Sawflies: A Truly Weird Garden Pest
Q. Nancy in Forks Township PA recently called into the show and left a message on our answering machine (610-694-8100) about sawflies on her hibiscus plants, but wasn’t available to talk about it on the show. Sawflies—possibly the strangest insect pest out there—are a topic that doesn’t come up often, and we haven’t discussed them in a while. And now we shall.
A. Although they look exactly like extremely creepy caterpillars, sawfly larva do not become moths or butterflies—nor do they turn into any kind of flies, as their name suggests.
I will now Quote from the Missouri Botanical Garden’s excellent website: “sawflies are in the same order as ants, wasps, and bees. Their name derives from the adult female's abdominal appendage, which she uses to insert eggs in foliage. Adult sawflies have 2 pairs of wings and are dark, wasplike, somewhat flattened insects, usually 1/2" long or shorter. Most surface-feeding larvae (the caterpillar look-alikes) have six or more pairs of prolegs on the abdomen and one large {quote} "eye" on each side of the head. An exception is the pear sawfly, whose larvae resemble a small, dark olive-green slug.”
It is difficult to adequately describe the caterpillar-like larvae, many of which are yellow with black dots, look like refugees from a horror movie, and tend to appear in large clusters, which makes them even creepier. But despite their uncanny resemblance, they are—once again—not caterpillars of any kind, and so the trusted organic caterpillar killer, Bacillus Thuringensis (aka Bt) has no effect on them.
(And no shame on those who have tried; it’s the first thing I would have done.)
And there isn’t just one type of sawfly. {quote}: “The bristly rose slug is, as the name implies, a sluglike sawfly larva, one of several types that feed on roses. It is shiny black to pale green; by maturity, it develops many bristlelike hairs on its body from which its name (‘bristly’) is derived. Young larvae skeletonize the lower leaf surfaces of their favored host, while mature larvae chew large holes in leaves.”
The good news is that sawflies generally don’t do much damage, and ignoring them is one option. (Ignoring things going wrong in the garden is often my favorite tactic.)
“Furthermore”, the Botanic Garden site continues, “a number of natural enemies keep most sawfly populations low, including parasitic wasps, birds, small mammals, and predaceous beetles, as well as fungal and viral diseases.” Organic gardeners should always remember that what’s a pest to you is a big delicious bag of protein to a predator, especially in this case, birds.
(Side note: When a single nest of tent caterpillars appeared in my crabapple tree this Spring, I told my assistant, The Awesome Ninja Sean to just poke at it with the longest handled hoe we had and make a tear in it. “And then what?” he asked. “That’s it; we’re done; the birds will take it from here.” The next day there wasn’t a single caterpillar left.)
Birds are a gardener’s best friend. To get them to work for you, do NOT feed them birdseed! They’ll fill up on the seed and leave your pests alone. Bird feeders are NOT good for birds for many reasons, including and especially the spreading of disease due to the unnatural crowding that feeders cause.
Feeders are 100 percent for YOUR amusement only; the birds do not need them! They have been feeding on pests like sawflies since sawflies were there for them to eat, and they will continue to do so unless you fill them up with junk food.
If you want to really lean into attracting flying friends to your garden, have birdbaths around your plants. Birds don’t need human-supplied food in the summer, but fresh water can be scarce at times. Give them lots of fresh water and they’ll return the favor by eating your pest insects.
Again, The Missouri Botanic Garden notes that “a few sawfly larvae on a large or mature plant may do little damage and can be tolerated. And when larvae become half grown or larger, they eat less. Leaving the larvae alone for birds to manage naturally could be a good and beneficial solution. Sawflies feeding mid-to-late summer also do less harm to plants.”
Other simple solutions include hand picking the little monsters (they don’t sting, like some venomous caterpillars) and the classic: spraying them off your plants with sharp streams of water, which I have come to realize is the single most misquoted and misunderstood of my organic suggestions.
We are talking about SHARP streams of water: S H A R P ! Not gentle sprays. You need to use an adjustable nozzle that has a laser-sharp setting. If you don’t see the pests go flying off the plant when you spray it, it is not sharp enough. If you have lousy water pressure, you can buy an inexpensive nozzle that funnels even a weak flow through a smaller diameter tube to create a powerful force. You can also just prune off any affected plant parts, trash them and go home early.
And finally, if spray something you MUST (I know who you are!) you can smother them with professionally made insecticidal soap or a horticultural oil designed for use in the summer. (NOT dormant oil; that’s only for use on fruit trees over the winter).
