May is an important month in the control of insect pests of pine trees. Pine needle scale, a common but variable pest, could still be controlled in the egg stage with a dormant oil spray before bud break (an unusual situation but a possibility in "late" springs like this one), though more typically we think of May as the time to control the crawler stage.
Pine needle scale eggs that spent the winter under the scale of the adult female normally hatch during mid-May. This crawler stage is vulnerable to sprays of contact insecticides such as soapy water, horticulture oil, diazinon, Orthene, malathion and Sevin, so timing treatment to coincide with the egg hatch is important for effective control.
According to the phenological indicator system we use to predict when important events such as scale egg hatch will happen, get ready to treat infested pine trees that warrant treatment when lilacs are in bloom and treat during bloom of Vanhouttei spirea, horse chestnut and Zabel's honeysuckle plants
The second, common pest of pines that should be watched for in May, (preferably mid-May) is the European pine sawfly. European pine sawfly larvae feed in clusters on the old growth needles of mugho, Scots and red pine trees and shrubs. Larvae are grayish-green with 2 light stripes and 1 dark stripe on each side of the body and shiny black legs and head. Full grown larvae, usually present by Memorial Day weekend, are about 1 inch long.
Because only old needles are eaten and not the new, emerging growth, defoliated trees are generally not killed. Damage may be aesthetically displeasing, especially in Christmas tree plantations, and growth of the tree may be stunted.
Control can be as simple as pruning off and discarding infested branches. Heavier infestations on larger trees may justify foliar sprays of Sevin, Orthene or Isotox. Spraying is of greatest benefit when done before the larvae become one-half grown.
This article originally appeared in the May 5, 1995 issue, p. 53.