General Comments

In the face of climate change, one could envision the invasion of new areas by pests formerly limited by one or more constraints. For example, as explained in section 16.6, the range of pink bollworm in North American cotton is limited by winter frost; hence milder winters would increase its range northward. Boll weevil is limited by desiccation of fruit buds in hot dry areas (DeMichele et al, 1976); hence increased summer rainfall might extend its geographical range in formerly dry areas. This occurred during the early 1980s, when a sequence of very wet years in Arizona and southern California, coupled with the cultivation of stub-cotton, temporarily increased the threat from boll weevil.

Some species may quickly reach outbreak proportions when conditions become favourable. In North Africa and the Middle East, desert locust numbers decline during periods of drought, but quickly explode from barely detectable numbers during prolonged region-wide rainy periods (e.g. Roffey and Popov 1968). This scenario is similar to that of the cowpea aphid in Australia described above (section 16.2). Under current weather, it might not be necessary to know the population dynamics of such species per se, but only whether conditions favour their increase and for how long. The growth index approach

Fig. 16.7. Assembly diagrams for lucerne (A), pea aphid (P), blue alfalfa aphid (B) and the pea aphid specific parasitoids A. smithi (S) and stenophagous parasitoid A. ervi (E) during (a) wet and (b) dry seasons. The fungal pathogen Pandora neoaphidis is very active during wet years and less so during dry ones. The arrows indicate the entry of species, with the heavy arrows indicating the evolution of the system. The dashed line indicates an infeasible path.

Fig. 16.7. Assembly diagrams for lucerne (A), pea aphid (P), blue alfalfa aphid (B) and the pea aphid specific parasitoids A. smithi (S) and stenophagous parasitoid A. ervi (E) during (a) wet and (b) dry seasons. The fungal pathogen Pandora neoaphidis is very active during wet years and less so during dry ones. The arrows indicate the entry of species, with the heavy arrows indicating the evolution of the system. The dashed line indicates an infeasible path.

worked well for the highly migratory cowpea aphid but would it work equally well for locust in the vast affected areas of North Africa and the Middle East, an area many times larger than southeast Australia? The current limitation to this approach is the lack of infrastructure for collecting the requisite weather data, and the unreliability of rainfall predictions using satellite remote sensing over this large landmass. If this data gap could be overcome, physiologically based models could provide a good way to evaluate the effects of weather and climate change on pest dynamics regionally using real-time weather.

What would happen to species such as desert locust in North Africa or cowpea aphid that are not regulated by natural enemies if the rainfall increased in response to climate change? Would cowpea aphid and desert locust population outbreaks be more frequent and prolonged? These are difficult questions to answer and require simplified but realistic models (e.g. equation 16.8). The models proposed by Gutierrez (1996), Schreiber and Gutierrez (1998) and in this chapter may be useful in answering such issues. However, field data and intuition suggest that the exotic cowpea aphid would become a more serious problem requiring new biological control agents, and possibly fungal pathogens might become more important in desert locust. In any case, the analysis would require a tritrophic perspective, and if feasible the results could easily be embedded in a GIS system (cf. Cherlet and Di Gregorio, 1991). In areas with greater infrastructure, linking of biologically rich models for pest/crop (e.g. cotton) systems in GIS would provide important information on their interactions as modified by weather, including the effects of climate change.

Lastly, from the point of view of global ecosystem sustainability, humankind remains the major pest of nature (Regev et al., 1998), and appears to be the major cause of climate change. Global human activities based on greed require regulation that is not provided by the current the world economy.

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