The water trap colour preferences of farmland sawflies.

Abstract

Studies of insects on farmland tend to concentrate on pollinators or major pest species and their natural enemies rather than insects that have little economic impact on agriculture, yet the latter may play an important role in agroecosystems. One such group of farmland insects comprises those sawfly species that feed on arable weeds or on grasses and cereals. Although the graminivorous species have occasionally been recorded as minor crop pests (e.g., Herold, 1951; Wetzel & Freier, 1980) most are not present in crops at sufficiently high densities to cause economic levels of damage (Freier & Wetzel, 1984). However the larvae of both weed-feeding and graminivorous species have been identified as an important part of the diet of young chicks of farmland birds (Potts, 1977; Green, 1984). There is evidence for a severe decline in the abundance of the graminivorous sawfly species (Aebischer, 1990) on farmland in southern Britain in the last 25 years. In order to conduct a study into the ecology of these sawflies and to investigate their decline, techniques were needed to sample adult sawflies, the few previous studies having concentrated on sampling larvae through sweep netting and vacuum sampling.
Coloured water traps are often used to sample flying insects. Insects are attracted visually by the colour of the traps and are then captured in the water in them. Studies have demonstrated the preferences of a variety of insect groups for particular trap colours. Many insects are attracted to yellow; the peak colour reftlectance of plants is in the yellow band at 500-580 nm, so this colour may act as a super-normal foliage stimulant to herbivorous insects (Prokopy & Owens, 1983). Previous studies using coloured traps to collect adult sawflies have looked at colour preferences among sawflies when given a choice between yellow and white traps (Ritzau, 1988), yellow, white and blue (Schuster, 1985) or white, green, grey, black, brown and blue traps (Anderbrant et aI., 1989). The general preference in each case was for the colour yellow (when offered) and then white, with darker colours such as blue being least preferred. No wider survey of colour preferences has been attempted for this group, nor has the possibility of differences in preference between different species of sawflies in the same habitat been considered. This study was therefore conducted to assess the responses of sawflies to differently coloured water traps and to identify the colour preferences of the species regularly caught on farmland. A broad range of colours and trap design modifications are known to affect trapping efficiency in other insects, particularly with the cabbage root fly (Delia radicum) (Finch, 1992; Kostal & Finch, 1996). However, the colour range and trap design were kept deliberately simple in this experiment because there was no prior information available on trapping techniques for sawflies on farmland.
The aim of the experiment was not to study between-habitat differences in catches, but to consider the colour preferences of the common species in a range of habitats. Colour preferences were therefore tested in a range of different fields to maximise the variety of the physical environment and of habitat type used. This allowed assessment of the consistency of any preferences between different fields for species caught in more than one, but was not designed to replicate habitat type.