Population dynamics of invasive thrips, Thrips parvispinus (Karny) infesting chilli (Capsicum annuum L.) in the coastal zone of North Tamil Nadu

Introduction

Chilli (Capsicum annuum L.) is one of the most economically important vegetable crops in the world [1] . It is also known as hot pepper and belongs to the family Solanaceae. The origin of chilli was found to be South and Central America [2] . It is used all-over the world and consumed in fresh, dried, and in powder form [3] . India is the world's leading producer of chillies with an annual yield of 4363 thousand metric tonnes [4] . The main chilli-growing states in India are Andhra Pradesh, Maharashtra, Karnataka, Orissa, Tamil Nadu, Bihar, Uttar Pradesh, and Rajasthan [5] . The chilli production is threatened by around 293 insects and mite species which damages the crop both in the field as well as in storage [6] . Additionally, the The survey was conducted during Kharif and Rabi seasons in the year 2022-23.

Survey location and cropping pattern

All the surveys were conducted in farmers' fields and they were well informed about the objectives of the survey. The crop was maintained by following recommended crop production practices without any crop protection techniques and insecticide spray. In each location (village), two fields each of 20 cents were earmarked for the fixed plot survey and to adopt two different scouting methods. There was half a kilometre distance between two fields. Surface irrigation was done to the crop which was raised in rows of 45cm apart and the distance within a row of 60cm. The details of cultivars raised during various cropping seasons at the selected locations were as follows.

The details of cultivars raised during various cropping seasons at the selected locations were as follows.

Location

Kharif

Scouting

Scouting was undertaken during morning or evening hours at weekly intervals from the flowering stage of the crop to record the occurrence of T. parvispinus. Then the mean data was correlated with abiotic factors and regression equation was derived. Two different scouting methods such as direct visual count and trapping by using blue sticky traps were followed separately. In the earmarked field, five spots of 5x5m were selected by criss-cross pattern and ten plants were observed in each spot randomly. In total 50 plants were audited. Mean number of T. parvispinus per flower was calculated. Each spot was considered as a replication. Thus, five replications were followed. Five rows along the borders were avoided while recording the data (Figure 3) [18] . For the direct visual count, all the flowers in the randomly selected 10 plants per spot were observed for the presence of thrips using hand lens (10x) (Figure 4). For the trapping, blue sticky traps of 24 × 20cm, engraved with 2.5 x 2.5 cm squares (6.25 cm 2 area) were used. The traps were mounted on bamboo poles, and positioned along the height of the plant. Five such traps were used in 20 cent field. A thin layer of castor oil was smeared over the blue board. Once in a week, the squares on the boards were observed and the thrips stuck over the surface were taken into account. Ten randomly selected squares on the board were counted and mean per square was worked out. The weekly average meteorological parameters were calculated from the daily meteorological data of the study area for minimum and maximum temperatures, relative humidity, wind velocity, and rainfall. The meteorological data were obtained from the Agro-meteorological Observatory, Department of Agronomy, Faculty of Agriculture, Annamalai University, Chidambaram.

Results

Population dynamics of T. parvispinus on chilli in the coastal zone of north Tamil Nadu 3.1.1 Kharif, 2022 (Direct-count)

The data recorded during the survey which was carried out in the coastal zone of Chidambaram, Cuddalore district, Tamil Nadu during Kharif, 2022, is presented in the Table 1. Very low incidence of T. parvispinus was found in the first two standard meteorological weeks (25 and 26 th SMW).

Thrips incidence was gradually increased from 27 th SMW in all the four locations where the surveys were conducted. The maximum thrips population was observed in Ayipettai (mean no. of 16.84±0.01 thrips/flower on 36 th S M W ) followed by Parampattu (10.76±0.04 on 34 th S M W ) , Sakkangudi (10.56±0.02 on 37 th SMW) and Parathurchavadi (9.32±0.01 on 37 th SMW). The thrips population was started to decline from the 39 th SMW, and reached the population size of about 0.36±0.01, 3.48±0.04, 0.92±0.04 and 1.96±0.02 at the end of 42 nd SMW in Parampattu, Ayipettai, Sakkangudi and Parathurchavadi villages, respectively. Thrips incidence started from the 25 th SMW and persisted up to the 42 nd SMW during Kharif, 2022. The peak incidence was recorded from 34 to 38 th SMW in all the four villages.

Rabi, 2022-23 (Direct-count)

During Rabi, 2022-23, a similar pattern of incidence was noticed as in the Kharif, 2022. However, very low incidence was reported in the first five weeks of flowering and the T. parvispinus population gradually increased from the 2 nd SMW in all the selected villages. Pest population was found to be below 5.00, up to 5 th SMW. The peak incidence (mean no. of 10.92±0.04 thrips/ flower) was recorded on the 12 th SMW in Ayipettai village, followed by 8.98±0.02 in Parampattu, 7.88±0.03 in Sakkangudi and 6.04±0.04 in Parathurchavadi on the 14 th SMW (Table 2). 3). The thrips population raised from the 27 th SMW in all four locations. On an average, the highest number of adult thrips was recorded in all four villages from the 32 to 37 th SMW. The highest number of adults (17.96±0.04 thrips/ square) was trapped on 35 th SMW at Ayipettai village in the Blue sticky trap followed by Sakkangudi (13.52±0.01 on 34 th SMW), Parathurchavadi (13.24 on 35 th SMW) and Parampattu (12.48±0.01 on 35 th SMW). After the 35 th SMW, the number of thrips declined, and reached the lowest level of 5.26±0.04, 6.34±0.01, 6.12±0.01 and 4.68±0.02 in Parampattu, Ayipettai, Sakkangudi, and Parathurchavadi respectively at 42 nd SMW.

Rabi, 2022-23 (Blue sticky trap)

During Rabi 2022-23, very low thrips were found during the first two weeks (49 th and 50 th SMW). The thrips population raised from the 51 st SMW in all four selected villages (Table 4). On an average, the highest number of adult thrips was recorded in all four villages from the 5 to 12 th SMW. The highest number of adult thrips (9.14±0.01 thrips/square) was trapped on 9 th SMW at Ayipettai village in the Blue sticky trap followed by Parathurchavadi (7.64±0.01 on 8 th SMW), Sakkangudi (7.54±0.01 on 7 th SMW), and Parampattu (7.48±0.01 on 8 th SMW). After the 12 th SMW, the number of thrips captured tended to decline, reaching the lowest thrips counts of 4.54±0.02, 6.14±0.02, 6.86±0.02 and 5.64±0.01 in Parampattu, Ayipettai, Sakkangudi and Parathurchavadi, respectively at 14 th SMW (Table 4).

Correlation between weather parameters and population dynamics of

T. parvispinus 3.2.1 Kharif, 2022

During the Kharif, 2022, a positive correlation was registered between the thrips population and maximum temperature (r= 0.485, 0.336, 0.278, and 0.487, respectively) at Parampattu, Ayipettai, Sakkangudi and Parathurchavadi. It was observed that T. parvispinus incidence in Parampattu, Ayipettai and Parathurchavadi exhibited a negative correlation with the minimum temperature (r =-0.436,-0.399 and-0.441, respectively). Thrips population in Sakkangudi village recorded a significant (P ≤ 0.05) negative correlation with the minimum temperature (-0.584) (Table 5). A positive correlation with humidity (r= 0.035, 0.161, 0.167 and 0.084) was observed in thrips population of Parampattu, Ayipettai, Sakkangudi and Parathurchavadi villages. Thrips population in Parampattu, Ayipettai, Sakkangudi and Parathurchavadi recorded a positive correlation with the rainfall (r= 0.106, 0.104, 0.151 and 0.155 respectively) and wind speed (r= 0.410, 0.355, 0.197 and 0.235 respectively).

Rabi, 2022-23

During Rabi, 2022-23, it was found that T. parvispinus incidence in Parampattu, Ayipettai, Sakkangudi and Parathurchavadi were recorded a significant (P ≤ 0.05) positive correlation with maximum (r= 0.802, 0.753, 0.740, 0.845), and minimum temperature (r= 0.675, 0.619, 0.612, 0.758) (Table 6). A positive correlation was registered in the thrips population of Parampattu, Ayipettai, Sakkangudi and Parathurchavadi with humidity (r= 0.246, 0.138, 0.069 and 0.314). A positive correlation with rainfall (r= 0.449, 0.694) was observed in Sakkangudi and Parathurchavadi villages, whereas thrips population in Parampattu and Ayipettai villages recorded a significant (P ≤ 0.05) positive correlation with rainfall (0.615, 0.590). T. parvispinus population in Parampattu, Ayipettai, Sakkangudi and Parathurchavadi r e c o r d e d a p o s i t i v e c o r r e l a t i o n w i t h t h e w i n d s p e e d ( r = 0.282, 0.370, 0.281 and 0.361) (Table 6).

Regression equation on population dynamics of T. parvispinus

Kharif, 2022

The regression equation on the population of T. parvispinus in chilli during Kharif, 2022 are presented in the Table 7. The mean population against weekly average meteorological parameters showed a good and statistically significant R 2 value for thrips population in Parampattu (0.628), Ayipettai (0.511), Sakkangudi (0.631) and Parathurchavadi (0.619) at 5 percent level.

Rabi, 2022-23

The mean population against weekly meteorological parameters showed a good and statistically significant R 2 value for T. parvispinus population in Parampattu (0.744), Ayipettai (0.684), Sakkangudi (0.626) and Parathurchavadi (0.844) at 5 percent level during Rabi, 2022-23 (Table 8). [7] in their study indicated the same. Incidence of T. parvispinus was gradually increased from 27 th S M W (Kharif, 2022) and 2 nd SMW (Rabi, 2022-23) in all the four villages (Figure 5 &6). However, the thrips occurrence in the above-mentioned SMW's coincided with the chilli flowering phase. This showed that T. parvispinus has a strong preference for chilli flowers, indicating that they are anthophilous thrips feeding on pollen. Similar behaviour was also observed in the flower thrips, Frankliniella occidentalis (Pergande), since; the pollen is a main component in the diet of adult thrips [19,20] . The maximum thrips population was recorded in peak flowering phase in all the four villages i.e., 34 to 38 th SMW during Kharif, 2022 and 10 to 14 th SMW during Rabi, 2022-23 (Figure 5 &6). Among the four villages, the maximum thrips population was observed in Ayipettai, followed by Parampattu, Sakkangudi and Parathurchavadi during both seasons. This might be attributed to the microclimate where Ayipettai is located near the riverbank. Ananthakrishnan (1993) [21] stated that the microclimate is very important in altering the population. The suitable environmental factors like high temperature and humidity supports the spread of thrips population [ 2 2 , 2 3 , 2 4 ] . It was also observed that the thrips population started to decline when the chilli plants entered the later stage of the reproductive phase. Similar trend was recorded at the later age of Cayenne pepper plants (C. frutescens) at Melinggih Kelod village in Bali, Indonesia [25] .

Blue sticky trap

The adult thrips started to get trapped gradually from the flowering phase of the chilli plants (Figure 7 &8). The highest number of adult thrips (17.96±0.04 and 9.14±0.01 thrips/square) was trapped on 35 th and 9 th SMW of Kharif, 2022 and Rabi, 2022-23, respectively at Ayipettai village. The thrips incidence was higher during the Kharif season than the Rabi, and it was confirmed by the observations obtained from the blue sticky traps [26,27] . The number of thrips captured tend to decline after the 35 th SMW of Kharif, 2022 and 12 th SMW of Rabi, 2022-23 in all the selected four villages (Figure 7 &8).

Correlation between weather parameters and population dynamics of

T. parvispinus

During Kharif, 2022 and Rabi, 2022-23, it was found that T. parvispinus registered positive correlation with humidity, rainfall, and wind speed. In Kharif, 2022, T. parvispinus is positively correlated with maximum temperature and negatively correlated with minimum temperature. Whereas, in Rabi, 2022-23, a significant positive correlation was observed in thrips population with maximum and minimum temperature. The present study showed that the population of T. parvispinus increased with increase in maximum temperature, humidity, rainfall and windspeed. The thrips population increased due to the presence of maximum temperature (∼35 °C) prevailed in 34 to 39 th SMW during Kharif, 2022. Similarly, the maximum temperature (∼30 to 33 °C) and minimum temperature (∼24 to 26 °C) favours the incidence of T. parvispinus, which was prevailed in 8 to 14 th SMW during Rabi, 2 0 2 2 -2 3 . Regarding windspeed, heavy winds influenced the highest dispersal of T. parvispinus (∼25 to 35 km/hr) because they are passive fliers. Many farmers used closer plant spacing and this made the chilli plants more likely to be infested by T. parvispinus. The reason for this was the changing microclimate above the crop canopy with increased humidity and temperature. The current findings align with the observations made by Dirgayana et al. (2021) [24] , who reported that an increase in plant spacing leads to a decrease in the population of T. parvispinus in treatment plots. Similar results were obtained by Rabeena et al. (2020) [28] , who found that temperature and rainfall influence the seasonal dynamics of thrips, Frankliniella schultzei incidences in tomato fields.

Regression equation on population dynamics of T. parvispinus

The mean population against weekly average meteorological parameters showed a good and statistically significant R 2 value for thrips population in Parampattu (0.628 and 0.744), Ayipettai (0.511 and 0.684), Sakkangudi (0.631 and 0.626) and Parathurchavadi (0.619 and 0.844) at 5 percent level during Kharif, 2022 and Rabi, 2022-23, respectively. The present findings are supported with earlier findings of Naresh et al. (2017) [29] and Akashe et al. (2016) [30] who reported that maximum and minimum temperature, humidity and rainfall had a positive impact on the T. palmi incidence.

Conclusion

The study reveals that the T. parvispinus population increased with increase in the temperature, humidity and windspeed. The maximum thrips incidence was on the 34 th to 38 th SMW during Kharif, 2022 and in the season Rabi, 2022-23, the highest infestation was on the 10 to 14 th SMW in all the four villages of Cuddalore district. The highest number of adult thrips trapped in the blue sticky trap was on 35 th (17.96±0.04 thrips/flower) and 9 th S M W (9.14±0.01 thrips/flower) of Kharif, 2022 and Rabi, 2022-23, respectively at Ayipettai village. Among the four villages, the maximum thrips population was observed in Ayipettai, followed by Parampattu, Sakkangudi and Parathurchavadi village during both seasons. In both the seasons, thrips population is positively correlated with the weather parameters such as maximum temperature, humidity, rainfall and windspeed. These findings highlight the influence of climatic factors on T. parvispinus population dynamics, emphasizing the need for weatherbased forecasting models and timely integrated pest management strategies to mitigate its impact on crop production.

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