Seasonal incidence and correlation studies of thrips infesting chilli with weather parameters

Introduction

Chilli (Capsicum annuum L.), a vital solanaceous vegetable crop, is a dietary and economic cornerstone in India, prized for its pungent fruits rich in vitamins A and C, minerals like potassium and magnesium, and capsaicin, which imparts distinctive pungency and flavour (Bhatt and Karnatak, 2020) [2] . Used as a vegetable, spice, condiment, and in sauces and pickles, chilli's nutritional and medicinal properties enhance its global demand across tropical and subtropical regions (Zainab et al., 2016) [18] . Its vibrant colour and versatility in culinary applications underscore its export potential (Borgohain, 2024) [5] . Chilli's role in traditional medicine further elevates its significance (Meena et al., 2013) [10] . In 2023-24, India led global chilli production, cultivating 418 thousand hectares to yield 4.46 million tonnes at 10.67 tonnes per hectare (National Horticulture Board, 2024). This dominance is driven by rising consumption in culinary and processed food industries (Zainab et al., 2016) [18] . Maharashtra, ranking fifth nationally, produced 415.32 thousand tonnes from 38.12 thousand hectares, reflecting chilli's critical role in the state's horticultural economy and rural livelihoods (National Horticulture Board, 2024) [11] . Thrips, notably Scirtothrips dorsalis Hood and Thrips parvispinus Karny, are major pests, attacking over 100 hosts and causing 50-90% yield losses through sap-feeding and virus transmission (Butani, 1976) [6] . Their feeding leads to leaf curling, silvering, bronzing, and stunted growth, impairing photosynthesis and plant vigour (Kumar, 1995) [9] . The short 10-15 days life cycle of thrips enables multiple generations per season, rapidly escalating infestations (Pathipati et al., 2014) [14] . This necessitates vigilant monitoring to curb economic damage (Bokan et al., 2015) [4] . Thrips transmit chilli leaf curl virus, triggering the "murda complex" with symptoms like leaf distortion, chlorosis, reduced leaf size, and fruit malformation (Nelson and Natarajan, 1994) [12] . These effects drastically reduce marketable yield and quality, sometimes causing complete crop failure (Samanta et al., 2017) [15] . Chemical controls, while initially effective, risk pest resistance and environmental harm (Tommasini and Maini, 1995) [17] . Sustainable and integrated pest management approaches are thus critical (Chinniah et al., 2016) [7] . Weather parameters like temperature, humidity, rainfall, wind velocity and sunshine hours critically influence thrips population dynamics across agro-climatic zones (Patel et al., 2009) [13] . These factors modulate thrips abundance and damage potential (Barot et al., 2012) [1] . This study examines the seasonal incidence thrips on Phule-Jyoti variety of chilli crop and their correlation with weather factors to devise weather-based integrated pest management strategies for sustainable chilli production.

Material and Methods

Site of Field Experiment

The present experiment was conducted at the PG Research Farm, Division of Entomology, Dr. Sharadchandra Pawar College of Agriculture, Baramati, Maharashtra, during the kharif season 2024-25. The geographical details of the site are 18°09' N latitude, 74°34' E longitude, and 538 meters above from mean sea level (MSL).

Experimental Layout

The present experiment was conducted on chilli crop var. "Phule-Jyoti" (Capsicum annuum L.). The seedlings were raised in nursery beds and 45 days old seedlings were transplanted in a plot at a distance of 60 cm between rows and 45 cm between plants on raised beds in the main field during 1 st week of September 2024. All recommended agronomic package of practices except the plant protection measures were adopted for raising the chilli crop.

Observations Recorded

The pest population observations were recorded at seven days interval, from the initiation of the thrips infestation and was continued up to harvest. Observations were recorded from each experimental plot having five plants which were selected and tagged further; detailed observations were made at weekly intervals on the incidence of thrips. The population of thrips, were recorded from three leaves one each from the upper, middle and lower position on five selected plants. For recording observations, magnifying lens (10 X) was used to visualise and collect thrips from chilli plants and then population count of collected thrips was recorded under stereo binocular microscope.

Meteorological Data

The meteorological data on different weather parameters viz. temperature (maximum & minimum in °C), relative humidity (morning & evening in %), total rainfall (mm), wind velocity (Km/hr) and bright sunshine hours (hr/day) during the period of investigation were collected from the Agro-meteorological Department, NIASM, Baramati.

Correlation studies

A simple correlation coefficient (r) method was adopted to work out the relationship between the incidence of the thrips population and the weather parameters. The data thus obtained were converted into the average number of thrips per plant and represented in Table 1 and graphically depicted in Fig. 1.

Results and Discussion

To study the seasonal incidence of thrips infesting chilli during kharif season, 2024-25

The seasonal incidence of thrips on Phule-Jyoti variety of chilli during kharif 2024-25 at Baramati, Maharashtra, demonstrated periodic fluctuating patterns in thrips bionomics. Thrips infestation was observed from the 37 th Standard Meteorological Week (SMW; second week of September) to the 2 nd SMW (second week of January 2025), with mean counts of thrips population ranging from 0.67 to 4.67 thrips/leaf/plant. The initial incidence of 0.67 thrips/leaf/plant was observed in the 37 th SMW, following the post-monsoon transition, when the maximum and minimum temperatures recorded were 32.7 °C and 21.3 °C with morning relative humidity 91% and evening relative humidity 56%. This rate gradually increased, peaking at 4.67 thrips/leaf/plant during the 44 th SMW where temperature recorded was 33.3 °C maximum temperature and 20.6 °C minimum temperature, whereas morning relative humidity recorded was 92% and evening relative humidity of 53%. A secondary peak of 3.93 thrips/leaf/plant occurred in the 48 th SMW with temperature 32.7 °C maximum and 18.6 °C minimum, relative humidity 87% morning and 40% evening. Further driven by declining temperatures and crop senescence, the population declined to 1.84 thrips/leaf/plant in the 2 nd SMW. This indicates highrisk windows of severe thrips infestation in the 42 nd SMW to 48 th SMW. The data regarding to this is illustrated in Table . 1 and Fig. 1.

Correlation studies of thrips infesting chilli with weather parameters

Correlation analysis of thrips populations with weather parameters during kharif 2024-25, is depicted in Table 1 and presented graphically in Fig 1 . The data revealed a significant positive correlation between thrips incidence and maximum temperature (r = 0.465) as well as sunshine hours (r = 0.614). however, there was a non-significant positive correlation with morning relative humidity (r = 0.019). On the other hand, the thrips incidence exhibited a nonsignificant negative correlation with minimum temperature (r = -0.071), evening relative humidity (r = -0.354), wind velocity (r = -0.357) and rainfall (r = -0.288). Tirkey et al. (2020) reported a biphasic trend in chilli thrips populations with November and December peaks, mirroring the surges in thrips population here due to post-monsoon warming. Patel et al. (2009) [13] observed maximum infestation in late October to early November and positive sunshine correlations with thrips incidence, corroborating the extended daylight role in November-December peaks. Barot et al. (2012) [1] documented dry-spell surges, consistent with the early upsurge amid low rainfall. Bokan et al. (2015) [4] noted major pest peaks in mid to late autumn, supporting the secondary December rise from prolonged sunshine hours. Samanta et al. (2017) [15] highlighted December secondary peaks linked to residual warmth, validating the pattern, while the January decline echoes their observed cooler-phase reductions and minor minimum temperature role. Bhede et al. (2008) [3] reported positive maximum temperature correlations, aligning with November buildup. Meena et al. (2013) [10] found non-significant negative effects of rainfall and humidity, supporting January declines. Havanoor and Rafee (2018) [8] described wind velocity's negative impact, matching the dispersive effect thrips incidence. Zainab et al. (2016) [18] corroborated strong sunshine correlations, validating December's secondary These insights affirm the study's biphasic dynamics and weather correlations, enabling to formulate better pest management stratergies.

Conclusion

The analysis of weather parameters in the current study revealed significant positive correlations between the seasonal incidence of thrips and maximum temperature and sunshine hours during kharif 2024-25 at Baramati, with the infestation initiating in the 37 th Standard Meteorological Week (SMW; second week of September) at an average of 0.67 thrips/leaf/plant. The thrips population progressively increased, attaining a primary peak of 4.67 thrips/leaf/plant in the 44 th SMW (first week of November) and a secondary peak of 3.93 thrips/leaf/plant in the 48 th SMW (first week of December), driven by optimal thermal and photoperiodic conditions. Subsequently, a decline to 1.84 thrips/leaf/plant was observed in the 2 nd SMW (second week of January 2025), attributable to decreasing maximum and minimum temperatures. Non-significant negative correlations with evening relative humidity, wind velocity and rainfall, alongside a non-significant positive correlation with morning relative humidity, further modulated this decline. These findings elucidate the critical influence of climatic factors on thrips proliferation, enabling the identification of vulnerable stages for targeted pest management. The study advocates for the integration of biological control measures, leveraging natural enemies, and minimizing the use of synthetic pesticides to preserve soil and environmental integrity, with the ultimate motto of fostering sustainable chilli production and ecological balance.

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