Target pest identification and area of origin

Lindroth (1957) described five criteria that could be used to identify a pest as alien: insect history, geography, ecology, biology and taxonomy. Pschorn-Walcher (1977) added a sixth: parasitology. In brief, the first of these criteria applies to the rare situation where the history of an introduction has been well documented; geographical criteria may be inferred when an insect has a very patchy distribution over the range of its host, perhaps because insufficient time has elapsed to allow colonization. Insects that have a strong association with the human environment such as cockroaches would be included as alien under the ecology criterion. Host specificity would be a character associated with biology while the taxonomic criteria would apply to species holding an isolated taxonomic position with no close relatives among the native fauna. The parasitological criterion would include pests that had no obvious natural enemy complex associated with them, suggesting recent colonization (in evolutionary terms).

Once a pest has been identified a literature review would be undertaken to find out as much as possible about the species, its origins and its native natural enemies. The basis on which a pest is identified as alien should provide some information about its centre of origin. The ever increasing knowledge of insect taxonomy and distribution makes it increasingly easy to identify the native area of an invading species (van den Bosch and Messenger, 1973). However, failure to identify correctly the centre of origin could mean the importation and introduction of less effective natural enemies than could potentially be available, since a mature, well-balanced complex of natural enemies would only exist in the native home area (Pschorn-Walcher, 1977).

It is not necessarily a simple matter to identify the centre of origin, often because the insect(s) concerned have such vast ranges; for instance, the alfalfa weevil (Hypera postica) has a range that extends across Europe, Africa, the Middle East and Asia, while the codling moth (Cydia pomonella) covers the Palaearctic region (van den Bosch and Messenger, 1973). The distribution of the target host plant or areas having appropriate climates may provide useful indicators. Information on the biology of the rice stem borers Chilo suppres-salis and Tryporyza incertulas provided some circumstantial evidence for the area of origin of these insects (Yasumatsu, 1976). C. suppressalis is distributed northward from the tropics into the temperate zone where it enters diapause during the winter. It seems to prefer well drained paddies and its larvae remain high in the stalk at harvest. By way of contrast, T. incertulas is distributed from the tropics north to the southern regions of the temperate zone. It appears to favour poorly drained situations and its larvae bore towards the roots even in deep water. Yasumatsu (1976) concluded that these differences suggested that C. suppressalis originated in the dryer temperate area and T. incertulas in a damp tropical situation.

0 0

Post a comment