Pest Name : commonly referred to as "slug" due to appearance, but is correctly the larva of a sawfly, Caliroa cerasi. (Order Hymenoptera).
Appearance of Pest : The adult sawfly is 7mm long, glossy and black. Females have a saw-like ovipositor at the end of the abdomen which is used to make a slit in leaves and deposit eggs.
The larva or "slug" is what causes the damage. The slug-like appearance comes from the smooth, green slimy exudate what it produces whilst feeding. When fully-fed, the slime is shed and they become orange-yellow. When fully grown, the larva measures 1.3cm in length. It has a body that it slightly enlarged in front and tapered at rear.
Life Cycle : Complete metamorphosis (the insect hatches from egg in a totally dissimilar from to adult), with 2 generations per year. Pest hibernates as larvae in soil, in spring (late Oct - Nov) adult emerges and lays its eggs in leaves. In two weeks the eggs hatch, larvae thus seen in Nov-Dec. Larvae feed for 3 weeks, then crawl or fall into soil to form pupae. Adult appear again in late Dec-Jan and lay eggs giving rise to the second generation of leaf chewing larva in summer through to autumn. They are generally much more numerous and destructive second time around. They overwinter in soil. Favours cool, moist conditions - adults can only emerge from soil if it moist and larvae can dry up quickly on leaf surface when dry. Spread is by adults flying or being carried by wind - adults can travel large distances.
Damage : Leaves only are attacked. In wet weather can fed on undersurface of leaves. The larvae feed mostly on upper surface, leaving veins and lower epidermis - creates the characteristic "window-paning." Leaves can turn brown, shrivel and fall. Continued heavy infestation can severely weaken trees and reduce cropping.
Host Plants : Ornamental trees, fruit trees, shrubs and climbers : eg. pear, cherry, plum, sometimes peach, nectarine, almonds, quince, hawthorn, cotoneaster, hardenbergia
Controls : non-chemical : Control first generation - avoid overhead watering so that leaves are not wettened, slugs may be squashed manually if only a few trees, fly netting on the soil under the tree can trap emerging flies. Cultivating soil under tree can disrupt and destroy pupae. There are no biological controls.
chemical : Soap-based or Oil sprays, eg Pestoil, botanical extracts eg. rotenone (Derris Dust) - no longer recommended due to a reported increase in Parkinsons Disease, carbamates (eg. carbaryl), organophosphates. Sprays are only justified if the tree is dying or severely affected or in times of drought when defoliation is not tolerated.
As the pest does not affect fruit, often nothing needs to be done. Natural predators include a paper wasp; birds like blue wrens, shrike thrushes, yellow robins
References : Kerruish A117
natural control of garden pests, Jackie French
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