This is a guest post by PhD student Adam Dale.
This weekend as I was walking my dog down the greenway I noticed several heavily defoliated river birch branches overhanging the trail. Upon closer examination I saw several groups of a dozen or so yellow-green bodies with black spots and black heads chomping away at the leaves. When I reached up to grab a branch and take an even closer look, almost in unison they all swung their rear ends away from the leaf margin forming little S-shapes. This is the dusky birch sawfly, Croesus latitarsus.
Sawflies are relatively common leaf-eating pests of many landscape trees and shrubs. Different species usually specialize on one plant species or group of plants. Although easily confused with Lepidopteran (butterflies and moths) caterpillars, sawflies are actually Hymenopteran (bees and wasps) pests. The larvae are the damaging stage, which pupate in the soil after getting their fill of leaves, and turn into a winged, wasp-like adult. Don’t worry, though. It can’t sting you.
Typically, sawflies are shinier than caterpillars. However, the best way to distinguish between sawfly larvae and caterpillars is by counting the number of prolegs (the suction cup-like nubs along the rear underside of the body). Caterpillars always have five pairs of prolegs or less, while sawflies have six or more. As you can see in the photo, dusky birch sawflies have six.
As its name suggests, the dusky birch sawfly feeds primarily on birch trees, most commonly river birch in North Carolina landscapes. It is found throughout the United States and has two generations per year in NC. Right now they are in the larval stage of their second generation. Since these pests feed in groups, they tend to defoliate entire portions of a canopy and can clean the foliage off of an entire small tree. Despite this, they seldom reach high enough numbers to cause any lasting damage to a tree and rarely require intervention.If you’re concerned about them damaging your trees and you can reach them, just pluck them off and toss them into some soapy water or prune the whole branch. If you can’t reach them, you probably didn’t see them until they were pretty far along. Therefore, they’ll be gone soon and killing them now won’t prevent much damage. If you catch them early, good coverage with horticultural oil is effective and populations of larger individuals can be controlled with reduced-risk insecticides like spinosad. Products like Bacillus thuringiensis (Bt) will not work on them since they’re not caterpillars.
One interesting note is that I only saw clusters of these leaf-eaters on branches overhanging the paved greenway trail. When I ventured off trail into the forest they were nowhere to be found. It appears that they prefer the trees along the edges of the forest over the trail, perhaps because it’s sunnier, warmer, easier to find, less dangerous, or tastier. Edge effects like this aren’t unusual and can be caused by various factors. Remember that when scouting for pests like this and look at the prolegs if you’re not sure what it is.