Pear Slugs and Fruit Trees

Order: Hymenoptera (bees, ants, wasps, sawflies, parasitoids)
Family: Tenthredinidae (common sawflies)
Species: Caliroa crasei (pear slug)

Identification & Life History

What the heck is eating my fruit trees? Is it a snail? A slug? A freakish tadpole? It’s a pear slug! This isn’t your normal slug; in fact it isn’t even a slug at all. It’s the larva form of a sawfly.

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 & Monitoring

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.

Management

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.

Pear slugs are wimpy and appear to be susceptible to about any insecticide. Be sure that the insect appears on the label or the host tree and uses are listed on the pesticide label. This is important to not confuse ornamental uses and edible fruit tree uses. If you look at the WSU recommendations, there very extreme differences between the two types of trees. In fact, WSU does not recommend any chemical controls for pear slug on edible pear trees. Be sure to follow all cautions to avoid bee poisoning and do not spray insecticides during bloom, unless stated otherwise on the label. Although this data did not come from WSU Cooperative Extension, I have viewed data that shows diatomaceous earth and insecticidal soaps increase mortality of pear slug larvae. If you chose to experiment with these types of products, follow the label carefully. Test the product on just a few leaves first to be sure that there are no phytotoxic effects to your tree.

To reach Todd Murray please call (360) 676-6736 or e-mail him at: tmurray@wsu.edu