Winter moth caterpillars are responsible for eating holes in the leaves of many deciduous trees and shrubs during spring.
What are winter moth caterpillars?
Winter moth caterpillars eat holes in the leaves, blossom and developing fruitlets of many tree fruits, ornamental trees and shrubs. Severe attacks can weaken plants. Extensive damage to fruit trees can affect crop yields and quality.
The main fruit trees attacked are apples, pears, plums and cherries. Many ornamental trees are attacked as well, including oak, sycamore, hornbeam, beech, dogwoods, hawthorns, Sorbus, roses, hazels and elms.
You may see the following symptoms:
- Attacks by winter moth caterpillars are usually first noticed in spring when emerging leaves are loosely bound together with silk threads and eaten
- The damage is particularly noticeable in mid-summer when the leaves are fully expanded and the small holes made during the spring have enlarged due to leaf growth
- Blossom and developing fruitlets can also be damaged
- Early damage on apple fruitlets causes a deep cleft in the side of the fruits to develop by the time they have reached full size in late summer
Egg laying can be reduced by placing a sticky grease band around the trunk and tree stake in late October to intercept the females. Garden centres stock ready-prepared strips for tying round the trunk or grease for direct application to the bark.
Many birds, especially tits, feed their chicks with large numbers of winter moth caterpillars during the spring.
Shortly after bud burst - but not during flowering - apples, pears and small ornamental trees can be sprayed with deltamethrin (Bayer Sprayday Greenfly Killer) or lambda cyhalothrin (Westland Plant Rescue Fruit & Vegetable Bug Killer).
An organic alternative is pyrethrum (Py Spray Garden Insect Killer, Scotts Bug Clear Gun for Fruit & Veg) to control the young caterpillars. Caterpillar damage on tall trees has to be tolerated.
Pesticides for gardeners (Adobe Acrobat pdf document outlining pesticides available to gardeners)
Wingless female winter moths emerge from pupae in the soil during November to January and crawl up trunks to lay eggs on the branches.
Eggs hatch at bud burst and the pale green looper-type caterpillars emerge and start feeding. The caterpillars are up to 25mm (about 1in) long and complete their feeding by early June. They then go down into the soil where they pupate.
In some years oaks and other deciduous trees are largely defoliated during the spring by the caterpillars of winter moth and other species. Such trees will survive and produce more leaves during the summer.
Common name Winter moth
Scientific name Operophtera brumata
Plants affected Fruit trees, oak, sycamore, hornbeam, Sorbus spp., roses and many other deciduous trees and shrubs
Main symptoms Holes eaten in leaves, blossom and apple fruitlets
Most active Bud burst to early June