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), which infected the cells of the host gut wall, caused mortality in laboratory experiments. This parasite was the most probable cause of population decline in field sites and, as it readily infected the host in laboratory or field, offered the possibility of a host-specific treatment. The polyembryonic parasitoid Ageniaspis fuscicollis Dalm. (Hymenoptera) had no detectable effect on population density. Solitary hymenopteran parasitoids also had little effect, although they tended to be proportionally more numerous as the host population declined and may have contributed to the die-out of weak populations. There was no indication of significant predation, although Agria mamillata Pand. (Diptera), whose larvae preyed on the pupal aggregates, may have had a disproportionately large impact on adult emergence in relation to the number of individuals actually eaten. No significant mortality occurred during the winter, when the first instar larvae were dormant within the egg capsule.