The role of weather and natural enemies in determining aphid outbreaks
The importance of climatic conditions and the action of natural enemies in the population dynamics of aphids are considered and it is illustrated how they might affect the likelihood of aphid outbreaks. It is concluded that climatic conditions, especially temperature and wind, are critical to outbreaks, prolonged favourable weather conditions allowing aphids to achieve their rapid population increase to epidemic levels. Weather conditions also affect the number of immigrants settling on crop plants and food-plant quality. The impact of natural enemies on aphid populations is variable: in some cases they may contribute to a reduction in aphid numbers around the peak of abundance, but in many crop systems yield losses will already have occurred. The ratios of natural enemies to hosts and prey have to be relatively high soon after aphid colonization of the food plant if outbreaks are to be prevented by their action; the effect of climatic conditions on natural enemies frequently prevents such ratios from being achieved.