Predicting spring migration of the damson-hop aphid Phorodon humuli (Homoptera: Aphididae) from historical records of host-plant flowering phenology and weather.
Historical data of the spring migration of the aphid Phorodon humuli recorded at Wye, Kent and Rosemaund, Herefordshire, UK, and the phenology of overwintering host-plant flowering, recorded at East Malling, Kent, were examined for possible associations. Relationships between mean temperature over a phenophase interval, defined by flowering phenology of two overwintering host plum species (Myrobalan Prunus cerasifera and Victoria Prunus domestica), which are overwintering hosts for Phorodon humuli, and the start of migration of the aphid, versus the reciprocal of the interval duration (days), were significant for Wye, (situated 32 km from East Malling) and at Rosemaund, a more north-westerly site 260 km from East Malling. Predictive sample reuse (PSR) methodology was used to validate the potential of derived regression equations to predict the start of migration of P. humuli at the two sites. Data for model determination and validation consisted of observations of P. humuli migration over 20 years (1967-86) at Wye and 15 years (1972-86) at Rosemaund. The predictive performance of the host-plant flowering-aphid migration phenology equations was compared with that of two other methods that use historical field data to predict insect life cycle events. The host-planting flowering-aphid migration phenology equation using the beginning of flowering of Victoria gave the best prediction of the start of migration at Wye using one PSR criterion. The empirical day-degree (DD) programme gave the best prediction using an alternative criterion. For the more distant Rosemaund site, the empirical DD programme performed best. The potential use of host-plant phenology to predict 50% migration of P. humuli at Wye and Rosemaund was also investigated, and compared with the empirical DD programme using the PSR method. The host-planting phenology equation gave good prediction of migration at Wye but poor prediction at Wye and only reasonable prediction at Rosemaund. The value of using historical data of host-planting and insect phenology to develop predictive equations is illustrated, and the desirability of comparing the performance of alternative phenology equations that may be used for prescriptive use is discussed.