Acyrthosiphon pisum (Harris) (Homoptera: Aphididae) infestation level and damage on grass pea, Lathyrus sativus L., in West Gojam, Ethiopia

      Once a minor pest, the pea aphid, Acyrthosiphon pisum has already become a major pest of grass pea (Lathyrus sativus), in Ethiopia, particularly in the northwestern region. The effect of two levels of chemical treatment on pea aphids and their natural enemies was investigated in a field experiment in Achefer, northwestern Ethiopia from September 2010 to March 2011.   Plant height, plant density per 0.25 m2, number of pea aphids and parasitoids per plant, ladybird beetles per plot, and finally grain yield per plot were recorded. Based on the results of the ANOVA, plant height was significantly different between treated and untreated plots especially after the vegetative stage. The density of the plant did not significantly vary between treatments (P=0.99). Pea aphid population significantly varied between treated and untreated plots and between the different growth stages of the crop (P<0.0001). many of the natural enemies of pea aphid, i.e., ladybird beetles and parasitoids, were recorded on untreated plots. Generally, natural enemies increased as number of aphids increased. Significantly more yield was obtained from treated than untreated plots but there was no difference in yield between doses. Yield and aphid density were inversely related and grain yield loss due to pea aphids increased as high as 82.4%. This loss is very high necessitating appropriate control measures