Illustrated notes on the biology of two European species of Euceros Gravenhorst (Hymenoptera: Ichneumonidae: Eucerotinae)

Abstract

• Observational notes and photographs taken in the course of rearing Euceros albitarsus Curtis and E. pruinosus (Gravenhorst) in captivity from the egg stage are given. The bizarre biology of these obligate hyperparasitoids involves a planidial larval stage that attaches to caterpillars or sawfly larvae, followed by a short phase of internal parasitism of a primary parasitoid if already or subsequently present, followed in turn by the principle growth period of external parasitism on the primary parasitoid, as shown by Tripp (1961). Observations on aspects of this that were not illustrated by Tripp are given and, although these transitions are not fully elucidated here, it is speculated that the number of larval instars had hitherto been overestimated. Investigations on the behaviour and fate of the planidial phase suggest that it is best able to transfer to primary parasitoids that are associated with the prepupal state of the lepidopteran host. Planidia on caterpillars that are not themselves parasitised end up in the host pupa, usually occupying the subalar cavity of the exuvial space (that is, just below the pupal cuticle at the wing cases), but do not develop further in the absence of a primary parasitoid. A summary of some aspects of the biology of Euceros that are not yet understood is given: the main uncertainties are the number and feeding positions of larval instars, the timing and nature of the planidium’s transfer to the primary parasitoid, and the func-­ tion of the unusual male antenna