Winter moth caterpillars

Winter moth caterpillars can be responsible for eating holes in the leaves of many deciduous trees and shrubs during spring.

Winter moth caterpillar

Quick facts

Common name Winter moth, Mottled umber moth and March moth
Scientific name Operophtera brumata, Erannis defoliaria, Alsophila aescularia
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

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 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 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

Non-pesticide 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, reply on these caterpillars to rear their chicks with large numbers of winter moth caterpillars during the spring. 

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.

Pesticide 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 trees can be sprayed with natural pyrethrum (considered organic e.g. Bug Clear Gun for Fruit & Veg, Ecofective Bug Killer) or the synthetic pyrethroids lambda-cyhalothrin (e.g. Westland Resolva Pest Killer) or deltamethrin (e.g. Provanto Ultimate Fruit & Vegetable Bug Killer), provided the label instructions relating to the food plant are followed.
  • Inclusion of a pesticide product does not indicate a recommendation or endorsement by RHS Gardening Advice. 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)

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. 


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