Pear slugs or pear sawfly (Caliroa cerasi), are also known as cherry slugs. They are pests of several fruit trees primarily pear and cherry (including purple leaved sand cherry), but also plum, quince, serviceberry, Juneberry and occasionally apple. They also feed on a few ornamentals such as cotoneaster, hawthorn, buttonbush, and mountain ash. Pear slugs are not actually slugs but rather the slug-like larvae of a type of wasp. The adult wasps are referred to as sawflies, due to the saw-like appearance of their ovipositor (a specialized organ for depositing eggs into plant tissue). The larvae feed by scraping the leaf tissue off of the upper surface of leaves leaving the veins intact. This type of feeding damage is called skeletonizing. In mass they can completely defoliate a tree. Pear slugs are native to Europe but occur almost worldwide. This species is not to be confused with another species of pear sawfly Hoplocampa brevis, which feeds exclusively in the fruit of pear trees during spring.

Meet the Pest

Larvae and Larvae Damage:

The pear slug larvae pass through 5 instar stages in all. In the first 4 stages they have a slug like appearance: club shaped (wider at the front and tapering towards the backend), with no distinctive legs. They are olive green in colour, but the first 4 instar stages are covered with a slimy mucus that makes them appear shiny and blackish in colour. This slimy substance is unpalatable to their predators. The 5th instar appears quite different from the others. It is a caterpillar-like larvae measuring up to 1.25cm (1/2 inch long), with 10 distinctive legs and is a greenish orange colour.

The larvae feed on the upper surface of the leaves, consuming the epidermis (outer layer of plant tissue) but leaving the veins intact. The underside of the leaf in this damaged area dries, and turns beige, giving it a scorched appearance. If their populations are high, they can defoliate the entire tree or shrub.

Adults:

The adult sawflies are a non-stinging type of wasp that appear shiny black in colour with 2 antennae that each have 9 segments. They have four clear wings, with the forewings having a pterostigma (a darker, thickened spot on the upper margin of the forewings that assists the insect with gliding). Size wise they are about .5cm (1/5 in) long, just slightly larger than a house fly. Males are rare as the females are able to reproduce without them (parthenogenically).

Life Cycle of Pear Slugs

The adult sawflies begin emerging from the soil either in late spring, in warmer zones, or from mid-June until mid-July in colder zones. The females lay their eggs parthenogenically (without fertilization), on the underside of leaves, which appear as small blisters on the upper leaf surfaces. These eggs hatch in about 1-2 weeks. The newly hatched larvae move to the top of the leaf to feed on the epidermis. They will go through 5 instar stages, after which they will drop to the ground to pupate in the soil. The new adults will begin emerging 2-3 weeks later or the 5th instar will remain in the soil to overwinter, enclosed in a cocoon. There are 1-2 generations per year depending on local, with the warmer regions having 2 generations. In those regions it is the second generation, that occurs in the fall, that is the most damaging.

Management and Control of Pear Slugs

Cultural Practices:

• Keep susceptible trees and shrubs properly watered, fertilized and well cared for. Healthy plants are better able to ward of insect attacks.
• Cultivate around susceptible plants in early spring and fall to help destroy the pupae in the soil.

Natural remedies:

• Hand remove the larvae, dropping them into a bucket of warm soapy water.
• Use a strong blast of water to wash them off the leaves.

Natural and Organic Insecticides:

• Like true slugs and snails, pear slugs are very susceptible to desiccation.  Applying a material such as diatomaceous earth to the leaves of the host plant will cut up the larvae’s belly causing them to die of dehydration. Other coarsely textured powders may also work such as wood ashes, sand or dirt.
• Safer® Soap can be sprayed directly on the insects, which serves to breakdown the insects’ protective outer layer leading to dehydration and death.
• Spinosad, which comes from naturally occurring soil bacteria that is fermented to create 2 liquid chemicals called spinosyn A and spinosyn D have tremendous insecticidal properties when combined. Spinosad affects the insect’s nervous system and leads to paralysis and eventually death. NOTE: this product is not available in Canada.
• Kaolin clay products can be used to help provide a barrier between the insect and the leaf.

Chemical insecticides:

• Most conventional insecticides ((such as carbaryl (Sevin, Sevimol) and malathion), are effective at controlling pear slugs. NOTE: in many regions of Canada these sprays are not permitted for homeowners.

Photo credits: all photos taken by the author unless otherwise indicated.

References:

British Columbia Ministry of Agriculture, (2020). Pear Slug/Pear Sawfly (Caliroa cerasi). https://www2.gov.bc.ca/assets/gov/farming-natural-resources-and-industry/agriculture-and-seafood/animal-and-crops/plant-health/phu-pear-slug-pear-sawfly.pdf

iNaturalist.ca n.d.). Cherry Slug Sawfly Caliroa cerasi. https://inaturalist.ca/taxa/386969-Caliroa-cerasi

Jones V.P., Ryan S. Davis R.S. (2011). Pear Sawfly. https://extension.usu.edu/pests/research/pear-sawfly

Murray M., and Davis R. (2019). Pear Fruit Sawfly (Hoplocampa brevis); Utah Pests Fact Sheet; Utah State University Extension and Utah Plant Pest Diagnostic Laboratory ENT-196-18-PR No. https://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=2901&context=extension_curall

Natural Resources Canada, (modified: 2015-08-04). Pear Sawfly (Pear slug). https://tidcf.nrcan.gc.ca/en/insects/factsheet/7688

Nature Spot, ((n.d.). Pear Slug Sawfly – Caliroa cerasi. https://www.naturespot.org.uk/species/pear-slug-sawfly

The Robert H. Smith Faculty of Agriculture Food and the Environment, (Updated 3.9.2015). Caliroa cerasi L. http://www.agri.huji.ac.il/mepests/pest/Caliroa_cerasi/

University of Saskatchewan, (2018). Pear slug or pear sawfly. https://gardening.usask.ca/articles-and-lists/articles-insects/article-pear-slug-pear-sawfly.php

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