PHENOLOGY AND PREDICTION OF PEA APHID INFESTATIONS ON PEAS

This study was designed to investigate the possibility of predicting the timing and level of infestation of the pea aphid, Acyrthosiphon pisum (Harris), a pest of economic importance in peas. The source of infestation is perennial legumes. We followed the life cycle of the pea aphid from egg overwintering sites in leys of alfalfa, Medicago sativa (L.), and clover, Trifolium pratense (L.), to the infestations on peas, Pisum sativum (L.). Survival of overwintering eggs in the field was approximately 20%. Development of fundatrix to reproductive stage took 134 day-degrees (DD) above a threshold of 5 degrees C. Average total fundatrix fecundity was 52.04 nymphs. Because a considerable proportion of fundatrix offspring are alatae, these are probably an important source of first emigrants to the peas. Taking into account the low overall densities of these migrants, we found it impossible, using suction traps, to detect the timing or amount of migration to the peas early in the season. Catches in suction traps coincided with maximum alate production in peas, otherwise few pea aphids were caught in traps. Population development was best described using accumulation of day-degrees in the field. The intrinsic rate of increase per DD in peas was 0.01547 (SE 0.0007295, r(2) = 0.88) which is a common estimate for all years and locations. Peak occurrence in peas occurred at about 700 DD. Our studies show that it is virtually impossible to predict size of migration and therefore a general forecast of pea aphid levels in peas is impracticable. However, prediction of the timing of aphid population development is feasible. Temperature measurement together with field counts and the use of the common estimate of intrinsic rate of increase per DD is an appropriate method for the pest manager.