Rev Iss Web Jpe 12709 53-6 1823..1830
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View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Epsilon Open Archive Journal of Applied Ecology 2016, 53, 1823–1830 doi: 10.1111/1365-2664.12709 Predator refuges for conservation biological control in an intermediately disturbed system: the rise and fall of a simple solution Anna-Sara Liman*, Karin Eklund and Christer Bjorkman€ Department of Ecology, Swedish University of Agricultural Sciences, P.O Box 7044, SE-75007 Uppsala, Sweden Summary 1. Managed systems harvested at intermediate time-scales have advantages over annual short-cycled systems in maintaining top-down control of insect herbivores, and the flexible harvest regimes in these systems provide opportunities for habitat management that can stabi- lize predator–prey population dynamics across harvests – resulting in reduced risk of pest outbreaks. 2. In a large-scale field experiment, we explored whether retaining refuges, that is preserving parts of the stand to reduce predator mortality, could reduce the risk of pest insect outbreaks in willow short-rotation coppice. Population densities of three omnivorous predator species and three outbreaking herbivorous leaf beetle species were monitored over four years after coppice (stem harvest) in eight stands with refuges (treatment) and eight stands without refuges (control). Predation pressure was estimated in years three and four. 3. Contrary to our predictions, leaf beetle densities were higher in stands with refuges and predator densities were higher in stands without refuges. Leaf beetle egg mortality increased with total predator density, but did not differ between stands with and without refuges. 4. These unexpected results can be attributed to interactions between dispersal and patch age. The altered phenology of coppiced stems may have triggered leaf beetle aggregation in refuges and migration from stands without refuges. A behavioural response to resource con- centration in retained old patches likely transformed the predator refuge from a ‘source’ to a ‘sink’. 5. Synthesis and applications. This study shows that retaining refuges in willow short-rotation coppice to facilitate predator population recovery after harvest can come at the cost of more attractive herbivore habitats – and thus increased pest problems. We conclude that crop refuges in systems with intermediate disturbance regimes pose new challenges for conservation biological control, in particular the need to consider how patch age affects dispersal and recolonization of both pest and predators. Key-words: conservation biological control, harvest, insect outbreak, omnivore, patch age, perennial crop, recolonization, refuge, short-rotation coppice Thorbek & Bilde 2004). Predators and parasitoids often Introduction follow in response to prey aggregations – which limit their Habitat stability is a well-recognized prerequisite for top- ability to establish in frequently disturbed cropping sys- down control of herbivorous insect pests (Southwood tems (Wissinger 1997). Generalist and omnivorous preda- 1977). Periodical disturbance such as harvest and tillage tors exhibit weaker aggregative and numerical responses in agricultural systems displaces the resident insect com- to abundance of specific prey species and tend to recolo- munity and leads to cyclic recolonization from refuge nize disturbed habitats relatively late (Symondson, Sun- habitats (Letourneau & Altieri 1999; Hossain et al. 2002; derland & Grennstone 2002). The so-called background or early-season control of insect pests provided by poly- phagous predators could thus be especially sensitive to *Correspondence author. E-mail: [email protected] frequent and intense disturbances and especially favoured © 2016 The Authors. Journal of Applied Ecology published by John Wiley & Sons Ltd on behalf of British Ecological Society. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made. 1824 A-S. Liman et al. by management for habitat stability (Symondson, Sunder- above the threshold for economic loss (population densi- land & Grennstone 2002; Welch, Pfannenstiel & Harwood ties >1 ind.* shootÀ1 or 10 000 ind.*haÀ1) and defoliation 2012). result in substantial reductions in biomass production in Complementary refuge habitats external to or inside willow SRC and in natural willow stands in northern Eur- annual crops can enhance the impact of arthropod gener- ope (Sage & Tucker 1997, 1998; Bjorkman€ et al. 2000, alist predators. An accumulation of studies in annual sys- 2004; Bell, Clawson & Watson 2006; Dalin 2006). Preda- tems shows that hedgerows and ditches or temporary tion by several species of omnivorous heteropteran bugs structures such as field margins, grass covered earth banks contributes to the control of leaf beetle populations in or strips with herbaceous ground flora can act as refuges both natural and managed willow stands (Bjorkman,€ to reduce winter mortality and improve predator repro- Dalin & Eklund 2003; Bjorkman€ et al. 2004; Dalin 2006). duction (Thomas, Wratten & Sotherton 1991; Dennis, Leaf beetle population densities and thus outbreak risks Thomas & Sotherton 1994; Collins et al. 2002; Olson & are lower in willow stands in agriculture-dominated land- Wackers€ 2007; Griffiths et al. 2008; Ramsden et al. 2015). scapes – partly due to the high density of omnivorous Conservation tillage provides refuges of previous years’ predators in these landscapes (Liman, Dalin & Bjorkman€ crop residues on the soil surface, which by similar mecha- 2015). nisms act to increase generalist predator density in annual In willow SRC, the stems are repeatedly cut back systems (Stinner & House 1990; Symondson, Sunderland (coppiced) and harvested at regular intervals of three to & Grennstone 2002; Tamburini et al. 2016). five years, through the crops life span of 15–25 years The intermediate disturbance regimes in many perennial (Ledin & Willebrand 1995). Winter coppicing causes high cropping systems make them potentially even more amen- mortality in the omnivorous predatory bugs that hiber- able to habitat management than annual crops (Landis, nate in the willow stand and disrupts the predator–prey Wratten & Gurr 2000). Perennial arboreal systems, for interaction the first years after harvest (Bjorkman€ et al. example orchards and short-rotation forestry, provide 2004). The herbivorous leaf beetles are not as sensitive structurally complex, heterogeneous semi-permanent habi- to the direct effect of coppicing, since they hibernate off- tats that favour insect community diversity, stability and site and recolonize the willow stands in spring (Kendall resilience (Simon et al. 2009). Orchards have a long his- & Wiltshire 1998; Sage et al. 1999; Peacock & Herrick tory of habitat management for conservation biological 2000; Bjorkman€ & Eklund 2006). Reduced predator pop- control, utilizing the low soil disturbance and multi-strata ulations after harvest allow for fast population growth design (tree canopy with herbaceous understorey ground rates in willow leaf beetle populations on resprouting cover) inherent to these systems and management of adja- willows the following spring (Bjorkman€ et al. 2004). cent hedgerows and grasslands to provide refuges for gen- Predator–prey modelling suggests that reducing average eralist predators (Altieri & Letourneau 1982; Simon et al. predator coppice mortality from the current 80% down 2009; Silva et al. 2010; Paredes et al. 2015). There are also to 40% would stabilize predator–prey dynamics and recent examples illustrating that preserving the under- reduce the risk of willow leaf beetle outbreaks (Dalin storey cover of weeds in short-rotation forestry can et al. 2011). One option to prevent severe reductions in improve generalist predator density in the tree canopy local predator abundance and facilitate predator popula- (Stephan et al. 2015). tion recovery in willow SRC would be to retain refuges Preservation of omnivorous predators with strong asso- of unharvested stems. ciation to specific host plants (e.g. for overwintering, The aim of this large-scale, four-year field experiment reproduction and alternative food resources) can only be was to empirically test the theoretical predictions that achieved by crop refuges, that is retention of parts of the leaving willow refuges would (i) prevent severe reductions field (Bjorkman€ et al. 2004). The role of crop refuges in in local predator population density and (ii) facilitate perennial agricultural systems has rarely been explored, predator population recovery and, thereby, (iii) increase with a few exceptions (Summers 1976; Hossain et al. predation pressure on willow leaf beetles. More specifi- 2002). Harvest regimes, such as strip harvest or dispersed cally, we expected resprouting SRC willow stands with retention, create a spatial mosaic of differently aged crop refuges to have higher densities of predators and lower patches that may reduce omnivore mortality and act as a densities of herbivores, than stands without refuges. Leaf source pool for recolonization. Crop refuges may, how- beetle egg mortality was expected to increase with preda- ever, also directly affect abundance of the target insect tor population density and, as a consequence, be higher in pest – a problem not necessarily associated with non-crop stands with refuges as compared to the control