Alfalfa Black Aphid, Aphis craccivora Koch (Hom.: Aphididae) Stage Preferences by Lysiphlebus fabarum Marshall (Hym.: Aphidiidae)

The host-stage preferences of alfalfa black aphid, Aphis craccivora, by parasitoid wasp, Lysiphlebus fabarum, were studied under two trials under growth chamber conditions (55±5 %, R. H., L:D 14:10 H. and Tem. 20 ±2° C ). In the first trial, all stages of aphid in equal ratios, and in the second trial, variable ratios of 5:15, 15:15 and 15:5 of 3rd and 4th instar nymphs were used for parasitisation. The objective was to identify the most preferred stage of aphid for parasitism by Lysiphlebus fabarum. The results showed that 3rd instar nymphs of alfalfa black aphid were the most preferred stage with 38.75% of parasitism. Whereas 4th instar nymphs and adults had 23.75% and 21.25% parasitism, respectively. The first instar nymphs were not parasitized at all. In the second trial with variable ratios of 3rd and 4th instars of alfalfa black aphid, parasitism of 3rd instar in ratios 25, 50 and 75 percent of total population were 35.9, 55.74 and 79.6%, respectively, while those for 4th instar nymphs were 20.36, 44.26 and 64.09%, respectively. So there was a positive preference for 3rd instar and a negative preference for 4th instar nymphs.