2.1. Plantations vs natural forests
2.2. Exotic vs indigenous tree species
2.3. Mixed vs single species plantations
Damaging pests and diseases are found in plantations and natural forests in both the temperate and tropical zones. A fundamental concept of ecosystem dynamics is that as diversity increases, so does stability. The greater the number of plants and animals that occupy an ecosystem, the greater are the checks and balances that prevent any one species from increasing to the point where other ecosystem components are threatened. With pests and diseases, the greatest single deterrent to population increase is the amount of available host material. For example, in a complex tropical rain forest, where as many as a thousand plant species might occupy a single hectare, a population of a host specific, phytophagus caterpillar will find only a limited amount of suitable host material. Therefore its numbers will remain fairly stable. On the other end of the diversity scale is the agro-ecosystem. Dominated by a single plant species, agro-ecosystems often cover large areas and provide a virtually unlimited amount of suitable host biomass. This can result in population explosions of organisms that use the plant as host material. Furthermore, the lack of ecosystem diversity will, at best, provide only a marginal habitat for the natural enemies of the organisms that are using the crop plant as host material.
In terms of diversity and stability, forest plantations tend to be more like agro-ecosystems than natural forests. This would lead one to conclude that plantation forests are more susceptible to damage by pests and disease than are natural forests. While this may be generally true, it does not necessarily mean that natural forests are immune to damage from these agents. The relative lack of pest outbreaks in mixed tropical forests is often cited as evidence for the importance of diversity in stabilizing plant communities (Speight and Wainhouse 1989). Many natural forests in the northern hemisphere boreal and temperate zones however, tend to be relatively simple ecosystems when compared to tropical forests and are susceptible to outbreaks of insects. Examples include:
Natural forests are also subject to damage by a variety of diseases caused by fungi, bacteria, virus or parasitic mistletoes (Manion 1991) and they are vulnerable to introductions of exotic pests and disease. The introduction of white pine blister rust, Cronartium ribicola, into North America (Manion 1991), the European gypsy moth, Lymantria dispar, into the mixed broadleaf forests of the eastern United States (Doane and McManus 1981), of two Juniperus infesting scale insects, Carulaspis visci and Lepidosaphis newsteadi, into Bermuda (FAO 1986, Waterston 1949) and the apparent introduction of the soil fungus Phytophthora cinnamomi into Eucalyptus marginata forests in Western Australia (Newhook and Podger 1972) are just a few examples. Moreover, the recent introduction of pitch canker disease, caused by the fungus Fusarium subglutinans f. sp. pini, into forests of Pinus radiata, along the California, USA coast, now is a source of concern (Storer et al. 1994, 1997). A more recent example is the discovery in 1994-5 of the pink or hibiscus mealybug, Maconellicoccus hirsutus, an insect capable of feeding on between 125 and 150 different plant species, in the Caribbean islands of Grenada, Trinidad and Tobago and St. Kitts-Nevis. This insect poses a severe threat to natural mixed tropical forests as well as vegetable crops and ornamental plants in the region (SCMA 1996).
2.2.1. Exotic species plantations
2.2.2. Indigenous species plantations
2.2.1.1. Indigenous pests and diseases
2.2.1.2. Exotic pests and diseases
A typical scenario in the use of exotics in forest plantation programmes, both in tropical and temperate zones, is that they enjoy a pest free “honeymoon” for a period of time. During this period, management practices often evolve that make the plantations especially susceptible to pests and disease once they begin to appear. These include:
1. Failure to give proper attention to species/site matching.When pests and diseases do begin to appear, they may come from two sources. A complex of indigenous agents may adapt to the new host or exotic pests may be accidentally introduced.2. Use of planting stock from a narrow genetic base.
3. Failure to maintain optimum stocking levels and tree vigor through intermediate cuttings.
4. Dependence on one or two species in a plantation programme, resulting in an unlimited supply of host material for potentially damaging organisms.
A number of examples of indigenous insects adapting to exotic forest plantations are known. In Colombia, indigenous insects have been detected in that country’s exotic Cupressus, Eucalyptus and Pinus plantations, with the most frequently encountered groups being defoliating caterpillars of the family Geometridae, leaf cutting ants and stem infesting insects (Madrigal C. 1993). Rodas P. (1998) reports 30 species of defoliating insects feeding on exotic forest plantations in the Andean region of Colombia. A similar situation exists in Chile, where several species of indigenous pests are now associated with exotic forest plantations including Ormiscodes cinnamomea, a large Saturniid that occasionally defoliates Pinus radiata (Ciesla 1997). Ohmart and Edwards (1991) report 96 indigenous insects feeding on various species of Eucalyptus in China, 94 in India, 223 in Brazil, 31 in New Zealand, 105 in Papua New Guinea and 62 on the island of Sumatra, Indonesia. Termites and leaf cutting ants are the major damaging agents.
Indigenous disease causing organisms are also capable of adapting to new hosts and causing severe damage. In Kenya, plantations of Cupressus macrocarpa have been severely damaged by Monochaetia unicornis, a stem canker causing fungus (Odera and Arap Sang 1975, 1980). During the early 1990s, several species of foliar and twig pathogens, including Cylindocladium leaf blight, Coniella leaf spot, Kirramyces leaf spot and pink disease, Corticum salmonicolor, damaged Eucalyptus plantings in Vietnam (Sharma 1994).
The greatest hazard of pest and disease damage to exotic forest plantations is from the accidental introduction of exotic pests and diseases. Exotic pests and diseases agents, in the absence of natural enemies, and provided with a large supply of suitable host material, that may have little resistance to the new pest or disease, can build up rapidly and cause devastating losses.
One of the best-documented examples of exotic pests and diseases causing damage to exotic plantations is to pine plantations in the Southern Hemisphere, especially Pinus radiata. P. radiata were first planted in the late nineteenth century in Australia, Chile, New Zealand and South Africa. The excellent growth and form of the tree in these locations encouraged additional plantings. These provided the basis for thriving lumber and paper industries, especially in Chile and New Zealand. One of the first exotic insects to appear in New Zealand’s radiata pine plantations was the European wood wasp, Sirex noctilio, during the early 1900’s. This insect was probably introduced on unprocessed pine logs imported from Europe. Native to southern Europe and North Africa, where it causes little or no damage, S. noctilio normally breeds in recently killed pines. Plantations established in New Zealand during 1920-30 stagnated because there was no market for small logs from thinning operations. This made the plantations susceptible to S. noctilio and its associated fungus, Amylostereum areolatum. By 1947, high levels of tree mortality were occurring, primarily in the unthinned plantations (McLean 1998). However, today S. noctilio is not a problem because of biological control methods and better stand management.
S. noctilio subsequently appeared in P. radiata plantations in Australia in 1952 (Gilbert and Miller 1952; Taylor 1981). In 1980 infestations were detected in South America where this insect is damaging plantations of P. elliotii and P. taeda in Argentina, southern Brazil and Uruguay (Rebuffo 1990; Iede et al. 1988). In 1994, infestations of S. noctilio were discovered in P. radiata plantations in the Cape Peninsula of South Africa (Tribe 1995).
Other damaging insects that have been introduced into P. radiata plantations include the bark beetle, Ips grandicollis, a North American species, into Australia (Morgan 1989) and the European pine shoot moth, Rhyacionia bouliana into Argentina and Chile. The latter is now regarded as Chile’s number one forest insect pest (Ramirez Grez 1993; Ciesla 1997). The European bark beetles, Hyalstes ater and Hylurgus ligniperda were introduced into Chilean radiata pine plantations during the late 1980s and are capable of killing young pine seedlings (Ciesla 1988).
Several introduced fungi have also caused severe damage to P. radiata plantations. In 1957, a needle fungus, Dothistroma septospora (D. pini), became a major concern in P. radiata plantations in Chile, parts of East Africa and New Zealand, although there are records of its occurrence in these areas as early as 1940 (Manion 1991). The stem disease, Sphaeropsis sapinea (Diplodia pini) has also been introduced in a number of countries. This fungus has been especially damaging in Kenya, where it first appeared in 1973 and proliferated on P. radiata, a species adapted to a winter rainfall regime and is especially susceptible to fungal attacks during periods of rain and warm temperatures (Odera and Arap Sang 1975, 1980).
Another example of damage to exotic forest plantations by an introduced pest is that of the leucaena psyllid, Heteropsylla cubana (Homoptera: Psyllidae) on Leucaena leucocephala, a fast growing legume native to Mexico and Central America. This tree was introduced into the Philippines as early as 1565 and was quickly established throughout the tropics where local people found it an excellent fuelwood and later as shade under which crops such as coffee, cocoa, pepper and vanilla could be grown. Genetically improved varieties were eventually introduced into the tropics where it was regarded as having the widest range of uses of all of the tropical legumes. In 1983, leucaena psyllid, a somewhat obscure insect of L. leucocephala in its natural range, was detected in southern Florida, USA and during the following year it was found in the Hawaiian Islands. The insect spread rapidly across the Asia and Pacific Region between 1985 and 1988. In 1991, infestations were found on the southern Indian Ocean islands of Reunion and Mauritius and one year later in several eastern African countries, including Burundi, Kenya, Tanzania and Uganda. This insect feeds on the shoots and young foliage of Leucaena causing wilting and growth loss (Banpot Napompeth 1994).
Ironically, attempts to introduce new genetic material into a country have occasionally resulted in introductions of damaging pests. In 1968, a pine woolly aphid, Pineus borneri (also reported as P. pini), was introduced into Kenya on Pinus caribaea grafts imported from Australia (Owour 1991). Twenty years later, a scale insect, Oracella acuta, was introduced into China on P. elliotii scion material introduced from the United States. By 1995, over 212,500 ha of P. elliotii plantations in Guangdong Province in southern China were infested by this insect (Jianghua Sun et al. 1996).
2.2.2.1. Indigenous pests and diseases
2.2.2.2. Exotic pests and diseases
Throughout much of Asia, Europe and North America, forest plantations are typically established with indigenous species (e.g. Pinus massoniana in China, Picea abies, Pinus sylvestris and Populus spp in Europe, Pinus taeda and Pinus elliotii in the southeastern USA and Pseudotsuga menziesii in western Canada and the USA). These plantations are also susceptible to damage by a complex of both indigenous and exotic pests and disease.
In China and Vietnam, the defoliating caterpillar, Dendrolimus punctatus, is a major pest of indigenous pine plantations. This insect has two generations per year in east-central China where it defoliates Pinus massoniana. In Vietnam, D. punctatus has four generations per year and outbreaks have occurred in plantations of P. merkusii, often established on steep deforested slopes where they are an important source of lumber, pulpwood, fuel wood and resin (Dao Xuan Troung 1990).
Nun moth, Lymantria monacha, is a defoliating caterpillar of both Picea abies and Pinus sylvestris in central Europe. This insect was originally a major pest of natural Picea abies forests but readily adapted to the extensive pine plantations. In Poland, plantations of P. sylvestris are established on low nutrient, sandy soils or abandoned marginal agricultural lands. Between 1978 and 1983, a major outbreak of this caterpillar necessitated aerial spraying of 2.5 million ha, over 25% of Poland’s forests (Speight and Wainhouse 1989). In 1991, this insect again reached outbreak proportions and large areas of forest were treated between 1992 and 1994. Other defoliating insects of P. sylvestris plantations in Europe include Bupalis piniarius, Panolis flammea, Dendrolimus pini and sawflies, Diprion spp. (Speight and Wainhouse 1989, Ciesla 1994).
Fusiform rust, caused by the fungus Cronartium quercuum f.sp. fusiform, has become a serious pest of P. elliottii and P. taeda plantations in the southeastern USA. Prior to 1900, the disease was rare, but because of intensive cultural practices associated with plantation forestry, including planting of infected seedlings, fire management, genetic selection of fast growing trees and expansion of the range of susceptible hosts, this disease has become a major problem. Recent estimates indicate that $US130 million in damage is occurring annually and the disease is increasing at the rate of 2-3% per year (Manion 1991).
In China, species of Paulownia, are widely planted either as ornamentals in villages or in intercropping systems. Pawlonias are commonly infected by a mycoplasma-like organism (MLO), which produces large witches brooms in tree crowns. This disease causes significant reductions in increment, leading to a gradual decline and death of infected trees. The wood of infected trees is of reduced quality (Tsai et al. 1988).
The pine wood nematode, Bursaphelenchus xylophilus is the cause of pine wilt disease. This disease has been a major cause of tree mortality in pine plantations in Japan since the early 1900’s. The nematode is probably native to North America and has been introduced into Japan and more recently China, where it is causing extensive tree mortality in plantations of Pinus massoniana. The nematode is spread from tree to tree by longhorn wood boring beetles, Monochamus spp, when the adult beetles feed on pine shoots (Manion 1991).
The concept of diversity leading to ecosystem stability would suggest that mixed species plantations would be more pest and disease resistant than single species plantation. Indeed, work by Klimetzek (1990) in Germany indicates that the inclusion of broadleaf trees in pine forests, either as a mixture or in the understory, tends to reduce the incidence of outbreaks of Lymantria monacha. On the other hand, planting Picea abies under a plantation of P. sylvestris, could intensify or prolong outbreaks of this insect since both trees are hosts.
Wormald (1992) indicates that data on relative susceptibility of mixed and pure species plantations in subtropical and tropical regions is, at best, inclusive and confusing. While examples of where pests and diseases have become problems in pure species plantations are cited, many result from accidental introductions or of indigenous pest species adapting to new conditions. Furthermore he cites examples where establishment of mixed species plantations has had little or no effect on reducing pest caused losses.
Some temperate zone pests and diseases have adapted to mixed-species ecosystems by having evolved life stages on alternate hosts. Many rust diseases fall into this category, including fusiform rust of P. elliotii and P. taeda, Cronartium quercuum f.sp. fusiform, which has alternate spore stages on oak, Quercus spp. Therefore, removal of oaks from the vicinity of pine nurseries in the southeastern USA is a common although not altogether effective practice (Manion 1991). Cooley spruce gall aphid, Adelges cooleyi, an insect indigenous to western North America, has alternating life stages on Picea and Pseudotsuga menziesii, trees that are often found growing in close proximity to one another in natural forests (Speight and Wainhouse 1989).
Perhaps a greater concern with regard to pests and diseases is not mixed vs. single species plantations, but the number of species used in a country’s plantation programme. Reliance on one or two closely related species may meet a country’s needs for wood products and foreign exchange. However, should a pest or complex of pests appear in these plantations, the results could be devastating. Yet, many countries have fallen into this dilemma. Examples include Kenya, where 46% of its industrial plantation area is composed of Cupressus lusitanica (Ciesla et al. 1995); Colombia with 66% of its plantations Pinus patula (Madrigal 1993); Poland with 70% of its plantation forests P. sylvestris (MEPNRF 1991); and Chile and New Zealand with 90% of their plantations composed of P. radiata (Ramierez Grez 1993; Ciesla 1997). New Zealand experience has been that deliberately planting a wide range of species to reduce risk from pests and diseases did not work (D. J. Mead pers. comm.). Rather, good site matching and silviculture, coupled with emphasis on disease prevention and control, backed by research, has been used as an alternative strategy.
