Mass-emergence devices: a biocontrol technique for conservation and augmentation of parasitoids

Natural enemies frequently prevent the build-up of large pest populations. The conservation and augmentation of antagonists can be an effective biocontrol strategy to increase their impact. We suggest the use of pest-infested plant material as a source for conservation or augmentation of parasitoids. Plant material containing the host and its parasitoids can be collected and stored in large closed devices from which the generally smaller parasitoids can emerge through a selective tissue filter of appropriate mesh size. We describe the construction and field test of mass-emergence devices to conserve or augment populations of indigenous parasitoids of the invasive horse chestnut leafminer, Cameraria ohridella (Lepidoptera, Gracillariidae). At a mesh size of 600 µm, 78% of the parasitoids passed through the tissue filter, whereas only 1.1–2.5% of the leafminers overcame the barrier. Mass-emergence devices significantly increased percent parasitism of C. ohridella at one of two field sites tested. The observed parasitism rates at both sites were about 2.5-times higher than in the previous two years and at neighbouring sites in the same year. Moreover, percent parasitism during the third generation of C. ohridella was among the highest ever observed in Central Europe. Mass-emergence devices may thus contribute to an integrated control strategy against C. ohridella. In general, mass-emergence devices are a promising biocontrol approach to manage insect pests with a dormant stage in or on plant residues and should be considered as a supplementary technique in cases where the use of insecticides is restricted.