Pear Sawfly

Description

All wasps undergo complete metamorphosis, meaning they have four life stages: egg, larva, pupa, adult. Damage to the tree is caused by larvae as they feed on leaf tissues. Pear sawfly larvae are often called “pear slugs” because of their resemblance to true slugs. Early instar larvae are grey to green in color and secrete a slime which gives them a slug-like appearance. Mature pear slugs are about 9.5 mm (3/8 inch) long and green to orange in color with a caterpillar-like appearance. 

Quick Facts

  • The pear sawfly (Caliroa cerasi) is a wasp (order: Hymenoptera, family: Tenthredinidae). Wasps are in the same order as bees and ants. 
  • Pear sawfly larvae are often called “pear slugs” because of their physical appearance (see image below). However, they are not true slugs. 
pear sawfly adult

Pear sawfly (Caliroa cerasi) adult. Despite the misleading common name, these insects are wasps, not flies.
Whitney Cranshaw, Colorado State University, Bugwood.org

Pear sawflies (Caliroa cerasi) attack the leaves of several fruiting trees including pear, plum, and cherry. Adults are yellow and black and are a little larger than a house fly. The larvae feed on leaf tissue between veins, leaving only a leaf skeleton behind.

pear sawfly larva

Pear sawfly (Caliroa cerasi) larva. Note the slug-like appearance.
Gyorgy Csoka, Hungary Forest Research Institute, Bugwood.org

Life History 

Pear sawfly adults emerge from cocoons in late spring or early summer. They are slightly larger than a common house fly and are yellow and black in color. The adult wasps begin laying eggs in the upper portion of leaves on preferred host plants. The larvae hatch one to two weeks later and begin feeding on leaf tissue for about four weeks. Pear sawfly larvae consume the tissue between leaf veins, a feeding behavior that results in skeletonization of the leaf. After feeding for about four weeks, larvae pupate in the soil. 

A second generation of pear sawfly adults emerge later in the summer and is usually the leading cause of high population densities and greater potential for crop injury later in the growing season. When populations become large enough, every leaf on the tree can be visibly injured, though this is not usually fatal. Larvae from this second generation will overwinter in the soil as pupae, giving rise to first generation adults the following year. 

Injury

Damage to the tree occurs from larvae feeding on leaves which turn brown. Leaf curling and defoliation can also occur. While not fatal to healthy and well-established trees, this can affect the plant’s physiology and cause stress or weakening. 

Monitoring

Trees should be monitored for slug-like larvae later in the growing season (August to September) when population densities tend to increase. 

peach tree sawfly injury

Peach leaves damaged by pear sawfly (Caliroa cerasi) larvae.
Jerry A. Payne, USDA Agricultural Research Service, Bugwood.org

Cultural Control

Larvae can be physically plucked from leaves and placed in a bucket with soapy water, or they can be flushed off the plant with a strong stream of water. 

Chemical Control

Bacillus thuringiensis (Bt) applications and soil drenches are not effective management options for this pest. 

Insecticides (see resources below for details) should only be used when larvae from either generation are present; insecticidal sprays will not affect larvae after they enter the soil to pupate. Avoid applying insecticides during bloom periods when pollinators are active. It is always recommended that you rotate insecticides that are applied each year in order to prevent insecticidal resistance from developing over time. 

References

Jones, V. & Davis, S. 2011. Pear Sawfly. Utah State University: Utah Pests Extension. Available https://extension.usu.edu/pests/research/pear-sawfly 

Pellitteri, P. 2010. Pear Slug (Pear Sawfly). University of Wisconsin-Madison: Wisconsin Horticulture, Division of Extension. Available https://hort.extension.wisc.edu/articles/pear-slug-pear-sawfly/ 

Iowa State University. 2022. Pear Sawfly or Pearslug. Iowa State University: Extension and Outreach, Horticulture and Home Pest News. Available https://hortnews.extension.iastate.edu/pear-sawfly-or-pearslug 

Smithsonian Institution. (n.d.). 1996. True Bugs (Heteroptera). Smithsonian Institution. Available https://www.si.edu/spotlight/buginfo/true-bugs