What’s Eating Your Pine Needles? Sawflies, Probably
Conifer trees are incredibly common across the U.S. landscape and can be found nearly anywhere, including in yards, recreation areas, roadsides, and natural and managed forests. Most of the time—and I’m speaking as a forest health specialist here—we are most concerned with bark beetles and several different types of fungi as the things most likely to impact conifer health. But, every now and again, sawflies make their appearance. Sawflies are one of the only folivores (i.e., foliage eaters) on conifers and are barely noticeable as adults, eggs, or young larvae, but older larvae can defoliate trees, and sometimes entire forests, seemingly overnight.
Sawflies are named because of their saw-like ovipositor, which females use to insert eggs into conifer foliage. While their larvae look like caterpillars, sawflies are in the order Hymenoptera, making them more closely related to bees and wasps. Adults are stingless wasps, and larvae feed on conifer foliage (needles). Young larvae feed on the needles like an ear of corn, only consuming about half the needle and leaving a dry sliver of the needle uneaten. Older larvae consume the entire needle by eating it like a carrot, leaving only a needle stub when they’re finished. They are gregarious, feeding together on foliage during most of their larval life stage.
Despite their ubiquity in conifer forests in the U.S., little research has been done on these pests even though 25 different species can be found in eastern U.S. forests. A new article published in July in the Journal of Integrated Pest Management changes that, as members of the Linnen Lab at the University of Kentucky, led by Jeremy Davis, Ph.D., summarize our sawfly knowledge, make recommendations for management strategies, and provide (to my knowledge) the first in-color larval key to identifying these species. As most keys focus on adult internal and external morphology, this key—which uses larval color and other easily distinguishable field characteristics—is likely to be much more useful to forest health personnel in the field.
Management for sawflies varies depending on the situation. If a single tree (or group of trees) is affected in a yard or in an urban or suburban area, sawfly larvae can sometimes be knocked off by hand or with a strong stream of water from a hose. High populations in natural or managed forests may require pesticide applications to knock populations back. However, it should be noted that rarely are chemical treatments required, as in all cases natural enemies typically help moderate populations.
In the overall scope of forest pests in North America, conifer sawflies typically don’t garner a lot of attention or generate much concern among land managers. While their populations can flare up on occasion, they do serve as food for birds and support a myriad of natural enemies. As such they’re far more likely to be present in smaller numbers throughout our forests and trees, nibbling away at conifer foliage and only sometimes even being noticed.
Journal of Integrated Pest Management