Past and present impacts of exotic forest pests on Asian forests

Psyllid epidemic
Leucaena leucocephala, a native of Southeast Asia and the Pacific Islands, is grown extensively in community forestry and agroforestry ecosystems for fodder and fuel. The tree was almost pest free until 1988, when a psyllid bug (Homoptera) Heteropsylla cubana, spread in an epidemic, resulted in the extensive death of young trees and severe defoliation of mature trees in India. Later, it dramatically spread across the Asia-Pacific region, causing widespread defoliation and tree death. This discouraged further planting of Leucaena in several countries. Since 1994, the incidence of the pest has declined but the damage done to Leucaena cultivation is irreparable. The original distribution of this psyllid appears to be limited to Central America, particularly, in the Caribbean around Cuba and the east coast of Mexico, where it was recorded initially in 1982.

Needle blight of Japanese cedar caused by Cercospora sequoiae
Japanese cedar, Cryptomeria japonica, is one of the most important sources of lumber in Japan. Seedlings of the cedar first exhibited signs of a needle blight around 1910, in central Japan. It has since spread throughout Japan as an epidemic disease. The causal fungus of the disease was confirmed as Cercospora sequoiae, which already had been found in the United States on the giant sequoia. Fortunately, appropriate control measures were established by spraying chemicals in nurseries.

Pitch canker
Caused by a Fusarium fungus is known as one of most serious diseases of pines in California, southeastern United States and Mexico. In Japan, the pitch canker was found on Ryukyu pine in Amamioshima Island, southern Japan, in 1989, and now the disease appears in wider areas in the Ryukyu Islands. Also, there is a recent Korean report (Lee et al., 1998) concerning an exotic pine. Careful monitoring of the extension of the damage is necessary because Japanese black pine is known to be very susceptible to the pathogen.

Exotic weeds
During the past two centuries, several exotic weeds have been introduced into Southeast Asia. During this time, some have attained the status of serious weeds in waste lands and forest ecosystems. Of particular note are a) Chromolaena odorata, a native of subtropical and tropical America from Florida to Argentina, a noxious weed in 23 countries that was introduced in India in 1840s, probably via the Botanical Garden in Calcutta; b) Lantana camara, a native of tropical and subtropical America, introduced as an ornamental plant in Calcutta Botanic Garden in 1809. In disturbed forests, it impedes natural regeneration of native species by covering the entire area with almost pure growth of the weed. About eleven insect species have been introduced to India as a biological control against Lantana, although none have been successful; c) Mikania micrantha, originally from tropical and subtropical zones of South America, it is widespread in a number of countries within the moist tropical zones of Southeast Asia and the Pacific. The weed can smother, penetrate crowns, choke, and pull over plants. Badly affected tree crops include teak, eucalyptus, bamboo, reeds, Ailanthus triphysa, and Acacia aurifuliformis.

Other pests
Fall webworm (Hyphantria cunea) is a typical exotic pest introduced to Japan from the United States. Since first found in 1947, widespread damage to ornamental trees had occurred. Fortunately, however, no damage has been found in forests.

Other examples of serious pests that are presumed to be exotic are Entomosporium leaf spot (Entomosporium mespili) of rosaceous trees and leaf blotch of poplars (Septotinia populiperda).

White pine blister rust and chestnut blight have caused serious damage to forests in North America and Europe. The white pine blister rust fungus, Cronartium ribicola, is native to Asia. In Japan, the rust fungus occurs only in alpine areas (Fig. 3) on a native five-needle pine, P. pumila, but damage is scant because the pine is resistant to the fungus. Damage by the rust only became a problem in the 1970s when it attacked introduced strobus pine (eastern white pine) planted in Hokkaido (2).

Figure 3. Pinus pumila forest in an alpine area.	Figure 4. A mature chestnut tree in a natural forest of broad-leaved trees.

It is not easy to find blighted chestnut trees in Japanese natural forests (Fig. 4), though the disease almost destroyed big chestnut trees in the eastern United States These examples clearly show how artificially introduced pests can be disastrous for forests in other continents.

Most of the forest products exported from Japan are wood-packing materials. The quarantine procedure for these differs, depending upon the importing country (http://www.jppn.ne.jp/pq/). Bonsai pines exported to the EU are first quarantined for two years at the growing site in Japan to check for rust diseases.

The import of seeds, living woody plants, and logs are subject to quarantine, and a phytosanitary certificate issued by the exporting country is required. If any pests are found at the inspection, they are sterilized or the materials are incinerated. Completely processed lumber and wood chips are not subject to quarantine.

Tree seedlings without soil, logs, and lumber can be imported from any country, except for certain tree species from some designated countries to guard against specific pests. Huge numbers of insects and some pathogens, however, are found during import quarantine procedures. For example, beetles were found in more than 13,000 cases of imported logs in 1999 in Japan, including dangerous bark beetle vectors of pathogenic fungi from such genera as Dendroctonus, Hylastes, Ips, Monochamus, Platypus, Scolytus, and Tomicus. Fumigation or other treatments are undertaken for such materials after detection. These incidents show the real risks of invasion by exotic pests hiding in logs and the importance of the plant quarantine.

1. Wingfield, M. J. (ed.) 1987. Pathogenicity of the pine wood nematode. American Phytopathological Society, St. Paul, Minnesota.

2. Yokota, S. and Uozumi, T. 1976. New developments in white pine blister rust in Japan. Proc. XVI IUFRO World Congress, Div. II, pp. 330-343. Oslo.