Back Issue
May/June 2000

Controlling White Grubs
by Pamela Weil

To rephrase Ben Franklin’s famous quote: In this world, nothing is certain but death, taxes, and grubs in the lawn. We all have ‘em — plump little white grubs dining on grass roots. These grubs are the larval (immature) stage of four beetle species: Japanese beetle, oriental beetle, European chafer and Asiatic garden beetle.

Connecticut’s beetle population has changed over the years. The June beetle, a native species, used to be the most common. These night flying brown beetles were replaced by Japanese beetles. Now, in many parts of the state, Japanese beetle populations have dwindled and oriental beetle populations have increased.

Richard Cowles, an associate researcher at the Connecticut Agricultural Experiment Station in Windsor, has studied the beetle population in Connecticut for 5 years. Cowles has observed a naturally occurring fungal disease (Entoderma) that infects Japanese beetle larvae. “The levels can build up in the soil and cause high infection rates,” he says. “It’s a plausible explanation for why the Japanese beetle has virtually disappeared in some areas in Connecticut.” Unfortunately, Entoderma does not infect oriental, European chafer or Asiastic garden beetles.

Killing the grubs is complicated by the fact that all four common grub species have slightly different life cycles and respond differently to chemicals. Before buying a product that will kill them, you should identify the grubs that are living in your lawn. Cut 5 or 6 one-foot squares where the turf is dead or damaged. Bring the larvae in to the Connecticut Agricultural Experiment Station to identify them, or, with a little effort, you can identify them yourself. Look with a hand lens at the pattern of larger bristles on the grub’s underside, near the posterior end. For complete grub identification information, go to www.umass.edu/umext/turf.

How many grubs are too many grubs? If you dig up healthy turf and find grubs, that’s okay. A healthy lawn can handle more grubs than a lawn of poor quality.

There are only two reasons to kill grubs: either your lawn is already damaged, or your otherwise healthy lawn is being torn apart by animals looking for the large soft-bodied grubs to eat.

Common Grubs in Connecticut
Japanese Beetle
One-half-inch-long adults are a coppery green with wing covers the color of a copper penny. Mature grubs overwinter deep in the soil and begin to feed on grass roots and move upward in the spring. The adult emerges about July 4 and causes great damage in the garden by eating leaves and flowers. Eggs are laid in late summer; the newly hatched larvae feed on grass roots and move deeper into the soil as the weather cools.

The good news about Japanese beetle grubs is that they are more susceptible to most insecticides than other species. The bad news is that they are no longer the most common white grub in Connecticut.

Oriental Beetle
The colors of this beetle are highly variable — from a light tan with darker brown patches to nearly black over the entire body. An easy indication of adult flight activity is their arrival in swimming pools, where they drown and are caught in skimmer filters. This species is mostly found in southern Connecticut and along the Connecticut River Valley within 5 miles of the river.

Asiatic Garden Beetle
Asiastic garden beetles are less than one-half inch long and cinnamon in color; they have an iridescent sheen in the sunlight. They are attracted to porch lights on summer nights and feed at night, chewing irregular holes in many different plants. During the day, they rest in the soil.

European Chafer
The adult is a dull yellow-brown in color and flies at night, aggregating on hot, humid evenings around tall objects (trees and chmneys) to mate. Fully grown European chafter larvae are more cold-tolerant than the other beetle species, allowing them to begin feeding earlier in the spring (April into June) and continue to feed later into the fall, perhaps even into December. “They feed closer to the soil surface, which is more damaging to the turf,” says Cowles. “And they feed earlier and later in the year, when the turf cannot repair itself and grow back readily.”

European chafer larvae are the largest and most destructive; they also used to be the most difficult to kill with conventional insecticides. This situation has changed with the introduction of new chemistries, however.

Cultural Controls
What can we do to make our lawns and gardens as unattractive to these beetles as possible?

The type of grass you plant doesn’t seem to make a difference. “I have three different types of grass in my research plots — hard fescues, Kentucky bluegrass and tall fescues — and there are no differences,” says Cowles.

Studies have shown that grub populations are usually found clustered in localized ‘hot spots’ in lawns. Grubs like sun, but soil moisture is the key factor. Adult females prefer to lay eggs in moist soils, eggs need moisture to hatch, and young larvae need moisture to develop. It is often suggested that withholding irrigation when the female is laying eggs will reduce grub population levels. “I don’t agree with this,” says Cowles. “Water would have to be withheld for so long that the lack of water would damage the turf more than allowing the beetles to lay eggs.”

Maintaining a healthy lawn and mowing the grass high (3 to 3-1/2 inches) are helpful cultural practices. “This is likely to cause quite a difference in the amount of egg laying,” says Cowles. “I’ve done some studies with turf blocks. When the grass is very vigorous, we end up with fewer oriental beetles. A lawn of poor quality attracts them.”

Take care not to kill the naturally occurring beneficials in the turf environment — such as bigeyed bugs, ground beetles, predaceous mites, spiders, tachinid flies and parasitic wasps. They can be effective allies in your battle against grubs. For example, the winsome fly (lstochaeta aldrichi), a tachinid fly, is a natural parasite of Japanese beetles. If you find a Japanese beetle with small white specks on its shiny green shoulder region, do not kill it; those specks are the eggs of the winsome fly.

Some pesticide applications can kill beneficials for six or more months before they can rebound. Spot treatment of pesticides helps maintain beneficial populations; tolerating a low level of pest infestation also attracts and maintains beneficials. “It’s very well recognized that the flowering vegetation surrounding lawns has a major impact on how these beneficial insects will perform,” says Cowles. “They can be killed by insecticides used in the grass and on flowering plants surrounding the lawns.”

If we have followed these approaches and still have serious grub problems, it’s time to act. But think before you buy. Several common products found in garden centers can be extremely harmful: Diazinon and Dursban can be toxic to geese and related wild fowl; Sevin and Turcam are extremely toxic to earthworms; all of these products plus trichlorfon (Dylox) will kill beneficial insects. “These materials are inappropriate for homeowners to use to control most pests,” says Cowles.

Instead, choose one of following environmentally friendly approaches. All of them must be watered into the thatch and soil following application, partly to move the product downwards, and partly to bring grubs upwards in the soil so that they will interact with the product. Follow label directions and safety precautions.

There’s a small window of opportunity when it comes to killing grubs. Grubs are most vulnerable around early August, just after they have hatched from the eggs and are still relatively small. In spring (from April into May) they are large and fat, after overwintering in the soil, and harder to kill.

Imidacloprid
Imidacloprid is approved by the E.P.A. as nontoxic to humans and earthworms and has been sold to the homeowner as GrubEx and to the professional applicator as Merit.

Check the label on the GrubEx bag to verify that its active ingredient is imidicloprid. “Any imidacloprid product with the GrubEx label was manufactured last year,” says Cowles. In the future, imadicloprid will be marketed under a different label and newer bags of GrubEx will contain the halofenozide active ingredient.

“Imidacloprid is by far the outstanding white grub material for homeowners,” says Cowles. “It is environmentally soft, can be used at a low rate, and is effective against all grubs (although less effective against Asiatic garden beetle).”

Imidacloprid starts working 2 to 3 weeks after application and remains active for 3 months. When to apply it? “Ideally, at the end of July or early August, just as the eggs are hatched,” says Cowles. “However, applying the product in May or June also gives excellent results.” If you’ve had grub problems in previous years, go ahead and apply imidacloprid in spring. Otherwise, wait until the larvae have hatched and you can assess their population before applying it.

Lawn services tend to use imidacloprid at unnecessarily high levels. “A grub kill of 100% may not allow our native beneficial nematodes to survive,” says Cowles. The best strategy may be to use a very low rate of imidacloprid that will kill 70%-80% of the white grub population and sicken the remaining grubs. “A sick grub is extremely susceptible to our native beneficial nematodes,” says Cowles. “This gives you the best bang for your buck.”

Halofenozide
Halofenozide, an environmentally soft pesticide known as Mach2 (Molt Accelerating Compound), forces a larva into a molt before its normal time. Currently it is only available to professional applicators.

“Mach2 works extremely well against Japanese beetle, moderately well against oriental beetle, is of limited use against European chafer, and doesn’t work at all against Asiatic garden beetle,” says Cowles. It will be available to homeowners for the first time this year as the “new” GrubEx product.

The two best times to apply Mach2 are in early July, as a preventive treatment, and in early September after the larvae have hatched. Don’t apply it in spring.

Milky Disease
Various strains of this bacterium cause milky disease of Japanese beetle, oriental beetle, and in some locations European chafer (unfortunately, not in Connecticut) The spores are ingested by the grubs, penetrate through the gut, and eventually multiply within the body of the insect to such an extent that the larvae appear to die of starvation.

Because the disease eventually becomes established, further treatment may not be necessary. However, milky disease has not always worked well in Connecticut, perhaps because our summer soil temperatures do not remain warm enough long enough for the organism to be established. The product quality has been unreliable, and milky disease can take 3 or 4 years under conditions of high grub populations to build up to soil levels needed for control.

“The jury is still out on the effectiveness of milky disease,” says Cowles. If you want to try it, the best time is in August or September.

Beneficial Nematodes
Beneficial (entomopathogenic) nematodes are microscopic worms which feed on insect pests. They contain a bacteria within their bodies. The nematode releases the bacteria upon entering the host insect, and the bacteria actually causes the death of the insect. “There are native populations of beneficial nematodes in many locations in Connecticut,” says Cowles.

No single nematode species can control all turfgrass pests. Different nematode species carry different bacteria, use different hunting tactics and have different temperature activity ranges. Against white grubs, the Steinernema group has shown moderate to poor results in field testing. However, the more aggresive Heterorhabditis (Hb) group has shown fairly consistent results of 70-80% control, at rates of 500 million to 1 billion per acre.

Order nematodes a few weeks prior to use; most require refrigeration. Apply them on a rainy day either 2 weeks before or after applying either chemicals or fertilizers. Look for more information on nematodes at www2.oardc.ohio-state.edu/nematodes.

When’s the best time to apply nematodes? “Early to mid May,” says Cowles, “because this assures that there will be nematodes present when the next generation of grubs hatches in August. Also, we’re more likely to have adequate soil moisture in May than in August.”

So look for an imidacloprid product at your garden center or purchase some Hb nematodes. Although it’s certain that grubs are in your lawn, it’s also certain that you can control them with safe, ecologically sound products. If only that were true for death and taxes!

Pamela Weil is a Master Gardener certified by UConn’s Cooperative Extension System. She is the editor and publisher of Connecticut Gardener.

Connecticut Gardener
P.O. Box 248
Greens Farms, CT 06436
1-800-600-0476
email: editor@ conngardener.com