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Diptera: Syrphidae) Eur. J. Entomol. 102: 539–545, 2005 ISSN 1210-5759 Landscape parameters explain the distribution and abundance of Episyrphus balteatus (Diptera: Syrphidae) JEAN-PIERRE SARTHOU1, ANNIE OUIN1, FLORENT ARRIGNON1, GAËL BARREAU2 and BERNARD BOUYJOU1 1Ecole Nationale Supérieure Agronomique de Toulouse, UMR Dynafor, BP 107, F-31326 Auzeville-Tolosane, France; e-mail: [email protected] 212, rue Claude Bizot, F-33170 Gradignan, France Key words. Syrphidae, Episyrphus balteatus, distribution, abundance, seasons, forest edges, landscape Abstract. We studied the importance of forest structure (shape, edge length and orientation) and the crop mosaic (percentage of crops in the total land cover, within 100 and 2000 m from the forests) to the dynamics of an aphidophagous hoverfly Episyrphus bal- teatus. Adults were collected by Malaise traps located within and on the south- and north-facing edges of 54 forests. In winter, E. balteatus was only found on south-facing edges because of the greater insolation and temperature. In summer, it was more abundant on north-facing edges because of the abundant presence of flowers. In spring, more adults were found on long and south-facing edges than on northern edges. The presence of shrubs within 2000 m also positively affected abundance. In autumn, abundance was positively associated with length of the north-facing edge and forest shape. Emergence traps revealed that in southern France, E. bal- teatus may overwinter in the larval or puparial stage in forest edges. Overwintering was earlier reported only in adults. Landscape structure, length of forest edges and probably presence of shrub fallows, influence abundance of Episyrphus balteatus. INTRODUCTION farmed landscape are area, floristic composition and also Because most of the natural enemies of crop pests do the shape of the largest features (Nentwig, 1988; not carry out their complete life cycle in cultivated fields, Molthan, 1990; Thomas et al., 1992). many studies focus on the important role of uncultivated In some landscapes characterised by forest fragments elements of the farmed landscape, such as hedges, field surrounded by crop mosaic, forests and their edges could margins, “beetle banks” and fallows, in increasing their play the role of natural enemy reservoirs for at least one populations and in improving their efficiency (Russel, phase of the life cycle of beneficial arthropods. If so, the 1989; Landis et al., 2000; Gurr et al., 2004) as control issue would be how forests and their edges in a rural agents. For example, in a structurally complex landscape, landscape should be distributed, shaped and managed in still characterised by several tens of metres of hedges per such a way as to favour natural enemies at both local and hectare, parasitisation of rape pollen beetle (Meligethes regional landscape levels. aeneus) larvae has been reported to be higher and the In the present study we analyse the effect of local and damage of this pest on oilseed rape (Brassica napus) crop regional landscape context, and forest fragment features, lower, than in a structurally more simple landscape with on the distribution and abundance of an aphidophagous larger fields and a high percentage of agricultural use beneficial species: Episyrphus balteatus (De Geer, 1776) (Thies & Tscharntke, 1999). Marino & Landis (1996) (Diptera: Syrphidae). The larva of this species is one of found similar results about parasitisation of armyworm in the most efficient predators of crop aphids (Ankersmit et maize fields. al., 1986; Poehling, 1988), and the adults feed on flowers At the landscape scale, patch isolation is often predicted both for energy and amino acids, needed particularly by to have a negative effect on population density and spe- females for oocyte maturation (Schneider, 1948). E. bal- cies richness, because many isolated patches have lower teatus is the most studied species among Syrphidae by immigration and emigration rates, thus reducing possibili- virtue of its efficiency as a predator and also its abun- ties of recolonisation and rescue (McArthur & Wilson, dance in most terrestrial habitats. This species is also 1967). Consequently, some studies have focussed on the known to overwinter as fertilised females in southern significance of degree of isolation on biological control in parts of Europe (Lyon, 1967), but also by migrating agroecosystems (Tscharntke & Kruess, 1999). Patch iso- southwards in the autumn throughout most of Europe lation may have a positive effect on population density (Aubert et al., 1976). for prey or host species which are controlled by enemies. It is well known that the sooner aphidophagous benefi- Indeed, it may disrupt the enemy’s ability to detect the cials establish in crop aphid colonies, the greater the prey (Kareiva, 1987) as Kruess & Tscharntke (1994) chance is of keeping pest populations below damaging showed with isolated fields of bush vetch (Vicia sepium) levels (Tenhumberg & Poehling, 1995); this fact has also which were less colonised by parasitoids of seed feeders. been demonstrated for syrphids (HonČk, 1983; Tenhum- The features that are most often cited as positively berg & Poehling, 1995; Corbett, 1998). Thus in southern influencing natural enemies in non-crop elements of the European regions where E. balteatus overwinters, it is 539 worthwhile to try to improve its overwintering conditions, Edge orientation and flower hypothesis in order that it will be present in aphid colonies as early Ten other forests were chosen according to their squat shape, and as abundantly as possible in the spring. their surface area (from 1,5 to 4 ha) and their northerly orienta- A previous study dealing with syrphid diversity in 54 tion and topographical location (with a south-facing edge rising forests in relation to geometric features of forests and on a hilltop or slope, and a north-facing one stretching into a landscape (Ouin et al., in press), encountered large num- basin bottom or close to it). These features were all exhibited by bers of adult E. balteatus (almost 20% of the more than the greater part of the forests in this region. 4900 syrphids collected), unevenly distributed among the Pre-imaginal overwintering stage hypothesis forests. We know that (i) forests and their edges are some Five of the ten forests described above were chosen so that of the most stable structures in rural landscapes, (ii) E. they covered the same general geographical area in the land- balteatus is a pollen and nectar-feeding insect in the adult scape (from north towards south, and from east towards west) as stage and probably overwinters as adult females at least in the larger set of ten. sheltered places, (iii) in different seasons, edges should Trapping methods thus be visited at different frequencies, dependent upon Local and wide landscape parameters hypothesis their particular characteristics, (iv) E. balteatus is almost We adjusted sampling intensity to the forest fragment area in ubiquitous (Speight, 2003) and mainly breeds in open an asymptotic manner: one trap for each 5 ha unit up to 15 ha, habitats (including forest edges) rather than within forest one trap more for each 10 ha up to 95 ha, and then one trap habitats (Rojo et al., 2003), and lastly (v) in spring and more for each 25 ha interval, so that the largest forest (171 ha) autumn, forest interiors (which are not optimal habitats had 15 sampling stations. Traps were installed in a total of 54 for the breeding of this species: Speight et al., 2003) can forests and samples were collected from 10th May to 14th June thus be supposed to act as filters and to be visited only by 2000 (hereafter called the spring) and from 13th September to occasional foraging individuals. Given these, we test here 19th October 2000 (hereafter called the autumn). The traps were set up at sparsely-vegetated and quite well-lit locations. The the following hypotheses: traps used were interception Malaise traps, composed of black (1) E. balteatus distribution and abundance in forests (non-attractive) vertical walls and roof (Marris House Nets depend upon both local and regional landscape parame- model). The sample bottles, part-filled with 70% ethyl alcohol, ters (peculiar to the forest and its surrounding crop were replaced every fortnight. mosaic, respectively), which vary in their influence Edge orientation and flower hypothesis according to the season; this hypothesis will subsequently Each of the ten forests was fitted out with one trap on its be referred to as the “Local and wide landscape parame- south-facing (180° ± 40°) and one on its north-facing edge (0° ± ters hypothesis”. 40°); a third was placed in its centre. All of the ten forest centres (2) Forest edges, south- as well as north-facing, and are quite densely vegetated and less well-lit than the edges. A flowers in their vicinity, act as factors determining E. bal- total of 30 traps were installed from the beginning of March teatus distribution and abundance, while forest interiors 2003 to the beginning of March 2004, and operated continu- have no significative effect. Indeed, we assume that, in ously. These traps were of the same Malaise-trap model, and the our study region, south- and north-facing edges act as sample bottles were managed in the same way. winter and summer shelters, respectively (Sarthou, 1996); Pre-imaginal overwintering stage hypothesis this hypothesis is subsequently referred to here as the Each of the five forests was fitted out with one emergence “Edge-orientation and flower hypothesis”. trap on the south-facing edge and another on the north-facing (3) E. balteatus overwinters also at a pre-imaginal edge. A total of 10 traps were in operation from the beginning stage, at least along forest edges, from which it spreads of February 2004 to the end of June 2004 and the sample bottles through the landscape in spring; this is referred to as the were checked every two weeks until the beginning of May, then “Pre-imaginal overwintering stage hypothesis”.
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