Colour pan-traps often catch less when there are more flowers around

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.