Impact and oviposition behaviour of Ageniaspis fuscicollis (Hymenoptera : Encyrtidae), a polyembryonic parasitoid of the apple ermine moth, Yponomeuta malinellus (Lepidoptera : Yponomeutidae)

The distribution and extent of parasitism of the apple ermine moth Yponomeu ta malinellus Zeller by the polyembyronic encyrtid parasitoid Ageniaspis fu scicollis (Dalman) were examined in a three year field study and related to oviposition behaviour in the laboratory. Ageniaspis fuscicollis attacks eg g batches of its host and kills the final instar larvae, which feed gregari ously from within tents. Host population densities in the field were low, f rom 1.5 to 2.2 tents per 100 leaf clusters, and parasitism increased from 7 .8% to 18% over the three year period. Parasitism was independent of host d ensity at the whole tree scale, but at the individual tent scale, the proba bility of a tent containing parasitized host larvae increased and percent p arasitism decreased with the number of host larvae per tent. Observations o n the oviposition behaviour of A. fuscicollis in the laboratory revealed th at parasitoids distributed their eggs randomly within host egg batches. On average, they spent almost two hours on an egg batch and laid 44% of their egg load of 132 eggs into the first egg batch visited, leading to a mean of 1.4 eggs laid per host egg through frequent self-superparasitism of hosts. The percentage of eggs receiving one or more ovipositions was independent of the size of arm egg batch, contradicting our field observations of inver se density dependence. Factors that might account fur the differences in ra tes of parasitism and attack distributions between laboratory and field dat a are discussed.