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Winter moth – additional information

Life cycle

  • Adults occur from October to January but are most numerous in November and December.
  • On emergence from pupation, the spider-like female crawls up the trunk of the tree and after mating, lays 100-200 eggs singly in crevices in the bark.
  • Eggs hatch in spring mainly from bud-burst to green cluster, but some hatch later.
  • The tiny caterpillars are often blown from tree to tree and infestations may also spread to fruit trees from adjacent woodland in this way.
  • Feeding continues until late May or early June.
  • When mature, the larvae drop to the ground to pupate.

Pest status

Important pest of apple and pear. Attacks fruit directly and so is damaging at low population densities, though can be controlled readily with insecticides.

Other hosts

A wide range of deciduous and coniferous trees, especially oak.

Varietal susceptibility

All varieties of apple and pear are susceptible and host plant resistance is not known to occur.

Distribution

Widespread and common, especially in wooded areas.

Damage

  • Foliage and buds are devoured indiscriminately in spring by larvae, which bite holes in developing fruitlets.
  • These either drop prematurely or develop into malformed fruits with corky scars.

Recognition

Adult male
Flies at night in winter. Wingspan 22-28 mm, forewings rounded, greyish brown and with darker wavy cross-lines.

Adult female
Found on tree trunks and branches in winter. Wings reduced to stubs; body 5‑6 mm long, dark brown mottled with greyish yellow.

Egg
Occur singly in bark crevices; 0.5 x 0.4 mm; oval, pale yellowish-green, soon becoming orange red with pitted surface.

Larva
Looper habit, up to 25 mm long. Pale green with dark green dorsal stripe and several whitish or creamish-yellow stripes along back and sides, including a pale yellow line passing through the spiracles. Abdomen has two pairs of prolegs.

Other pests with which winter moth may be confused

Tortrix moth caterpillars

  • Tortrix moth caterpillars often occur in blossom trusses in spring and, when tiny, can be confused with winter moth caterpillars.
  • However, tortrix moth caterpillars have 5 pairs of abdominal prolegs.

Clouded drab moth caterpillars

  • Clouded drab moth caterpillars occur commonly in the shoots of apple post blossom and have a similar green colour appearance to winter moth but are quite different if examined closely.
  • They gave 5 pairs of abdominal prolegs.

Monitoring

Pre-blossom visual assessment

  • Inspect at least 100 trusses (e.g. 4 on each of 25 trees) per orchard for signs of damage or infestation by winter moth larvae at the green cluster to pink bud growth stage of apple.
  • Inspect trees at the edge of the orchard adjacent to woodland or hedgerows where the risk of infestation is high, as well as those in the centre and other parts of the orchard where the risk of infestation is lower.
  • If damage is seen, open up truss to see if a winter moth larva is present.
  • A hand lens should be used to distinguish small caterpillars of the winter moth (2 pairs of prolegs) from those of tortrix or noctuid (e.g. clouded drab) moths (5 pairs of prolegs).
  • Treatment with an insecticide is justified if 5% or more of trusses are infested.

Late blossom visual assessment

  • The same methods are used as for the pre-blossom visual assessment.
  • However, the treatment threshold is 3% of trusses infested, lower than the pre-blossom threshold.

Late blossom assessment using the beating method

  • Beat at least 20 branches per orchard over a beating tray.
  • The treatment threshold is 1 caterpillar or more per 20 beats.

Damage at harvest

  • The percentage of fruits with corky scars, characteristic of early caterpillar feeding, should be monitored at harvest and during grading.
  • If the percentage exceeds 0.1%, this is an indication that  control methods that season were not optimal and more effective measures are likely to be needed the following year.

Pheromone traps for adults

  • The female-produced sex pheromone of winter moth has been identified and lures containing synthetic pheromone can be acquired from specialist manufacturers and suppliers.
  • Pheromone traps placed in orchards from October to January often catch large numbers of males but these may be attracted over considerable distances from woodland and hedgerows and may have little significance in terms of populations in the orchard. For this reason they are not used commercially.

Grease banding of trunks

  • A band of a recommended grease may be pasted round the lower trunk of a sample of trees in September to October to monitor the number of ascending females in winter.
  • Note that grease bands may be phytotoxic to young trees.

Forecasting

Useful forecasting models for winter moth have not been developed.

Chemical control

Chemical control is the principal means of control in UK orchards. A wide range of insecticides are approved for control of caterpillar pests on apple. The pest is sensitive to insecticides and can be controlled cheaply and effectively.

  • It was common practice for growers to apply a pre-blossom spray of a broad-spectrum insecticide, usually chlorpyrifos (no longer approved on apple), to control early season caterpillars and aphids.
  • A spray of an approved insecticide should be applied at the green cluster to pink bud growth stage.
  • Later spraying is preferable because late hatching larvae are more likely to be killed.
  • Broad-spectrum chemicals are harmful or dangerous to bees and should not be applied during blossom.
  • Many other insecticides when applied before blossom will also control winter moth and have varying degrees of activity against different pests.
  • Diflubenzuron (Dimilin), indoxacarb (Steward) and methoxyfenozide are selective materials which are likely to control caterpillars only, and have little effect on aphids.
  • Indoxacarb (Steward) may give some control of capsids. Spinosad (Tracer) may also be effective.
  • Fenoxycarb (Insegar) is not a good choice because it only controls caterpillars in their latter stages of development and will not prevent early damage to buds, blossoms and fruitlets.
  • The synthetic pyrethroid deltamethrin (Decis) is also highly effective against winter moth but its use should be avoided because it is harmful to important orchard natural enemies including the orchard predatory mite.
  • Bacillus thuringiensis is the only material approved for organic orchards. It can be fairly effective, providing that temperatures are warm at and shortly after application so that caterpillars are feeding actively.

Insecticide resistance

  • As winter moth is abundant in woodland and wild places that are not treated with insecticides and which provide the source of infestation for the pest and because the pest has only one generation per annum, the probability of insecticide resistance developing is very low.

Cultural control

Spatial isolation

  • Winter moth is a denizen of woodland trees, especially oak.
  • The pest is much less of a problem in orchards which are isolated from such woodland and larger hedgerows.

Grease banding of trunks

  • A band of a recommended grease may be pasted round the lower trunk of each tree in September to October to prevent females ascending the tree in winter.
  • This method is effective, but labour intensive, and is not usually done in large-scale commercial orchards.
  • The band has to be renewed annually so that a sticky surface to the band is maintained.
  • To reduce labour costs, it might be appropriate to treat the trees round the periphery of the orchard only, or areas close to woodland where the risk of infestation is high.
  • Grease banding is harmful to some predatory insects, such as earwigs, which climb the tree from the soil in spring.
  • Also, it can be phytotoxic to young trees.

Natural enemies

Predators

  • Insectivorous birds and many species of polyphagous predatory insects feed occasionally on winter moth larvae.
  • However, their impact on populations of winter moth larvae in orchards is limited.

Parasitoids
Many species of parasitoid attack the larvae or cocoons of the winter moth and these are its most important natural enemies.

  • The tachinid fly Cyzenis albicans is one of the most common.
  • It lays up to 1000 eggs which are attached singly to leaves that already have some feeding damage by winter moth larvae.
  • The eggs are ingested with the food and the parasite larva soon begins to consume the host tissue.
  • Parasitism rates can be high (30-60%) when the density of winter moth larvae is high, but are much lower (<5%) in commercial orchards.
  • The ichneumonid parasitic wasp Agrypon flaveolatum, which attacks winter moth larvae, is another common species which may play a role in reducing populations.
  • Parasitic wasps are sensitive to broad-spectrum insecticides, which are especially harmful to adults.

Bacteria

  • Bacillus thuriengiensis is a pathogen of winter moth larvae but infections are normally associated with applications of the bacterium as a biological control agent.

Viruses

  • Nucleopolyhedroviruses and cypoviruses have been recorded from winter moth.

Biological control

One or more sprays of Bacillus thuringiensis (Bt) before blossom will control winter moth larvae, providing temperatures are high enough for the caterpillars to be feeding actively.

  • The bacteria and the crystal toxin which it produces, have to be ingested in order to act.
  • The main problem is that caterpillars are often feeding in or amongst the buds or in furled rosette leaves where they are inaccessible to sprays.
  • The bacterium is degraded by heat and UV light so is of short persistence.
  • For these reasons, it is probable that more than one spray will be required for a high standard of control.
  • Bacillus thuringiensis is harmless to bees and may be applied during blossom if necessary.

Further reading

Briggs, J. B. 1955. Notes on the biology and identification of some allies of the winter moth (Operophtera brumata (L.). Report of East Malling Research Station 1955, 141-146.

Briggs, J. B. 1957. Some features of the biology of the winter moth (Operophtera brumata (L.)) on top fruits. Journal of Horticultural Science 32(2) 108-125.

Hand, S. C., Ellis, N. W. & Stoakley, J. T. 1987 Development of a pheromone monitoring system for the winter moth, Operophtera brumata (L.), in apples and in Sitka spruce. Crop Protection. 6: 3, 191-196

Hardman, J. M;. & Gaul, S. O. 1990. Mixtures of Bacillus thuringiensis and pyrethroids control winter moth (Lepidoptera: Geometridae) in orchards without causing outbreaks of mites. Journal of Economic Entomology.83: 3, 920-936

Holliday, N. J. 1977. Population ecology of winter moth (Operophtera brumata) on apple in relation to larval dispersal and time of bud burst. Journal of Applied Ecology 14: 3, 803‑813.