These gregarious larvae will wave in unison as you approach and resume munching on your Mugo pine as soon as you leave. This joint reaction is thought to deter predators from the feeding colony.
The European Pine Sawfly, Neodiprion sertifer, is the larvae of a non-stinging relative of bees and flies. It is as its name would imply a native of Europe and was accidentally introduced into North America in the late 1920’s.
Young first instar, the larvae as it emerges from the egg, feed on new soft growth on conifers in late April to early May. As they grow they will move to feeding on older last year’s growth. The preferred host plants are two needle pines like, Jack pine, Scots pine, and Japanese two needle pines. They have been known to feed on red pines as well as white pines.
The larvae feed in large groups of 40 to 200 individuals; this limits the effect of predators as well as creating local areas of intense feeding. This feeding can disfigure and even kill individual branches in specimen trees. Though seldom fatal too host plants repeated infestation can reduce the vigor of healthy trees and leave them susceptible to other diseases and insects.
Once the larvae grow and develop through a series of instars, larval stages, they migrate down the tree to the soil where they spin a cocoon. In the cocoon they transform into wasp like creatures. The adult forms emerge in September and October, quickly mating and laying eggs on the needles for next year’s generation.
The fertilized female lays groups of the small yellow, rice shaped eggs. These are laid or glued as a sticky elongate coating on needles in the upper branches of the tree. The eggs overwinter, emerging to start the cycle next spring.
Control can be as easy as hand picking needles with eggs off the pines in the fall, or spraying them with a blast of cold water in the spring. Insecticidal soap is effective if applied to the early stages of larvae. Once the larvae are in the final instar, last stage before they descend, they have stopped feeding. This makes control ineffective. Chemical control is available and should be applied according to label direction.
If you have ornamental pines ISA Certified Arborists are industry recognized experts in plant insect control. Contact your I.S.A. Certified Arborist to see if you have this damaging plant pest.
For more information on this subject or any plant health care related questions please feel free to ask Professor Tree at the link in the right margin.