When assessing changes in populations of species it is essential that the methods used to collect data have some level of precision and preferably also good accuracy. One commonly used method to collect pollinators is colour pan-traps, but this method has been suggested to be biased by the abundance of surrounding flowers. The present study evaluated the relationship between pan-trap catches and the frequency of flowers on small (25 m2) and large (2-6 ha) spatial scales. If pan-traps work well, one should assume a positive relationship, i.e. more insects caught when they have more food. However, in contrast, we found that catches in pan-traps were often negatively affected by flower frequency. Among the six taxa evaluated, the negative bias was largest in Vespoidea and Lepturinae, while there was no bias in solitary Apoidea (Cetoniidae, Syrphidae and social Apoidea were intermediate). Furthermore, red flowers seemed to contribute most to the negative bias. There was also a tendency that the negative bias differed within the flight season and that is was higher when considering the large spatial scale compared to the small one. To conclude, pan-trap catches may suffer from a negative bias due to surrounding flower frequency and colour. The occurrence and magnitude of the negative bias was context and taxon dependent, and therefore difficult to adjust for. Thus, pan-traps seems less suited to evaluate differences between sites and the effect of restoration, when gradients in flower density is large. Instead, it seems better suited to monitor population changes within sites, and when gradients are small.

STUDY SITES: Data were collected in 2015 in the province of Östergötland, southern Sweden. The landscape in the study area consists mainly of coniferous forest, but there are also bogs, lakes, small patches of seminatural grasslands and arable fields. Twelve clear-cuts were selected (2-6 ha and logged 4-6 years prior). Six of them had been marked as coniferous forests on maps from the 1870s when the other six were marked as meadows. Clear-cuts on former meadows have higher amounts of herbs than clear-cuts which were formerly forests. Hence, our site selection strategy covered the wide range of flower abundances that occur on clear-cuts in the study area. 

PAN-TRAPS: The pans used to collect pollinators were painted in one of the following colours: blue, white and yellow with UV-reflecting-colour (Soppec, Sylva mark fluo marker, Nersac, France). The pans had a diameter of 8.7 cm, a volume of 0.5 L and were filled with non-toxic propylene glycol (40% concentration), to conserve the pollinators and to decrease the surface tension. A small opening (4 mm in diameter) at the top of each bowl was made to ensure that rainwater could drain. One set of pan-trap consisted of three pans, one in each colour, placed on a steel stick.

Four sets of pan-traps were placed in each clear-cut, in the same height as the vegetation and in places that were considered representative for the clear-cut. During the main flight period of most pollinating insects, three sampling periods were conducted: in the beginnings of June, beginning of July and beginning of August. Each period lasted for one week and had at least some days with more than 17°C and wind velocity less than four on the Beaufort scale. The pans were covered with caps between collecting periods. When a new collecting period started, some sets of pan-traps were moved - at most 30 cm - or some of the vegetation was removed to prevent overgrowth.

In total there were 48 set of pan-traps collecting during each period, but a few sets of pan-traps had been knocked down by animals and were therefore excluded (1, 1 and 2 during the first, second and third period, respectively). In addition, a single blue pan went missing during the second period. 

Four taxonomic groups dominated the catches – Aculeata, Lepturinae, Cetoniidae and Syrphidae – and they were identified to species-level. Aculeata was subdivided as solitary Apoidea, social Apoidea (Bombus spp. Apis mellifera), and Vespoidea (including one species of Chrysidoidea). Other insects caught, that are not identified, were mainly small Coleoptera, small Lepidoptera and Symphyta.

FLOWER FREQUENCY: The clear-cuts were photographed in conjunction with each collecting period to estimate flower frequency. A 1 m² square was placed on the ground and photographed from above (from c 160 cm height). Around each pan-trap, at least 25 such 1 m² squares were photographed reflecting small-scale flower abundance (“trap scale”). An additional at least 100 pictures were systematically distributed along transects over the whole clear-cut reflecting large-scale (“clear-cut scale”) flower abundance. Photos were taken during each collecting period, or at most 5 days before or after. 

All 8048 photos taken were inspected to see if they held flowers within the 1-m² square and if so of which colours (red, yellow, blue and white). The frequencies of colours (i) around set of pan-traps, and (ii) on clear-cuts were expressed as the odds for the colour occurring in a square metre plot: (0.5+p)/(0.5+(1-p)), where p=frequency of photos with the colour. Also, we calculated the odds for flower of any colour occurring in a plot.