Predation of codling moth eggs is affected by pest management practices at orchard and landscape levels
Introduction
Despite concerns over the environmental impact and the sustainability of pesticides, the control of agricultural pests is still predominantly based on chemical insecticides. Among more environmental friendly approaches, conservation biological control programs aim at increasing the abundance of naturally occurring pest enemies by managing their habitat within and around fields. Such programs may have to be implemented at the landscape level (Kruess and Tscharntke, 1994). However, fruit growers, seldom rely on biological control possibly because of the variability of its success and the lack of definitive knowledge about factors influencing the efficacy of natural enemies. This is particularly true in apple orchards that are heavily treated worldwide (Blommers, 1994, Jones et al., 2009). In Southern France, over 35 pesticide treatments are applied yearly in apple orchards, among which 8–15 are targeted against the codling moth (Cydia pomonella, Lepidoptera: Tortricidae) (Sauphanor et al., 2009). A number of its natural enemies have been assessed and used in classical, augmentative or conservation biological control strategies, including predators (Glen, 1977) and parasitoids (Athanassov et al., 1997, Mills, 2005, Rosenberg, 1934). The egg stage is the most targeted to avoid damages caused to the fruits. Both egg parasitism (Yu et al., 1984, Cossentine and Jensen, 2000, Pinto et al., 2002) and egg predation may contribute to this early-stage biological control. Predator activity can be very efficient, leading to the rapid disappearance of sentinel codling moth eggs (Glen, 1977). Among other arthropods, earwigs (Forficula sp) are particularly efficient predators in undisturbed habitats (MacLellan, 1962, MacLellan, 1972, Glen, 1975). Most studies, however, were conducted in untreated or organic orchards and little is known about the potential for predation in commercial conventional orchards. It is suspected that broad-spectrum insecticides would reduce the abundance and the diversity of natural enemies in treated orchards, and consequently, their contribution to pest control (Blommers et al., 1987, Suckling et al., 1999, Epstein et al., 2000, Pekar and Kocourek, 2004, Simon et al., 2007).
In treated orchards, the level of pest control by natural enemies depends both on local insecticide use and on the immigration of natural enemies in the orchard. Such immigration may be favored by the presence of untreated semi-natural areas nearby the crop that function as a source of enemies, providing them refuge, shelter, complementary food resources or alternative hosts or preys (Landis et al., 2000, Cronin and Reeve, 2005). The plant diversity in such semi-natural area may, in particular, enhance the survival of generalist predatory species when the free transit in different landscapes for feeding, oviposition and overwintering is preserved (Gurr et al., 2003). The importance of semi-natural areas was already reported in apple and pear orchards, where the abundance of beneficial arthropods was indeed dependent on the distance from adjacent natural habitat and on the presence of specific host plants particularly in hedgerows (Miliczky and Horton, 2005, Debras et al., 2007, Simon et al., 2010). However, an increased abundance of beneficial arthropods does not necessarily translate into increased pest control, in particular when generalist arthropods are considered, because of possible intra-guild interferences (Letourneau et al., 2009). Here we thus assess the influence of the crop protection programs and of the landscape composition directly on the ecological service expected from beneficial arthropods, i.e. on the predation and parasitism of C. pomonella eggs, in a set of commercial orchards from the Durance valley, France. We furthermore ask whether a description of landscape composition that accounts for land use, including crop protection practices in surrounding orchards, is more relevant than a more classical description of land cover.
Section snippets
Studied orchards
The study orchards were located in the low Durance valley, South-Eastern France, at 18–150 m altitude (coordinates in the WGS84 system from 43°46′27′′N to 43°51′23′′N and from 4°51′12′′E to 4°57′34′′E). In this 70 km2 area, orchards represented 39% of the total area. The main species grown were apple and pears (87% of orchards), apricots (7%), cherries (1.3%), peaches (1.2%) and there were some quince, walnut, fig and plum orchards (Ricci et al., 2009). The orchards averaged 0.85 ha and were
Characterization of pesticide pressure in orchards
The pest control practices and the treatments applied on the studied orchards are summarized in Table 2 and Appendix A. A mean of 32.3 treatments were applied in 2008, and the mean treatment frequency index (TFItot) was 31.7 indicating that most treatments were applied at full dose by the growers. The relatively high number of insecticides (mean TFIinsect: 12.7) is a characteristic of apple production in Southern France.
In the conventional orchards, neurotoxic insecticides were mostly targeted
Discussion
The rate of predation of codling moth eggs in commercial apple orchards was explained by protection programs within and around the sampled orchards and we did not find any effect of other landscape elements. One reason for such importance of protection strategies may be the intensity of crop protection practices in the study area. The protection programs of the studied orchards involved high frequency of pesticide applications. The total TFI is very high in French apple orchards as compared to
Acknowledgments
This work was supported by the ECOGER program (‘Ecco des Vergers’), by a doctoral grant to B. Ricci funded by INRA and the PACA region, and by a sabbatical grant at INRA to L. Monteiro funded by CAPES (Brazil). We specially thank all the farmers and technical advisers from the study area and the EPI staff for help in carrying out this study.
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