Laboratory tests
Insects. A laboratory population of T. tabaci was established from individuals that were collected from the fields of Welsh onions located in the Kameoka City, Kyoto Prefecture, on 4 August 2015. The laboratory culture was maintained on broad beans, Vicia faba, at 25°C under a 15L– 9D photoperiod regime, as described by Sogo et al. (2015) 49 and Aizawa et al. (2016) 21.
Insect nets. In the experiments, the following six types of colored insect nets were tested: a plain-woven insect net with red polyethylene yarn in the warp and transparent yarn in the weft with (0.8, 1.0, and 2.0 mm mesh sizes; see red-white net in Fig. 1A), net with red yarn in the warp and black yarn in the weft (only 0.8 mm mesh size; see red-black net in Fig. 1A), net with red yarn in the warp and red yarn in the weft (only 0.8 mm mesh size; see red-red net in Fig. 1A), net with black yarn in the warp and transparent yarn in the weft (0.8, 1.0, and 2.0 mm mesh sizes; black-white net), net with black yarn in both warp and weft (only 0.8 mm mesh size; black and black net), and net with transparent yarn in both warp and weft (0.8, 1.0, and 2.0 mm mesh sizes; white net). Figure 1B shows the spectral reflectance of each type of net measured with a spectrometer via optical fiber connected with an integrating sphere (HS1000S, Asahi Spectra Co. Ltd., Japan).
Experiments. The following experiments were conducted in the laboratory at 25°C under a 15L– 9D photoperiod regime. The skeletons of cylinders (150 mm dia., 250 mm high) for holding up the nets were made using wire and covered with one of the six types of colored insect nets described above. For controls, the skeletons were not covered with any type of net. One seedling of Welsh onion was planted in 200-ml plastic cups (100 mm dia., 45mm high) containing vermiculite. Fourteen-day old potted plants (with two leaves each) were placed in each of the six types of colored net and set in a plastic ventilated cage (304 mm wide, 250 mm dia., 280 mm high) for 24h. 20 female adults were then released into each ventilated cage. After another 24 h, the number of adults and the feeding marks (abrasion marks) on Welsh onion leaves were counted. The experiment was replicated 10 times. In addition, two each of the five types of 0.8 mm mesh colored insect nets (except black and black type), for a total of 10 test combinations, were selected and placed together 10 cm apart inside a plastic ventilated cage (304 mm wide, 250 mm dia., 280 mm high) with a 10 cm opening. One seedling of Welsh onion was placed inside the insect net, as in the above experiment. In the center of the plastic ventilated cage, 20 female adults were released. The number of adults on Welsh onion leaves was counted after 24 h. This experiment was replicated 6 times.
Field trials. Two experiments were conducted in a Welsh onion field at the Kyoto Prefectural Agriculture, Forestry, and Fisheries Technology Center in Kameoka City, Kyoto Prefecture from June to August in 2016 (Trial 1) and in 2017 (Trial 2).
Trial 1. The Welsh onion field was divided into four experimental plots (each 21.6㎡, 5.4 m X 4 m). For each of the three test plots, the entire surface of the greenhouse framework (4.0m width, 5.4m depth, 2.0m height) was covered with 0.8 mm mesh red-white, red-red, or white-white types of insect net (Fig. S1). The control plot was not covered with insect net. This experiment was repeated twice. Welsh onion plants were planted on 15 June 2016. The cultivars planted were ‘Shinkujohosonegi’ and ‘Ryokusyu’ with plant and row spacing of 14cm and 25cm, respectively. In all test plots, no chemical insecticides were used during the study period.
Plants were monitored once every two weeks, from 5 July to 15 August 2016. We counted the number of T. tabaci individuals and leaves damaged by thrips (except on 5 July) on 10 plants in each experimental plot. The numbers of leaves with necrotic spots by IYSV were also counted. Infection of IYSV was confirmed by DAS-ELISA when chlorotic spots were observed on the Welsh onion plants in each treatment. Leaves with chlorotic spots were counted as diseased leaves.
Trial 2. As in Trial 1, the field was divided into four experimental plots (each 21.6㎡, 5.4 by 4 m): plot with only the framework (4.0m width, 5.4m depth, 2.0m height) covered with 0.8 mm mesh red-red insecticidal netting; plot with only the ceiling covered; plot with only the sides covered, and plot not covered with any netting (Fig. S1). This experiment was replicated twice. Welsh onion plants were planted on 13 June 2017. The cultivars planted were ‘Taibyosobutori’. The plant and row spacing was the same as in 2016. Due to the high densities of T. tabaci in the untreated area, all experimental plots were sprayed with spinetoram wettable powder on July 20, 2017, and with dinotefuran water-soluble powder on August 15, 2017.
The surveys were conducted as in Trial 1, at approximately two-week intervals during the period of 3 July to 14 August 2017.
All the experiments were performed in accordance with relevant institutional, national, and international guidelines and legislation.
Analysis. In the single-invasion experiments in the laboratory, the invasion rate through the insect nets was statistically analyzed by Tukey-Kramer multiple comparison test using arcsine transformed values. For the number of feeding marks by thrips, Steel-Dwass multiple comparison test was used. The generalized linear model 50 was used to analyze the effect of the yarn color on the invasion rate. The invasion rate, r, was set as the dependent variable that is following the binomial distribution. Six types of nets were quantitively systematized based on the amount of red and black fibers that used to make them (SI Appendix, Fig. S2) and used as explanatory variables (Table S1). Logit function, Logit (r) = ln{r / (1–r)}, was used as the link function between invasion frequency and linear expression of explanatory variables. The significance of the regression coefficients was assessed by the Wald test. In the two-nets-choice experiments, the choice rate among the two nets was analyzed by the GLM as well. The choice rate c was set as the dependent variable that is following the binomial distribution. Only the intercept was used as the explanatory variable. The link function and the test of the significance of the intercept were the same as in the single-invasion experiments. In the field trials, thrips populations in each treatment were analyzed for both adults and larvae using the Steel-Dwass multiple comparison test. The rate of leaf damage by thrips and IYSV infection in each treatment were analyzed using the Tukey-Kramer multiple comparison test with arcsine transformed values. Statistical analyses in the multiple comparison were performed using Social Survey Research Information Co., Ltd., and analysis in the GLM was performed using R, which is the language and environment for statistical computing 51.