Quick facts…
- Pear slugs chew the leaves of many common trees and shrubs such as cotoneaster, cherry, and related plants.
- Pear slugs are not true slugs. They are a type of insect known as sawflies.
- Two generations of pear slugs occur. Most damage usually occurs from the second (September) generation.
- When severe defoliation is threatened, pear slug injury should be controlled.
Introduction
Pear slugs (also called “cherry slugs”) feed on the leaves of several commonly planted trees and shrubs during mid to late summer. Cherry, cotoneaster, plum, apricot, pear, hawthorn, and mountain ash are highly susceptible species.
Pear slugs are dark green to orange, swollen at the head end, and slimy. When full-grown, pear slug larvae may reach 1/2 inch. They tend to lighten in color as they grow older. Pear slugs can be confused with common garden slugs, however, garden slugs are not insects, while pear slugs are. They develop into small, dark, nonstinging wasps (sawflies) that are rarely noticed.
History and Damage
Pear slugs overwinter underground in the pupal stage. During late spring, the adult insects emerge to mate and lay eggs. Adult pear slugs are dark colored, nonstinging wasps, called sawflies, approximately 1/4 inch long. They lay eggs in slits made in the leaf.
Young pear slugs feed on leaves and mature to their adult stage in approximately three weeks. Pear slug larvae feed on the upper leaf surface. They avoid the larger leaf veins and rarely penetrate the lower leaf surface. The result are “skeletonized” leaves that have a characteristic appearance. Chewed areas of the leaf turn brown. When heavily damaged, the entire leaf falls prematurely.
Pear slug injury occurs in two peaks during the growing season. This injury coincides with the presence of full-grown larvae, which do most of the feeding. In Colorado, damage by the first generation often peaks in late July. The second generation of larvae are usually the most active in September. This later generation is often more damaging and can completely defoliate susceptible plants.
Control
Severe pear slug injuries most often occur late in the season and often have little impact on plant health. However, the damage may be quite unattractive. Occasionally, heavy infestations can reduce plant vigor. In these situations, pear slugs may need control.
Most insecticides easily control pear slugs when used at labeled rates listed for control of caterpillars or leaf beetles on trees and shrubs. Effective insecticides include carbaryl (Sevin, Sevimol), malathion, and Spinosad. Malathion and carbaryl can be used on fruit trees if waiting periods (1 to 14 days) are observed.
Pear slugs can often be controlled using a forceful jet of water to dislodge them. Soaps have been effective for controlling pear slugs but can injure plants such as cotoneaster, plum and cherry. Pear slugs are also readily killed by applications of wood ashes.
Reference: CSU Fact Sheet 5.560