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

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

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

A. Tony: I don't work forGardens Alive; they simply host my Question of the Week. But I did askthe folks at GA—who seem to carry just about every other natural pestcontrol—why they don't carry milky spore. They explained that they hadheard great things about its ability to control the grubs of Japanesebeetles in turf grass, but had also heard about recent tests indicatingit may only work in the lab. A little checking around revealed thatthere's quite a bit of disagreement about this stuff in the researchworld.

So I called THE authority on Japanese beetles and their grubs, Dr.Michael Klein, Adjunct Professor of Entomology at Ohio State Universityand former Lead Scientist for what was known for many years as the USDA"Japanese beetle lab" and is now called the "Horticultural InsectsUnit". Dr. Klein explained that when Japanese beetles entered thecounty (on a shipment of plants to Riverton, New Jersey sometime priorto their discovery in 1916) they were rare in their native country, andconsidered 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. Kleinexplains that that's actually a brand name; the correct generic term,he says is "milky disease". Anyway, it appears that this naturallyoccurring soil organism was already in the Jersey dirt, rather thancoming over with the beetles. (Until very recently, nobody had evenfound 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 Japanesebeetle babies, some strains have been shown to infect other whitegrubs—which is good, because other beetle grubs are learning how muchfun it is to live in turf.

Misconception #2: "The diseasejust has to be in the soil to work."
Dr. Klein explains that very specific conditions must exist for thedisease to do its job: To become infected, a grub has to be activelyfeeding in warm soil and ingest some spores. Just being in the samedirt 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 andgardens already look pretty milky, grubs that are infected with thedisease look even milkier, he explains. If you want to be sure, clipoff a leg; the fluid will run clear from a healthy grub and milky whitefrom an infected one. Sounds like you're checking to see if a turkey isdone.

Anyway, although the number of variables involved makes it somewherebetween 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 thenecessary requirements of soil temperature and grubs.

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

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

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

Don't worry about existing grubs in the Spring. Any nibbling they maydo after rising to the surface in preparation for their finalmetamorphosis into the flying defoliators we know so well is prettyinconsequential, AND the Northern grasses that house the vast majorityof beetle grubs (at least so far) are growing at a rapid pace in theSpring. 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 areally hot and dry July and August) is weak from summer heat stress andthe grubs are truly voracious.

If you want to eliminate grubs now in the hope of reducing adult beetledamage this summer, two of Dr. Klein's favorite non-chemical treatmentsare beneficial nematodesand the legendary Spikes of Death. We'll detail those options and talkabout the different adult forms and the damage they cause in Part Twoof this Special Report next week.

You Bet Your Garden   Question of the Week ©2006 MikeMcGrath

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