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 in early spring. Severe attacks can weaken young trees. 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 all 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 feed on most types of tree fruit and many deciduous trees and shrubs. The main fruit tree host are apples, pears, plums and cherries. Many ornamental trees are also hosts, including oak, sycamore, hornbeam, beech, dogwoods, hawthorns, Sorbus, roses, hazels and elms.

Symptoms

You may see the following symptoms:

• Winter moth caterpillars feeding is usually first noticed in spring when emerging leaves are eaten
• Feeding 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

Control

On non fruiting trees these caterpillars can be tolerated as any leaf damage does not affect the long term health of the tree. In addition many birds, especially tits, rely on these caterpillars to rear their chicks during the spring.

When choosing control options you can minimise harm to non-target animals by starting with the methods in the non-pesticide control section. If this is not sufficient to reduce the damage to acceptable levels then you may choose to use pesticides. 

Non-pesticide control

• Where possible tolerate populations of winter moth caterpillars, as moths are an important part of the garden ecosystem
• Encourage predators and other natural enemies in the garden such as birds, hedgehogs and ground beetles. These caterpillars are an important food source for nesting birds
• Egg laying on fruit trees 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. Grease bands and barrier glues must not include glues that are strong enough to entangle larger animals such as birds, bats or mice

Pesticide control

• Organic contact action sprays containing natural pyrethrins (e.g. Bug Clear Gun for Fruit & Veg, Neudorff Bug Free Bug and Larvae Killer)
• More persistent contact insecticides include the synthetic pyrethroids lambda-cyhalothrin (e.g. Westland Resolva Bug Killer), deltamethrin (e.g. Provanto Ultimate Fruit & Vegetable Bug Killer, Provanto Sprayday Greenfly Killer) provided the label instructions relating to harvest interval, spray interval and maximum number of applications are followed

Download

Pesticides for gardeners (pdf document)

Biology

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 with no long term effects on their health.