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What is Milky Spore Disease?

Grubs of Japanese beetles can wreak havoc on turf grass--and several natural control methods are available. Milky spore, also called milky spore disease, is a naturally occurring organism in the soil. When grubs ingest the milky spore bacteria, they die from the infection. Read below to learn more about what is milky spore, milky spore treatment, how to apply milky spore powder and other tips for milky spore grub control.

What are grubs and how do I know I have them?

Japanese beetle grubs are soft-bodied, c-shaped, off-white and up to 1 inch long. Because they live in the soil, most people discover grub infestations by the damage that they do. The Japanese beetle grubs feed on grass roots, causing the grass to brown and die.

Common signs of grub infestation are dead patches of grass. The turf may also have a spongy feel to it. Another sign may be birds, including robins and crows, and raccoons hunting and digging for Japanese beetle grubs in the lawn.

When To Apply Milky Spore Disease?

Because grubs must ingest the milky spore bacteria, milky spore powder should be applied when the grubs are most actively feeding—and when soil temperatures are 60-70 degrees F. In most areas, the best time to apply milky spore is in late summer to early fall.

How To Apply Milky Spore Disease

Apply 1 teaspoon of milky spore powder every 4-feet in a grid pattern in your lawn. Water gently and thoroughly. You want the powder to soak into the soil and not wash away. If using a hose, make sure it is on the gentle setting.

Be patient. Milky spore doesn't kill grubs instantly. They have to ingest it—and it may be weeks, months or even a few years before you see noticeable results.

Milky spore disease remains in the soil for years, so a repeat treatment isn't needed for several years.

The Cons of Milky Spore Treatment

While milky spore treatment can be effective for Japanese beetle grub control, it has some downsides.
  • The treatment can take time before you see noticeable results. Grubs must ingest the bacteria and be infected by it.
  • The soil temperature must be 65 degrees or above for the treatment to work. In some areas, the soil temperature may not be at the ideal temperature during prime grub-feeding season.
  • The treatment only works on juvenile grubs.

Advice from Our Expert

Q. OK, you have convinced me to wait until the fall to plant new grass seed. But how about the grass I have now? Is it too late to put down Milky Spore? When is the best time to assure it will 'take' in the soil for the long term? Thanks,
            ---Jaan in Yardley PA

Our lawn was so infested with grubs that we dug the whole thing up and are having new sod put in. Is there something I should put in the soil to kill any remaining grubs before the sod is laid? Thank you,
            ---Joan in Ottawa, Ontario; CANADA

Mike: I have neighbors who are going to apply milky spore on their lawn to kill grubs; they say applying it three times a year will rid my yard of grubs after three years. But I couldn't find it for sale at the Gardens Alive website, so I'm thinking it might not be that good. What IS the story with this stuff?
            ----Tony in McLean, Virginia

A. Tony: I don't work for Gardens Alive; they simply host my Question of the Week. But I did ask the folks at GA—who seem to carry just about every other natural pest control—why they don't carry milky spore. They explained that they had heard great things about its ability to control the grubs of Japanese beetles in turf grass, but had also heard about recent tests indicating it may only work in the lab. A little checking around revealed that there's quite a bit of disagreement about this stuff in the research world.

So I called THE authority on Japanese beetles and their grubs, Dr.Michael Klein, Adjunct Professor of Entomology at Ohio State University and former Lead Scientist for what was known for many years as the USDA"Japanese beetle lab" and is now called the "Horticultural Insects Unit". Dr. Klein explained that when Japanese beetles entered the county (on a shipment of plants to Riverton, New Jersey sometime prior to their discovery in 1916) they were rare in their native country, and considered good luck because of their beautiful green and gold'finery'.

Their famed natural enemy was discovered—also in New Jersey—in the1930s. Although many of us call this stuff "Milky Spore", Dr. Klein explains that that's actually a brand name; the correct generic term,he says is "milky disease". Anyway, it appears that this naturally occurring soil organism was already in the Jersey dirt, rather than coming over with the beetles. (Until very recently, nobody had even found it in Japanese soils.)

The name isn't the only thing we've been getting wrong, says Dr. Klein;a lot of misinformation has been whispering down the lane here…  

Misconception #1: "Milky spore(disease) ONLY works on JAPANESE beetle grubs.
Dr. Klein explains that although it does work best against Japanese beetle babies, some strains have been shown to infect other whitegrubs—which is good, because other beetle grubs are learning how much fun it is to live in turf.

Misconception #2: "The disease just has to be in the soil to work."
Dr. Klein explains that very specific conditions must exist for the disease to do its job: To become infected, a grub has to be actively feeding in warm soil and ingest some spores. Just being in the same dirt as the disease doesn't harm grubs, and if the soil is cooler than65 degrees, the spores just pass right thru without harm.

Although the distinctive crescent shaped grubs we find in lawns and gardens already look pretty milky, grubs that are infected with the disease look even milkier, he explains. If you want to be sure, clip off a leg; the fluid will run clear from a healthy grub and milky white from an infected one. Sounds like you're checking to see if a turkey is done.

Anyway, although the number of variables involved makes it somewhere between hugely difficult and totally impossible to prove conclusively,Dr. Klein feels that milky disease DOES work naturally in many areas,and should be able to be introduced successfully in areas that meet the necessary requirements of soil temperature and grubs.

And at least one piece of information people have been dispensing about milky disease IS correct—it lasts as long as its reputation.Researchers have found the disease—which affects no other creatures besides grubs—still active in soils that were treated decades ago.

The more grubs in the soil when you apply it the better, as infected grubs breed more of the disease. The best time to infect large numbers is in early Fall, when the grubs are in nice warm dirt, chewing grassroots madly to put on fat for the wintertime. So applying a concentrated form of the disease (isolated from actual grubs and available in bags and shaker cans at most garden centers) anytime over the summer would seem best. Just don't use any other grub-killers,warns Dr. Klein, or the milky disease spores won't have anything to infect.

Repeated applications shouldn't be necessary if there are a good number of grubs in the soil to become infected. Three times total seems excessive, much less three times a year. As you've always heard, it takes several years to build up enough disease spores in your soil to make a noticeable difference—around three in the Philly-DC area; five up in New England and Canada.

Don't worry about existing grubs in the Spring. Any nibbling they may do after rising to the surface in preparation for their final metamorphosis into the flying defoliators we know so well is pretty inconsequential, AND the Northern grasses that house the vast majority of beetle grubs (at least so far) are growing at a rapid pace in the Spring. The real damage is done to these cool-season turfs in the Fall,when the grass (which thrives in cool weather but can barely tolerate a really hot and dry July and August) is weak from summer heat stress and the grubs are truly voracious.

If you want to eliminate grubs now in the hope of reducing adult beetle damage this summer, two of Dr. Klein's favorite non-chemical treatments are beneficial nematodes and the legendary Spikes of Death. We'll detail those options and talk about the different adult forms and the damage they cause in Part Two of this Special Report next week.

You Bet Your Garden   Question of the Week ©2006 Mike McGrath

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