The eff ect of diff erent seeding densities of linseed ( Linum usitatissimum L . ) on fl ax fl ea beetles ( Coleoptera : Chrysomelidae )

Linseed, one of the oldest cultivated crops, is again gaining in importance, mainly due to its nutritional benefi ts and biomedical applications. Th erefore, it is expected that herbivores will also exist in greater abundance. Among them the fl ea beetle, Aphthona euphorbiae Schrank and Longitarsus parvulus Paykull are considered to be serious pests of fl ax grown for fi bre and seeds in Europe. Th e aim of this study was to determine fl ax fl ea beetles’ abundance, species richness and seasonal dynamics on linseed grown at diff erent densities. It was expected that linseed seeding density can signifi cantly aff ect fl ea beetle populations. Th e experiment was carried out in Lower Silesia, Poland, from 2011 to 2013. A genetically modifi ed type of linseed overproducing fl avonoids was used. Flea beetles and the damages they caused were determined on plants and also a sweep net was used for the collection of adult beetles. During the three years of the study 15 species of fl ea beetles were identifi ed from oil fl ax plants, with A. euphorbiae and L. parvulus being dominant. In terms of the total catch, the tendency was for beetle numbers to decrease with increasing plant density. Flax fl ea beetles feeding on linseed plants, irrespective of plant density, had two peaks of abundance. Th e fi rst peak was lower and occurred in June, when plants were at the blooming stage. Th is peak was caused by overwintering adults who colonized crops in spring. Th e second, higher peak of abundance was recorded in the second half of July, when plants were at the ripening stage. Th is peak was formed by adults of the new generation. Each year, at the higher population peak of abundance, the fl ea beetles were most numerous on plants grown at the lowest density. Th ere was one period, lasting either from mid-May to the fi rst few days of June, or from the beginning of June to mid-June, during which the number of holes and damage on plants of each treatment were highest. During the three years of the study there were several cases of signifi cantly higher numbers of fl ea beetle feeding symptoms on plants grown at the lowest density as compared to the medium and highest densities.