Diel Periodicity of Thrips (Thysanoptera: Thripidae) Dispersion and the Occurrence of Frankliniella williamsi on Onions

Abstract

Diel periodicity of the intraplant dispersion of thrips on onions, Allium cepa L., was determined by taking 10-plant samples at hourly intervals on each of four dates from late May to late June 1988, and recording the kinds and numbers of thrips in four onion growth zones (bulb, or portion below ground; neck from soil surface to the point of leaf divergence [i.e., distal end of sheath]; basal half of leaves above the neck; and the remaining apical half of leaves). Thrips tabaci Lindeman was significantly most abundant on the basal half of leaves from late May to mid-June. In late June, T. tabaci was significantly most abundant on both apical and basal halves of leaves, and density declined almost 5-fold during early afternoon hours, indicating flight activity was taking place. Frankliniella occidentalis (Pergande) was associated mostly with basal half of leaves in late May. In early June, it exhibited a nocturnal pattern of random dispersion, followed by an afternoon aggregation on the apical half of leaves, and subsequent evening return to random dispersion. Although it is not known as an onion pest, Frankliniella williamsi Hood was found consistently throughout the study at a density of ≈1.2 thrips per plant. Numbers of immature thrips of each species were pooled because of difficulty with identifications. In late May and early June, immatures were most abundant in the neck; whereas, in mid- and late June, significance was exhibited for the basal half of leaves.