As you read this newsletter, European pine sawflies could be active in your landscape or Christmas tree plantation. On May 13, 2002, on the ISU campus, these larvae were1/2 inch long and had just started producing noticeable damage to mugho pines. Other favored hosts include Scots, red, and jack pines. Eastern white and Austrian pines are also fed upon when they occur in mixed plantings with favored species.
Newly hatched larvae feed on the outside green tissue of a pine needle, leaving the inner portion intact. This weakened needle turns yellowish orange and curls or twists, much like a string ribbon on a gift package. This "straw stage" is one of the first indications of European pine sawflies feeding. Closer examination of the pine branch reveals small grayish green larvae feeding in clusters. Each larva has a black head, black legs and a dark stripe bordered by white stripes down the side of the body.
As the larvae grow, they consume the entire needle down to where it attaches to the twig, leaving the paper sheath intact at the base of the needle cluster. Full-grown European pine sawfly larvae are about 1 inch long. At cessation of feeding, larvae drop to the ground and pupate in the soil. Adults (small wasp-like insects) appear in the fall, and females deposit eggs in current year needles using a saw-like appendage, thus the common name of this insect. This insect overwinters as eggs within the pine needles.
Defoliation by this insect pest generally lasts four to five weeks, usually peaking at the end of May. Removal of last year's needles can be quite extensive, resulting in isolated bare branches with a tuft of current season needles at the end, much like a lion's tail. Damage is variable from tree to tree and within a given tree. Affected trees appear unhealthy, but are not killed. On occasion, stunted growth of branches results.
Pruning off and discarding the clustered larvae can accomplish control of European pine sawfly on landscape trees. Shaking the branch over a bucket of soapy water, thus dislodging larvae can be equally effective. For plantings with multiple trees (e.g. windbreaks, golf courses, large corporate grounds or Christmas tree plantations) or with very heavy sawfly populations, sprays are available. Foliar applications of horticultural oil, insecticidal soap or Sevin will control this pest; wherever possible, spot treat clusters of sawflies and not the entire tree. Sprays will be most effective when larvae are about 1/2 inch long. Please read and follow all label directions.
This article originally appeared in the May 17, 2002 issue, p. 64.