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It's a Caterpillar! It's a Slug! No--It's a Sawfly!


You Bet Your Garden
Question of the Week © 2020 Mike McGrath

Note: Hundreds of Mike's informative articles are available (in alphabetical order!) right here at the Gardens Alive website. To find Mike's answers to your specific garden problem, Click here and find your topic (like Weeds, Worms, Rhododendrons...) in our complete alphabetical archive of Questions of the Week.

It's a Caterpillar! It's a Slug! No--It's a Sawfly!

Q. Earlier this month, Paul in Ardmore, PA emailed us a picture of what looked like a big cluster of black and white caterpillars and wrote: "I'm wondering what these are; they only attack my yellow twig dogwood. They attack it every year and will eat every leaf if I don't collect them quickly'"

A. I replied that they look like some kind of crazy caterpillar and proceeded to try and ID them--but no matter how hard I searched, I couldn't find a match. So I turned to our Facebook friends. My newly married daughter Amanda posted the photo and asked for help and help we got. Facebook Friend Susan was the first to ID them as sawfly larvae and correctly noted that they are specialists, each sub-species feeding only on a specific plant, making this 'the dogwood' sawfly. Another Friend chimed in to add that they are currently attacking her red twig dogwood. But my favorite response to "what are they?" came from J. M. who simply answered, "bird food".

Sawflies are one of the weirdest pests out there. Most of the adults look like wasps, but don't have a thin, 'waspish' waist like true wasps. Some look like bees and even appear to have a stinger--but it's actually their ovipositor; the body part that females use to inject their eggs into the target plant.

Sawflies are related to wasps and bees--and ants; and they've been damaging plants for an estimated 250 million years. Although the adults are all flying insects with wings, the larvae are unique. Some look--and act--exactly like caterpillars, while some look exactly like slugs. (The unlucky pear tree has one of each!)

But roses may have the most: 'the rose slug' feeds on leaves. The 'rose stem sawfly' burrows into the stems of roses like a cane borer. And one that looks like a caterpillar--'the rose leafroller sawfly'--rolls up the leaves its eating so you can't see it. There's even a sawfly that builds a cocoon around itself as it feeds on evergreens; doing a darn good imitation of a bagworm!

There are an estimated 8,000 to 10,000 different species, and each one is a specialist. The pine sawfly and the iris sawfly are especially damaging in forestry and professional horticulture.

Some of the adults may look like they have a stinger, but they don't sting. Some of the caterpillar-like larvae look like they have stinging hairs--like the nasty saddleback caterpillar--but they don't sting either. Their biggest defense is to secrete a nasty irritating liquid when they're attacked--which must work well if they've been hanging around for hundreds of millions of years.

The leaf-eaters typically eat only the soft parts, leaving behind the veins, which makes the leaves look lacy and skeletonized--exactly like the damage caused by Japanese and other scarab beetles, making proper identification difficult. But proper ID is important, because Bt--the old original organic caterpillar killer--has no effect on sawflies, no matter how much they look like caterpillars. To try and make an accurate ID, type the name of the plant being attacked into your search engine and add the word 'sawfly' after it if the pest looks like a caterpillar; use the word slug if it looks like...well...a slug! Or pick one up and count the number of {quote} 'legs' it has. (They're not real legs.) Anyway, sawflies have six of these phony legs; caterpillars have fewer.

OK--so what can we do about them?

• Handpick and drop them into a bucket of soapy water.
• Handpick and feed them to your chickens.
• Spray them off with laser-sharp streams of water.
• Hang suet cakes near affected plants in the winter and remove them when Spring arrives. Don't put out birdseed; many birds feed on sawflies, especially members of the chickadee family. These birds love suet; and when it disappears, they'll go hunt sawflies.
• Cultivate the soil in the Fall and again in the Spring underneath the plants that were attacked in Summer. The sawflies are pupating down there and would otherwise emerge in their wasp-like form in May or June.
• If your sawfly look like slugs, dust them with diatomaceous earth (DE); the mined remains of ancient sea creatures called diatoms. DE looks like flour to us but is razor sharp on a microscopic level and dehydrates slugs and other soft-bodied slug-like creatures. Be careful not to inhale any; wear a dust mask.
• Spray them with a professionally made insecticidal soap. Home-made soap sprays can easily become accidental herbicides.
• Spray them with a light horticultural oil designed for use in the summer--not dormant oil; that's for winter.
• Whether its insecticidal soap or horticultural oil, you have to soak the pest with it; these products smother the pest directly and have no residual action.
• If no flowers are nearby, spray one of the new Spinosad products. But don't let it hit any flowers as it is toxic to bees.
• Spray concentrated neem or neem oil on the pests and the leaves.
• If rose canes wilt, find the hole the sawfly made, prune the cane out, expose the caterpillar-like pest inside and taunt it to death.

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