DOI: 10.5061/dryad.wm37pvmmd
title: Colour pan-traps often catch less when there are more flowers around

description: 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.
description:   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 (<i>Bombus</i>
 spp. <i>Apis mellifera</i>), 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 m<sup>2</sup> square
 was placed on the ground and photographed from above (from c 160 cm
 height). Around each pan-trap, at least 25 such 1 m<sup>2
 </sup>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-m<sup>2</sup> 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.