Lawn Double Trouble: Grubs and Crabgrass

March 24th, 2005

   Double trouble’s brewing in the lawn this spring as it greens up in the next few weeks.

   Grubs soon will emerge from their winter siesta to resume feeding on lawn roots, while last year’s bountiful crop of crabgrass seeds is about to sprout and create a potentially uglier sequel to last year’s weedfest.

   Now’s the time to plan your attack so these two lawn villains don’t abuse you once again.

A Japanese beetle grub.

   First the grubs.

   These are those fat, white, C-shaped wormy-like critters that ultimately turn into beetles. In this larval stage, they can kill off whole patches of lawn by eating the roots out from under it.

   That’s precisely what happened to many area lawns early last fall.

   “It was a big problem that snuck up on us,” says Jim Welshans, Penn State Extension’s Capital Region turfgrass educator. “A lot of people were asking, ‘How come we had such a grub problem? I didn’t see that many Japanese beetles.'”

   One reason was the plentiful rain. Beetle eggs hatch best in damp soil. And when each female lays 40 to 60 eggs, it doesn’t take that many beetles to produce a hefty grub colony.

   The other sneakier explanation is that many of the grubs weren’t Japanese beetle grubs at all but the larval stage of the similar masked chafer beetle.

   “This is the one I found most of,” says Welshans. “This critter comes out at night as an adult beetle. Masked chafers don’t feed on plants, and they’re only out for about two weeks in July, so people don’t notice them.”

   Masked chafers also are prolific egg-layers and produce gobs of grubs that look nearly identical to Japanese beetle grubs. Both also can chew up a lawn in short order.

   If that’s not bad enough, skunks often move in and start digging for grubs, which are a favorite skunk delicacy.

   “Once skunks get in there and start tearing away,” says Welshans, “that area’s probably shot, and it’s probably going to have to be redone.”

   Fortunately, treatments for these two grubs are also similar, with the exception that the organic milky-spore treatment works only on Japanese beetle grubs, not masked chafer grubs. (See the sidebar below for more on that.)

   If you had enough grubs last fall to notice lawn damage and didn’t deal with them then, it’s going to be even more difficult to kill the full-grown grubs when they begin feeding again in April. (Grubs spend the winter about 2 feet underground, then move back up to near the surface as the soil warms before pupating in May and finally emerging as new adult beetles in July.)

   “You’re going to have to use the higher rate (on the insecticide label),” says Welshans, “and use a lot of moisture to get the material down into the roots where the grubs are feeding.”

   Welshans suggests Dylox (trichlorfon) as the grub-killer of choice for quick action, although carbaryl (Sevin) also is fairly effective. He also points out that if you’ve got a thatch layer of more than three-quarters of an inch, it’s going to be nearly impossible to get the chemical to where it’s needed. (Thatch is the layer of dead, matted grass and roots between the live grass and the soil. It can be raked up or torn up with a dethatching machine.)

   As a practical matter, Welshans says he’d probably skip trying to kill this year’s grubs and focus on knocking out next year’s. “The spring feeding time is fairly short anyway,” he adds.

   The best way to deal with next year’s grubs, he says, is by applying a treatment of either Merit (imidacloprid) or Mach 2 (halofenozide) sometime late May through June. (July is still OK, too.)

   Once those products are watered or rained in, they’ll be in the root zone when the new crop of beetle eggs hatches. The down sides: Merit and Mach 2 have to be re-applied each year, and neither is very effective if you wait until you see damage in the fall to use them.

   For organic gardeners, a new option is a type of ground-dwelling nematode (Heterohabditis bacteriophora) that feeds on grubs. These also are effective only for one year and are best applied in early fall. (One source: the Gardens Alive catalog at www.GardensAlive.com or 513-354-1482.)

   Once your grub battle plan is in place, think crabgrass.

Crabgrass that's already produced seedheads.

   This also was a weather-related nightmare last year. Warm temperatures and consistent summer moisture kept crabgrass seeds germinating even beyond the normal effective period for most crabgrass-preventing chemicals.

   “It was the perfect storm for crabgrass,” says Nancy Bosold, Welshans’ Extension counterpart in the Southeast Region.

   The really bad news is that crabgrass seed can last for 15 years, and each plant can put out tens of thousands of new crabgrass seeds. A small outbreak can quickly turn into a big problem.

   The good news, though, is that crabgrass is an annual weed that dies out over winter. All of the new crabgrass we’ll get this year will come from prior years’ seed.

   Bosold says that gives us our best offense – using a pre-emergent herbicide to go after the seeds as they germinate.

   “Pre-emergents all need to be watered into the top quarter inch of the lawn,” she says. “When the seed tries to emerge in this coating, the chemical inhibits that.”

   Many products are available to prevent crabgrass, such as the long-acting Barricade (prodiamine), Balan (benefin), bensulide (Betasan), Team (benefin and trifluralin) and Pendulum (pendimethalin).

   For organic gardeners, corn gluten meal also is a decent option. Bosold says it’s about 50 percent effective in the first spring but becomes more effective if two applications are used each year (early spring and late summer).

   No matter which weed preventer is used, it’s critical to get it down and watered into the surface before the crabgrass seeds begin germinating.

   Bosold says a good timing indicator is when forsythia is in full bloom. That’s toward the end of March in a typical Harrisburg year.

   “The main reason pre-emergents fail is timing,” she says. “It’s applied either too early or too late.”

   Once crabgrass is up, it’s much tougher to kill. Hand-pulling is effective for a few plants here and there, but after-the-fact crabgrass killers are expensive, more toxic than pre-emergents and not as effective.

   The arsenic compounds that homeowners can buy (MSMA and DSMA) also tend to discolor lawns even if you apply them soon enough to work.

   Commercial applicators have several other less-toxic and more effective options (such as Drive and Acclaim Extra), but these are usually more expensive and less effective than early-spring crabgrass preventers.

   The best defense of all, says Bosold, is a thick lawn. If you cut the grass high, fertilize correctly, overseed and follow other good lawn-care practices, she says, there won’t be much room for crabgrass to take hold.

   That’s also assuming the grubs haven’t already eaten all of your grass.

 SIDEBAR 1

   Ticked off that your expensive and supposedly long-term milky-spore treatment for grubs didn’t work last fall?

   It could be that you were going after the wrong grub.

A masked chafer beetle in a compromising, raster-exposed position.

   Milky-spore disease is an organic granular product that’s harmless to birds, pets and people but deadly to Japanese beetle grubs – supposedly for 10 or more years with one application.

   The problem is it doesn’t work on masked chafer beetle grubs, a similar insect that also had a banner year last year.

   Masked chafers are brown beetles with a little shield over the back of their heads – kind of like mini Darth Vaders. They come out at night for about two weeks in July and don’t feed on plants, so people seldom notice them.

   Japanese beetles, on the other hand, have shiny coppery shells and feed heavily on 300 different types of landscape plants in broad daylight. So they’re VERY noticeable.

   As grubs, masked chafers and Japanese beetles look alike and do similar damage to lawns.

   The only way to tell the difference at the grub stage is by looking at the hairy projections on the grub’s “raster” (grub-speak for “butt”). Japanese beetle grubs have V-shaped hair patterns while masked chafers have no pattern.

   Evaluating grub butt-hairs with a magnifying glass isn’t necessary if you’re planning to kill them with chemicals or nematodes. Those controls are the same for both types of grubs.

   But if you’re using the milky-spore approach, that product is effective on Japanese beetle grubs but not masked chafer grubs.

   Penn State Extension turfgrass educator Jim Welshans also cautions that it can take three to five years for milky spore to become fully effective. “If you want quick relief from grubs,” he says, “this is not the way to go.”

   And if you’ve got masked chafers instead of Japanese beetles, it’s also not the way to go.

 SIDEBAR 2

   Got grub damage?

   If grubs and ensuing skunks tore up sections of lawn last fall and you didn’t repair the damage then, the next few weeks are ideal for patching.

   Remove the dead turf (it’ll pull up like carpet) and loosen the top 4 to 6 inches of the soil with a digging fork.

   Then rake smooth and either lay new sections of sod or sprinkle new grass seed over the surface.

   If you’re using sod, tamp it down or roll it to ensure good root contact with the soil.

   If you’re seeding, lightly rake the seed into the surface and cover with a light layer of straw or chopped leaves.

   With both methods, make sure the soil is consistently moist for at least the next three weeks.

   As an added precaution against grubs feeding on the new grass roots in April as the overwintering grubs return to the surface, consider applying a treatment of Dylox (trichlorfon) to the soil’s surface. Dylox is a quick-acting chemical grub-killer.