Winter moth

Identification

A moth with marked sexual dimorphism: the male has grey-brown, finely banded wings with a span of about 25 mm, whereas the female is practically wingless and flightless and therefore climbs up the trunk in autumn to lay its eggs in the crown. In spring green looper caterpillars with pale lengthwise stripes hatch, moving with the characteristic "looping" gait (arching the rear of the body) and eating the developing buds, young leaves and fruitlets, often webbing them together with threads. It feeds on apple, pear, plum, cherry and many deciduous trees and shrubs. The mature caterpillar reaches about 25 mm in length and is yellow-green with a darker dorsal line and pale stripes along its sides; as a typical looper (Geometridae) it has only two pairs of prolegs at the end of the abdomen, hence its looping gait. The eggs are laid singly near the buds and in bark crevices, where they overwinter, and just before the spring hatch they change colour from greenish to orange-red. The species has one generation a year: the moths fly in late autumn (October-December, which is the origin of the Polish name "przedzimek", meaning before-winter), on cool, frost-free evenings, and the flightless females then climb the trunks. The spring hatch of the caterpillars is closely synchronised with bud burst (usually April); after about 4-6 weeks of feeding the caterpillars lower themselves to the ground on threads and pupate shallowly in the soil, where the pupa spends the summer. The signs of feeding are holed and ragged young leaves, eaten flower and leaf buds, and deformed fruit covered with corky scars that develop from the damaged fruitlets; under heavy pressure the caterpillars can almost strip the crown bare.

Treatment

The key and proven measure is fitting glue bands to the trunks in autumn (from October), which catch the wingless females climbing into the crown - this barrier greatly reduces egg-laying. In spring, sprays of Bacillus thuringiensis var. kurstaki are used against young caterpillars, ideally at bud burst. Protecting and attracting insectivorous birds (nest boxes) and other natural enemies also helps; chemical control is reserved for orchards under heavy pressure and is carried out before flowering. Fit the glue bands low on the trunk and keep them in place from October right through to spring, renewing the sticky surface regularly, because once it dries out or becomes dusty it stops catching; it is worth banding stakes and supports too, since the females also use them to reach the crown. The bands double as monitoring - the number of females caught indicates the scale of the threat, while the flight of the males can be tracked with pheromone traps. Bacillus thuringiensis acts selectively on the young caterpillars once they eat the sprayed tissue; it is safe for bees and predators but has to hit the early stages and needs temperatures above about 15 degrees C; the treatment is aimed at the bud-burst and green-bud stage and is repeated if the hatch is prolonged. Natural enemies genuinely limit the population - tits and other insectivorous birds, parasitic wasps and tachinid flies (Tachinidae) that destroy the caterpillars, and predatory ground beetles that eat the pupae in the soil - but this is preventive support, not emergency firefighting. Shallow loosening or digging over of the soil beneath the crowns in late summer also helps, damaging the pupae that are then resting shallowly in the ground (the winter moth overwinters as an egg on the bark and spends the summer in the soil as a pupa). Chemically, only under heavy pressure, spraying is done at the pink-bud stage and strictly before flowering (the products are toxic to bees): with contact pyrethroids or with more selective insect growth regulators (chitin synthesis inhibitors) acting on the young larvae. In home orchards glue bands and the bacterial product are usually sufficient, without resorting to chemicals.

How to tell it apart from look-alikes

Control — organic and chemical methods

We give active-ingredient groups and modes of action, not brand names — specific products and their approval vary by country and are sometimes withdrawn. Always check and follow the current label.