Correlations between Colonization of Onion Thrips and Leaf Reflectance Measures across Six Cabbage Varieties

János Bálint,József Fail,Balázs Vince Nagy,Murad Ghanim

doi:10.1371/journal.pone.0073848

Abstract

The main purpose of this study was to reveal if the UV-A, and visible light reflection of leaves of white cabbage varieties is correlated to resistance against onion thrips. The antixenotic resistance (AR) against onion thrips and thrips damage differed between varieties Balashi, Bloktor, Riana – considered resistant – and Green Gem, Hurricane, Quisor – considered susceptible. The solar UV-A (340–400 nm) and visible (401–650 nm) light reflection of white cabbage leaves were recorded. Correlation between AR against onion thrips and reflection of leaves in UV-A and visible range of the studied white cabbage varieties were computed. According to the AR evaluation onion thrips density was always higher on susceptible than on resistant varieties. The UV-A light reflection of head forming leaves and the contrast between head and exterior leaves (H/E) was negatively correlated with onion thrips host preference at an early stage of cabbage head formation. The visible light reflection of both head forming and exterior leaves was also negatively correlated with onion thrips host preference. Susceptible varieties had greater damage ratings at harvest than resistant ones and positive correlations were observed between AR and damage. AR against onion thrips may be affected by differences in reflection of cabbage leaves at an early growth stage. It is suggested that more intensive reflection of leaves and/or higher contrast values between the reflectance intensity of head versus outer leaves made the resistant varieties less attractive to onion thrips. Our results reported here provide the first evidence of negative correlation between UV-A and visible reflection of leaves and AR of white cabbage against a dangerous insect pest, opening new perspectives for understanding the role of reflection by plant leaves in pest management.

Highlights

One of the most important pests of cabbage is the onion thrips (Thrips tabaci Lindeman, 1889) [1,2]
Antixenotic Evaluation We considered antixenotic resistance (AR) as the number of thrips colonizing white cabbage varieties at the beginning of the vegetation period during the cabbage head formation
The AR assessment yielded significant differences between the studied cabbage varieties Significantly more thrips adults were found on susceptible varieties (Green Gem, Quisor and Hurricane) than on those varieties (Bloktor, Riana and Balashi) that were categorized resistant in previous studies [12] (Figure 1)

Summary

Introduction

One of the most important pests of cabbage is the onion thrips (Thrips tabaci Lindeman, 1889) [1,2]. Damage is produced by individuals from head forming leaves, here for susceptible varieties thrips number was twice high as on exterior leaves [4]. The most common method to control onion thrips populations is the use of foliar insecticides, but T. tabaci is difficult to control because insects are found mainly in the narrow spaces between the inner leaves [5,6,7]. The use of natural enemies (predatory mites) does not give adequate control either [8]. Other authors have stated that the primary control of onion thrips damaging cabbage may be the selection of tolerant varieties [9,10]. The term antixenosis was proposed by Kogan and Ortman [11] and defines host plant features (for instance leaf structures, light reflectance, chemical volatiles) that can positively or negatively influence the colonization of phytophagous insects

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