The effect of predators and hymenopterous parasites on population fluctuations of alfalfa aphids.

Among alfalfa pests in Iran three aphid species, green alfalfa aphid Acyrthosiphon pisum Harris spotted alfalfa aphid Therioaphis trifolii forma maculata Buckton and blue alfalfa aphid, Acyrthosiphon kondoi Shinji are important pests. The green alfalfa aphid can be observed all along the growing season particularly from late May to mid June at Karaj climate conditions. During this period, the mean monthly maximum temperature and relative humidity were about 28 degrees C and 60-65% respectively. This aphid overwinters as nymph and viviparous female. Sexual forms and eggs could not be seen under field conditions. Spotted alfalfa aphid, Therioaphis trifolii fonna maculata is the most prevalent aphid in summer time, when the mean monthly maximum temperature and relative humidity are about 33-34 degrees C and 44-58% respectively. Sexual individuals have been observed in the laboratory but not in the field. Among predators (Coccinella septempunctata, Adonia variegata, Syrphus cinctus, S. corolae, S. grassulariae, Chrysoperla carnea and Nabis capsiformis) one coccinellid species, C. septempunctata, had greatest impact on fluctuations of population. Among hymenopterous parasitoids two species have been collected from alfalfa field they were Aphidius ervi and Praon palitans. These parasitoids destroyed a good percent of aphids and statistically proved to lower aphid populations significantly.