Dogwood Sawfly

If you have either silky dogwood, or red osier dogwood (the latter also called red twig dogwood*), you are likely to see powdery white caterpillars nibbling on their leaves in August. These are not in fact ”caterpillars,” which are technically the larvae of butterflies and moths, but the juvenile or larval form of the dogwood sawfly (Macremphytus tarsatus), a native dogwood species herbivore. Not only are immature sawflies not caterpillars, they are not ”flies” either. Sawflies are wasps, in the same biological Order — Hymenoptera — as bees, ants and other wasps. And, although they are wasps, they are stingless, and harmless, except to red osier dogwood leaves! 

Dogwood sawfly females lay eggs in early spring along dogwood leaf veins. The ”sawfly” name comes from the tiny set of saw teeth that they use to cut slits into the leaves. You may see the egg sites as small brown dots on the leaf undersides. They hatch in July and begin life as light yellow-green larvae. This is the first of three instars, or stages of growth. Second instar larvae are covered in a white powdery or waxy coating and this stage is likely the first time observers will notice them. This coating has two purposes –- it conserves moisture and defends against predators – apparently it tastes awful. The third instar larvae gradually molt from yellow-green to a darker color; typically yellow with black rectangles crossed by a short white line on each body segment. Because they hatch and feed communally, it is sometimes difficult to determine that there is only one sawfly species, since the color variations make it appear as several different larvae.  

Fully mature larvae will look for a site in soft decaying wood or loose soil to pupate and overwinter.  Adults emerge in spring and lay about 100 eggs, starting the life cycle all over. 

There are nearly 10,000 different sawflies, each one a specialist and each one unique. You can tell the difference between a caterpillar and a sawfly by the number of fleshy legs on the abdomen. Caterpillars have five or fewer pairs of short, fleshy prolegs, while sawflies have a pair of legs on each abdominal segment. For dogwood sawfly, with 10 abdominal segments, that’s 10 pairs of legs, four pairs of true legs and six pairs of prolegs. A proleg is not a true leg, but a fleshy stub that provides a way for a larva to attach to a surface.  

Sawfly adults feed on nectar and pollen and are generalist, though undervalued, pollinators. Interestingly, adult dogwood sawflies do not pollinate dogwoods, because dogwoods bloom before the adults emerge. As leaf-feeding juveniles, they can cause damage to the dogwood by defoliating the plants. It is important to remember, however, that the sawfly and the dogwood have co-evolved, making each generally tolerant of the other. Overall, the damage to the dogwood, while unsightly, does not cause significant harm to the plants. 

*Dogwood nomenclature can be a bit confusing; these common names are generally botanically known as Cornus amomum and Cornus sericea, respectively.

Article by James Arnold, FCFB member