PHENOLOGY AND PREDICTION OF PEA APHID INFESTATIONS ON PEAS

This study was designed to investigate the possibility of predicting t he timing and level of infestation of the pea aphid, Acyrthosiphon pis um (Harris), a pest of economic importance in peas. The source of infe station is perennial legumes. We followed the life cycle of the pea ap hid 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 a pproximately 20%. Development of fundatrix to reproductive stage took 134 day-degrees (DD) above a threshold of 5 degrees C. Average total f undatrix fecundity was 52.04 nymphs. Because a considerable proportion of fundatrix offspring are alatae, these are probably an important so urce of first emigrants to the peas. Taking into account the low overa ll densities of these migrants, we found it impossible, using suction traps, to detect the timing or amount of migration to the peas early i n the season. Catches in suction traps coincided with maximum alate pr oduction in peas, otherwise few pea aphids were caught in traps. Popul ation development was best described using accumulation of day-degrees in the field. The intrinsic rate of increase per DD in peas was 0.015 47 (SE 0.0007295, r(2) = 0.88) which is a common estimate for all year s and locations. Peak occurrence in peas occurred at about 700 DD. Our studies show that it is virtually impossible to predict size of migra tion and therefore a general forecast of pea aphid levels in peas is i mpracticable. However, prediction of the timing of aphid population de velopment is feasible. Temperature measurement together with field cou nts and the use of the common estimate of intrinsic rate of increase p er DD is an appropriate method for the pest manager.