Activity Patterns, Population Dynamics, and Spatial Distribution of the Stick Tea Thrips, Dendrothrips minowai, in Tea Plantations

Simple Summary We studied the activity patterns, population dynamics, and spatial distribution of Dendrothrips minowai Priesner, one of the most destructive pests of tea plants in tea plantations. A large proportion of D. minowai individuals were caught in traps placed at heights ranging from 5 cm below to 25 cm above the position of tender leaves at the top of the tea plant, with most captures at a height of 10 cm above this position. The flight activity of D. minowai was highest from 10:00 to 16:00 h on sunny days in the spring and from 06:00 to 10:00 h and from 16:00 to 20:00 h in the summer. The distributions of D. minowai females and nymphs on leaves were aggregated according to Taylor’s power law and Lloyd’s patchiness index. The D. minowai populations were dominated by females, and the density of males was high in June. The seasonal prevalence of tea thrips captured with sticky traps in the field was bimodal; adult thrips overwintered on the bottom leaves. The peak periods of activity were from April to June and from August to October. This work provides new insights that have implications for enhancing the efficacy of measures to control D. minowai. Abstract The stick tea thrips, D. minowai Priesner (Thysanoptera: Thripidae), is one of the most economically significant thrips pests of tea (Camellia sinensis (L.) O. Ktze.) in China. Here, we sampled D. minowai in tea plantations from 2019 to 2022 to characterize its activity patterns, population dynamics, and spatial distribution. A large proportion of D. minowai individuals were caught in traps placed at heights ranging from 5 cm below to 25 cm above the position of tender leaves at the top of the tea plant, and the greatest number of individuals were captured at a height of 10 cm from the position of tender leaves at the top of the tea plant. Thrips were most abundant from 10:00 to 16:00 h in the spring and from 06:00 to 10:00 h and from 16:00 to 20:00 h on sunny days in the summer. The spatial distribution of D. minowai females and nymphs was aggregated on leaves according to Taylor’s power law (females: R2 = 0.92, b = 1.69 > 1; nymphs: R2 = 0.91, b = 2.29 > 1) and Lloyd’s patchiness index (females and nymphs: C > 1, Ca > 0, I > 0, M*/m > 1). The D. minowai population was dominated by females, and male density increased in June. Adult thrips overwintered on the bottom leaves, and they were most abundant from April to June and from August to October. Our findings will aid efforts to control D. minowai populations.


Introduction
Thrips (Insecta: Thysanoptera) are major pests of agricultural and horticultural crops around the world [1][2][3]. Many thrips are pests of commercial crops due to the direct damage Insects 2023, 14, 152 2 of 15 they cause by feeding on developing flowers, fruits, vegetables, or leaves, affecting yield or cosmetic appearance [4,5]. Thrips may also serve as vectors for plant diseases, such as tospoviruses [6,7]. In most annual vegetable and row crop production systems, along with climatic variables, the seasonal availability of host plants proudly impacts thrips population dynamics [8,9]. However, in perennial plantation crops where the host is available throughout the year, thrips population dynamics and their dispersal patterns are largely influenced by climatic variables [4]. Therefore, in perennial agroecosystems, understanding yearly thrips population dynamics in the field and their dispersal patterns on host plants are very important in developing effective integrated pest management strategies [10]. The information can be used to predict thrips abundance on host plants in the field, and this may suggest further ways of developing their potential for pest management.
Thrips have become a threatening pest to tea plants (Camilla sinensis (L.) O. Ktze.) in China, which is one of the most important tea-producing and tea-exporting countries in the world [11,12]. A total of 28 thrips species have been documented in tea plantations in China, including D. minowai Priesner, Scirtothrips dorsalis Hood, and Mycterothrips gongshanensi Li, Li, and Zhang [13][14][15]. The stick tea thrips, D. minowai has become an increasingly significant pest of tea plants in China in recent years [16,17]. D. minowai induces direct damage to tea plants by sucking nutrients from the leaflets; whether it can also be a vector of viruses remains unknown. The presence of stripes and scarring along the leaf veins and blades on the abaxial and adaxial leaf surfaces is a sign of feeding damage, and heavy D. minowai infestations can lead to the gradual loss of leaf color, leaf stiffness, and decreases in tea yield and quality [18,19].
Insecticides are often used to control D. minowai in conventional tea plantations, and the use of insecticides can have negative environmental effects, decrease the abundance of beneficial natural enemies, and favor the evolution of resistance in thrips populations [20][21][22]. Thus, the frequency and timing of insecticide applications are critically important for the sustainable management of D. minowai populations. Knowledge of the abundance of thrips on tea plants is important for understanding seasonal variation in the activity of thrips, including the timing of infestations [23]. Knowledge of the spatial distribution of thrips on tea plants is also important for the development of strategies to control their populations [24]. The aims of this study were to monitor the flight heights of thrips, characterize their daily activity patterns, clarify the spatial distribution and population dynamics of D. minowai in tea fields, and provide data that will aid integrated thrips management programs in China.

Materials and Thrips Identification
Blue sticky traps made from PVC (10 × 25 cm) purchased from Hangzhou Yihao Agricultural Technology Co., Ltd., Zhejiang Province, China, were used to characterize the flight height and diurnal activity patterns of thrips. Actually, we identified D. minowai based on their morphological characteristics reported in the literature.

Flight Height Observations
The flight heights of D. minowai adults were evaluated using commercially available colored sticky traps. Blue sticky traps were used because they have been shown to be effective in attracting various other thrips species [25,26]. Traps were hung on branches at various heights below (negative numbers) and above (

Materials and Thrips Identification
Blue sticky traps made from PVC (10 × 25 cm) purchased from Hangzhou Yihao Agricultural Technology Co., Ltd., Zhejiang Province, China, were used to characterize the flight height and diurnal activity patterns of thrips. Actually, we identified D. minowai based on their morphological characteristics reported in the literature.

Flight Height Observations
The flight heights of D. minowai adults were evaluated using commercially available colored sticky traps. Blue sticky traps were used because they have been shown to be effective in attracting various other thrips species [25,26]. Traps were hung on branches at various heights below (negative numbers) and above

Diurnal Activity Patterns
Following previous studies, observations of the diurnal activity patterns of D. minowai were made on sunny, cloudy, and rainy days [27]. Thrips were not active at night; the number of thrips caught at night was counted once; however, the number of thrips captured during the day was counted every 2 h (specifically, counts were conducted at 6:00, 8:00, 10:00, 12:00, 14:00, 16:00, 18:00, and 20:00). Sampling was conducted at organic tea plantations of Shaoxing Royal Tea Village Co., Ltd., in the spring (24 April 2022: sunny day; 25 April 2022: rainy day; 26 April 2022: cloudy day) and the summer (24 June 2022: sunny day; 26 June 2022: cloudy day; 28 June 2022: rainy day). Blue sticky cards were placed 10 cm away from the surface of tea leaves, and there was a distance of 5 m between each trap to minimize interference between traps. Five traps were randomly placed at the study sites during each sampling period. The number of thrips caught per sticky trap was recorded.

Diurnal Activity Patterns
Following previous studies, observations of the diurnal activity patterns of D. minowai were made on sunny, cloudy, and rainy days [27]. Thrips were not active at night; the number of thrips caught at night was counted once; however, the number of thrips captured during the day was counted every 2 h (specifically, counts were conducted at 6:00, 8:00, 10:00, 12:00, 14:00, 16:00, 18:00, and 20:00). Sampling was conducted at organic tea plantations of Shaoxing Royal Tea Village Co., Ltd., in the spring (24 April 2022: sunny Figure 2. Schematic diagram of how the traps were placed at different heights from below (negative numbers) to above (positive numbers) the position of tender leaves at the top of the tea plant in tea plantations (a) and the numbers (mean ± SEM) of D. minowai adults captured in traps (b). Different lowercase letter denotes a difference at the p < 0.05 level, while the same lowercase letter was not significantly different (p > 0.05) (ANOVA followed by LSD).

Spatial Distribution and Sex Ratio
The spatial distribution and sex ratio of D. minowai on leaves were studied in tea plantations at four sites (Hangzhou Fuhaitang Tea Ecological Technology Co., Ltd.; Zhejiang Camel Jiuyu Organic Food Co., Ltd.; Shaoxing Royal Tea Village Co., Ltd.; and Shengzhou Tea Comprehensive Experimental Base) in Zhejiang Province, China. To characterize the spatial distribution of D. minowai, we visually inspected tea plants for morphological indicators of female and nymph D. minowai presence [28]. Our previous observations suggest that male D. minowai adults are rarely found on tea leaves and spend most of their time hiding in tea bushes. This suggests that the above sampling method of visually inspecting plants is not effective for detecting D. minowai males. Therefore, to evaluate the sex ratio, we visually inspected plants for the presence of female and male D. minowai adults using knockdown techniques [29], which involved holding tea branches over a rectangular 40 × 20 × 10 cm white-colored pan and striking the branch five times; the numbers of females and males that fell into the pan were then counted [28].

Seasonal Abundance
The seasonal abundance of D. minowai on tea leaves was monitored at weekly intervals between April 2019 and October 2022 in organic tea plantations of Shaoxing Royal Tea Village Co., Ltd. To measure thrips abundances, we divided the study area into plots of 20 × 30 m. We then sampled D. minowai adults from the 100 tea leaves at five random points within each plot. The numbers of D. minowai adults on the upper (second leaf under the tender shoot), middle, and bottom leaves were estimated using the method described above.

Statistical Analyses
All data were checked for normality and equality of variances prior to statistical analysis. Datasets that did not meet assumptions were square-root transformed to meet the requirements of equal variances and normality. Differences in the numbers of thrips per trap, at different heights, and during different periods were determined using analysis of variance (Minitab 13, Minitab Inc., State College, PA, USA).
The means (m) and variance (V) of the densities of thrips were calculated. Means and variances of D. minowai were modeled according to Taylor's power law (TPL) [lg(V) = lga + blg(m)], where a is a sampling factor, and b is the aggregation parameter. The distribution is considered regular if b < 1; random if b = 1; and aggregated if b > 1 [30]. The spatial distribution of the thrips was analyzed using density data and Lloyd's patchiness index [31,32]. Parameters were obtained using the following model: , index of clumping I = V/m − 1, mean crowding intensity M* = m + V/m − 1, and aggregation index M*/m. C < 1, I < 0, Ca < 0, M*/m < 1 represents a regular distribution, C = 1, I = 0, Ca = 0, M*/m = 1 represents a random distribution, and C > 1, I > 0, Ca > 0, M*/m > 1 represents an aggregated distribution.
Graphs of the flight height, daily flight activity, and seasonal distribution of D. minowai were made using GraphPad Prism 7.0, and graphs of the linear relationships between variances and means of D. minowai were made using OriginPro 2021. All analyses were conducted using SAS 9.4.

Flight Height
The numbers of D. minowai captured on blue sticky traps varied at different heights (F 14,60 = 385.65, p < 0.001) (Figure 2b). Overall, traps ranging from 5 cm below to 25 cm above the position of tender leaves at the top of the tea plant had a high number of thrips. Most thrips were captured at a height of 10 cm above the position of tender leaves at the top of the tea plant (Figure 2b).

Diurnal Activity Patterns
The daily flight activity of the stick tea thrips D. minowai was examined using blue sticky traps in tea plantations. We found that thrips flight activities were affected by both weather and temperature (Figure 3). In the spring, thrips were most abundant between 10:00 and 16:00 h on sunny days, and their abundances declined from 16:00 to 20:00; they were largely inactive from 20:00 to 08:00 h. The number of thrips captured in traps was significantly lower on rainy or cloudy days than on sunny days between 10:00 and 16:00 h; the daily flight curve was unimodal (Figure 3a).  In the summer, the daily flight curve was bimodal on hot, sunny days ( Figure 3b). Specifically, thrips were more abundant between 06:00 and 10:00 h and between 16:00 and 20:00 h; only a few thrips were found between 12:00 and 16:00 h and between 20:00 and 06:00 h. On cloudy days, thrips were captured on sticky traps during the entire sampling period from 06:00 to 20:00 h; a bimodal daily flight curve was also observed on these days. Few thrips were captured in traps throughout the sampling period on rainy days. In the summer, the daily flight curve was bimodal on hot, sunny days (Figure 3b). Specifically, thrips were more abundant between 06:00 and 10:00 h and between 16:00 and 20:00 h; only a few thrips were found between 12:00 and 16:00 h and between 20:00 and 06:00 h. On cloudy days, thrips were captured on sticky traps during the entire sampling period from 06:00 to 20:00 h; a bimodal daily flight curve was also observed on these days. Few thrips were captured in traps throughout the sampling period on rainy days.

Spatial Distribution and Sex Ratio
Variances and means were significantly related according to Taylor's power law (females: R 2 = 0.92, p < 0.0001, b = 1.69 > 1; nymphs: R 2 = 0.91, p < 0.0001, b = 2.29 > 1), indicating that the distribution of D. minowai females and nymphs in the four different tea plantations was aggregated (Figure 4a,b). Lloyd's patchiness index indicated that the distribution of D. minowai females on tea leaves in Yuecheng District, Shaoxing, China, was aggregated from April to June 2021 and, in Xihu District and Yuhang District, Hangzhou and in Shengzhou County, Shaoxing, from April to June in 2022 (C > 1, Ca > 0, I > 0, M*/m > 1) ( Table 1). Lloyd's patchiness index indicated that the distribution of D. minowai nymphs on tea leaves was aggregated in Yuecheng District, Shaoxing, from April to June 2022 (C > 1, Ca > 0, I > 0, M */m > 1) ( Table 2). In general, the spatial distributions of D. minowai females and nymphs were relatively stable within different fields and different periods, respectively. Variances and means were significantly related according to Taylor's power law (females: R 2 = 0.92, p < 0.0001, b = 1.69 > 1; nymphs: R 2 = 0.91, p < 0.0001, b = 2.29 > 1), indicating that the distribution of D. minowai females and nymphs in the four different tea plantations was aggregated (Figure 4a,b). Lloyd's patchiness index indicated that the distribution of D. minowai females on tea leaves in Yuecheng District, Shaoxing, China, was aggregated from April to June 2021 and, in Xihu District and Yuhang District, Hangzhou and in Shengzhou County, Shaoxing, from April to June in 2022 (C > 1, Ca > 0, I > 0, M*/m > 1) ( Table 1). Lloyd's patchiness index indicated that the distribution of D. minowai nymphs on tea leaves was aggregated in Yuecheng District, Shaoxing, from April to June 2022 (C > 1, Ca > 0, I > 0, M */m > 1) ( Table 2). In general, the spatial distributions of D. minowai females and nymphs were relatively stable within different fields and different periods, respectively.    The proportions of female and male D. minowai adults in different periods at different sites varied ( Table 1). The proportions of female thrips in tea plantations were higher than those of males throughout most of the sampling period, indicating that D. minowai populations were dominated by females in the tea plantations, especially as the density of the thrips population increased (from April to the end of May). However, the density of D. minowai males increased in early June and eventually outnumbered females by late June.

Seasonal Abundance of D. minowai
Annual cycles were observed in the D. minowai female population in tea plantations, with a bimodal type of occurrence, the two highest densities occurring in April to June and August to October on tea leaves, regardless of the cultivars and years in Zhejiang Province, China ( Figure 5). However, in 2022, the number of D. minowai females was near zero, different from the months of August to October 2019-2021. The abnormal phenomenon was caused by the continuous extremely high temperature and drought from June to August 2022 (Table S1).

Discussion
Characterizing the flight heights and diurnal activity patterns of thrips is important for accurately estimating their densities and dispersal patterns, as well as developing pesticide application strategies. The flight activity patterns of insects are related to their responses to sunlight, temperature, and relative humidity [24,31,33]. In our study, the flight activity patterns of D. minowai on sunny days differed in the spring and summer. D. minowai flight activity peaked from 10:00 to 16:00 h in the spring; however, the peaks of their flight activity were from 06:00 to 10:00 h and from 16:00 to 20:00 h in the summer (Figure 3). They appear to avoid flying in temperatures below 20 • C or above 30 • C (Table S1). No flight activity was observed at night (Figure 3). Frankliniella occidentalis Pergande females are immobile at midday and at night [34], and Thrips imaginis Bagnall, T. hawaiiensis Morgan, S. dorsalis, Megalurothrips usitatus Bagnall, and F. schultzei Trybom seek refuge on their host plants during the hottest times of day, which corresponds to the period when their densities are highest [35,36]. The number of thrips captured on traps was significantly lower on rainy days than on sunny days. The decreased abundance of thrips on rainy days might stem from the effect of temperature, solar radiation, or an effect of humidity.
In our study, the distribution of D. minowai females from April to June at all sites was aggregated according to Taylor's power law (b > 1) and Lloyd's patchiness index (C > 1, Ca > 0, I > 0, M*/m > 1) ( Table 1). These results indicate that the aggregated distribution of females was not affected by tea variety and geographic region. The distributions of other thrips species have also been shown to be aggregated. For example, the distribution of F. occidentalis was significantly aggregated on cucumber, cotton, tomato, and strawberry [37][38][39][40], the distribution of F. schultzei was significantly aggregated on cucumber [41], the distribution of Pezothrips kellyanus Bagnall was significantly aggregated in citrus groves [42], and the distribution of S. dorsalis was significantly aggregated on chili plants [43]. The distribution of nymphs was more aggregated than that of females (nymphs: b = 2.29 > females: b = 1.69) in tea plantations ( Figure 4). This pattern has been observed in many other thrips species; nymphs aggregate during the early nymphal stages mainly because of their limited mobility, and they become less aggregated as their mobility increases [44]. These findings are consistent with the results of previous studies showing that the distribution of nymphs is more aggregated than that of females in F. occidentalis on tomato flowers and on greenhouse cucumber leaves, as well as in T. hawaiiensis, T. palmi Karny, and S. dorsalis on their host plants [24,42,45].
Our study of the population dynamics of thrips across four years revealed two key periods in which the abundance of D. minowai and, thus, the damage that they induced to tea plants were highest in Zhejiang Province ( Figure 5). This information can aid the management and control of thrips on tea leaves in different seasons. In addition, D. minowai adults overwintered on the bottom leaves. The full-bloom stage of tea plants runs from mid-October to late November, and most tea flowers are present on the lower to middle parts of tea plants [46]. Some D. minowai adults began to colonize the lower middle leaves in tea plantations starting in October. D. minowai adults overwinter on the bottom leaves from November until the following March. During this stage, volatiles such as beta-ocimene, farnesene, and methyl benzoate have been identified [47]. According to our previous research, D. minowai is attracted to the above three volatiles [17]. Thus, the presence of overwintering adult thrips in the lower middle part of the tea plants might stem from their attraction to the volatiles of tea flowers. However, more laboratory studies and fieldwork are needed to clarify the overwintering mechanism of D. minowai. In any case, the bottom tea leaves merit increased attention, given that many of them serve as overwintering sites for these thrips.

Conclusions
Our data on the flight heights and activity patterns of thrips indicate that blue sticky traps hanging at 10 cm above the position of tender leaves at the top of the tea plant were more effective on sunny days. The distribution of D. minowai females and nymphs was aggregated, and a bimodal type of occurrence was observed in the female population in tea plantations. Adult thrips overwinter on the bottom leaves. Thus, the application of pesticides on old bottom leaves during the winter months could reduce population densities and pesticide residues in the following year. The results of this study provide new insights that will aid the management of D. minowai populations in tea fields, as well as the development of integrated pest management programs to control D. minowai infestations.