Walking behaviour and population density of adult linyphiid spiders in relation to minimizing the plot size in short term pesticide studies with pyrethroid insecticides.

Temporal changes in spatial distribution and population density of linyphiid spiders were studied in relation to physical factors and deltamethrin spraying in the field. The results of the study have been used to set the conditions necessary when plot size is reduced in field trials. The results showed independence of observations at 7 m from an unsprayed reservoir, with respect to migration, in the week following pesticide application. Soil moisture had a positive effect on trapping success, without having apparent effect on trapping success, without having was found for deltamethrin spraying on wet and dry soil. Shifts in the spatial distribution of spider activity showed a close correlation with crop microclimatic conditions. The effect of trapping on population density was shown to be limited. The results suggest that small plots surrounded by a border of 7 m are appropriate for assessing short term pesticide effects on adult linyphiid spiders in winter wheat.

A dose-effect relationship for the effect of deltamethrin on a linyphiid spider population in winter wheat.

The relationship between application rate and effect of deltamethrin (Decis) on a linyphiid spider population was studied in relation to ambient physical factors. A field experiment using five application rates and a control was conducted in winter wheat plots. The aim was to determine application rates that allowed for determination of negative as well as positive effects of physical factors on arthropod toxicity using pitfall traps. Several physical variables were measured in the crop canopy and deltamethrin deposition was determined at ground level. The use of Before-After-Control-Impact design (BACI) time series analysis for characterization of instantaneous pesticide effects and velocity of recovery, relative to the application rate sprayed was evaluated. An initial reduction in spider catches ranging from 58% to 85% percent for sprayed concentrations of 0.25 to 4 g active ingredient per hectare (ai ha-1) was observed, followed by exponential recovery with a half-life time of 2.8 days for all treatments. The effects observed in this study were relatively mild compared to other field-experiments. It was concluded that an application rate near 0.25 g ai ha-1 deltamethrin (i.e., one twentieth of the recommended rate) would be sufficient to study the relationship between physical factors and the effects of deltamethrin. BACI time series analysis was shown to be a useful technique for characterizing short-term pesticide effects.