16.1.2. Why insects become pests
Insects may become pests for one or more reasons. First, some previously harmless insects become pests after their accidental (or intentional) introduction to areas outside their native range, where they escape from the controlling influence of their natural enemies. Such range extensions have allowed many previously innocuous phytophagous insects to flourish as pests, usually following the deliberate spread of their host plants through human cultivation. Second, an insect may be harmless until it becomes a vector of a plant or animal (including human) pathogen. For example, mosquito vectors of malaria and filariasis occur in the USA, England, and Australia but the diseases are absent currently. Third, native insects may become pests if they move from native plants onto introduced ones; such host switching is common for polyphagous and oligophagous insects. For example, the oligophagous Colorado potato beetle switched from other solanaceous host plants to potato, Solanum tuberosum, during the 19th century (Box 16.5), and some polyphagous larvae of Helicoverpa and Heliothis (Lepidoptera: Noctuidae) have become serious pests of cultivated cotton and other crops within the native range of the moths.
A fourth, related, problem is that the simplified, virtually monocultural, ecosystems in which our food crops and forest trees are grown and our livestock are raised create dense aggregations of predictably available resources that encourage the proliferation of specialist and some generalist insects. Certainly, the pest status of many native noctuid caterpillars is elevated by the provision of abundant food resources. Moreover, natural enemies of pest insects generally require more diverse habitat or food resources and are discouraged from agro-monocultures. Fifth, in addition to large-scale monocultures, other farming or cultivating methods can lead to previously benign species or minor pests becoming major pests. Cultural practices such as continuous cultivation without a fallow period allow build-up of insect pest numbers. The inappropriate or prolonged use of insecticides can eliminate natural enemies of phytophagous insects while inadvertently selecting for insecticide resistance in the latter. Released from natural enemies, other previously non-pest species sometimes increase in numbers until they reach ETs. These problems of insecticide use are discussed in more detail below.
Sometimes the primary reason why a minor nuisance insect becomes a serious pest is unclear. Such a change in status may occur suddenly and none of the conventional explanations given above may be totally satisfactory either alone or in combination. An example is the rise to notoriety of the silverleaf whitefly, which is variously known as Bemisia tabaci biotype B or B. argentifolii, depending on whether this insect is regarded as a distinct species or a form of B. tabaci (Box 16.1).
