Exotic Forest Pests—A Global Issue

L. D. Dwinell

One of the unintended consequences of the globalization of trade has been an increase in the frequency of introductions and the number of exotic (alien, introduced, foreign, non-native, or non-indigenous) species intercepted at ports of entry. This is largely due to the increase in volume of trade and a broadening of trading partners, especially with the Pacific Rim and Asia. During 1985 through 1998, the U.S. Department of Agriculture (USDA) Animal and Plant Health Inspection Service (APHIS) made 6,952 interceptions of exotic insects on wood articles at port locations (nearly 100) throughout the United States. The intercepted insects originated from at least 95 countries worldwide. In a 1997 survey by the Canadian Forest Service, numerous exotic species were intercepted in wood articles from Asia, Europe, and South America at Canadian ports of entry. In 2000, European countries intercepted the pinewood nematode in crating lumber from the United States, Canada, Japan, and China.

Exotic forest pests can enter a country through a number of pathways including logs, chips, unseasoned lumber, solid wood packaging material (SWPM), propagative material, and as hitchhikers. SWPM includes wood dunnage and packing material, such as boxes, crating, cable spools, attached pallets, drums, and spacers imported with cargo. The SWPM pathway is currently in the spotlight. Although most introduced organisms fail to become established, history has shown that the potential for damage is large if they do.

During the past century, exotic forest pests have destroyed the ecologic, aesthetic, and economic value of many of the world’s most versatile and important trees. Countries in North America and around the globe have suffered the consequences from the introduction of pests into highly vulnerable tree populations. Tree diseases introduced into North America include white pine blister rust, chestnut blight, Dutch elm disease, beech bark disease, and scleroderris canker (European race). The European gypsy moth, balsam woolly adelgid, and pine shoot moth are among the exotic insects causing irreparable damage to the forested ecosystems in Canada and the United States. Other examples include the introduction of the European woodwasp into Uruguay, Brazil, and New Zealand, bark beetles into Australia and China, and the European shoot moth into Chile. The pinewood nematode, the causal agent of the pine wilt disease, was introduced into Japan, South Korea, Taiwan, China, and, most recently, Portugal.

Epidemics and outbreaks
A tree disease can result from the introduction of a pathogenic fungus or nematode into a susceptible host population with which it has not co-evolved. Since the host population has little or no resistance to the pathogen, an epidemic may occur if the environmental factors are favorable. The long-term control of introduced diseases, such as white pine blister rust, chestnut blight, and Dutch elm disease, usually involves breeding for disease resistance. Because the host has little natural resistance to the pathogen and the virulence of the causal agent can change, this can be a daunting task.

An insect outbreak can occur if an insect is introduced into a host population without its natural enemies (parasites, predators, viruses, and pathogenic nematodes and fungi). The long-term management of an exotic forest insect frequently involves biological control by reassociation of the insect with its natural enemies. In Brazil, for example, the potential of using a parasitic nematode and parasitoids for the biological control of the European woodwasp is being evaluated. Chile has developed a biological control program for the European shoot moth using a parasitoid introduced from Europe that parasitizes the larvae of the moth.

Hitchhiker
A prime example of a hitchhiker is the Asian gypsy moth that rides the waves on ships to North America from ports in far east Russia. In 1991, an accidental introduction of the Asian gypsy moth occurred in Vancouver, where larvae were blown ashore after hatching on ships in the harbor. Concurrently, the insect was intercepted in Portland, Oregon, and Seattle, Washington. Vancouver, Portland, and Seattle were sprayed aerially with a biological insecticide to eradicate the Asian gypsy moth. The combined cost of the spray program was 20 million U.S. dollars.

The consequences of the establishment of the Asian gypsy moth in British Columbia, for example, are twofold: ecologic and economic. The most direct effect of the Asian gypsy moth is defoliation and mortality of trees and shrubs. If British Columbia were to become infested, the province’s multi-billion dollar export trade could be in jeopardy as noninfested countries attempt to protect their forested ecosystems from accidental introductions of the Asian gypsy moth. The current priority for British Columbia is to exclude this insect from the province. In the United States, USDA APHIS has established inspection guidelines to prevent the artificial spread of the Asian gypsy moth from far east Russia ports to North America. APHIS has the authority to order infested vessels to leave U.S. waters.

SWPM
A longhorned beetle native to China and Korea brought the issue of pests associated with SWPM to the forefront of the exotic pest debate. Established populations of the Asian longhorned beetle were first discovered in New York in 1996 and then in Illinois in 1998. In Chicago and New York City, the Asian longhorned beetle has attacked maple, the preferred host, as well as other hardwoods. Because the Asian longhorned beetle is a significant threat to the hardwood forests of the United States, it was decided in 1996 to initiate an eradication program. During approximately the past 4 years, 6,050 trees have been cut in Chicago and New York City. It is estimated that the total expenditures on the Asian longhorned beetle from 1996 to 2001 will be nearly 35 million U.S. dollars. Although the Asian longhorned beetle has not become established in Canada, it has been intercepted in recent years on wood articles from China.

The United States imposed stricter regulations on SWPM from China beginning in December 1998 that required all SWPM used in association with exports to the United States to have been heat-treated, fumigated, or treated with a preservative prior to export.

Impact on trade
A native of North America, the pinewood nematode has become a destructive pest in the forests of Japan and other Asian countries where it causes the pine wilt disease. The pinewood nematode is vectored by species of Monochamus, a longhorned beetle. As a result of interceptions of the pinewood nematode by the Finnish Plant Quarantine Service and the pine wilt disease in Japan, the European Union and other countries regulate the import of all coniferous chips, sawn wood, and logs to protect their forested ecosystems from the pinewood nematode and other exotic forest pests. Despite these efforts, the pinewood nematode was discovered in Portugal in 1999. The embargoes have had a negative economic impact on the North American softwood export trade. Because environmental, business, regulatory, and supply issues also affect wood exports, the economic impact has not been accurately determined. It is thought to be substantial, however. Because of interceptions of the pinewood nematode in SWPM by Finland and other European counties in 2000, the European Union is in the process of writing new regulations for SWPM.

Mitigation
To ensure global transport of wood fiber and SWPM without causing ecological disaster or economic damage, the wood must be free of viable pests. Mitigation options include prevention, host selection, and treatment by fumigation, irradiation, chemical dips, and elevated temperatures. Scientists in a trilateral study involving Canada, the United States, and the European Union concluded that heat-treating unseasoned coniferous lumber to a core temperature of 56ºC for 30 minutes eradicated the pinewood nematode and its vectors. This pasteurization treatment has become the standard for export of most coniferous wood to the European Union and China. This pasteurization treatment, however, does not control many of the fungi that inhabit wood.

Summary
The impact of the spread of exotic forest pests is increasingly recognized as a global issue. Albeit slowly, a global strategy to address the exotic pest problem is beginning to evolve. The International Plant Protection Convention (Food and Agricultural Organization of the United Nations) has begun the process of harmonizing standards for the treatment of SWPM to minimize the movement of exotic pests without adversely impacting global trade. Their task is to be completed in the spring of 2002.