Within-Plant Distribution of Onion Thrips (Thysanoptera: Thripidae) in Onions

Abstract

Two aspects of the within-plant distribution of Thrips tabaci Lindeman (Thysanoptera: Thripidae) on onion, Allium cepa L., plants were investigated: 1) diurnal variations in the distribution of adults and larvae between basal and upper sections of onion leaves, and 2) between-leaf and within-leaf distribution of the eggs. The diurnal investigations showed that higher proportions of larvae than of adults congregated at the basal sections of plants, particularly when plants were young and thrips density was low. As plants matured and thrips density increased, the larvae became more dispersed. Regardless of plant size, there were always more adults in the upper than basal plant sections. There were no clear time-windows during the 24-h diurnal cycle when more thrips were in the upper plant parts. T. tabaci eggs were laid everywhere in the plant. Leaves of intermediate ages had more eggs than older or younger leaves. Within leaves, the white leaf sheath received the least eggs and leaf tips received slightly more eggs than leaf sheaths. The highest egg density was found between the green leaf base and the leaf tips. Regardless of plant size, more than half of all eggs were laid above the basal sections. The percentage increased to >95% in mature plants. Except when plants were small the outer leaves were preferred over inner leaves and upper leaf sections preferred over lower leaf sections as egg-laying sites by adults. Implications of the results in the management of T. tabaci are discussed.

Onion Thrips (Thysanoptera: Thripidae): A Global Pest of Increasing Concern in Onion

“…On a within-plant level, Sites et al (1992) observed that during the onion vegetative stage, T. tabaci populations were concentrated on the center leaves but progressively populations became equally distributed all over the leaves. Mo et al (2008) observed that when onion plants were young and T. tabaci densities were low, higher numbers of larvae than adults gathered at the base of plants. When onion plants matured and thrips densities increased, the larvae dispersed to other parts of the leaves.…”

Nonrandom Distribution of Cabbage Aphids (Hemiptera: Aphididae) in Dryland Canola (Brassicales: Brassicaceae)

“…Many insect pests are known to be nonrandomly distributed within single plants (Kennedy and Booth 1951, Liu and Sengonca 1997, Mo et al 2008, Kumar et al 2014). Cabbage aphid colonies in the small plot trial were spatially aggregated vertically, starting on the abaxial side of leaves in the bottom portion of the canopy and moving to the racemes over the 7-wk period.…”

Investigating the Effect of Invasion Characteristics on Onion Thrips (Thysanoptera: Thripidae) Populations in Onions With a Temperature-Driven Process Model

“…According to the bestÞt invasion scenarios, onion thrips adults started invading onion crops at 6 Ð98 d after onion emergence or from mid-June (mid-winter) to mid-September (early spring), depending on site and year. Invasions starting as early as 6 d after onion emergence are surprising but plausible, because trapping data showed that adults were ßying in onion Þelds in small numbers long before the Þrst thrips were detected on onion plants (Mo 2007), and egg monitoring data showed that eggs were laid in onion plants as early as the single-leaf stage (Mo et al 2008). Two of the Þve seasonal patterns were best described with the daily invasion rates of 0.002Ð 0.008 adults/plant, one by a daily invasion Table 2 Numbers in parentheses show the relative widths of the ranges as percentages of the widths of the tested ranges.…”

“…For species identiÞcations, subsamples of thrips were slide-mounted and checked under a stereo microscope. Compared with the alcohol-wash method, the in situ inspection method generally underestimates larval densities because of the small size of young larvae and the larval habit of hiding in unexposed areas of the leaves (Mo et al 2008). The degree of underestimation is likely to vary because of variations in levels of concentration of the inspectors and skill differences between different inspectors.…”