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Question. Mike: We have a beautiful hedge of Knockout roses across the front of our house, with a sidewalk between the roses and the lawn. Over the last couple weeks the roses have been ravaged by something. We looked for the culprit but couldn't detect anything. Then I was walking down the sidewalk and noticed three "blobs" that were moving. Upon closer inspection, the blobs consisted of hundreds of tiny larvae that seemed to be covered with some kind of slime. I used a commercial insect spray to prevent them from getting to the roses, but the past two mornings, new blobs have shown up that I've had to dispatch. Any idea what these critters are? How do I get rid of them and save our roses? Thanks,
---Bob in Huntingtown, MD
Answer. Well, my first thought was that 'the blobs' might be a common, but harmless, nuisance. So I asked Bob: "Are the roses or nearby areas mulched with wood or bark?
He answered "no." Hmmm—that rules out the various slime molds that colonize chipped-up wood. And he did say that he saw lots of little larvae, so I took a guess: "Look online at images of armyworms", I suggested. He quickly emailed back: "I believe they are armyworms. The pictures online look exactly like what I'm dealing with. Any suggestions?"
Yes—if the 'worms' (which are actually caterpillars) are still small in size use Bt, the old original organic caterpillar controller. (Technically it's the BTK version of Bt, but it's the Bt family member that everybody just calls 'Bt' because it was the first.) But if the caterpillars have gotten big—say an inch to an inch and a half apiece—use one of the spinosads instead. This new generation organic pesticide works better than Bt on full-sized caterpillars.
Armyworms are a strange pest. The moths responsible for mobilizing the 'army' don't overwinter in the North, just down in milder climes, where the armies of caterpillars are major pests of turfgrass. There are several generations a year down South, where infestations can occur on such a massive scale, reports our turf grass specialist Dr. Nick Christians of Iowa State University, "that you'll see them reported on the evening news. The lawns appear to actually be moving, and severe outbreaks can cover a hundred mile stretch."
They feed as they march, eating the tops of grass blades, and can be very damaging—especially down South, where the warm-season grasses that predominate get a much shorter cut than our Northern cool-season grasses. "The lawn looks severely burnt and drought stressed after they've passed over it," explains Nick. If the weather is decent and rains come, the grass can recover, he continues, because—unlike lawn grubs—they don't damage the root system. "But if armyworms feed on a lawn in the middle of a hot and dry stretch, they can destroy large sections."
So how do these Southern menaces appear in the North? Our homeowner lives in the 'somewhat Southern' Washington DC region, but armyworms have been reported as far North as Massachusetts! "Wind", explains Nick. "You'll have the first generation of moths in the air somewhere down South, a big storm whips up around them and they can be blown incredible distances. The next thing you know there's a report of a 'moving lawn' in New Jersey." And while a marching army can cover vast distances back home, these Oz-borne invasions tend to be highly localized.
But one thing still bothered me—the ravaged roses. "They can eat other plants," Nick explained. "Typically they prefer grass or grains—they're a serious pest of corn crops—and roses would be highly unusual. But the description of a large mass of larvae moving across a lawn is pretty specific. And we've had the kind of weather that typically blows them up North early in the season."
Nancy Bosold, Turfgrass Management Educator for the Berks County Extension in Leesport, PA, agrees. Although they had several reports of armyworms "devouring lawns" in the Reading area some years back, "it would be highly unusual for them to cross over to roses. And we haven't had any armyworm reports yet this season", she adds, "but we have had a lot of complaints about rose sawflies (a pest we covered in depth a few shows back), and we had a grass sawfly report this Spring. Although the sawfly isn't a caterpillar, they look similar and both insects DO march across lawns." (http://ag.udel.edu/extension/IPM/ExtensionFactSheets/SawflyandArmywormIPM-6.pdf)
So what did Bob see, and what ravaged his roses? It's not that unusual for a highly visible insect to get the blame for damage done by a sneaky one, so I'm going to suggest that our listener did see colonies of armyworms. (If that IS the case, look for lots of night flying moths going to outdoor lights and coming into the house in a few weeks, warns Nick, followed by a late run of more 'worms' attacking lawns and corn crops). My second guess would be grass sawflies.
But the rose damage? Nancy Bosold tells me that a planting of Knockout roses outside her office—the exact type named by our listener—were "40% eaten by rose sawflies this season."
So I asked Bob a few final questions. "No, we never actually saw the creatures from the blob ON the roses," he replied—"but something ate 40 to 50% of the top growth. Oh, and you asked if there was any damage to the lawn: Yes; there are quite a few bare spots."
So I say armyworms (or maybe grass sawflies) ate the lawn; rose sawflies ravaged the roses. A tough one-two punch!