Pear slugs are not true slugs, they're actually larvae of the sawfly (Caliroa cerasi). In their larval stage sawflies eat tree leaves, especially the leaves of fruit trees like pears and cherries. Although easy to get rid of, pear slugs are considered a pest because of the minor damage they inflict on foliage.
Pear slugs got their name because of their slimy, squishy bodies and propensity to feed on fruit trees. These larvae are only half an inch long when fully grown. They are green or orange in color and have bulbous heads compared to the rest of their bodies. Adult sawflies are very small, only about a quarter-inch long, and totally harmless. They are shiny and black, and look similar to houseflies, but with much longer antennae.
Caliroa cerasi is an extremely widespread species. It originates from Europe but has been introduced to North America, South America, Eurasia, Asia and Australia. Sawflies can live wherever pear (genus Pyrus), cherry (Prunus), cotoneaster, hawthorn (Crataegus), Chaenomeles, Cydonia, mountain ash (Sorbus), maple (Acer), and Ceanothus trees can be found. The adults can fly, but the pear slugs stay on the same tree for the duration of the larval stage.
Pear slugs emerge from their pupae as adults after winter. In late spring the adults mate, the females lay eggs in slits made in the leaves, and the eggs hatch into pear slugs. The female's ovipositor is modified to cut slits into the leaf for egg deposition, which is how the sawfly got its name. It takes about three weeks for the slugs to reach their full size. Sawfly larvae feed through the summer until winter approaches, when they burrow into the ground, form their pupae, and hope to survive the coldest months underground. Larvae come in two waves, with one brood reaching full size in July and the next in September. In the northernmost parts of the sawfly's range only one brood is produced, late in the summer.
Unlike caterpillars, sawfly larvae don't eat the whole leaf—in minor infestations only the leaf's top side is consumed, and the leaf veins are avoided. Pear slugs leave brown spots on leaves where the juices have been sucked dry. Despite their choosy eating patterns, in more extreme cases pear slugs eat all of the leaf except the veins, causing it to die prematurely. Extensive foliage damage can stunt fruit growth. However, in most cases trees are not adversely affected by a C. cerasi infestation.
Purging Pear Slugs
Sawfly infestations aren't a serious threat to trees and aren't difficult to control. However, the unsightly brown spots these larvae leave behind are a nuisance to some. Pesticides used for leaf beetles and caterpillars are also effective on pear slugs, for instance carbaryl, malathion and spinosad. You can also remove C. cerasi larvae by spraying them off the tree's leaves with a strong burst from the garden hose, washing the leaves with soap and water, or coating foliage with wood ash. Care should be taken when using these methods on fruit-bearing trees to avoid contaminating the fruit.
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