Population dynamics and integrated pest management of Thrips tabaci on leek under field conditions in northwest Italy

Thrips tabaci Lindeman (Thysanoptera: Thripidae) is a major pest of leek, Allium porrum L. (Alliaceae), in Piedmont, northwest Italy, and to control its infestation the leek crop is sprayed intensively with insecticides during the summer period. In order to find the most efficient and environment-friendly method of thrips control, research was conducted on six commercial farms during 2005-2006 to assess thrips population composition and infestation levels, and in an experimental field during 2005-2007. Biological and chemical control were compared during 2005-2006, whereas integrated pest management was adopted during 2007. During the growing season, thrips and natural enemy populations were monitored at 14-day intervals by beating plants; new leaves of plants were also visually inspected for thrips-feeding symptoms. Furthermore, in the experimental field at harvest-time, the level of thrips injury to plants was assigned to one of five classes, depending on the percentage of leaf area damaged. Over 99% of phytophagous adult thrips found were male and female T. tabaci. Infestations were very variable in the crops surveyed, partly due to broad-spectrum chemical treatments against the leek pests, which often failed to control thrips. In general, populations peaked in September, when they reached the maximum mean values ranging between 1.7 and 33.1 thrips per plant. At harvest, none of the surveyed farms experienced quality losses due to thrips injuries. In the experimental field during 2005-2006, the mean number of thrips per plant was greater in the chemical than in the biological control treatment, even though damage indices showed no significant differences between the two treatments. Predatory thrips of the genus Aeolothrips (Thysanoptera: Thripidae) and predatory bugs of the genus Orius (Heteroptera: Anthocoridae), mostly Orius majusculus Reuter, were particularly abundant during 2007, supporting the importance of management with selective insecticides to encourage natural colonization by predators.