Slug-like sawfly larvae feed on leaves and skeletonize them, leaving only a framework of veins. Here’s how to manage them using organic methods.
The pear sawfly (Caliroa cerasi), also known as the cherry or pear slug, is widely distributed throughout the United States and Canada. It is a common pest of mountain ash, hawthorn, cotoneaster, flowering cherry, flowering plum and pear and is occasionally found on quince and shadbush. High populations can defoliate entire trees.
Young larvae (1/2 inch long) are greenish-black, elongated, slim and slug-like, with very little evidence of legs. As the slugs grow, they become lighter colored. When fully mature, pear sawfly larvae resemble green-orange caterpillars. The adult (1/5 inch long) is a black and yellow, 4-winged non-stinging wasp (sawfly) that is rarely noticed.
The winter is passed in the soil inside a cocoon. In the late spring, shortly after trees have come into full leaf, the adults emerge and deposit their eggs in the leaves. These hatch a week or more later, depending on temperature. Larval development is completed in less than a month, and pupation takes place in the soil. Adults emerge during late July and August and lay eggs for the second generation of slugs. This generation usually causes the greatest amount of injury, especially on young trees, which they may completely defoliate. When this second generation of larvae becomes fully grown, they go into the ground and remain as larvae until the following spring, when they pupate. There is usually only one generation per year, but there may be a partial second.
Cultivate around trees and shrubs in the early spring and again in the fall to help reduce the overwintering population. Wash pear slugs off leaves with a strong jet of water from the Bug Blaster; larvae may also be sprayed with a commercial insecticidal soap. Diatomaceous earth is an effective organic insecticide that works well when dusted over slug infested areas. Many growers are reporting great success with Monterey Garden Insect Spray (Spinosad). A relatively new insect killer, it can be applied to a large number of ornamental and garden plants. If pest levels become intolerable, apply least-toxic botanical sprays as a last resort.
Photo Credit: Romana Plackova