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The effects of air pollution on vegetation, in particular reduction in the economic yield of crops have been studied for many years. Various estimates have been made of the economic impact of different pollutants in this respect in both USA and Europe, while more recently interest has started to grow in developing countries (Ashmore et al., Shamsi et al., this volume — Chapters 3 & 6, respectively). The best estimate of such effects was produced by the USA National Crop Loss Assessment Network (NCLAN) programme, carried out in the 1980s. This employed a rigorous common protocol in which experimental plants were grown to maturity in open-top chambers ventilated with air containing a range of 03 concentrations in order to generate dose/response relationships. On the basis of the NCLAN study it was estimated that the current direct economic impact of O3 alone on the ten most important crops in the USA amounted to about $ 3 x 109 per year (Adams et al., 1988). This was equivalent to about 2.8% loss on annual production over the country as a whole. However, comparison with losses due to other stresses indicates that the latter are of a considerably greater magnitude, e.g., a 33% reduction in the annual production in USA resulting from attack by fungal pathogens and insect pests (Bell et al., 1993).

For decades, the existence and importance of sympatric speciation has remained a controversial issue in speciation research. Very strong opinions exist according to which sympatric speciation could, at most, be a rarely occurring exception, restricted to special types of situations, such as shifts in host preference by herbivorous insects (e.g., Mayr 1982, 1988; Futuyma & Peterson 1985).

Aerial pollution frequently modifies population densities of herbivorous insects, and this is often attributed to changes in host-plant quality. The role of indirect pollution mediated by changes in host plant quality can be elucidated only by monitoring both insect density and plant quality in natural environments. A major problem with this approach is how to identify parameters of plant quality which directly estimate its effects on insects, and which can be evaluated in the field.

The workshop on specialization in herbivorous insects, held by the European Science Foundation Network on Insect-Plant Interactions at Oslo in May 1991, brought together scientists working on ecology, population genetics, behaviour and physiology. The presentations provided a forum to review and discuss our knowledge about specialization in herbivorous insects. Areas within this topic that need further study were identified. It was recognized that although many European laboratories have an experience in the study of specialization, the subject would benefit from more collaborative projects between these laboratories and colleagues in other parts of the world. The topics covered included: A. factors influencing the driving force in the evolution of insect host plant specificity (“Why” questions), and B. mechanisms underlying specialization vs polyphagy (“How” questions). The discussions centered mainly on topic B.

Plant biologists long dismissed the importance of herbivorous insects in plant communities on the assumption that the amount of tissue they consume in non-outbreak years is inconsequential. In the last 15 years, the role of insects in driving the evolution of plant chemical and physical defenses has become appreciated. Other roles that insects may play are less well established: some feel that the impact of herbivores is small enough to ignore while others consider their affects on physiology, population and community dynamics as too important to dismiss.