Landscapes Forest and Global Change - ESA - Escola Superior ...

A. Ouin et al. 2010. Do wooded elements in agricultural landscape contribute to biological control in crops
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hoverfly abundance (eggs+larvae) in wheat fields between the wooded and non wooded
landscapes. Compared with poorly wooded landscapes, landscapes with 27% of wood landcover
seemed to shelter higher hoverfly abundance during one of the key-periods for the aphid
population dynamics (early spring), thus providing them with higher potential regulation
capabilities. Afterwards, the difference between hoverfly population abundances decreased
during the season (Figure 1).
Density of overwintering ground beetles in a woodlot
We collected a total of 2014 ground beetles during the whole trapping period, belonging to 48
species. The total density of emerging ground beetles was four to five times higherin edges than
in the inner areas of the woodlot and the mean number of species per trap was three times
higher in edges than in the inner areas. Open habitats species overwintered only in the edges and
not in the inner areas while the two other groups of species overwintered in all the woodlot but
with a marked preference for the edges (Figure 2).
4. Discussion
Our results show that woodlot edges are favourable for insect biodiversity in agricultural
landscapes, by (i) favouring abundances of aphidophagous hoverflies in early spring in fields,
and (ii) sheltering both high species richness and abundances of ground beetles. Positive
influence of woodlot edges on these insect populations seems to occur mainly during the winter
period, probably by offering local environmental conditions favourable to their overwintering.
Identifying such landscape elements which favour overwintering of beneficial insects is of a
particularly interest from the agronomic point of view of biological control of pests, for which
early arrival of a natural enemy is required in spring (Honek, 1983; Tenhumberg & Poehling,
1995; Corbett in Pickett & Bugg, 1998). Aphidophagous hoverflies distribution and abundance
prove to be dependent both on landscape parameters (Molthan, 1990), peculiar to forests and
crop mosaic respectively, which act at different periods through the year and sometimes with a
lasting effect. Indeed south forest edges proved to be important landscape elements for the
overwintering of adult females of the beneficial aphidophagous hoverfly Episyrphus balteatus,
particularly when spatially coupled with grasslands and fallows rich in floral resources
(Arrignon et al., 2007). As for north forest edges, they also proved to enhance populations of
adult E. balteatus emerging in spring since they are preferential overwintering sites for the
larvae, probably because of higher density of aphid populations in the fall (Sarthou et al., 2005).
For ground beetles, forest edges appeared as a major reservoir of specific diversity (numerous
species and their individuals) potentially able to control some pest species in the nearby crop
fields, even the forest species, which are the biggest ones and are reported to prey on slugs
(Kromp, 1999; Symondson et al., 2002). Moreover, woodlot edges sheltered ground beetles
species whose adults live and exert beneficial influence in crops but are not usually found in
wooded habitats. The impacts of edge management on these populations and their ability to
move from forest to crop have to be investigated in the future. These results offer new insights
for pest regulation through landscape management, particularly by pointing the forest edges as
other important landscape elements to add to well-known set-asides and hedges in the design of
semi-natural element network in agricultural landscapes (Russel, 1989; Landis et al., 2000; Gurr
et al., 2004). Undoubtedly, this 'landscaping' management of agroecosystems will benefit from a
better ecological services-based knowledge of other landscape elements (natural grasslands,
fallows, ditch edges…) at different spatial scales, and a better integration of this knowledge in
crop management.
Forest Landscapes and Global Change-New Frontiers in Management, Conservation and Restoration. Proceedings of the IUFRO Landscape Ecology
Working Group International Conference, September 21-27, 2010, Bragança, Portugal. J.C. Azevedo, M. Feliciano, J. Castro & M.A. Pinto (eds.)
2010, Instituto Politécnico de Bragança, Bragança, Portugal.