Insect defoliation and damage

Insect larvae are effective herbivores that can defoliate - and kill - entire trees and stands. In North America, insects and pathogens affect an area almost 50 times larger than fire with an economic impact nearly five times as great (Dale et al., 2001). Insect activities in this respect have been observed and studied in many parts of the world. In Britain, forests suffered severely in 1979, 'the year of the caterpillar', when defoliation of oak and other hardwoods affected whole hillsides in North Wales, mid-Wales, Cheshire, Lancashire, Devon and Dorset. Harding (1992) conducted a long-term study of the oak canopy of Chaddesley Woods National Nature Reserve (NNR) near Kidderminster, Worcestershire. This followed severe defoliation of broadleaves in 1979 and especially 1980. At the end of May 1980, following a very warm month with virtually no rain, numerous oaks, hazels and even ash were completely defoliated, though some oaks which had only just begun to flush escaped attack (Harding, 1992). Falling frass (insect faeces) could be heard hitting the dry litter, while trunks were festooned with silken skeins produced by caterpillars descending to pupate or find food. The insects involved were mainly 'looper' caterpillars (Geometridae, including the winter moth Operophtera brumata, and the mottled umber Erannis defoliara), which together with the green oak roller Tortrix viridana are well known for causing severe devastation, often for several successive years (Evans, 1984).

Defoliation in oaks is greatly influenced by the order in which individual trees produce their first leaves. At Roudsea Wood NNR, Cumbria, for example, Satchell (1962) found that certain individual trees retained relatively undamaged canopies in years of generally severe defoliation; his conclusion was that this resistance to attack was based on the phenology of oak bud flushing relative to insect egg hatch. Early trees can produce more foliage so they can 'grow away' from insect attack, while buds of late flushers open too late for the young larvae. Severe defoliation occurs most commonly on trees that flush at the same time as the larvae emerge. Such phenological sequences reflect genetic variation between individual trees, a phenomenon confirmed by Crawley and Akhteruzzaman (1988) who reported that the flushing order of 36 oaks at Silwood Park was consistent from year to year of a 7-year study.

Leaves of the dominant trees of temperate woodlands become tougher as they age, and also tend to possess increased concentrations ofsuch compounds as resins and tannins which are distasteful to herbivores. Figure 5.3 illustrates both the phenology of insects that feed on oak and beech in the UK and the way that the tannin concentration of oak leaves develops during the season. Most forest Lepidopteran pests feed on new foliage in spring; interestingly winter moth caterpillars (b in Fig. 5.3) reared on oak leaves picked at the end of May developed into smaller - and thus less fertile - adults than those fed on younger leaves which had less tannin and so more available nitrogen. Figure 5.4 illustrates the caterpillar counts in 24 frass traps under six of the Chaddesley oaks during a period which included the major outbreak of 1990. Defoliation was total in 1980, and ranged from 30-78% between 1982 and 1990. Subsequent values failed to reach 10% until 2002, and rose to 60% in 2005. The large peak expected in 2002 failed to materialize.

Harding (1996, 2000, 2002) provides data regarding defoliation in eastern Europe. In Hungary, where oaks form 35% of the total forest area, forest owners report occasional major defoliation by Lepidoptera, notably by winter moth Operophtera brumata, mottled umber Erannis defoliaria and E. aurantiaria. The cyclic nature of these attacks which, during the period 1961-1998, showed really major peaks in 1962 and 1963 and lesser ones in 1972, 1983, 1993-5 and 2004, is compared with those in Poland and at Wytham Woods, Berkshire, UK in Fig. 5.5. In the Strict Reserve of the BialowieZa Forest, Poland, there are in some years areas of lush nettles Urtica dioica, one reason for this being nutrient enrichment from geometrid caterpillar frass. This increased growth of herbs affords shelter to small mammals, leading to adverse effects on songbirds subjected to more than usual attacks by birds of prey attracted by the small mammals. Defoliation of broadleaves, of which hornbeams were a major component, was near 80% in 1992/3 and though no significant damage was reported in the period 1996-99, the cycle then resumed and peaked in 2003. As in Hungary and Wythan Woods (a and c in Fig. 5.5),

Mall Images Insects

Figure 5.3 Phenology of foliage-feeding insects on oak and beech. (a) Tannin content of leaves of pedunculate oak Quercus robur.(b)-(e) Phenology of four species of Lepidoptera on oak (from Varley, 1967). (f) Rhynchaenus fagi; beech leaves are mined by the larvae and skeletonized by the adult weevils (data from Nielsen, 1978, Natura Jutlandica 20, 258-72.) E, egg; P, pupa; A, adult; L, larval growth periods, shaded. (From Packham and Harding, 1982. Ecology of Woodland Processes. Edward Arnold.)

Figure 5.3 Phenology of foliage-feeding insects on oak and beech. (a) Tannin content of leaves of pedunculate oak Quercus robur.(b)-(e) Phenology of four species of Lepidoptera on oak (from Varley, 1967). (f) Rhynchaenus fagi; beech leaves are mined by the larvae and skeletonized by the adult weevils (data from Nielsen, 1978, Natura Jutlandica 20, 258-72.) E, egg; P, pupa; A, adult; L, larval growth periods, shaded. (From Packham and Harding, 1982. Ecology of Woodland Processes. Edward Arnold.)

there were approximately 10-year intervals between peaks. Only a small proportion of temperate forest insects go through these regular cycles: what causes them to do so? Berryman (1996) and others conclude that regular cycles of forest Lepidoptera, including winter moth, are most likely the result of interactions with parasitoids, predators or diseases, rather than through delayed induced defences in the tree host, but there is still much to learn about this complex and important phenomenon.

Not all cycles are regular, however. The eastern spruce budworm Choristoneura fumiferana, a lepidopteran native to North America, normally occurs at low densities but periodically (at approximately 35-year intervals) it

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