Living where the food is: web location by linyphiid spiders in relation to prey availability in winter wheat.
Abstract
Spiders form a major component of the generalist predator fauna, potentially able to restrict pest population growth, but their populations may be food-limited under current farming regimes. This study aimed to quantify food availability to spiders in winter wheat and to determine whether spider web locations are positively associated with available food resources. Mini-sticky traps (availability rate per 24 h, including prey falling from the crop) and mini-quadrats (instantaneous density on the ground by day) were used, in combination, to monitor the availability of potential prey to web-building species of money spider (Linyphiidae) in fields of winter wheat in Warwickshire, UK, during 1997 and 1998. These methods were applied to web sites of individual spiders and to non-web sites located randomly up to 30 cm away from each web. A total of 18 546 invertebrates were captured using these methods. Overall, significantly more potential prey were available in web sites than in non-web sites (both on sticky traps and in quadrats). Prey availability in May and July was about a third of that in June (both on sticky traps and in quadrats) and may have been below that known to be necessary for spiders to realize their maximum population growth rate. The peak rate of capture of linyphiid spiders on mini-sticky traps was 0.6 trap-1 day-1 at web sites, and approximately half this value at non-web sites. Numbers of spiders captured by mini-sticky traps and mini-quadrats increased exponentially as the season progressed. The high capture frequency in relation to population density, and the differential between web and non-web sites, points to a dynamic and aggregated distribution of spiders in winter wheat, which is consistent with what is known about mate-searching and web site abandonment rates by the Linyphiidae. The combination of techniques described here is recommended for monitoring prey availability in prey-enhancement programmes and may prove useful in quantitative studies of both intra- and inter-specific interactions between spiders.