Diversidad y control biológico de insectos - CyberTesis UACh ...

fields that are sprayed with pesticides such as lambda-cyhalothrin often have lower spider
populations (Wehling and Heimbach, 1991; Maloney, 2002), as a consequence of the high
spider susceptibility to this kind of insecticide (Krause et al., 1993).
Lycosid spiders prey on lepidopteran pests in several crops: corn (Laub and Luna, 1992),
rice (Wilby et al., 2005), soybean (Pearce et al., 2004), apple orchards (Allen and Hagley,
1989). Although the diet of spiders in the Chilean pastures is unknown, top-down effects of
spiders are evident even when they do not actually feed upon the target pest (Greenstone,
1999). Often, insect herbivores reduce their feeding when in the presence of spiders, and
disperse or abandon high quality patches (Sunderland, 1999). It has been stressed that an
assemblage of spider species is more effective at reducing prey densities than a single
species of spider (Greenstone, 1999; Sunderland, 1999). As suggested by Greenstone
(1999), it is important to have an assemblage of spiders rather than just one species so
predators of appropriate size classes and foraging modes will be present to prey upon
different prey life stages throughout the growing season. Therefore, it would be desirable
for growers to have a more selective tool than lambda-cyhalothrin if conservation of spider
assemblage is attempted.
Indeed, it seems that spider species abundant in pastures (agrobionts and agrophiles) are not
the same present in surroundings (Martin and Major, 2001; Alderweireldt, 1989).
Therefore, edges would not be primary migrant sources for the agrobionts and agrophiles
spider species, making more difficult for local populations recovering from depressed
levels caused by insecticide spraying. Lycosid spiders migrate basically by cursorial
movement (Bishop and Riechert, 1990). Ballooning, the colonization mean over long
distances, is possible just for juveniles less than 5 mm (Pearce et al., 2004; Bell et al.,
2001), then the capacity of spiders to recolonize areas where they were removed is limited
over long distances. Therefore, to avoid spider local extinctions in pastures is highly
desirable and this can be accomplished if, for instance, lambda-cyhalothrin spraying is
restricted to crucial periods in the pest life cycle, spraying at midday when many wandering
spiders are inactive and in sheltered location (Riechert and Lockley, 1984) or preferring
other less disruptive tools such as the B. bassiana spores tested in this study.
In small-scale within-field trials, the impact of pesticides may be underestimated (Duffield
and Aesbicher, 1994; Prasifka et al., 2005), because individuals can migrate from the
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