Climatic signals in the life histories of insects: the distribution and abundance of heather psyllids (Strophingia spp.) in the UK

1. The population density and age structure of two species of heather psyll id Strophingia ericae and Strophingia cinereae, feeding on Calluna vulgaris and Erica cinerea, respectively, were sampled using standardized methods a t locations throughout Britain. Locations were chosen to represent the full latitudinal and altitudinal range of the host plants. 2. The paper explains how spatial variation in thermal environment, insect life-history characteristics and physiology, and plant distribution, intera ct to provide the mechanisms that determine the range and abundance of Stro phingia spp. 3. Strophingia ericae and S. cinereae, despite the similarity in the spatia l distribution patterns of their host plants within Britain, display strong ly contrasting geographical ranges and corresponding life-history strategie s. Strophingia ericae is found on its host plant throughout Britain but S. cinereae is restricted to low elevation sites south of the Mersey-Humber li ne and occupies only part of the latitudinal and altitudinal range of its h ost plant. There is no evidence to suggest that S. ericae has reached its p otential altitudinal or latitudinal limit in the UK, even though its host p lant appears to reach its altitudinal limit. 4. There was little difference in the ability of the two Strophingia spp. t o survive shortterm exposure to temperatures as low as - 15 degrees C and l ow winter temperatures probably do not limit distribution in S. cinereae. 5. Population density of S. ericae was not related to altitude but showed a weak correlation with latitude. The spread of larval instars present at a site, measured as an index of instar homogeneity, was significantly correla ted with a range of temperature related variables, of which May mean temper ature and length of growing season above 3 degrees C (calculated using the Lennon and Turner climatic model) were the most significant. Factor analysi s did not improve the level of correlation significantly above those obtain ed for single climatic variables. The data confirmed that S. ericae has a I year life cycle at the lowest elevations and a 2 year life cycle at the hi gher elevations. However, there was no evidence, as previously suggested, f or an abrupt change from a one to a 2 year life cycle in S. ericae with inc reasing altitudes or latitudes. 6. By contrast with S. ericae, S. cinereae had an obligatory 1 year life cy cle, its population decreased with altitude and the index of instar homogen eity showed little correlation with single temperature variables. Moreover, it occupied only part of the range of its host plant and its spatial distr ibution in the UK could be predicted with 96% accuracy using selected varia bles in discriminant analysis. 7. The life histories of the congeneric heather psyllids reflect adaptation s that allow them to exploit host plants with different distributions in cl imatic and thereby geographical space. Strophingia ericae has the flexible life history that enables it to exploit C. vulgaris throughout its European boreal temperate range. Strophingia cinereae has a less flexible life hist ory and is adapted for living on an oceanic temperate host. While the geogr aphic ranges of the two Strophingia spp. overlap within the UK, the psyllid s appear to respond differently to variation in their thermal environment.