The Ecology of Gramnivorous Sawflies (Hymenoptera: Symphyta) on Lowland Farms in Scotland.

Abstract

Gramnivorous sawflies are an important food source for the chicks of some farmland bird species. Numbers of gramnivorous sawflies on lowland farmland have been declining since the 1970s. A few studies into the ecology of sawflies on lowland farmland have taken place in Southern England. This research was designed to complement the English studies and to increase our knowledge of the ecology of gramnivorous sawflies in Scottish farmland.
In semi-field studies of sawfly life history, larvae appeared to survive better on Poa annua and Dactylis glomerata than on spring barley but the statistical support for this was weak. In overwintering experiments, the number of larvae recovered from the soil was very low. All the individuals recovered were found in the top 16cm of the surface.
In distribution studies of adult sawfiies, there appeared to be some differences in the species composition of across Scotland but this was not statistically supported. There was no variation in species composition between similar grass habitats on a farmwide scale. Sawflies collected from within 1 field were either aggregated or randomly distributed. There were extreme male biases on all geographic scales for the most commonly captured species, Dolerus aeneus.
Adult capture-recapture experiments did not provide enough data for statistical analyses, but the percentage of recaptures indicated that adult Dolerine sawflies are fairly sedentary. The prevailing weather conditions did not have a significant affect on the flight of sawflies.
In farm-wide collections of sawfly larvae, numbers were higher in grass fields than in any other habitat. There appeared to be a general trend of greater larval numbers in field edges than in the middles, but this was not always statistically supported. The mean number of larvae at a site could be positively correlated with percentage grass cover. Some larval species can be associated with particular host plant presence or absence.
In laboratory experiments, larvae moved freely between host plants. They were more likely to be found on a preferred host, and spent more time on a preferred host before moving away. This was independent of whether nearby plants were of the same species or not. Survival of larvae was better on their preferred host plants.
In molecular analyses, ITS primers enabled discrimination between 2 common species of Dolerus. Cutting with ITS primers alone did not appear to show differences within a species but restriction enzyme SOU3 cut PCR product from the ITS short region for Dolerus aeneus. It may be possible to use this restriction enzyme to identify within species differences in future. The use of flow-cytometry analysis of sawfly brain cells was used to attempt to detect ploidy of individuals but was unsuccessful.
The research indicated that gramnivorous sawflies on farmland are inbred, probably due to fragmentation and manipulation of their habitats. High male biases indicated the presence of CSD in populations, which if present, would increase the problems associated with inbreeding depression. If the decline of sawflies on farmland is to be reduced, conservation measures will need to be introduced. Suggestions include:
Planting of grassland strips around arable fields.
Where possible connecting viable habitats for gramnivorous sawflies, this could be done by careful placement of the margin strips so that they adjoin existing grass habitats.
Reducing or delaying ploughing of grassy areas to avoid damaging overwintering larvae.