Host stage preference and reproductive fitness of Aphidius eadyi (Hymenoptera:Aphidiidae) on Acyrthosiphon pisum (Hemiptera:Aphididae)

Host stage preference by Aphidius eadyi Stary, Gonzalez & Hall on pea aphid, Acyrthosiphon pisum (Harris), and its effects on the reproductive fitness of A. eadyi, were studied in the laboratory at 20 +/- 1 degrees C and RH 60-70% with a photoperiod of 16 h light:8 h dark. Aphidius eadyi females accepted aphids of all stages but preferred fourth instar nymphs and adults for oviposition. Females developed increasingly faster with the increase of host stages, and males developed faster than females in all host stages. The sex ratio of resulting parasitoids was close to 1:1 from all host stages. The egg load and body size of A. eadyi progeny at emergence increased with increasing host stage at the time of parasitisation. The host stage had nonlinear relationships with the number of eggs laid and aphids parasitised by A. eadyi, and the body size of its resulting progeny. Both host stage and body size of resulting parasitoid progeny affected the egg load of newly emerged parasitoids but the latter had more effect. The fourth instar aphids appeared to be the most appropriate hosts for the mass-rearing programme because they gave the best fitness return for the parasitoids. The field release of parasitoids may be better timed when fourth instar aphids are the most abundant because aphids parasitised at this stage produce few progeny.