Comparative phenologies of two migrant cereal aphid species

1. The flight phenologies of the cereal aphids Sitobion avenae and Metopolophium dirhodum, as indicated by suction trap catches at 18 sites throughout Great Britain between 1975 and 1984, were compared using multivariate methods. 2. Generalized Procrustes analysis identified trends in the phenologies of both species that were not clear from principal coordinate analysis alone. 3. The 18 sites were grouped into the same eight regions for both species, which confirmed broadly that sites relatively closer had more similar flight phenologies. 4. The flight phenologies of both species followed a general south-north trend but with separate components each side of latitude 54-degrees-N, that to the south being oriented from south-south-west to north-north-east, that to the north from south-south-east to north-north-west. There were slight orientation differences between species. An explanation for this is proposed based on winter temperature clines. 5. For M. dirhodum the date of first catch and date of achievement of 25, 50 and 75% of total seasonal catch were generally later with increasing distance north; for S. avenae these trends were present but weaker south of latitude 54-degrees-N. For both species these trends were slightly more variable south of latitude 54-degrees-N. The duration between achievement of 25 and 75% of total seasonal catch was shorter at sites north of latitude 54-degrees-N for both species, but more markedly so for S. avenae. 6. The range in the above phenological events averaged over years was greater for M. dirhodum than for S. avenae. The range of date of first catch was greater than the range of date by which 25 or 50% of the total catch was achieved, for both species. The ranges of date by which 75% of the total catch was achieved were markedly different in the north and south of Britain for both species. 7. The consistency of the flight phenologies between years varied independently for each species and was greater for S. avenae. However, for both species 1978 and 1981 were unusual, the date of first catch being earlier and the season extended later, especially at southern sites. 8. The most important implication of these findings for pest forecasting is that forecasts based on phenology, and developed from data for a single site, are likely to have an application locally but cannot be used accurately in other regions.