The pear slug, or cherry slug, is not a slug at all, but instead the larvae of the pear-leaf sawfly (Caliroa cerasi). In its larval stage, C. cerasi feeds on the leaves of such trees as hawthorn, cotoneaster and ash, as well as fruit trees such as pear, cherry, plum and apricot. Usually benign, the pear slug nevertheless has the potential to impact fruit harvests and should be monitored closely.
The pear slug feeds only on the upper surface of the leaves, and only in the area between the veins. These chewed-on regions turn brown while the veins remain green and intact, resulting in a tell-tale "skeletonized" appearance.
Two generations of pear slugs appear over the course of a year, and it's the second generation whose damage is the most noticeable. That late in the season, pear slugs pose little threat to tree health. Though the skeletonized leaves may be unattractive, those leaves will drop in the normal course of autumn anyway.
Damage from pear slugs feeding can cause leaves to drop prematurely. A severe infestation of pear slugs can potentially defoliate the tree entirely. This can have a negative effect on the tree's health.
If skeletonized leaves start appearing in greater numbers than usual, or if damage becomes noticeable earlier in the summer than usual, you'll need to take action to prevent defoliation. A forceful stream of water is enough to dislodge the pear slugs without harming the tree. An application of wood ash will kill C. cerasi larvae without fail.
Negative Impact on Harvest
Extensive defoliation caused by the second generation of pear slugs may reduce the amount of blooms you'll see next year. That's bad enough in ornamental trees, but in fruiting trees it means a reduced harvest. And both generations of C. cerasi larvae can have an impact on the current season's crop, resulting in smaller fruit. If you're counting on a sizable fruit harvest, you can't afford to ignore the pear slug.