Action: Use mass-emergence devices to increase natural enemy populations
Parasitism: One randomised, replicated, controlled study in Switzerland found higher parasitism at one site but no effect at another site when mass-emergence devices were used in urban areas.
Pest damage: The same study found no effect on pest damage to horse chestnut trees
Mass-emergence devices are containers giving natural enemies a sheltered environment and a food or prey source (such as pollen or pests on infested foliage), enabling enemy numbers to establish before emerging from the device and dispersing into the crop. Designs may include size-selective exits, preventing pests but allowing natural enemies (such as parasitoid wasps) to leave and disperse. Conventional practices of removing and destroying pest-infested crop foliage can reduce natural enemy numbers, but using the foliage in mass-emergence devices instead may relocate natural enemies back into the crop.
We found no studies testing this action in a farmed environment, but one study of urban trees is presented here as preliminary evidence.
Supporting evidence from individual studies
A randomised, replicated, controlled study in 2003 at two urban sites in Bern, Switzerland (Kehrli et al. 2005) found higher parasitism of horse chestnut leafminers Cameraria ohridella in trees with mass-emergence devices (averaging 5-16% leafminers parasitised) than control trees without devices (3-10%) at one site and for a March (rather than May) application date. There was no effect of mass-emergence devices (or timing of application) at the second site (4-14% leafminers parasitised in treated trees vs. 5-15% in controls). Leaf loss caused by leafminers was similar in mass-emergence (3-54% defoliation) and control (3-63%) trees at both sites. Devices were placed in horse chestnut Aesculus hippocastanum trees to control leafminer damage using parasitoid wasps (Hymenoptera). Devices were 200 l plastic tubs with four openings covered in a tissue filter with 600 µm mesh size – allowing wasps (but not leafminers) to develop, emerge and disperse into the trees. Horse chestnut leaf litter containing leafminers and parasitoids was placed inside the tubs (10 kg/device). Ten blocks of horse chestnut trees were selected (five at each site) and devices were hung in three trees/block. Two trees had devices (1 device/tree, applied 20 March and 23 May, respectively) and a control tree had no device.