NOW is the time to be checking pine trees for needle feeding damage by the larvae of the European pine sawfly. Pine sawfly larvae can be present any time after mid-May. The small larvae can only chew the edge from the needle, but as they grow, they consume more and more of the needle until eventually they are eating the entire needle down to the stub. Defoliation lasts for four to five weeks and usually peaks near the end of May.
European pine sawfly defoliation is common on Scots, Mugho and Austrian pines, though other pine species may be damaged. The feeding activity causes the old needles to disappear, resulting in isolated bare, branches with a tufts of new growth at the end. Damage is highly variable from tree to tree and from branch to branch within the same tree.
European pine sawfly larvae are grayish-green with 2 light stripes and 1 dark stripe on each side of the body. The legs and head are shiny black. Full grown larvae, usually present by Memorial Day weekend, are about 1 inch long. A curious but distinctive characteristic is that the larvae are gregarious and stay together in a cluster as they feed, moving as a group from one stripped branch to the next.
Pine sawfly larvae feed only on old needles that emerged in previous seasons and not on the new needles that are emerging now. Affected trees are not killed. There will be new growth to keep the tree alive and functioning, though the aesthetic value may be diminished and growth may be stunted.
Control of the European pine sawfly on pine trees in the landscape can be as simple as pruning off and discarding infested branches. Shaking branches to dislodge larvae is often surprisingly effective as well.
If you have too many trees to inspect or treat by hand (such as a wind break or Christmas tree plantation), or if trees are too large and clusters can not be safely reached, or if infestations are unusually heavy, sprays are available. It is necessary to treat only the clusters and not the entire tree. Use foliar applications of horticulture oil, Sevin, Orthene or Isotox. Read and follow all label directions. Spraying is of greatest benefit when done before the larvae become one-half grown.
This article originally appeared in the May 15, 1998 issue, p. 59.