INSECTS AND DISEASES
Outbreaks of insects and diseases in forests have resulted in substantial economic losses and environmental damage, even though they may be less visible and less dramatic than fires and ice storms. For the purposes of this report insects and diseases are analysed together, as they are often co-dependent.
Insects and diseases are integral components of forest ecosystems and normally are present at a relatively low density, causing little damage and having negligible impact on tree growth and vigour. From time to time, however, some species may quickly reach damaging numbers, spatial distribution may increase and the outbreak may persist for a variable time before subsiding. Such large populations may have adverse effects on many aspects of forests, such as tree growth, survival, yield and quality of wood and non-wood forest products, and soil and water conservation. Such outbreaks are costly to control and may cause considerable damage, compromise national economies, local livelihoods and food security, and result in trade restrictions on forest products.
The types of problems caused by introduced insects and diseases have changed rapidly in recent years. Movement of insects and diseases has been facilitated by intensified long-range air travel and reduced travel time, increased international trade of agricultural and forest products and the exchange of plant material. Local climatic fluctuations may facilitate the establishment of introduced insects in previously hostile environments. Introduced forest pests can be extremely destructive, as seen in recent years in the impact of the cypress aphid (Cinara cupressivora) in Eastern and Southern Africa and more recently in South America.
As mentioned, the International Plant Protection Convention (IPPC) is a major international treaty that aims to secure action to prevent the transboundary spread and introduction of plant and plant-product pests (FAO, 1999b). The International Standards for Phytosanitary Measures (ISPM) (FAO, 1995–2005), developed within the framework of the IPPC, include a basic framework for risk analysis and development of phytosanitary measures to minimize such transboundary movement. Particularly relevant to forestry are ISPM No. 15, Guidelines for regulating wood packaging material in international trade (FAO, 2002b), and the ISPMs relating to risk analysis and pest reporting and status. Data on the movement of and disturbances by introduced insects are essential in the development of risk management strategies for transboundary pests.
Despite the significant adverse impacts of forest insects and diseases, and indications that outbreaks are on the increase in some regions, insects and diseases are often not considered in the planning of forest and forest-conservation programmes. There has been no attempt to systematically gather and analyse comprehensive information on the type, scale and impact of such outbreaks at the global level.
Identification of insects and diseases as causal agents of damage to forests is a highly specialized technical area. The insect and disease data in FRA 2005 indicate the overall extent of forest affected, but offer minimal details in most cases on the underlying causes.
A system that enables data to be reported on a continuous as well as an ad hoc basis could encompass the complexity of information required – so as to have data useful to the development of risk management strategies for forests and other wooded land.
Insect and disease problems are often either cyclical or chronic. Thus they require long-term investment in data collection and technical resources in order to fully assess the complexity and extent of the issues. A chronic disturbance by insects and diseases may be caused by a complex of species rather than by a single entity. The complex can vary not only in the species involved but also in the impact of each individual species within that particular disturbance. Thus defining the beginning and end of a disturbance event can be a challenge.
There are further complications in recording data: (i) some insect life cycles overlap or are significantly longer than one year (e.g. the Siberian caterpillar – Dendrolimus sibiricus); and (ii) other cyclical disturbance events caused by insects last more than a year. For example, gypsy moth (Lymantria dispar) outbreaks of several generations can be every 7–10 years. More recently, however, the period between outbreaks has apparently been becoming shorter. Capturing data for such long-term cyclical events is difficult, particularly when there is variability in the length of cycles. The information supplied by countries for insect disturbances has been reported for annual averages over five years. With long cyclical outbreaks, five-year reporting periods do not adequately reflect the status of these events.
Moreover, due to the longer duration of some disturbance events, it is difficult to accurately assess the area affected annually. Some countries appear to have reported the cumulative area affected in a given year, rather than the additional area of forest affected within that year. Thus the figures for the different types of disturbances are not directly comparable.
The status of the data on insect and disease disturbances is poor, mainly owing to a lack of clarity in interpreting what constitutes a ‘disturbance’. Globally, the quantifiable data on insect incidences and their effects on forests and forest products are limited. Insect and disease outbreaks in developing countries are primarily surveyed and reported for forest plantations and planted trees only, and corresponding surveys of forest decline and dieback are rare in these countries. Serious outbreak situations may be recorded, but details of causative agents and the quantifiable impact on forest resources often are not. In some instances, there may be a reluctance to record such severe outbreaks because management jobs or even forest products trade can be put at risk.
Data on insects and diseases are collected and reported in a variety of ways. In some instances, data provided on the area of forest affected by diseases and insects (and other biotic disturbances) are not separated.
For insect infestations, of the 229 countries included in FRA 2005, 48 countries provided data for both the 1990 and 2000 reporting periods; a further 18 countries provided data for the 2000 reporting period only. These 66 countries represent 65 percent of the world’s forest area. Reports from East Asia, Europe and North America covered more than 90 percent of their forest areas, while those from Africa and Oceania covered less than 1 percent of the forest area in their respective regions (Figure 4.3).
For diseases, 42 countries provided data for both the 1990 and 2000 reporting periods. A further 15 countries provided data for the 2000 reporting period only.
For the 2000 reporting period, East Asia and Europe provided data for over 80 percent of the forest areas within the region, while North America, South America and South and Southeast Asia each provided information for more than 50 percent of the forest area in their respective region or subregion. Data from Africa, the Caribbean and Oceania were largely missing (Figure 4.4).
For some regions, more data exist but were not readily accessible for this report owing to a lack of information exchange among sectors, individuals and government agencies or a lack of awareness of the existence of data.
To complement existing information and facilitate documentation on forest health at the country level, FAO is compiling data, with the cooperation of experts from member countries, for a global information system on the impact of insect and disease outbreaks on natural and planted forests, other wooded land and trees outside forests. The system is intended for national forest services, research and academic institutions and technical officers dealing with forestry and pest management. It should help improve planning and decision-making, increase awareness of the severe problems related to forest insects and diseases worldwide, and provide up-to-date baseline information to support risk assessment and the design and implementation of effective forest protection strategies (www.fao.org/forestry/site/18748/en ). A two-tiered questionnaire has been sent to in-country technical specialists in an attempt to obtain more detailed information. The results of this study are available in a separate thematic report (Box 4.2).
Globally, the combined forest area adversely affected by insects and diseases for the 2000 reporting period was approximately 68 million hectares. In most cases, there are no details indicating the causative agent(s), so the data provided may reflect combined insect and disease disturbances. The highest area of insect disturbance reported for a single country was 14.2 million hectares (Canada), and of disease disturbance, 17.4 million hectares (United States) – both countries within the top five in terms of forest area and with good data-collection systems. Tables 4.3 and 4.4 present a summary of results for the 2000 reporting period.
The data reflect differences in the two periods, but as only two reporting periods are compared, they should not be construed as trends. The raw data indicate a very large increase in the level of disease reported and a decrease in the level of insect damage reported between the 1990 and 2000 reporting periods. However, this is primarily because more countries reported for the 2000 period than for 1990.
Analysing data only for those countries that have provided information for two points in time, the area affected by diseases shows a slight increase globally (from 4.4 to 4.7 million hectares per year), despite a significant decrease reported by Africa and East Asia (Table 4.5). The increase in South America is particularly noticeable and is primarily due to the fact that Chile has reported a very large increase in the area of forest affected by diseases.
The area affected by insects, on the other hand, shows a decrease (from 45.7 to 35.7 million hectares per year), owing to a substantial decrease in affected areas reported by Canada and the United States. Most other subregions and regions reported an increase in the area of forest affected by insects (Table 4.6). In Europe, the large increase in the area of forest affected by insects in the 1998–2002 period compared with the 1988–1992 period may be due to increased attacks following the storms of December 1999. This may also be the reason behind the increase in the area affected by diseases in this region.
It should be noted that this information is indicative as, again, there are only the two data points in time and data are missing for a large number of countries. Conclusions cannot be drawn from the data as to the causative agents or trees species involved and the effects on trees and the forest ecosystem as a whole.