Pear Slug

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This integrated pest management (IPM) fact sheet provides information useful for identifying and combating a particular pest while minimizing environmental impact. It was originally written by Philip Dickey of the Washington Toxics Coalition and designed by Cath Carine of CC Design for the Green Gardening Program which is funded by Seattle Public Utilities. It has been modified for use on Toxipedia.org.

Note: This fact sheet is not intended for commercial fruit growers.

Species


Caliroa cerasi.

Host/Site


Especially pear and cherry, but also plum, apple, quince, hawthorn, buttonbrush (Cephalanthus spp.), and mountain ash.

Identification


Pear Slug Larva and Damage
Photo by R.D. Akre.

Pear slugs aren't slugs at all, but rather the larval stage of a sawfly. The adult fly is about 5 mm (1/5-inch) long and shiny black in color. Its wings are smoke colored but somewhat translucent. The larva is about 1/4-inch long when mature, with a body larger in front and tapering towards the rear. They appear olive green or blackish in color and are covered with a slimy material.


Life Cycle


Pear Slug Adult
Photo by K. Gray

The pear slug pupa overwinters underground in a cocoon 2 to 3 inches under the soil. The life cycle has two generations per year. Beginning in early summer, the adults emerge and the females insert eggs into the leaves of the host plant. When the eggs hatch one or two weeks later, the larvae begin to feed on the upper surface of the leaf. After feeding for 2 or 3 weeks, the mature larvae crawl or drop to the ground, where they burrow into the soil to pupate. About two weeks later, usually early August in the Northwest, the second generation adults emerge. The second generation proceeds similar to the first, and in September the mature larvae again move to the soil to pupate.


Natural Enemies


Natural enemies exist but their identity is currently unknown. Without their presence, damage would be much more extensive.

Monitoring


Pear slug damage is visible as a gradually enlarging series of holes or spots on the leaf where the green pigment has been eaten, leaving a yellow or red lower layer. In severe cases, only the skeleton of veins remains. Larvae should be visible on leaves in late June through July and again in late August into September.

Action Threshold


Physical control can be done whenever larvae are present. Chemical controls should only be used if the larvae are present and the damage is serious. Most plants can tolerate a 25 to 30% loss of leaf surface. Pear slug is not usually a problem in unsprayed backyard trees. Because the larvae feed during narrow intervals and are not on the tree at other times, sprays during other parts of the life cycle will be ineffective.

Cultural and Physical Controls


Trees can be sprayed with water from a garden hose or sprayer to dislodge pear slugs. On small trees, the larvae can easily be removed by hand from the surface of the leaves and destroyed.

Biological Controls


Natural parasites and predators often keep pear slugs under sufficient control.

Chemical Controls


Pear slug larvae are easily controlled with almost any insecticide. Insecticidal soap is the least-toxic chemical control and is effective if spray contacts the larvae directly. Be aware that larvae will be found on the upper surfaces of leaves. Avoid using broad-spectrum insecticides that are toxic to bees and other beneficial insects. If peak defoliation from the second generation occurs in fall, chemical control won't gain much, since leaf drop will occur shortly, and may destroy internal parasites.

References


Antonelli, A. Pear Slug. WSU Cooperative Extension Bulletin EB 1369.

University of California Statewide Integrated Pest Management Program. Pear Slug

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