The Ecology of Pests
What Makes a Pest a Pest?
Species are not created as “pests.” Whether a species constitutes a pest is a direct result of the preferences of humans. Animals or plants that are in places or at times where humans do not want them to be are considered pests. For example, there is nothing inherent to a particular plant that makes it a weed. It is the fact that a plant is not where humans want it that makes it a weed. Additionally, species that cause some form of harm either to humans or to other organisms, or things that humans value, are also considered to be pests. Pests can directly harm humans. For example, biting flies and mosquitoes cause pain, itching, and annoyance. Some pests are merely nuisance pests. For example, certain ants, which cause no particular harm and may be perfectly acceptable when outside, become pests when they find their way into human kitchens. Some pests, such as termites and carpenter ants, may cause harm by damaging human structures. Other pest species may not harm humans directly, but instead may be vectors that transmit human diseases, such as malaria, dengue fever and other insect-borne diseases. Similarly, pests can transmit diseases to domestic animals, whether pets or livestock. Of course, one of the most significant forms of pest are those of agricultural crops and products.
Virtually every crop and animal species has pests associated with it. Some pests are generalists and will feed on a wide range of species. Other pests are more host-specific and will only be attracted to certain taxa of organisms. For example, with crop pests, the level of attractiveness of specific host taxa to specific pest species is frequently due to specialized secondary chemicals produced by the host species, to which the pest species has evolved an attraction.1
There are literally hundreds of species that are considered “major” pests. Major pests exist for humans, animals, and plants. One of the most significant human medical pests includes the Anopheles mosquito, which carries and transmits malaria throughout vast regions of the world. Fleas have caused substantial harm to humans by transmitting the deadly plague, which has been responsible for multiple waves of pandemics killing millions of people throughout human history,2 and lice transmit typhus. Livestock has a number of major pests that transmit disease, including certain ticks and flies.
It is estimated that there are more than 300 major crop pests.3 Some of the most serious of these include locusts, which can devastate virtually any crop, and a number of plant and leaf hoppers, and bollworms, which are significant pests of major crops, like grain and cotton. Some pests, such as locusts, are major pests because of the large mass of plant material they consume. Others, such as the hoppers, are pests because they are vectors of plant diseases, which cause substantial crop loss.
Because pests are defined as such by being in a place and time where they are not desired, they can be distributed geographically and temporally. Some pests are localized, whereas others may be ubiquitous and distributed over vast areas of the earth. Some pests are only problems temporarily during certain seasons or under certain conditions, whereas others persist through a range of seasons and conditions.4 Jason Clay describes this phenomenon as follows: “[b]y providing vast areas of food and by eliminating the diversity that would support competition from a wider array of species, modern agriculture has reduced the number of species that can live in large areas of the landscape while allowing a few to become dominant.”5 It is these dominant “few” that we consider to be pests.
The Role of Pests in Ecological Systems
Because many of the species that we consider to be pests, and thus that we use pesticides to control, are invertebrate species, to understand their role in ecological systems it is first necessary to understand the role of invertebrate species in ecosystems. It is estimated that there are as many as 8 million species of insects, a large category of invertebrates, on earth, making up four-fifths of all metazoan species.6 In fact, in evolutionary terms, insects are considered to be one of the most successful forms of animal, due to their ability to rapidly adapt to changing conditions. They are able to thrive in virtually all types of ecosystem. Samways has described the insects as an “immensely successful life form that has taken … many diverse evolutionary paths ….”7 Insects’ ability to adapt to so many different environments, including man-made ones, has made them one of the most significant categories of pest.
While any living organism can be considered a pest if it is found in a place and at a time it is unwanted, or if it causes economic loss or health problems, invertebrates, particularly insects and arachnids, are second only to weedy plants as the most significant targets of pesticide use. Crop damage from insects typically results from the sheer numbers of insects feeding on a crop. In most cases, small numbers of pest insect species can be tolerated without significant crop damage. However, when insect populations reach a threshold level, cumulative damage from many thousands of insects becomes unacceptable. It is estimated that there could be as many as 25 million insects per hectare of soil and 25,000 in flight over a hectare at any given point in time.8 Of course, most of these are not significant pests, and in fact many may be beneficial. Nevertheless, even the most insignificant pest can become a major problem if present in high enough numbers. For example, as many as 22 million fruit flies may be found in one hectare of oats and an estimated 200 million black bean aphids in a hectare of sugar beets during infestation.9
Long before humans were faced with the challenges of agricultural pests, hunter-gatherers had to deal with nuisance and disease-carrying biting insects.10 Even today, a major category of what we refer to as pests includes species such as the tsetse fly and numerous species of mosquitoes, which act as vectors transmitting diseases to humans and domestic animals. The other major category is agricultural pests, which include, among many others, the well-known locusts, armyworms, bollworms, and diamond-back moths. These kinds of pest have existed since the beginning of agriculture. Six thousand-year-old Chinese cave paintings and 4,300-year-old Egyptian artifacts depict pests attacking crops.11 The Bible references the “plague of flies” and the “plague of locusts.”12