Not a Slug, Not a "Worm"—it's a Sawfly!
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Q. My roses are being decimated by little green worms. Some new branches have no leaves left at all. After a bit of research I found out they are sawfly larvae; I can't find any mention of them on your site. Other websites suggest wildly different ways of dealing with them; and some say that they will eventually go away on their own (as flies, I assume). But will there be anything left of my roses at that point? Thanks,
- ---Kathy in North Philly
- ---Steve in El Portal, CA
- ---Roy; just outside Lambertville, NJ
- ---Pamela in Asheville, NC
Sawfly larvae are known for attacking roses, and for doing so every which way—some types tunnel into the stems, some skeletonize the leaves like Japanese beetles, and others are 'leaf rollers'—they curl a big leaf around themselves to hide as they chew away. But roses are far from their only prey—there are a large number of different species and they are pestiferous on a wide range of plants. (The pine sawfly is an especially pesky member of this family.)
I'll bet that some of the "caterpillars" I've seen on my roses over the years have actually been sawflies. And pity the poor organic grower who sees what certainly appears to be a pest caterpillar chomping away and, in response, sprays the safe and natural organic caterpillar controller, Bt, over and over—to no avail. Because it don't matter how much you LOOK like a caterpillar, you have to actually BE a caterpillar to be done in by Bt.
Most experts feel that the feeding done by these pests is indeed 'self-limiting'; that is, they tend to disappear before extensive damage can be done. And, of course, strong plants that are fed organically, have lots of airflow, and aren't mulched with wood or watered to death stand the best chance of perking right back up after the sawflies…well, eh—fly.
Different types of sawflies go through different life cycles, so if you have a plant they're being persistent about season after season, read up on your exact foe. Many kinds can be controlled by cultivating the soil at the base of attacked plants at the right time of year to destroy their overwintering forms. Cleaning the orchard floor at the end of the season is essential to controlling the kinds of sawflies that attack apples and other fruits (and it prevents lots of other problems as well).
When ANY kind of tree is the target, hang suet feeders from the branches over the winter to attract chickadees and other birds that specialize in eating such pests. If your sawfly foe overwinters in parts of the tree—like inside some pine needles—spray dormant oil on the trees in the dead of winter. In warmer months, use a lighter-weight 'all season' horticultural oil. (The product mentioned by one of our listeners, Pyola, is a lightweight horticultural oil with a small amount of natural pesticide added.) If you actually see the pests being active on or in trees or shrubs, prune out and destroy the infested branches as early in the season as possible.
On roses and similar plants, handpick the pests, spray them off with sharp streams of water early in the morning (knock them down and they can't get back up), or spray neem or one of the new spinosad products. You can spray either of these low-toxicity pesticides on the leaves or directly on the pests; but don't spray any on the flowers or you could hurt bees. In fact, if a rose bush is covered with flowers and sawflies, harvest all the roses for cut bouquets and then spray to kill the weird little wasps. You'll get rid of the sawflies and encourage a new set of blooms to appear quickly.
When you use Pyola and/or other lightweight horticultural oils, spray directly on the pest to smother them. Same with insecticidal soap. If you do choose soap, spray first thing in the morning and then rinse the plants off an hour later—just to make sure the soap residue doesn't damage any leaves if the day turns blindingly hot.