Beetles and Grub Butts

June 26th, 2012

   We’re at the end of June already, and for gardeners, that means one thing — it’s Japanese beetle time.

Japanese beetles swarming on a rose -- their favorite.

   This shiny, copper-colored, leaf-eating pest usually shows up around now and feasts on upwards of 300 different plant species for about 6 weeks.

   Japanese beetles are particularly annoying because they’re a double-duty yard pest. They’re actually more destructive in the larval stage, when the grubs eat the roots out from underneath the lawn.

   I get a lot of questions about what to do to head off beetle/grub damage. Lately, people have been asking about whether an organic approach — using milky spore powder — really works.

   From what I’ve seen and read, milky spore has had spotty results. It seems to work fairly well sometimes in some areas but hardly at all in other cases. One little-known problem that can explain why it doesn’t work is having the wrong kind of grub to start with.

   Milky spore is a disease that’s effective only against the Japanese beetle grub. If your lawn damage is due to the grub of the masked chafer beetle — also fairly common around here — that would explain why your treatment has been ineffective.

   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 as adults.

   Japanese beetles have shiny coppery shells and feed heavily on landscape plants in broad daylight. So they’re VERY noticeable. Some years they aren’t as bad as others. Last year, for example, I didn’t hear of too much trouble.

   At the grub stage, masked chafers and Japanese beetles look alike and do similar damage to lawns. They’re both fat, white, C-shaped wormy-like things. The only way to tell the difference 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. So to be sure which you’ve got, you’ll have to capture one, get out a magnifying glass and have a closeup look at the butt hairs.

   If it turns out you’ve got masked chafer grubs, you’ll have to switch to another treatment. If you’ve got Japanese beetle grubs and you’ve given milky spore at least three years to take hold, then it’s just not working.

What kind of grub is this? Only its butt hairs know for sure.

    The most effective alternative is a grub-preventing chemical applied to the lawn, ideally in June or early July to stop the next generation.

   Merit (imidacloprid) and Mach 2 (halofenozide) are the two you’re most likely to find at the garden center and home stores. You might also run into a product containing thiamethoxan. And newer still is an insecticide called chlorantraniliprole, which takes longer to become effective and is probably best applied in early May. I have no experience on either of the newcomers.

   To kill existing grubs, you’ve got two chemical choices. One is Dylox (trichlorfon), which is a quick-acting chemical insecticide that kills both kinds of grubs. The other is Sevin (carbaryl). Both of these work best in fall when the grubs are still small. It’s pretty hard to kill big, fat, adult grubs in spring no matter what you use (short of a hammer).

   What effect do these products have on earthworms and the environment in general? That depends on whom you believe. As with practically all pesticides, you’ll find a variety of studies, and even more confusing, a wide range of opinions on what the studies mean.
   From what I can decipher, imidacloprid isn’t as toxic as many other insecticides. It rates fairly low in toxicity to mammals (one study showed thyroid lesions in rats), but has been shown to be acutely toxic to earthworms in lab tests (deformities in sperm and DNA). However, another study showed that in field tests, regular applications of imidacloprid did NOT have much impact on the worm population (although another one showed they didn’t burrow as deeply).
   A bigger concern lately is imidacloprid’s effect on honeybees. That’s not a big threat when you’re applying the chemical to lawn soil, but in a flower bed, some imidacloprid probably will make its way into the pollen of the flowers. Bees and butterflies may consume it that way.
   I’ve only seen one study on halofenozide and earthworms, and it determined there’s little risk. I don’t know about the other two preventers. Carbaryl is believed to be toxic to earthworms, trichlorfon supposedly is not.

   Pretty confusing, eh? Personally, I’d rather not get confused or take a chance on something that may be more destructive than helpful. What really gets me is when a product is sold for years, and then it’s later yanked from the market as being too risky after we’ve all dumped it everywhere. That’s why I pretty much skip all of this and just strip off any dead turf and reseed it when damage happens. Most years I get little to no damage anyway.

   One other anti-grub option is biological control. There’s a microscopic nematode called Heterohabditis bacteriophora that feeds on grubs. I don’t have any first-hand experience with them, but I’ve seen reports and studies claiming these work reasonably well against both chafer and Japanese beetle grubs.

   Nematodes are significantly more expensive and will need to be applied late each summer. On the plus side, they’re a targeted organism that won’t pollute or harm people, pets, earthworms or other beneficial soil organisms.

   A few local garden centers carry them (I think Highland Gardens in Camp Hill, for one), or you can order them from organic-products catalogs, such as Gardens Alive (see www.gardensalive.com/article.asp?ai=811 for details).

   And as for what to do about the adult Japanese beetles if they start swarming shortly in your yard, here’s a piece I wrote earlier on that: https://georgeweigel.net/favorite-past-garden-columns/battling-japanese-beetles.