Plant Eating Beetles & Grimy Grubs
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Question. Dear Mike: Every year, we have an invasion of Japanese beetles; they destroy everything! I have heard there is an oil that may work against them. Can I get it in a store or do I have to order it from a catalog? Sincerely,
---Jennie, on the Eastern Shore of Virginia near the Maryland border
Mike: When I turned over the soil in one of my raised beds, I found more than a dozen grubs. Should I be concerned about my crops? Grubs have successfully devoured most of the lawns in my rural neighborhood.
---Jo Ann; Washington, NJ
Mike once described a homemade insect repellent for roses. I think it had garlic in it. Anyway, it really worked, but I've lost the recipe. Can you help before the Japanese beetles arrive? Thanks, ---Jane in Wilmington, Delaware
Answer. I wouldn't worry about grubs harming garden crops. These soil dwelling larval forms of a variety of different scarab beetles (Japanese, June, rose chafer, etc.) did most of their eating last year—in the late summer and early fall. Right now, they're mostly just hanging out until they morph into their flying defoliator adult form.
If, however, you still wish to dispatch them—perhaps in the hope of preventing damage by adults this summer—beetle grub expert Dr. Michael Klein, Adjunct Professor of Entomology at Ohio State University, suggests beneficial nematodes. Sold mail order by the multi-millions, they're packaged in little sponges or as water dispersible granules. Water them into moist soil with a watering can or sprayer early in the evening (NEVER in the heat of the day!) and they should decimate the grub population below within a few weeks—perhaps in time for those of you in the North to prevent some of this year's beetles.
The always-popular "Spikes of Death" are a more immediate and certain option. As I've often noted, those familiar 'lawn aerating sandals' are totally useless for aerating lawns—you need to remove plugs of earth to do 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 to the surface. Dr. Klein explains that you only have to nick a grub with the spikes, and those spikes can nick a lot of grubs; studies have shown a better knock down from Dirty Dancing than chemical insecticides.
The technique is so effective that New Zealand has approved 'spiking machines' for the control of what they call "pasture/lawn grubs". And that's an important term. Once upon a time, you could be confident that grubs feasting on grass roots were Japanese beetle babies. But so many other types of grubs have migrated into lawns, explains Dr. Klein, that in some places, Japanese beetles only make up 25 % of the population. "Grass death by grub", once only a Northeast worry, is now an equal opportunity scourge all across the country.
But again, as with purely Japanese beetles, the real damage to lawns is done in late summer/early fall, when the grubs are at their biggest, hungriest stage and feeding heavily on grass roots. This is doubly hard on the cool-season grasses grown in the North, because those lawns are already under lots of stress at that time of year from the summer heat they so despise. Just as in the Spring, you can use nematodes or Spikes of Death to protect your turf…
…or you can avoid the problem by making your lawn less of a target this summer. Dr. Klein explains that bad lawn habits—scalping and "compulsive over watering"&mdashare often to blame for grub damage. Cutting grass at the recommended height—on average, two inches down South and three in the North—makes it more attractive to the eye, and less attractive to egg laying females. And keeping it as dry as possible in late summer will cause lots of beetle eggs and grubs to desiccate and die.
(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 long term for the grass to let it take that summertime nap—it will green up again as soon as the weather cools and the rains return. Unfortunately, warm-season lawns need to be watered in late summer because they're actively growing. Try to have as light a hand as possible with the water and cut at the maximum suggested height to discourage egg laying.)
To control dastardly adults, like the legendary Japanese beetle (which continues to extend its range into warmer and more western climes), Dr.Klein urges you to do everything you can to prevent plant damage early in the season; that first feeding attracts LOTS of other beetles. Drape spun polyester row covers over your roses and other preferred plants when you see the first beetle (these diaphanous blankets allow light and water through, but block insect attack; "Reemay" is one brand name). Or try spraying the plants with a repellent; I'm thinking that those garlic sprays sold for mosquito control might work well.
And here's that home made insect repellent recipe:
In a blender, whiz up 1 hot pepper and 1 clove of garlic in a pint of water, strain, pour into a sprayer that has never held chemicals of any kind, add a drop of dish washing soap and a drop of vegetable oil and spray on the plants, shaking repeatedly. Always spray early in the morning.
Beyond repellents? Dr. Klein thinks that insecticidal soap is one of the best non-toxic pesticides for beetles; just remember—soap has to coat an insect to kill it; spraying the leaves just gives you soapy leaves. And if you go this route, I strongly recommend you BUY a commercial insecticidal soap instead of trying to make your own; homemade versions often kill more plants than pests. The situation is similar with "Oil sprays"; they work the same as soap—the oil has to coat the bug to put it to bed. Be sure you use a light, vegetable oil-based spray if you go this route; the basic variety of "horticultural oil" you'll find on store shelves is a petroleum based product designed to be sprayed on trees while they are dormant in the dead of winter—not on tender plants in the summertime.
Don't underestimate the low-tech method of knocking the beetles off your plants with a broom and into a big pan with a little soapy water in the bottom first thing in the morning, when they're sluggish. (This is almost as emotionally rewarding for rose growers as the Spikes of Death). You can also vacuum them off your plants.
Remember that beetle traps should never be placed anywhere near the plants you wish to protect or any plants that are especially attractive to beetles; you'll wind up drawing more of the pests to your landscape. Dr. Klein feels that traps may have value intercepting beetles when placed around the perimeter of your property, but warns against this if there's a golf course or other huge grassy area over there; you'll simply attract too many beetles.
And finally, here's a link to last week's discourse on milky spore disease for grub control; and a previous Question of the Week detailing lots of other adult beetle options.
You Bet Your Garden Question of the Week © 2006 Mike McGrath