Pear Slug and other Skeletonizers

Problem type: Insect

Name of problem:
 Pear Slug and other Skeletonizers

Plant name(s): Mountain ash, hawthorn, cotoneaster, pin cherry, oak, birch, crabapple and other fruit trees

Symptoms / Characteristics:
Initially, leaves have a spotty bleached appearance, quickly turning brown and papery. If enough of the leaf surface is consumed, the leaf will drop prematurely. Larvae generally feed on only one leaf surface, leaving a thin layer of tissue making the leaf appear translucent. Occasionally, leaves may be completely consumed with only leaf veins remaining, creating damage resembling that of other defoliators, such as caterpillars and certain sawflies (e.g., spiny ash sawfly and Nematus sawfly on willows) not usually categorized as skeletonizers. Symptoms typically appear in late summer. Repeated attacks may reduce annual growth and cause mild branch dieback.

The pear slug, Caliroa cerasi, is actually a sawfly but is named after the slug-like larvae. In late June, the adult sawfly deposits eggs in the lower leaf layer. The larvae hatch two weeks later and begin to skeletonize the upper leaf surface. After feeding for about a month, they drop to the ground to pupate. A second generation may occur before winter if the weather is favourable. Immature larvae have a pear shaped body, are dark green to black and covered in slime. At maturity, they lose their mucous coating and turn light yellowish-green. Adults are small black sawflies.

Other skeletonizers include moths that have similar life cycles as the pear slug. Immature larvae are white, turning yellowish green as they mature. All larval stages spin small silken tents and finally a cocoon in which to pupate to emerge as adult moths.

Control / Preventions:
Many natural predators usually keep these pests under control, not warranting the need for chemicals. Monitor trees regularly beginning in mid July. A high pressure water spray can be used to remove the larvae from the leaves. A mix of soap and water can also provide effective non-chemical control. Horticultural soaps are available as well as diatomaceous earth which is a dust applied to the foliage. Almost any insecticides will effectively control the pear slug, but must be applied according to manufacturer instructions and precautions.