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 yield and quality.

Winter moth is a name that can be used for a number of species that have adult moths that emerge and lay eggs between November and April. These moths have wingless females that emerge from pupae in the soil and crawl up trunks to lay eggs on branches. The most important are the winter moth (Operophtera brumata), mottled umber moth (Erannis defoliaria) and March moth (Alsophila aescularia). The caterpillars of these moths hatch in the spring as  buds are opening and they will attack most types of tree fruit and many deciduous trees and shrubs.  The main fruit trees attacked are apples, pears, plums and cherries. Many ornamental trees are also hosts, 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 eaten
• The damage can be particularly noticeable in mid-summer when the leaves are fully expanded and the small holes made during the spring have enlarged with leaf growth, at which point the caterpillars have left the tree
• Blossom and developing fruitlets can also be damaged
• Early damage on apple fruitlets can cause a deep cleft in the side of the fruits to develop by the time they have reached full size in late summer

Non-chemical control

Egg laying can be reduced by placing a sticky grease band or barrier glue around the trunk and tree stake (if present) in October to intercept the wingless females. This needs to be kept sticky and free of detritus until mid-April. Many birds, especially tits, feed their chicks with large numbers of winter moth caterpillars during the spring.

Chemical control

• The need for chemical control can be reduced or eliminated by the use of grease bands and tree barrier glues 
• Caterpillar damage on tall trees has to be tolerated
• If necessary shortly after bud burst - but not during flowering due to the danger to pollinators – some fruit and small ornamental trees can be sprayed with deltamethrin (e.g. Sprayday Greenfly Killer) or lambda-cyhalothrin (e.g. Westland Resolva Bug Killer), provided the label instructions relating to the food plant are followed. Ornamental trees can also be sprayed with cypermethrin (e.g. Py Bug Killer). An organic alternative is pyrethrum (e.g. Bug Clear Gun for Fruit & Veg or ecofective Bug Killer (also contains fatty acids)) to control the young caterpillars, this can be used on all fruit trees provided instructions are followed
• Inclusion of a pesticide product does not indicate a recommendation or endorsement by the RHS. It is a list of products currently available to the home gardener

Download

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 April 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.

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