawn area, remain active in the soil for years. The bacteria need to be present in the soil BEFORE the grubs begin feeding to be most effective, as the disease can take months to kill established grubs.
Beneficial Nematodes are microscopic, nonsegmented worms that occur naturally in soil all around the world. Steinernema carpocapsae and Steinernema feltiae prey on ants, termites and the larval and grub stages of various beetles, weevils, armyworms, cutworms, chafers, webworms, borers, maggots, fleas, fungus gnats (sciarid flies). Once they are released, the nematodes seek out host insects and enter their prey through body openings and emit an endo-toxin that results in death for the host insect within 48 hours. The nematodes reproduce and their offspring feed on the insect cadaver and emerge to seek out new hosts.
Nematodes are easy to use. They are shipped in a formulation that you mix with water. The solution can be applied using a watering can; hose end, backpack, or pump sprayers; or through irrigation or misting systems. Always release early in the morning or late afternoon when temperatures are cooler. Generally, 1 vial of Beneficial Nematodes will effectively treat approximately 900 sq. ft. of conventional garden rows. Make releases every 3-6 weeks or until infestation subsides. Nematodes can be stored in the refrigerator (do not freeze!) for up to 2 months.