I have received this information sheet from Christine Tilbury via Sandra Denman
sandra.denman@forestry.gsi.gov.uk
OAK DEFOLIATION
Every year there is some defoliation of oak trees by caterpillars but this phenomenon varies greatly in severity;
sometimes quite large areas may be affected for several years in succession. Both Quercus robur and Q.petraea
(pedunculate and sessile oak) are affected but other oaks may be less susceptible. The damage happens soon after
the the trees have flushed and there can be complete defoliation by late May or early June in south-east England, a
little later in the north and west.
THE INSECTS RESPONSIBLE
Very few of about two hundred species of lepidopterous larvae ( moth caterpillars) that feed on oak cause serious
defoliation. When extensive defoliation is reported the most frequently encountered species are Operophtera
brumata (Winter moth), Erranis defoliaria (mottled umber moth) and Tortrix viridana (oak leaf roller moth).
Operophtera brumata larvae are green with a darker dorsal stripe and two white and one yellow line on either side.
They have a brownish head and are about 15mm long when fully grown. The larvae of Erranis defoliaria are
slightly larger, up to 20mm long, and are brown on top with dark wavy lines and are yellowish underneath. Both
species belong to the family Geometridae; characterised by larvae that walk with a characteristic looping action due
to them only having one pair of legs and a pair of claspers at the hind end rather than the four pairs plus claspers of
most other Lepidoptera. The larvae eat the expanding leaves in the spring, spinning several leaves loosely together
to conceal themselves when not feeding. They are fully grown by late May or early June, rather later in the northern
Britain, when they descend to the ground on a silken thread and pupate in the soil. The brown adult moths emerge in
late autumn but only the males fly. The females are flightless, with only vestigial wings, and crawl up the trunks to
lay their eggs near buds on oaks and many other species of tree and shrub. Eggs hatch at around the time oak buds
begin to burst open.
Tortrix viridana larvae, which are grey-green with black spots and a brown to blackish head, also hatch and start
feeding in oak buds as they open. Later the larvae roll or fold over part of a leaf, fixing it with silk to form a
protective cover within which they feed and later pupate. When the populations are high crowding often forces
larvae to move to another leaf and they descend on silk threads to the lower branches. When defoliation is total some
may drop down onto other plants beneath the oaks and may even feed there before pupating in a folded leaf. The
adult moths have distinctive green forewings, and pale grey hindwings with a 17-24mm wingspan. They fly from
late June to mid-July, or earlier in advanced seasons, and, although mainly nocturnal, are easily disturbed from the
green foliage during the day. The females lay their eggs on twigs high in the crown of the tree where they remain
until they hatch the following spring.
THE DAMAGE CAUSED
Operophtera brumata, Erranis defoliaria and Tortrix viridana are frequently found feeding in the same trees
although the species can occur singly. They can cause complete defoliation that may occur for several years in
succession before larval numbers decline due to the action of insect parasites, predators and diseases. Trees are not
killed by these insects and, despite the bare wintry look, defoliated oaks will reflush in early summer and regain a
green canopy. While individual mature trees that have been defoliated on many occasions in the past have suffered
no permanent damage there is a reduction in growth and trees the chance that trees may become weakened and more
susceptible to damage by secondary agents e.g. oak mildew. Treatment of larger trees is neither practical or
necessary but young trees, not yet established, especially when growing underneath mature oaks supporting a high
population might need an application of an approved insecticide.
Operophtera brumata and Erranis defoliaria can be significant pests on fruit trees. Egg laying can be reduced by
placing a sticky band around the trunk and tree stake in late October to intercept the females. Ready prepared sticky
bands or grease for direct application to the trunks is available in Garden centres.
Christine Tilbury
Tree Health Division
Plus information from Sandra Denman for which I am grateful and would like to pass on to your forum
Thank you for your e-mail. I have sought the advice of our entomologist diagnostician. In her opinion the caterpillars (larvae) were likely to be the winter moth (Operophtera brumata). I attach a note from Christine on this topic.
It is worrisome that you say the trees did not re-sprout or put out summer foliage. We would expect them to. I think we need to monitor the situation. Although winter moth is a polyphagous species I am not sure whether it has been recorded from all the tree species you mention.
It would be helpful if you could monitor and record the situation this year. Take careful not of first appearance (date and tree species) of the caterpillars. Take good photographs. Catch some from the different host species and place them in vials (a separate one for each tree). This should be done in spring and again in summer if the caterpillars persist throughout the season. Please send them to Christine at Alice Holt so that we can keep a record of the site. Also it would be useful to know if powdery mildew appears on the leaves of the oak trees and if so when you notice this.
Many thanks
Sandra
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