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Question. Dear Mike: Every year, wehave an invasion of Japanese beetles; they destroy everything! I haveheard there is an oil that may work against them. Can I get it in astore or do I have to order it from a catalog? Sincerely,
---Jennie, onthe Eastern Shore of Virginia near the Maryland border
Mike: When I turned over the soil in one of my raised beds, I foundmore than a dozen grubs. Should I be concerned about my crops? Grubshave successfully devoured most of the lawns in my ruralneighborhood.
Mike once described a homemade insect repellant for roses. I think ithad garlic in it. Anyway, it really worked, but I've lost the recipe.Can you help before the Japanese beetles arrive? Thanks, ---Jane inWilmington, Delaware
Answer. I wouldn't worry about grubsharming garden crops. These soil dwelling larval forms of a variety ofdifferent scarab beetles (Japanese, June, rose chafer, etc.) did mostof their eating last year—in the late summer and early fall. Right now,they're mostly just hanging out until they morph into their flyingdefoliator adult form.
If, however, you still wish to dispatch them—perhaps in the hope ofpreventing damage by adults this summer—beetle grub expert Dr. MichaelKlein, Adjunct Professor of Entomology at Ohio State University,suggests beneficialnematodes. Sold mail order by the multi-millions, they're packagedin little sponges or as water dispersible granules. Water them intomoist soil with a watering can or sprayer early in the evening (NEVERin the heat of the day!) and they should decimate the grub populationbelow within a few weeks—perhaps in time for those of you in the Northto prevent some of this year's beetles.
The always-popular "Spikes of Death" are a more immediate and certainoption. As I've often noted, those familiar 'lawn aerating sandals' aretotally useless for aerating lawns—you need to remove plugs of earth todo that. But they are perfect for dancing on your lawn to kill grubs,especially in late spring, when the big fat beetle babies are close tothe surface. Dr. Klein explains that you only have to nick a grub withthe spikes, and those spikes can nick a lot of grubs; studies haveshown a better knock down from Dirty Dancing than chemicalinsecticides.
The technique is so effective that New Zealand has approved 'spikingmachines' for the control of what they call "pasture/lawn grubs". Andthat's an important term. Once upon a time, you could be confident thatgrubs feasting on grass roots were Japanese beetle babies. But so manyother types of grubs have migrated into lawns, explains Dr. Klein, thatin some places, Japanese beetles only make up 25 % of the population."Grass death by grub", once only a Northeast worry, is now an equalopportunity scourge all across the country.
But again, as with purely Japanese beetles, the real damage to lawns isdone in late summer/early fall, when the grubs are at their biggest,hungriest stage and feeding heavily on grass roots. This is doubly hardon the cool-season grasses grown in the North, because those lawns arealready under lots of stress at that time of year from the summer heatthey so despise. Just as in the Spring, you can use nematodes or Spikesof Death to protect your turf…
…or you can avoid the problem by making your lawn less of a target thissummer. Dr. Klein explains that bad lawn habits—scalping and"compulsive overwatering"—are often to blame for grub damage. Cuttinggrass at the recommended height—on average, two inches down South andthree in the North—makes it more attractive to the eye, and lessattractive to egg laying females. And keeping it as dry as possible inlate summer will cause lots of beetle eggs and grubs to desiccate anddie.
(Don't worry if your cool-season lawn turns brown from lack of water;that's its natural summer dormancy response, and its better in the longterm for the grass to let it take that summertime nap—it will green upagain as soon as the weather cools and the rains return. Unfortunately,warm-season lawns need to be watered in late summer because they'reactively growing. Try to have as light a hand as possible with thewater and cut at the maximum suggested height to discourage egg laying.)
To control dastardly adults, like the legendary Japanese beetle (whichcontinues to extend its range into warmer and more western climes), Dr.Klein urges you to do everything you can to prevent plant damage earlyin the season; that first feeding attracts LOTS of other beetles. Drapespun polyester rowcovers over your roses and other preferred plantswhen you see the first beetle (these diaphanous blankets allow lightand water through, but block insect attack; "Reemay" is one brandname). Or try spraying the plants with a repellant; I'm thinking thatthose garlic sprays sold for mosquito control might work well.
And here's that home made insect repellant recipe:
In a blender, whiz up1 hot pepper and 1 clove of garlic in a pint ofwater, strain, pour into a sprayer that has never held chemicals of anykind, add a drop of dishwashing soap and a drop of vegetable oil andspray on the plants, shaking repeatedly. Always spray early in themorning.
Beyond repellants? Dr. Klein thinks that insecticidal soap is one ofthe best non-toxic pesticides for beetles; just remember—soap has tocoat an insect to kill it; spraying the leaves just gives you soapyleaves. And if you go this route, I strongly recommend you BUY acommercial insecticidal soap instead of trying to make your own;homemade versions often kill more plants than pests. The situation issimilar with "Oilsprays"; they work the same as soap—the oil has to coat the bug toput it to bed. Be sure you use a light, vegetable oil-based spray ifyou go this route; the basic variety of "horticultural oil" you'll findon store shelves is a petroleum based product designed to be sprayed ontrees while they are dormant in the dead of winter—not on tender plantsin the summertime.
Don't underestimate the low-tech method of knocking the beetles offyour plants with a broom and into a big pan with a little soapy waterin the bottom first thing in the morning, when they're sluggish. (Thisis almost as emotionally rewarding for rose growers as the Spikes ofDeath). You can also vacuum them off your plants.
Remember that beetletraps should never be placed anywhere near the plants you wish toprotect or any plants that are especially attractive to beetles; you'llwind up drawing more of the pests to your landscape. Dr. Klein feelsthat traps may have value intercepting beetles when placed around theperimeter of your property, but warns against this if there's a golfcourse or other huge grassy area over there; you'll simply attract toomany beetles.
And finally, here's a linkto last week's discourse on milky spore disease for grub control;and a previous Questionof the Week detailing lots of other adult beetle options.
You Bet Your Garden Question of the Week © 2006Mike McGrath