Identification/definition:
Pear slugs (botanical name caliroa cerasi), also called ‘cherry slugs’ or ‘cherry tree slugs’, are not true slugs but the slug-like larvae of a type of insect, the black and yellow sawfly.
The ‘slugs’ are dark green to orange, swollen at the head end, and slimy, and when full-grown may reach 1/2 inch in length. They tend to lighten in colour as they grow older. They emit a slimy, olive green substance which gives them the appearance of shiny slugs (it is this material that the larvae exude which makes some of them appear dark green rather than the more common orange).
They are found on the leaves of many common trees and shrubs such as cotoneaster, cherries, hawthorn, apples and pears, in mid-late summer.
Life cycle:
The adult lays eggs in slits made in the leaf, which become larvae. After feeding on leaves (they become fullgrown in about three weeks), the larvae drop to the ground and dig into the soil, where they pupate. Adults emerge and fly to the leaves to mate and lay more eggs.
There are two generations a year: the first larvae appear on trees in early summer and become adults in December and January; the second generation (usually more numerous and widespread) is in March.
Damage:
Pear slug larvae feed on the upper leaf surface. They avoid the larger leaf veins and rarely penetrate the lower leaf surface. The resultant ‘skeletonised’ leaves have a characteristic appearance. Chewed areas of the leaf turn brown. When heavily damaged, the entire leaf falls prematurely. Damage is worst as larvae become fullgrown.
If/when to control:
Although some claim ‘when severe defoliation is threatened, pear slug injury should be controlled’ others note that, ‘severe pear slug injury’, while unattractive, ‘most often occurs late in the season’ and though it has the potential to completely defoliate susceptible plants, usually ‘has little impact on plant health’.
Where heavy infestations might reduce vigour, and lead to poor stone fruit yield and quality, slug control seems warranted.
Suggested solutions:
- encourage natural predators such as paper wasps, hoverflies, lacewings, spiders and insect-eating birds
- pick off (from individual plants)
- hose them off with a strong jet of water
- fence bantams/chooks under the tree during winter to destroy overwintering pupae
- trap emerging pupae by inning fine fly netting at base of tree to prevent them reaching leaves (if no vegetation under tree)
- band the tree with a horticultural glue to prevent slugs climbing back up
- dry them out using, for example
- ash (stand upwind and throw over)
- flour (stand upwind and throw over)
- derris dust (sprinkle over, if small tree)
- talc powder
- Dolomite lime (but consider soil pH)
- builders lime (easily mixed with water for spraying)
- vacuum cleaner dust
- spray with soaps or detergents (but some can injure plants such as plum and cherry)
- spray with insecticide
- Place a good, big handful of builders' lime into a bucket, wet it to a thin paste then fill the bucket with water to create a white milk of lime.
- Pour this through a piece of shade-cloth into another bucket to filter out all lumps.
- To spray, you can use a device with a small flexible tube that fits to the end of a garden hose and goes into the bucket of mix. When the tap is turned on, the lime mixture is sucked up by the passing water and sprays over the trees. Water pressure is sufficient to keep nozzles unblocked.
- Normal spraying devices can also be used, but the mix must be constantly agitated to prevent blocking spray nozzles.
Aussies Living Simply (forum)
W.S. Cranshaw and D.A. Leatherman, ‘Pear Slugs’
Colarado State University Extension
http://www.ext.colostate.edu/pubs/insect/05560.htmlLauri Jean Crowe, ‘Fruit tree pests: Pear slugs’
Peter Cundall ‘Slug it to those pests’
Weekly Times
Green Harvest, ‘Pear and Cherry Slug’
My Garden (forum)
Sophie Thomson (presenter), ‘Garden Woes’ fact sheet
Gardening Australia,
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