Predation rate was assessed using ‘prey surrogates’ comprising
25 cm2 water-proof sandpaper squares pinned to the
soil surface by wooden toothpicks (Merfield et al., 2004; Frank
et al., 2007). Live aphids (dorsal side uppermost) were glued
onto the sandpaper (P150, Norton) using 3 M repositionable
glue in a grid pattern with 1 cm between aphids. The blowfly
eggs were not glued onto the surface but were placed in a
similar pattern. The sandpaper sheets were pinned at the field
boundary, the field centre and midway between the two in two
transects (5 m apart) in each field and had a 225 cm2 metal
plate supported 10 cm above to protect them from rain.
Predation rate was calculated from the removal of ‘prey’
types per 24 h period during the two study periods. At each
site, each type of ‘prey’ at both densities (minimum and
maximum) was positioned 1 m apart at the locations described
above. For each prey type for each period, overall mean
prey disappearance was calculated separately from the means
of the two prey densities.