FALLING APHIDS ENHANCE IMPACT OF BIOLOGICAL-CONTROL BY PARASITOIDS ONPARTIALLY APHID-RESISTANT PLANT-VARIETIES

Cage experiments, in which the population increase of Metopolophium di rhodum was measured either in the absence or presence of the parasitoi d Aphidius rhopalosiphi, showed reductions of peak populations in the presence of the parasitoid of 30% on the susceptible wheat cultivar Ar mada, but of 57% on the partially resistant Rapier. On both cultivars, aphid population curves in the presence and absence of the parasitoid diverged before the first mummies were formed. The number of aphids l eaving plants was measured, and was found to increase in the presence of the parasitoid, particularly with Rapier (almost double). Most aphi ds (about 75%) which left plants fell, and a smaller proportion regain ed plants when parasitoids were present than when they were absent. A field experiment with Brevicoryne brassicae and natural predation on B russels sprouts also showed an increase in aphids leaving a less susce ptible compared with a more susceptible cultivar.