NATURAL CONTROL OF THE SMALL ERMINE MOTH YPONOMEUTA-PADELLA (L)

The natural mortality of the Small Ermine Moth Yponomeuta padella (L.) was investigated. A previously-unknown microsporidium (Microspora), w hich infected the cells of the host gut wall, caused mortality in labo ratory experiments. This parasite was the most probable cause of popul ation decline in field sites and, as it readily infected the host in l aboratory or field, offered the possibility of a host-specific treatme nt. The polyembryonic parasitoid Ageniaspis fuscicollis Dalm. (Hymenop tera) had no detectable effect on population density. Solitary hymenop teran parasitoids also had little effect, although they tended to be p roportionally more numerous as the host population declined and may ha ve contributed to the die-out of weak populations. There was no indica tion of significant predation, although Agria mamillata Pand. (Diptera ), whose larvae preyed on the pupal aggregates, may have had a disprop ortionately large impact on adult emergence in relation to the number of individuals actually eaten. No significant mortality occurred durin g the winter, when the first instar larvae were dormant within the egg capsule.