Pear Slug

Pear slug damage on hawthorn

Pear slug adult sawfly

Pear slug larva

Host Plants And Distribution

Saskatoon, pin cherry, Cotoneaster, hawthorn and possibly chokecherry. This insect is found throughout Canada and the northern United States.

Biology

The pear slug is the larva of a leaf-skeletonizing sawfly. These insects overwinter as fully grown larvae in small cocoons in the soil near the base of the host plant. Pupation occurs in the spring and adults emerge from mid-June to mid-July. Adults are 5 mm long, shiny black sawflies. Females deposit eggs in slits cut in the lower surface of the leaves. Eggs hatch after two weeks and larvae move to the upper surface of the leaf to feed. Larvae are black, slug-like and slightly enlarged at one end during most of their development. Mature larvae are about 10-12 mm long and change to a yellowish-green colour. Occasionally, the pear slug may pupate in the same year, leading to a second generation present in late summer and fall.

Symptoms And Damage

Larval feeding causes leaf discolouration, mottling and loss of leaf material. Initially leaves have yellowish spots due to feeding on upper leaf surface. As feeding progresses, the affected areas enlarge, leading to a bleached appearance of the leaf. Finally the foliage turns reddish-brown in late July and August, giving the tree a burnt appearance. Larvae are present and visible on affected leaves. The burnt appearance of the trees reduces their aesthetic value. Healthy trees can withstand several years of moderate to severe damage because the damage occurs late in the season.

Scouting Techniques

Larvae of the pear slug and the mottled leaves are readily visible. Weekly inspection of the plantation should reveal any developing problems. Affected leaves may be removed (with larvae) and disposed of.

Economic Thresholds