STATUS OF AGENIASPIS FUSCICOLLIS (HYMENOPTERA: ENCRYTIDAE), AN INTRODUCED PARASITOID OF THE APPLE ERMINE MOTH (LEPIDOPTERA: YPONOMEUTIDAE)

The apple ermine moth, Yponomeuta malinellus Zeller, is a univoltine pest species that defoliates apple, Malus domestics (Borkh) (Rosaceae), in the temperate region of the Palaearctic. First instars overwinter within a communal hibernaculum beneath the covering of the egg batch (Kock 1998). In spring, larvae emerge to initially mine apple leaves and subsequently feed externally within a communal tent (Menken et al. 1992). During heavy infestations, the communal tents may envelop the entire apple tree, resulting in total defoliation (Parker and Schmidt 1985). There have been several accidental introductions and subsequent eradications of the apple ermine moth in eastern North America (Hewitt 1917; Parker and Schmidt 1985) but, by 1989, the pest was found in the Fraser River Valley in British Columbia, in Whatcom county, Washington, and in northwestern Oregon (Antonelli 1991; Unruh et al. 1993).

Impact and oviposition behaviour ofAgeniaspis 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 mothYponomeuta malinellusZeller by the polyembyronic encyrtid parasitoidAgeniaspis fuscicollis(Dalman) were examined in a three year field study and related to oviposition behaviour in the laboratory.Ageniaspis fuscicollisattacks egg batches of its host and kills the final instar larvae, which feed gregariously from within tents. Host population densities in the field were low, from 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 density at the whole tree scale, but at the individual tent scale, the probability of a tent containing parasitized host larvae increased and percent parasitism decreased with the number of host larvae per tent. Observations on the oviposition behaviour ofA. fuscicollisin the laboratory revealed that 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 an egg batch, contradicting our field observations of inverse density dependence. Factors that might account for the differences in rates of parasitism and attack distributions between laboratory and field data are discussed.

Quantifying the impact of insect predators and parasitoids on populations of the apple ermine moth, Yponomeuta malinellus (Lepidoptera: Yponomeutidae), in Europe

Life tables were developed to assess the significance of natural enemies on the dynamics of apple ermine moth, Yponomeuta malinellus Zeller, in southwestern Germany and to select parasitoid species for use in the biological control of this pest in Canada. During the study from 1993 to 1995 the abundance of Y. malinellus varied from 1.5 to 4.3 tents per 100 leaf clusters indicating that this was a non-outbreak population. From the life tables it was evident that the impact of egg predators accounted for 25–43% of the total generational mortality of Y. malinellus, more than any other known mortality factor. Percent parasitism varied from 18 to 30%, but the impact of parasitoids in relation to the total generational mortality of Y. malinellus from the life tables was remarkably constant at 11–14%. The loss of potential fecundity had an important influence on the generational mortality of Y. malinellus, but declined from 27% to 15% over the course of this study. This decline corresponded with a rise in the net rate of increase R0 from 1.35 in 1993 to 6.8 in 1995, despite the impact of insect predators and parasitoids on the generational mortality. Yponomeuta malinellus was attacked by five different obligate primary parasitoids, a single obligate hyperparasitoid, and three facultative hyperparasitoids. Of these, the oligophagous egg-larval parasitoid Ageniaspis fuscicollis Dalman (Encyrtidae) and the oligophagous larval-pupal and pupal parasitoid Herpestomus brunnicornis Gravenhorst (Ichneumonidae) were selected as potential biological control agents for Canada due to a minimal degree of interspecific competition.