OVERWINTERING OF CEREAL APHIDS (HOMOPTERA, APHIDIDAE) ON DURUM-WHEAT IN A MEDITERRANEAN CLIMATE

A 7-yr study on field plots in Lleida, northeastern Spain, was carried out to determine the factors that play a major role in the population dynamics of cereal aphids in winter under Mediterranean conditions. P lots were monitored at 7-10-d intervals from crop emergence until the first fortnight in April using a D-vac vacuum net set. Several weather and host plant parameters were related to the following aphid populat ion changes: (1) seasonal net rates of aphid increase (from the first sampling in October or November to 31 March), (2) net rates of increas e, and (3) k mortality values between consecutive samplings. Populatio n dynamics of Rhopalosiphum padi (L.) and Sitobion avenae (F.), the sp ecies found most frequently, were affected, not only by low temperatur es causing aphid mortality but also by other factors. Dry and cold wea ther, through the effect they have on host plant phenology and quality , reduce aphid developmental rates. Winter minimum temperatures and ra infall, the latter by its effect on host plants, seem to be the major weather parameters to consider to forecast aphid population levels at the beginning of spring.