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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.

Several insecticides, when applied to a hawthorn hedge by hydraulic sprayer at approximately 85 ml m−2 to control feeding larvae of small ermine moth, compared favourably with the only currently approved insecticide (trichlorfon), and some may be preferable for environmental reasons. The control provided by diflubenzuron (0.005%) and some pyrethroids (0.001%), which are arthropod-specific or nearly so, was so high as to suggest that further reduction in spray concentration or volume is possible. Bacillus thuringiensis, whose effect is confined to the larvae of Lepidoptera and some Diptera, and dimethoate, which gave a rapid and curtailed kill, also performed comparably with trichlorfon. When applied in a Potter's Tower, DNOC, trichlorfon and fenitrothion killed larvae within their dormant-season covering, but fine-droplet hydraulic spraying in February was unsuccessful, apparently because of inadequate target coverage.