Conservation Evidence strives to be as useful to conservationists as possible. Please take our survey to help the team improve our resource.

Providing evidence to improve practice

Action: Use mass-emergence devices to increase natural enemy populations Natural Pest Control

Key messages

Read our guidance on Key messages before continuing

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

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.

Referenced papers

Please cite as:

Wright, H.L., Ashpole, J.E., Dicks, L.V., Hutchison, J., McCormack, C.G. & Sutherland, W.J. (2019) Some Aspects of Enhancing Natural Pest Control. Pages 603-625 in: W.J. Sutherland, L.V. Dicks, N. Ockendon, S.O. Petrovan & R.K. Smith (eds) What Works in Conservation 2019. Open Book Publishers, Cambridge, UK