Population Fluctuations and Spatial Distribution of Onion Thrips (Thrips tabaci Lindeman, 1889) in Different Cuttings of Alfalfa (Medicago sativa L.)

In order to study the population fluctuations and spatial distribution of onion thrips, Thrips tabaci (Lindeman) (Thysanoptera: Thripidae) in alfalfa fields of Eyvan city (Ilam province), sampling were done weekly in different growth stages of alfalfa from the beginning of April to late September 2017. The results of population fluctuations showed that the highest population of adult and immature thrips stages were average 6.20 ± 0.85 and 1.35 ± 0.44 in 20 plants in August and September, during the 3th cutting of alfalfa. The study of the relationship between the population dynamics of adult and immature stages with temperature showed that there was a significant and positive correlation between population changes and temperature. Based on Taylor’s power law, the spatial distribution for both adult and immature stages was randomized, but the total stages were aggregated. Whereas, based on Iwao's patchiness, regression between Log S2 and Log m in immature stage was not statistically significant (P> 0.05). The spatial distribution pattern of adult stages was randomized and for both adult and immature stages was aggregated. The parameters derived from population fluctuations and the spatial distribution pattern of this species can be used in the sampling program and estimates of population density of this pest in alfalfa fields.