No one wants to hear about a new insect pest. There are already so many of them and it seems new ones appear every year. Well, sorry to have to tell you, but here’s one more, especially interesting to me because I (sort of) helped discover it.
Three years ago, a reader from Quebec City, Canada (where I live), sent me a photo of the damage done to her goat’s beard (Aruncus dioicus), a popular perennial reputed to be particularly easy to grow and free of insects and diseases. Certainly, I’ve grown it for years and have never seen a “bug” touch it. But on hers, the leaves were skeletonized, with only veins remaining. She described the predator as being a little green caterpillar.
I was baffled. I’d never heard of a goat’s beard suffering an insect attack, at least not in North America. On the other hand, considering the damage (I had her send me a photo), the fact that it only fed on goat’s beard and the description of the insect, it really sounded a lot like the goat’s beard sawfly (Nematus spiraeae). Yet this insect is strictly European in distribution.
I explained to the reader that I wasn’t sure, but it sure sounded like sawfly damage and explained what to do about it. Still, I had my doubts. What insect was it really?
A Scientific Confirmation
I kept getting emails about this pest, all very local, both in 2016 and 2017 and again this year. So I checked the Internet again a few times … and last week, eureka! I discovered the pest had been officially identified.
The report A Palearctic Sawfly on Aruncus (Rosaceae) New to North America, by Joseph Moisan-De Serres of the Quebec Department of Agriculture, Fisheries and Food and David R. Smith, Department of US Agriculture, was published in August 2017. It reports the discovery of this European insect in a private garden in Stoneham, Quebec (just north of Quebec City) in 2016. According to the owner of the garden, the insect had been present for “four or five years,” so since 2011 or 2012. The report makes no suggestion as to how the pest got from Europe to Canada.
How far will this insect spread?
For the moment, it seems to be confined to the Quebec City region in eastern Canada, but is spreading locally. Given that goat’s beard is not only native throughout much of North America* (other than in the more arid Prairie states and provinces), but is also widely grown as an ornamental throughout temperate areas of the continent (even in the Prairies!), the sawfly can readily find its host plant just about everywhere and it’s therefore probable it will expand its range over time, likely throughout North America.
*Goat’s beard (Aruncus dioicus) is native throughout the temperate Northern Hemisphere, thus not only North America, but Europe and Asia as well.
So, here’s what to be prepared for:
Description and Life Cycle
The goat’s beard sawfly (N. spiraeae) is a rather svelte flying insect with black to brown head and a yellowish abdomen. It’s about 5 to 6 mm long. Despite its wasplike narrow waist and long antennae, it’s a sawfly, perhaps distantly related to ants and wasps, but not a true wasp and no, it will not sting.
The larva looks much like a caterpillar, but that term applies strictly to butterfly and moth larvae, so it is sometimes called a false caterpillar. It’s light green and somewhat translucent with a greenish to brown head. It can measure up to 20 mm in length.
The female reproduces by parthenogenesis (without fertilization) and in fact, no male of this species has ever been found. It emerges in late spring and lays small white eggs under goat’s beard leaves, its exclusive host. Larvae, very gregarious, feed on the tissues between the veins of the leaves. After a few weeks, they drop to the ground, dig in, weave a shelter of silk and become pupae, then remain quiescent for about a month. A second cycle then begins. In mild climates, there is often a third generation in early fall. The larvae of the year’s final generation drop to the ground in the fall and overwinter underground as larvae, becoming pupae only in the spring. Then the cycle starts all over.
Goat’s beard sawfly populations are very irregular: there are years when you’ll scarcely notice the damage and others where the whole plant is defoliated.
The easiest and most effective way of controlling them is to keep an eye open for the larvae, then when they appear in early summer, knock them off the plant into a bowl of soapy water. Since they are gregarious, at least they tend to concentrate on the same part of the plant, making harvesting easier.
Or spray with a pyrethrum-based insecticide, preferably in the evening. Although it’s an organic insecticide, it has a broad spectrum and can therefore harm bees and other pollinating insects, so avoid applying it when your goat’s beard is in bloom. Note that although the pyrethrum treatment works well on young sawflies, more mature ones seem to be fairly resistant to insecticides.
Note that you would be wasting your time applying BTK (Bacillus thuringiensis kurstaki) to control goat’s beard sawfly. This popular organic pesticide is only useful for controlling true caterpillars, not false ones!
Well, let’s all hope that the goat’s beard sawfly doesn’t spread any further, but I’d be surprised if it didn’t. Otherwise, it will end up knocking yet one more plant—goat’s beard—off the list of the truly easy perennials anyone can grow!
If you find sawflies on your goat’s beard plants, please share the information with Joseph Moisan-De Serres of the Quebec Plant Protection Expert and Diagnostic Laboratory. He’s trying to track the progress of this new invader. He can be reached at Joseph.Moisan-DeSerres@mapaq.gouv.qc.ca.