Control of a glasshouse pest through the conservation of its natural enemies? An evaluation of apparently naturally controlled shore fly populations Luke A.N. Tilley a,b,⇑ , Pat Croft b , Peter J. Mayhew a a Department of Biology, University of York, P.O. Box 373, York YO10 5YW, UK b Stockbridge Technology Centre Ltd., Cawood, Selby, North Yorkshire YO8 3TZ, UK article info Article history: Received 6 February 2010 Accepted 9 September 2010 Available online 22 September 2010 Keywords: Algae Aphaereta debilitata Conservation biological control Glasshouse Kleidotoma psiloides Natural enemy Pest management Scatella tenuicosta Tritrophic abstract There are few examples of naturally controlled pest populations in glasshouses. Evaluating the occur- rence of these systems may provide information about conditions that can promote natural enemy densities helpful to glasshouse growers. We evaluated the incidence of apparently naturally controlled populations of the algal-feeding shore fly Scatella tenuicosta Collin (Diptera: Ephydridae) over 8 weeks, in eight glasshouses in England. Across glasshouses, shore fly numbers were negatively correlated with the numbers of its parasitoid Aphaereta debilitata Morley (Hymenoptera: Braconidae), suggesting that A. debilitata may control shore flies naturally. Counterintuitively, the amount of algae present was negatively correlated with shore fly numbers, but positively correlated with numbers of the parasitoid. We therefore suggest that if growers want to encourage natural control of shore fly through A. debilitata then, contrary to conventional wisdom, algal growth should be encouraged within a glasshouse. This conservation approach to biological control could then be part of an integrated pest management pro- gramme for shore flies. This interrelationship between the pest, its food, and its natural enemy also high- lights the importance of considering all trophic levels within a biological control system, even where the pest is not a direct pest of the crop. Ó 2010 Elsevier Inc. All rights reserved. 1. Introduction Conservation biological control (CBC) is the modification of the environment or existing practices to protect and enhance specific natural enemies or other organisms to reduce the effect of pests (Debach, 1974; Eilenberg et al., 2001). A conservation approach does not rely solely on the application of commercially available control agents but makes use of the existing knowledge growers have of their cropping environments, and the insect species within them (Croft, 1990). The goal is to identify ecological constraints that limit the effectiveness of natural enemies, and to adjust prac- tices to lessen these constraints where possible. Therefore, CBC also allows practitioners to target any cultural modifications towards the specific conditions of their own sites in order to con- serve, and even enhance, naturally occurring control agents (Van Driesche and Bellows, 1996). However, in order for conservation strategies to be successful, prior knowledge of the pest, and its nat- ural enemy, is essential (Landis et al., 2000). A useful way to obtain such information is to evaluate existing cases of natural control of pest populations. In this paper we evaluate apparent cases of natural control of the glasshouse pest Scatella tenuicosta Collin (Diptera: Ephydridae), and relate the degrees of control to the envi- ronmental conditions and existing practices. Previous research on the ability of CBC to reduce pest popula- tions naturally has been mostly centred on outdoor crops and gen- eral habitat conservation (Van Driesche and Bellows, 1996; Bellows and Fischer, 1999; Gurr and Wratten, 1999; Landis et al., 2000; Gurr et al., 2003; Wilkinson and Landis, 2005; Wratten et al., 2007; Zehnder et al., 2007; Cullen et al., 2008; Wade et al., 2008). Examples of natural control within protected cultures, such as glasshouses, are limited. A notable example of the success of natural control establishment, however, has been shown in the tomato leaf miner, Liriomyza bryoniae Kalt (Diptera: Agromyzidae). Here, conservation measures to preserve two parasitoid wasp spe- cies, Dacnusa sibirica Telenga and Opius pallipes Wesmael (Hyme- noptera: Braconidae), provided adequate control of the pest (Woets and Van der Linden, 1982). The shore fly, Scatella tenuicosta, is an algal feeder (Zack and Foote, 1978; Foote, 1995), and is therefore found in most glass- house crops. The algae commonly grow on the surface of growing media and in standing water in glasshouses (Lindquist et al., 1994). Shore flies have a short generation time and are highly fecund, so populations can increase quickly when algal growth is continuous, such as in commercial glasshouses (Jacobson et al., 1999; Vanninen, 2001; Vanninen and Koskula, 2004; Ugine et al., 2007). Large populations of shore flies are a nuisance pest to glasshouse 1049-9644/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.biocontrol.2010.09.003 ⇑ Corresponding author. Address: Area 18, Department of Biology, University of York, P.O. Box 373, York YO10 5YW, UK. E-mail address: lant500@york.ac.uk (L.A.N. Tilley). Biological Control 56 (2011) 22–29 Contents lists available at ScienceDirect Biological Control journal homepage: www.elsevier.com/locate/ybcon