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Several organic landscaping professionals in the Northeast have reported that they have little to no grub problems in organically managed lawns. Scientists have yet to determine why this is. How do you know if you have a grub problem? As your lawn begins to green up, do you see bare, brown patches of soil here and there? Maybe you noticed them last fall, too. Can you lift up whole tufts of grass with no roots attached? These areas are a sign that grubs (beetle larvae) have been feeding on the grasses in your lawn, literally eating them out by their roots. Additional signs of grub activity may be crows gathering on the lawn to dig up the grubs, or mole tunneling. Moles are voracious eaters of grubs, and will quickly excavate large areas in search of their prey. If grubs are the culprit, you will find them in the first couple of inches down in the dirt near the brown patches. Grubs are C-shaped, white, with six legs near the darker head and range in size from 1/4" to 1" depending on the species.
If you find grubs, don't panic! Rake up the dead grass areas to expose the grubs to natural predators like birds and pick out and dispose of visible grubs. Reseed, cover with 1/4" compost, and water. Adding some white clover seed and using a mix of grub resistant grass varieties such as tall fescue, fine fescues, Kentucky bluegrass and perennial ryegrass will help prevent future damage. Good turf management (infrequent but deep watering, moderate fertilization, soil amendments based on soil tests, mowing high at 3"-4", and a diversity of grass and plant species) will result in a vigorous turf with a deep extensive root system that can tolerate relatively high grub densities without damage. Nature itself often can keep grub populations in check through the checks and balances of weather conditions and predator-prey cycles.
If the grubs are from Japanese beetles, you can use milky spore bacteria (Bacillus popilliae) for safe, long-term control of the white Japanese beetle grubs. While it may take two to three years to achieve complete control, milky spore is worth using due to its simple application method and effective long-term results. The spores multiply inside the grubs, and when the current infestation subsides, the spores lie dormant in the soil for up to 15 years, waiting to attack subsequent grub populations. Be sure to follow the label instructions regarding application rate and use when the soil has warmed up to at least 55 degrees. It is most important to apply the milky spore powder on an overcast day and to water it in well to the lawn and surrounding garden bed areas where beetles have been a problem.
For short-term, fairly fast-acting control of Japanese beetle grubs, try Heterorhabditis bacteriophora and/or Steinernema feltiae, or a mixture of both. These are beneficial nematodes, microscopic soil organisms that attack deep-dwelling lethargic white grubs. Beneficial nematodes also need to be applied when direct sun will not dry them out, and they need to be watered in well immediately prior to application and immediately after application. Researchers are still exploring the use of other beneficial nematodes that show promise for control of other grub species in lawns. Nematodes are most effective on the early stages of larval development, so are best when applied in mid-August to mid-September.
Watering during adult beetle activity in late spring and early summer attacts egg-laying females (especially when the soil in surrounding areas is dry). This is one of several good reasons to minimize lawn watering.
For ornamental plants, handpicking is an ideal way to keep the adult beetle population under control if there are a limited number of plants being attacked. This is easiest to do in the early morning or late afternoon when the adult beetles are more dormant – you can just push them off the leaves. Use a drop of biodegradable soap in a jar of water to collect the feeding adults. Contrary to some traditions, there is no need to use oil or kerosene in the jar. Once they have died in the soapy water, just dump the whole jar in your trash, or better yet, your compost pile.
A more detailed discussion of grub identification, behavior, and organic management can be found in The NOFA Organic Lawn and Turf Handbook available for purchase in the store.
Contributions by Priscialla Williams, Mike Nadeau, and Sarah Little