CLASIFICACION DE LAS COMPETENCIAS Y SUS CARACTERISTICAS

Habitat manipulation, defined as a series of environment manipulations to provide natural enemies with the necessary resources to improve their effectiveness at combating agricultural pests, emerges as a unifying theme to mitigate the negative impact of agricultural practices (Landis et al. 2000). The underlying concept of habitat manipulation is that provision of supplementary and complementary food, microclimate modification and existence of refuge habitats in close association with crop fields might help natural enemies to cope with the detri- mental impact of agricultural practices. For example, many species of adult parasitoids use wildflowers and aphid honeydew as food resources that are not provided in agricultural fields in which weeds are controlled (Jervis et al. 1993; Idris and Grafius 1995; Dyer and Landis 1996; Jervis et al., ch. 5 this volume). Practices such as no tillage and conservation tillage, cover cropping, crop residue conservation, intercropping and establishment of herbaceous strips in close spatial association with crop fields represent viable within-crop field approaches to provide the necessary resources to enhance parasitoid survival (Van Driesche and Bellows 1996; Khan et al. 1997; Tscharntke 2000).

As explained in the following sections, molecular tools can be integrated with an in-depth knowledge of insect biological and ecological requirements to help entomologists improve habitat manipulation practices for parasitoid conservation. The integration of these tools and knowledge could be done at different spatial scales.

Within-crop field activities and molecular based – techniques

Pesticide applications can be the most important factors reducing parasitoid survivorship within crop fields. In the short term, pesticides can kill large numbers of parasitoids, which may lead to an outbreak of secondary pests. In the long term, repeated pesticide applications may select resistant biotypes and influence parasitoid population dynamics (Tolstova and Atanov 1982). Ruberson et al. (1998) described different approaches to integrate pesticide applications with natural enemies. Among the recommended tactics are periodic scouting of crop fields, use of selective pesticides and sublethal doses, and spatial and temporal separation of pesticides and natural enemies.

Molecular technologies can help biological control practitioners to integrate pesticide use with natural enemies through the selection of pesticide-resistant strains of natural enemies. Although this approach appears to be more successful with predators than with parasitoids, recent progress in parasitoid selection programs could reverse this situation (Johnson and Tabashnik 1994). For example, RAPD-PCR has proved to be a useful technique to distinguish between two populations of the walnut aphid parasitoid, Trioxys pallidus (Hymenoptera: Aphidiidae), differing in resistance to pesticides (Edwards and Hoy 1995). Although the experi- ments were performed in the laboratory, the authors suggest that RAPD-PCR could be used to determine the fate of pesticide-resistant parasitoids released in the field.

Conserving parasitoids at the farm and landscape level

While practices at the within-field level might enhance parasitoid survivorship, agricultural practices occurring at larger scales might negate such conservation efforts. Several studies have shown that field size, crop rotation and presence of non-crop habitats play a critical role in determining within-field parasitoid abundance. For example, Landis and Haas (1992) deter- mined that parasitism of the European cornborer, Ostrinia nubilalis (Lepidoptera: Pyralidae) by its larval parasitoid, Eriborus terebrans (Hymenoptera: Ichneumonidae) was significantly higher at the borders of maize (also known as corn) fields than in field interiors. They further deter- mined that the greatest levels of parasitism were observed at wooded field edges. In accordance, it has been observed that access to plant nectar, aphid honeydew or sugar improved E. terebrans survival in crop and non-crop habitats (Dyer and Landis 1996; Landis and Marino 1999a). These studies suggested that the absence of food resources in large maize fields, combined with high temperatures before canopy closure, was responsible for the low abundance of parasitoids observed in field interiors (Dyer and Landis 1997).

Similarly, Langer (2001) determined that short rotation coppice hedges and clover/grass grazing areas spread among crop fields forming a series of annual, biannual and semi-perennial habitats correlated with an increase in parasitism of the pest aphid Sitobion avenae (Homoptera: Aphididae). These and other studies (e.g. Höller 1990; Duelli 1997; Lee et al. 2001) highlight the importance of semi-natural habitats established in close association with crop fields for natural enemy conservation. Ideally, agricultural landscapes should contain a series of non-crop habitats interspersed with crop fields to provide shelter and food for parasitoids. Mid- and late-succes- sional habitats provide parasitoids with adult food resources, alternative hosts, overwintering sites and shelter for adverse conditions (Marino and Landis 1996; Landis and Menalled 1998; Menalled et al. 1999; Schmidt et al., ch. 4 this volume).

Because parasitoids use semiochemicals in host location, some of which emanate from plants (De Moraes et al. 1998; Khan and Pickett, ch.10 this volume), non-crop vegetation interspersed with crop fields can also affect parasitoid behavior. Understanding changes in parasitoid behav- iour to plant assemblage composition is a key element in the development of habitat practices aimed at increasing vegetational diversity of agricultural systems (Landis et al. 2000). Yet due to their small size, vagility and the inherent difficulty of mark and recapture experiments, very little is known about parasitoid dispersal behaviour (Hastings 2000).

Molecular-based techniques may aid entomologists to assess the impact of agricultural landscape structure on parasitoid population and community structure (Loxdale and Lushai 1998). Vaughn and Antolin (1998) used RAPD-PCR markers visualised by SSCP analysis to study the population structure of Diaretiella rapae (Hymenoptera: Braconidae), a parasitoid of several aphid species. The authors found that larger genetic variation occurred between fields separated by short distances than between areas separated by longer distances. They argued that the reduced genetic exchange among subpopulations indicates that released D. rapae would not disperse between fields.

In another study, Althoff and Thompson (2001) used the mtDNA cytochrome oxidase 1 (CO1) gene sequence and nuclear rDNA RFLPs to compare patterns of host search behaviour among six populations of Agathis n.sp. (Braconidae: Agathidinae) located in a relatively large geographic area in south-eastern Washington, USA. The results showed no isolation by distance, suggesting long-distance dispersal among populations. The authors argued that phenotypic differences in host–parasitoid interactions such as time allocated to searching, ovipositor length and place of searching appear to be driven by local plant characteristics. Although this study was not developed within the context of annual cropping systems, it provides a framework that could be used to foster the understanding of how local, mid- and large-scale agricultural landscape affect parasitoid abundance, searching behaviour and distribution.

Conclusion: integrating molecular techniques with field studies