Pear Slug

The pear slug, Caliroa cerasi, resembles a slug in appearance, but it is actually a sawfly. It is rarely a pest in commercial pear orchards but may appear as sprays for other insects are reduced.

Adults are small (1∕5 inch), black sawflies with transparent wings. Larvae are shiny black and sluglike with seven pairs of prolegs in addition to three pairs of true legs, and reach 1∕3 inch long at maturity.Pupae overwinter in the soil. Adults emerge in the spring and begin oviposition by inserting eggs into small slits in the leaf, laying two to five eggs per leaf. Females produce eggs without mating. Larval feeding results in skeletonization of the upper leaf surface, leaving only leaf veins uneaten.

Mature larvae drop to the ground and burrow into the soil about 4 inches to pupate. A second generation emerges midsummer and continues skeletonizing leaves. Second-generation damage is usually more severe, sometimes retarding tree growth the following year.

Life cycle:Like all hymenopterans, sawflies have two pair of wings in the adult stage. Adult sawflies are different from other wasps and ants and bees because their abdomen is broadly joined to the thorax. All other members of hymenoptera have that characteristic constricted waist.Pear slugs overwinter as a pupa. Adults emerge in spring to begin laying eggs. Adult sawflies are small (about 1/5 inch) shiny black flies. Females are equipped with a small ovipositor used to lay eggs but not sting people. Eggs are deposited into leaves. Upon hatching the newly emerged sawfly larva feeds on the upper surface of the leaves. Newly hatched larvae are yellow to green in color but soon turn slimy dark green, almost appearing black. This is what gives them their name: pear slugs. After the larvae fully develop, the larvae drop down to pupate in the soil. Soon after, adults emerge in August to do the whole thing over again. The second generation is much quicker and during warmer years there can even be a third generation.

Damage:Pear slugs feed on the upper surface of leaves, and typically avoid feeding on lower leaf surfaces. When feeding, pear slugs eat the tissue between leaf veins, but leave the veins themselves intact. This feeding pattern is known as skeletonization and gives leaves a lacy appearance. If feeding is extensive, leaves may brown and shrivel. In general, damage from pear slug is cosmetic and will not harm the overall health of a plant.

The pear slug larvae feed on the upper surface of leaves causing very distinct damage. The upper epidermis and mesophyll layer of the leaf is removed during feeding while the lower epidermis layer of the leaf and the leaf veins are left intact. This gives the appearance of brown or silvery leaves and is referred to as leaf ‘skeletonization.’ Pear slugs do not limit themselves to just pears, but will attack other similar fruit trees. Pear slugs will feed on cherry, apple, plum, hawthorn and mountain ash trees. Damage can be seen on ornamental flowering trees and are not limited to fruiting types. Scout for damage to leaves in the late summer and beginning of the fall. Look for the slug-like larvae feeding on top of the leaf surface.

Control:Generally pear slugs do not become a pest. If they do, it isn’t until the second generation. Even though damage may appear dramatically, there may be no need to treat for this pest. This is where you need to put on your old English horsehair wig and pass judgment on these pests. If your tree has a high infestation but the tree is showing signs of the fall season, then kick back and relax. Your tree will soon drop its leaves anyway and managing this pest is not necessary. By doing so, you are not dooming your tree to another infestation next year. High mortality can occur in the overwintering pupae. Make a note to yourself to monitor for pear slug activity next season. This is where keeping a garden journal is useful. I don’t remember what I did yesterday let alone remember how many pear slugs where on my cherry tree last year. Keep a record of your observations; they will always be useful and maybe entertaining.

If your tree is carrying a high infestation of pear slugs and fall is a little further off, then you need to grab your old English gavel and start hammering the larvae. Handpicking pear slugs off small trees is your 100% effective treatment. If you don’t want to do the dirty work by hand, try a forceful stream of water from you garden hose. For trees that are larger, you can resort to an insecticide. Do so only if you are sure that the pest is still present and feeding and has not dropped to the ground.

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