TDA 132 Not a shower of rain
Tree Damage Alert No 132
10th June 2009
Not a Shower of Rain.
If you go down in the woods this month be sure to take an umbrella! It is a number of years since we have had hot summer days with clear cloudless blue skies, but a sound of ‘rain’ falling through the trees. No its not global warming but the droppings of many thousands of herbivores – that is leaf eating caterpillars.
There have been predictions that the unusually cold weather last winter will have killed many of the insect pests. Insects that over winter above ground as an adult or larva are likely to have been damaged by the cold (e.g. Green spruce aphid – Elatobium abietinum). In contrast, any insect that was over wintering as an egg or pupa (e.g. Winter moth – Operophtera brumata) in the soil or on the plant above ground (e.g Green oak roller moth – Tortrix viridana) will have been better equipped to survive the cold.
Defoliation caused by caterpillars of a number of moth species has been locally very intense this year, and has affected a range of broadleaved tree species. Most species of such trees are well able to produce a second flush of foliage. An exception to this is Beech (Fagus sylvatica) which does not have a ready supply of dormant buds. In contrast, Oak (Quercus robur) is so well adapted to early season defoliation that, in a British summer, it regularly produces a second flush of growth with the shoot extension often exceeding that of the spring flush.
The visual impact of defoliation of Oaks and other species has, in some areas, been severe. Tree owners are asking questions about the future of these trees, some even believing that their tree has died. The likely result of the intense damage to broadleaved trees this year will be reduced growth – that is an unnoticeable effect. If the damage is repeated for several consecutive years individual trees could become more susceptible to other forms of damage (e.g. diseases).
What about the birds that rely on a supply of caterpillars to feed their young in the nest? Raising a brood of young birds of many species is dependent upon hatching in the nest coinciding with young caterpillars being available. Casual observations suggest that this year fledging of young birds has been very successful.
What about leaf miners, and particularly Cameraria ohridella, are we to expect a serious infestation this year? Pupation occurs in the fallen leaves and so there should have been some protection from the cold weather. There are already early signs of larval activity on the leaves of trees in Wimbledon where the pest was first recorded in 2002, and other locations. Reports that mines in Horse chestnut (Aesculus species) leaves may be appearing later than in previous years are unfounded.
By the time you read this TDA the caterpillars in some locations will have finished their feeding cycle and have already pupated. So the shower of frass will have ended and you should be able to enjoy walks in the woods without taking an umbrella during what remains of the typical British summer.
Derek Patch, Ben Holding (AAIS) and Christine Tilbury, Forest Research, Farnham.
AAIS l Alice Holt Lodge l Wrecclesham l Farnham l Surrey l GU10 4LH
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