Causes of Pest Outbreaks

The history of pest management is a subset of the history largely of agriculture and while pests have been a chronic problem in agriculture since the beginning, many of today's serious pest problems are the direct consequence of actions taken to improve crop production (Waage, 1993). The intensification of agriculture has created new or greater pest problems in a number of ways:

1. The concentration of a single plant species/variety in ever larger and more extensive monocultures increases its apparency to pests and the number of pest species which colonize it (Strong et al., 1984).

2. Generally, high yielding crop cultivars can provide improved conditions for pest colonization, spread and rapid growth.

3. Reductions of natural enemies around crops means that natural enemies of pests must come to the crop from increasingly small and more distant non-crop reservoirs, entering crops too late or in too little numbers to prevent pest outbreaks.

4. Intensification results in a reduction of intervals between plantings of the same crop, or overlap of crops, which provides a continuous resource to pests.

5. The search for better cultivars and accelerated movement of plant material around the world and with it the movement of pests. Plant breeders, commercial importers, distributors of food aid and general commerce inadvertently introduce pest species.

6. Virtual reliance on chemicals leading to an increase in pest problems particularly for insects.

One of the factors influencing the increased introduction of exotic insect species has been the increase in importation of foreign products and materials. In the absence of their normal natural enemy complex or of environmental constraints these introduced species may become pests and cause extensive damage to crops or livestock. Pests may also be transported to countries in which they are not indigenous by the introduction of new crop types or animal breeds, with similar results. In general, it is such changes in agricultural practices as the introduction of new crop species or enlargement and aggregation of fields, use of monocrops and plant density, that have been held responsible for causing many pest problems. Risch (1987) cited:

1. The changes in crop cultivars relative to those of wild relatives.

2. The simplification of agroecosystems compared with natural ecosystems as being the most important contributing factors.

The former occurred because the successful control of insect pests with insecticides in the 1950s and 1960s allowed plant breeders to concentrate on developing new, high yielding cultivars in the safe knowledge that the insects were taken care of cheaply and effectively with insecticides. Hence, the breeders focused their attention more on attaining outstanding yields than on producing insect resistant cultivars (Ferro, 1987). Insect outbreaks then occurred on cultivars having no natural levels of insect resistance when the insecticide umbrella was removed (brought about by insects resistant to insecticides).

The second important contributing factor, the reduction of diversity in large crop monocultures, has long been associated with reasons for pest outbreaks. The reasons for this are that a monocrop is thought to provide a highly suitable habitat for a pest, but a highly unfavourable one for the pest's natural enemies, thus creating conditions appropriate for outbreaks. However, more recently it has generally been recognized that outbreaks are not an inevitability of such trophic simplicity (Redfearn and Pimm, 1987), an idea long recognized in forest entomology. Monocultures do occur in natural ecosystems, e.g. bracken, heather or natural forest. The establishment of species in monocrop plantations is certainly not a radical departure from this and should not automatically make them more vulnerable to pests (Speight and Wainhouse, 1989). In forest systems in particular it has been shown that cyclical pest outbreaks can occur as a consequence of natural changes in the physiological condition of the host, with weather often playing an important role (Berryman, 1987; Speight and Wainhouse, 1989).

Insect outbreaks, especially of migratory pests, are often associated with particular weather patterns, e.g. outbreaks of the desert locust and Spodoptera spp. The weather can also directly affect population development, if temperatures are favourable for population growth at an appropriate period during the insect's life cycle then outbreaks can occur, e.g. mild winters in the UK are associated with outbreaks of cereal aphids. Weather can produce a differential development of pests and their natural enemies causing a decoupling of their association and thereby permitting an unregulated pest population increase.

In effect, the goals of agricultural intensification are being undermined by pest problems that are now inadvertently the result of that very process. Sustainable agriculture and hence, sustainable pest management required finding solutions to these pest problems which protect the goals of intensification (Thomas and Waage, 1996).

Insect outbreaks can be triggered through intervention by man, i.e. through the use of insecticides, irrigation, fertilizer or cultivars lacking resistance to insects. Of these, it is insecticides that have had the most widespread influence on insect pest outbreaks. They can be an indirect cause of insect pest outbreaks by a number of means including reduction of natural enemies, removal of competitive species and secondary pest outbreaks, and through the development of insecticide resistant insects. The use of broad spectrum insecticides (which is now on the decline) can cause the destruction of both pest and natural enemy populations, but the ability of the pest to then rebound in the absence of the natural enemy can lead to an outbreak. Secondary pests occur when an insecticide differentially affects the major pest relative to minor ones. With the subsequent destruction of the major pest, competition is removed, allowing the minor pests to exploit more effectively the resource, and an outbreak ensues. The situation where insect outbreaks occur because of the development of insecticide resistance is well known. Insecticides that are given repeated widespread application create a situation in which there is intense selection pressure for resistant individuals. These individuals can then proliferate in the insecticide treated area and cause subsequent outbreaks.

In addition to all of these factors the changes in consumer requirements for insect free produce in the USA and Europe have been one driving force behind the need to control insects at lower densities than before. Such shifts in consumer standards have meant that some insects are controlled beyond the point at which they are causing physical losses to the crop or animal. More recently demand for 'organic' or 'green' products has created a market at the other end of the spectrum. These are items of food, etc. that have been produced using only environmentally friendly techniques, excluding the use of chemicals such as insecticides or herbicides. With premium prices being paid for such 'green' or 'blemish-free' products then economics or aesthetics determine the level of infestation at which an insect is regarded as a pest.

Thus, consumerism and the needs of the consumer influence pest management practices. There are a whole range of stakeholders as well as consumers who influence pest management, how it has developed in the past and how it will develop in the future.

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