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How to make your lawn grub-proof

Friday - 5/24/2013, 9:23am  ET

Try BT or BTK to rid your trees of caterpillars, without harming the tree or your pets. (Thinkstock)

Don't cut the grass this weekend

WTOP Garden Editor Mike McGrath


WASHINGTON - The big holiday weekend is getting off to a somewhat soggy start, and that calls for a special lawn care warning alert: Do not cut your lawn while the grass is still wet.

Cutting a wet lawn tears the grass blades apart, instead of cutting them cleanly, and that makes the grass much more vulnerable to injury when summer heat arrives. So let the lawn dry out for at least a day before you mow. And don't mow first thing in the morning when the grass is wet with dew. If cut grass is sticking to your shoes, the lawn is too wet to mow.

Equally important is to cut with a super, sharp blade. A dull mower blade tears the grass up just as badly as cutting when wet. So if you haven't sharpened or replaced your blade this season, do so before the next cut.

Cut sharp and cut dry in the spring, and your lawn will stay green and bright when summer heat settles in.

Milky spore vs. lawn grubs

Matt in Laurel has a lot of questions about lawn grubs. He writes: "Late last summer I discovered a bad grub infestation in my lawn and treated it with a combination of milky spore powder and Dylox (Bayer's 24-hour grub killer). Now, I would like to try and preventů"

Whoa! Let's stop right there, Matt.

Milky spore is a naturally occurring soil organism that's deadly to lawn grubs if they ingest it while feeding on the roots of your grass. So you did apply it at the right time of year, as grubs only feed in the summer. (Although stores sell milky spore powder in the spring, it doesn't work in the spring.)

But to achieve its legendary decades of future protection against grubs, milky spore must have lots of live grubs to infect. But you killed those grubs with that toxic chemical grub killer, negating any positive effect from the milky spore.

Springtime grub control is cultural, not chemical

OK. We'll let Matt in Laurel continue his grubby tale. He writes: "Last summer I discovered a grub infestation (after much of my grass had died), and dealt with it with a combination of milky spore and Dylox (Bayer's 24-hour grub killer). I would like to try to prevent an infestation this year and have heard that Scott's GrubEx (chlorantraniliprole) is helpful? I was wondering if you have any suggestions for the timing of application."

Yes, Matt. How about never? Toxic, chemical grub killers are incredibly dangerous and totally unnecessary. The best way to prevent female Japanese beetles from infesting your turf with their grubby little babies is to make your lawn unattractive as a nursery. That means cutting your grass at the correct height, never lower than 3 inches, and watering deeply but infrequently during weeks when we don't get an inch of rain.

Female beetles look for scalped, wet lawns in which to lay their eggs. They want to be able to reach the soil easily, and they must have nice moist earth in which to lay those eggs, which are prone to dehydration. Short cuts and frequent watering virtually guarantee that your turf will play host to lots of bouncing baby beetles.

But a turf that's correctly cared for won't appeal to a female beetle's motherly instincts. And that lawn will look better, and stay greener this summer to boot.

Make your lawn grub-proof this season

Matt in Laurel continues: "I would like to apply milky spore powder again this season, as I've heard it takes several years of consistent applications to build up a population in the soil. When is the best time to apply it?"

Milky spore powder, a concentrated natural soil organism that kills grubs while they are actively feeding on lawn roots, should be applied about a month to six weeks after you notice the adult beetles feeding on nearby plants. That's when any eggs the female beetles have deposited in your lawn will have hatched, and the grubs will actively feed on the roots of your turf.

Apply the powder as directed on the packaging and don't use any other insecticides on your lawn. Milky spore needs to be ingested by live grubs to be able to spread throughout your lawn and build up the "population" to which you refer. Every live grub that ingests a single spore of this disease (a disease that only affects the larval stage of beetles) becomes a kind of milky spore factory, spreading this amazing natural substance throughout your turf, and maintaining a healthy population of the beneficial organism for years to come.

Apply the powder in mid to late summer to an otherwise insecticide-free lawn and your turf might become grub-proof for decades to come. That's right, decades. Some lawns that were treated back in the 1950s still test positive for milky spore and negative for grubs.

Bt is the answer to caterpillar problems

Susan in Greenbelt writes: "I need something to treat pests that are attacking a young river birch, something that won't hurt my dogs. The pests might be stink bug larvae, as they look like very small caterpillars. They line up around the edge of a leaf and eat their way in until the leaf is gone."

Then they are small caterpillars, Sue. Baby stink bugs look like, well, baby stink bugs.

And no matter what kind of caterpillar is doing the munching, the answer is easy: Bt. One of the oldest and safest organic pest controls, it is also known as BTK, and sold under brand names that include Dipel, Thuracide, and Green Step. Bt quickly kills any caterpillars that eat the sprayed leaves, but harms nothing else — not you, your dogs or wildlife. It doesn't even harm adult butterflies. Bt only affects caterpillars, and it only affects caterpillars that chew on the sprayed leaves. It may well be the safest and most targeted insecticide ever discovered.

Note: BTI, the organic control that prevents mosquito breeding, is a different product. BTI will not stop caterpillar infestations. And Bt won't stop mosquito breeding.

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