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Figure 5.4 Numbers of geometrid caterpillars found in a 3-week peak activity period at the end of May/mid-June in 24 frass traps under six oak trees in Chaddesley Woods, Worcestershire, UK, over a 15-year period 1987-2002. (Drawn from data of Harding, 2002. Quarterly Journal of Forestry 96.)

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20000

10000

70000

60000

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10000

Imagines Ecology Center

rv> oS c$> ri> rv & S> <& <& rp rp

Figure 5.5 Long-term monitoring of geometrid caterpillars. (a) Annual areas (ha) of Hungarian forests defoliated between 1961 and 2005. (Csoka, pers. comm.); (b) Index of abundance of geometrids - Operophtera fagata, O. brumata, Erannis defoliaria, E. marginaria, E. aurantiaria, Phigalia pedaria, - on hornbean Carpinus betulus in May, BialowieZa Forest, Poland between 1975 and 2005 (Jaroszewicz and Wesolowski, pers. comm.). This figure also shows a caterpillar descending to pupate; (c) Numbers of winter moth O. brumata caterpillars trapped beneath five oaks at Wytham Woods, UK (mean no. m~2) from 1952-2005 (Cole, pers. comm.). No data for 1974, 1977-82. (Data compiled by David J. L. Harding.)

rv> oS c$> ri> rv & S> <& <& rp rp

Figure 5.5 Long-term monitoring of geometrid caterpillars. (a) Annual areas (ha) of Hungarian forests defoliated between 1961 and 2005. (Csoka, pers. comm.); (b) Index of abundance of geometrids - Operophtera fagata, O. brumata, Erannis defoliaria, E. marginaria, E. aurantiaria, Phigalia pedaria, - on hornbean Carpinus betulus in May, BialowieZa Forest, Poland between 1975 and 2005 (Jaroszewicz and Wesolowski, pers. comm.). This figure also shows a caterpillar descending to pupate; (c) Numbers of winter moth O. brumata caterpillars trapped beneath five oaks at Wytham Woods, UK (mean no. m~2) from 1952-2005 (Cole, pers. comm.). No data for 1974, 1977-82. (Data compiled by David J. L. Harding.)

200

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erupts in epidemics. These defoliate the local conifers (mostly firs and spruces) for periods as long as 5-10 years; in Quebec these epidemics are known to have occurred back to 1704.

Global warming is also likely to have an effect. In Holland investigations of the response of winter moth Operophtera brumata egg hatching and oak Quercus robur bud burst to changing seasonal temperature patterns during the period 1975-1999 appear to show a disruption of the synchrony of their

phenology (Visser and Holleman, 2001). This was ascribed to the pattern of spring temperatures and winter frosts. The first stage in oak bud burst is initiated by the build-up of a 'chilling sum' by the trees in a time period beginning on 1 November, a form of stratification similar to that seen in seeds (Section 4.6.2). After this first threshold value has been reached they commence to build a 'warmth sum'; the buds burst only after this second threshold has been reached. In years when the mean winter and spring temperatures were high in relation to the number of frost days, bud burst in oak is earlier, but egg hatching is up to 3 weeks earlier than bud burst and many caterpillars starve. Warmer springs also lead to mistimed reproduction in Dutch great tits Parus major (Visser et al, 1998), where caterpillars developed earlier but the time tit eggs were laid did not. During egg laying great tits forage in larch Larix decidua and downy birch Betula pubescens, whose dates of bud burst are much less temperature-dependent than that of oak and so have not advanced in the same way. In consequence, many more geometrid caterpillars feeding on oak are likely to survive the crucial early period.

Monitoring of a site in Surrey (Sparks and Smithers, 2002) shows an advance of nearly a month in oak leafing dates since records began in 1950, a greater response by the oaks than that expected if air temperature was the sole trigger. Harding (2002) found no evidence of such an advance at Chaddesley and thought this might be due to a shorter observation period of two decades. Is global warming the cause of the recent decline in episodes of severe defoliation by oak-feeding caterpillars in England but not in Scotland (interestingly, also seen in such things as declining cycles of vole numbers in Scandinavia)? Whatever the reason, although earlier leafing out, when it occurs, seems desirable to foresters as active photosynthesis continues for longer and tree-ring growth is less disrupted so timber production is greater, it does have negative effects: fewer caterpillars means less food for rearing the young of woodland birds.

Insects not only defoliate trees, they can damage shoots and cause damage by burrowing under bark (Fig. 5.6). They also often act as vectors of disease, spreading fungi through the wounds they make. As can be seen in Table 5.1, a number of serious pests in eastern USA are insects and many of the major fungal diseases are aided by insects, as discussed later in this chapter. Non-native insects inadvertently introduced may be even more devastating than their native counterparts if their normal population regulators (predators, climate) are left behind or their new food source is particularly suitable. Examples of widespread and rapid damage by introduced insects are legion. Two examples are given below; the first is of global concern, the second of a more localized but severe insect introduction.

Asian Longhorned Beetle Damage Chart

Figure 5.6 Common European beetles that cause damage to trees. (a) Large pine weevil Hylobius abietis: larvae develop in stumps and logs; adults damage bark of young pines. (b) Pine-shoot beetle Tomicus piniperda: larva and adult live under bark, a potential source of fungal infection of weakened standing trees. (Drawn by David J.L. Harding from Cannock Forest, Staffordshire, UK. From Packham and Harding, 1982. Ecology of Woodland Processes. Edward Arnold.)

Figure 5.6 Common European beetles that cause damage to trees. (a) Large pine weevil Hylobius abietis: larvae develop in stumps and logs; adults damage bark of young pines. (b) Pine-shoot beetle Tomicus piniperda: larva and adult live under bark, a potential source of fungal infection of weakened standing trees. (Drawn by David J.L. Harding from Cannock Forest, Staffordshire, UK. From Packham and Harding, 1982. Ecology of Woodland Processes. Edward Arnold.)

5.3.1 Asian longhorned beetle

The Asian longhorned beetle is a large (up to 35 mm long) and remarkably attractive black and white beetle with blue feet (Fig. 5.7), aptly named the starry sky beetle in its native Asia. In China, however, it is a particularly troublesome native pest of hardwood trees such that 40% of all poplar plantations have been damaged and around 50 million trees were felled in a 3-year period just in Ningxia Province in central China (an area twice the size of Belgium or equal to West Virginia). The larvae burrow under the bark and girdle branches and trunks (i.e. cut through the phloem around them, depriving the roots of sugars from above - see Section 1.3.2 for details of tree morphology) leading to death over a few years. Unfortunately, the Asian longhorned beetle is becoming a world pest. In 1996 it arrived in the USA from China carried as larvae/pupae inside solid-wood packing material. A year later and populations were established in New York City and Chicago. Since then it has been found in warehouses at ports around North America, and small populations have been found in Austria (probably arriving in 1998) and in Germany (2004). The beetle has also been found in numerous ports around Europe, including the UK, with consequent worries about it escaping into the wild.

Table 5.1. List of exotic pests and pathogens that have caused or threaten to cause dramatic tree declines in forests ofeastern USA.

Pest/pathogen

Type

Date introduced

Species affected

Gypsy moth (Lymantria

Insect

1869

Quercus; other

dispar)

hardwoods

Beech bark disease (Nectria

Fungus

1890

Fagus grandifolia

coccinea var. fraginata)

Balsam woolly adelgid

Insect

1900

Abies balsamea,

(Adelges piceae)

A. fraseri

Chestnut blight (Cryphonectria

Fungus

1904

Castanea dentata

parasitica)

White pine blister rust

Fungus

Early 1900s

Pinus strobus

(Cronartium ribicola)

Dutch elm disease (Ophiostoma

Fungus

1930

Ulmus americana

ulmi)

Hemlock woolly adelgid

Insect

1950s

Tsuga canadensis,

(Adelges tsugae)

T. caroliniana

Butternut canker (Sirococcus

Fungus

1967

Juglans cinerea

clavigigenti-juglandacearum)

Dogwood anthracnose

Fungus

1978

Cornus florida

(Discula destructiva)

Asian longhorned beetle

Insect

1996

Acer, Betula, Populus,

(Anoplophora glabripennis)

Ulmus spp.

Source: Based on Orwig, 2002. Journal of Biogeography 29.

Source: Based on Orwig, 2002. Journal of Biogeography 29.

Linda Orwig

Figure5.7 FemaleAsianlonghornedbeetle (Anoplophoraglabripennis). (Photograph by Linda Haugen, USDA Forest Service, www.forestryimages.org.)

So far outside Asia, the Asian longhorned beetle has been confined to urban and ornamental trees, particularly maples. Along the eastern seaboard of North America there are worries about the maple syrup industry and autumn colour tourism if the beetle becomes common. Moreover, there is a real threat that the Asian longhorned beetle could also become a serious global landscape pest, so much so that it has been included as one of the world's worst 100 invasive species in the Global Invasive Species Database. Visit the database (www.issg.org/database) for further details of this and other world pests.

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