Bermudagrass stunt mite in lawns

The tufted witches broom symptoms of BSM in
are seen in the stem on the left.  Normal
bermudagrass stem on the right. Photo: M. Merchant.

I don't often address pest issues in lawns and on ornamental plants; but many Insects in the City readers do include lawn care services. So I thought I would address a lesser known turfgrass pest problem that seems to be on the increase.

Bermudagrass stunt mite (BSM), Eriophes cynodoniensis, is one of our tiniest arthropod pests of ornamental plants.  It lives inside the leaf sheaths of grass and is a common (but relatively minor and spotty) pest of home lawns. On golf course greens, where expectations of smooth putting surfaces are high, BSM is a serious pest throughout the southern U.S. According to some experts, the incidence of this pest appears to be on the rise--possibly because of the loss of older insecticides, a trend toward higher mowing heights and less irrigation, and possibly the use of newer, more susceptible grass varieties.

The BSM feeds only on bermudagrass (though there are closely related species that feeds on buffalograss and zoysiagrass). When the mite feeds under the leaf sheaths the leaves start to yellow and twist. As the grass tries to grow, the gaps between the leaves get shorter and shorter, resulting in a bunchy, "witches broom" appearance. Eventually the leaves and stems die, probably as a result of a toxin injected into the grass by the mite.  Look for areas of stunted, green to brown grass, and dead spots in a lawn. You can identify BSM damage by the stunted, tufted appearance of the grass around the edges of the dead spots.

Damage often seems to occur in grass that is stressed from not receiving enough water.  In my own yard for several years I commonly saw BSM damage in the median strip between sidewalk and street. Since upgrading my sprinkler system, however, I see fewer signs of these mites.

Dr. J.C. Chong, of Clemson University, got interested in this mite about 10 years ago, and found no one else studying it.  Today he has devoted as much time as anyone to studying these tiny pests.  His knowledge about BSM is in especially high demand this year, he says.

"I am getting more requests for diagnosis and confirmation from golf courses and high-end landscapes in Florida, the Carolinas and Texas in the past 3 months than the entire [2016] combined."

Damage from BSM appears as brown, dead patches.  Examine the borders of
these spots closely to look for the tufted, stunted plants typical of the mite.

According to Chong, the last time anyone bothered to studying control of BSM was in the early 1980s. At the time, the best insecticide by far was diazinon. Now that diazinon is no longer available due to environmental concerns, we have few comparable products.

Part of the problem is that there are few researchers with time or funding to study BSM.  Another problem common to turfgrass pests is that it is difficult to know if and where mites will show up in a large enough area to design a good insecticide trial.

Bifenthrin is a standard go-to miticide for many in the pest control industry. But experts vary in their opinions about its effectiveness against BSM.  Bifenthrin, lambda-cyhalothrin, and deltamethrin products all carry labels for mite control in turfgrass, and may provide some control.  However, if you are attempting control with one of the pyrethroid insecticides experts advise using a surfactant to help the insecticide penetrate deeper into the leaf sheath. Dr. Eric Rebek, at Oklahoma State University, suggests using Dispatch at 3/4 oz. per 1000 ft sq. Rebek also noted that Dursban provides slightly better control of BSM than the pyrethroid products, but can only be used on sod farms, in road medians, around industrial plants or on golf courses (brand products differ, so check the label).

Chong has had little success with bifenthrin, regardless of timing.  In trials conducted between 2011 and 2015, he found avermectin to be the best treatment after diazinon. He used Avid 0.15 EC at 28 fl oz/acre, at two week intervals. When testing weekly applications, Chong found the best mite suppression came from four weekly applications in June compared to four weekly applications in April.

Another suggestion passed on by many experts is to scalp, or mow the grass to be treated very short, before applying insecticide. The reasoning is that this prunes off and removes many of the infested grass tufts, plus opens the grass canopy to better spray coverage. Bagging your clippings and disposing of them off site will ensure that the mites are not just spread around by the mowing operation.

On golf courses Chong now recommends Divanem (8% abamectin, by Syngenta).  A Restricted Use nematicide, Divanem has a 2ee registration for bermudagrass mite control on greens, tees and fairways (March 2017). Rate is 3.125 to 6.25 fl oz/acre.  He recommends using the high rate if economically possible, and repeating every 2-4 weeks.

Despite several years of field tests, Chong notes that there is still a lot to learn, especially when it comes to combining insecticides with different cultural practices like mowing, irrigation and varietal selection. Experience with BSM demonstrates that, at least for some pests, it's not always easy to come up with reliable management recommendations.  For a pest like BSM, one pest can lead to a career's worth of work for some lucky entomologist.