Outbreaks of the Tentcaterpillar (Malacosoma neustria L.) on elm trees in Amsterdam and the efforts to control them with derris-powder

A review is given of the recurrent outbreaks of the tentcaterpillar, Malacosoma neustria L., affecting the elm trees in the city of Amsterdam in the period 1930—1957. An outbreak of the brown tail moth, Euproctis chrysorrhoea L., in 1935—1936 is also mentioned and some occasional loss of foliage through the caterpillars of the vapourer moth, Orgyia antiqua L. Large fluctuations in the size of the Malacosoma populations were found. After some years in which the damage was negligeable, the increase always started in the N.W. part of the centre of the city. In the following years the outbreaks moved to adjacent parts of the centre and finally sometimes even to more eccentric parts (see Fig. 6—9). However, the occurrence of really harmful numbers of tentcaterpillars was mostly restricted to the centre of the town. The merits of the control measures which were carried out with derris/talcum powder spread by motordusters are discussed. The insecticide was thought to be the only sufficiently harmless for its use in the centre of a city. Some cases of defoliation and other inconveniences caused by the presence of crowds of caterpillars could apparently be reduced by the control measures. In several instances, however, the efficacy of the method remained doubtful. The urgency is stressed of further studies of the bionomics of Malacosoma under local conditions and some suggestions are made for a better organization of the measures applied.