Host-plant specialization in needle-eating insects of Sweden

It has been suggested that the enormous diversity of phytochemicals within the plant kingdom makes it impossible for one and the same insect species to exploit all plant species (Dethier 1954, Fraenkel 1959). Not surprisingly, the number and diversity of host plants utilized by different phytophagous insects are highly variable, and the specific selective pressures acting on them are still poorly understood (Bemays and Graham 1988, Strong 1988). Many hypotheses have been proposed to explain variations in host specificity among phytophagous insects. For example, there are those based on plant traits: plant defenses (Ehrlich and Raven 1964), plant apparency (Feeny 1976, Rhoades and Cates 1976), plant abundance (Root 1973), plant nutritional value (Mattson and Scriber 1987); those based on insect traits: degree of intimacy with the host plant (Mattson et al. 1988), neuronal capacity of the insect (Lains and MacArthur 1969) and finally those based on natural enemies' traits, e.g. the impact of generalist predators (Bernays and Graham 1988). In this paper we examine the predictions resulting from hypotheses based on plant characteristics and insect intimacy. We have used the data available in the literature on host ranges of Swedish needle-eating insects reported to feed on the three major conifer species native to Sweden: Scots pine (Pinus sylvestris), Norway spruce (Picea abies), and juniper (Juniperus communis). These species account for almost all of Sweden's coniferous flora.