Thrips densities in organic leek fields in relation to the surrounding landscapes. Landscape management for functional biodiversity

The study assessed the effects of hedgerow networks (line elements containing shrubs less than 2 m), woodlots (height > 2 m), other natural areas, and agricultural and horticultural land (polygons) in the landscapes on the abundance of a generalist thrips species in organic leek fields. Landscape maps were compiled from two GIS frameworks (LGN3plus and Top10Vector Analysis) and supplemented with information from interviews with farmers on farming practices. Landscape structure was analysed at two spatial scales, at a radius of 1000 and 5000 m respectively from the centre of the leek fields. A multi-habitat species, onion thrips (Thrips tabaci) was monitored in 21 organic leek (Allium porrum) fields during the growing season and their densities were related with landscape features and the density of natural enemies on the leek plants. Results show that the length of the hedgerow network (height <2 m) in the landscape surrounding organic leek fields had no effect on the number of onion thrips in leek while there was a significant negative effect of amount of woodlot in the landscape on onion thrips densities within the leek fields. These results imply that non-crop elements in the landscapes surrounding commercial organic leek fields can have different effects on herbivorous thrips populations.