Introduction
Chilli is one of the most important spice in Nepal. India is the world’s largest producer, consumer and exporter of chilies in the world. India also has the largest area under chilies in the world. Chilies are the most common spice cultivated in Nepal and other Asian countries. Chilli or red pepper Capsicum annum L. belonging to family solanaceae is an important spice cum vegetable crop commonly used in Nepali dietary and are widely used in many cuisines to add spiciness to dishes. Though there are many factors responsible for low yields, the major constraint is the regular occurrence of insect pests. The crop is ravaged by many insect pests right from nursery till harvest. The pest spectrum of chilli crop is complex with more than 293 insects and mite species debilitating the crop in the field as well as in storage (Anonymous, 1987).
Thrips (Scirtothrips dorsalis) and fruit borer (Helicoverpa armigera) are the most important recurring pests in chilli (Reddy & Puttaswamy, 1983). Recently, the incidence of thrips and its damage is increasing and cause heavy economic losses to farming community. Weather variables including rainfall, temperature, relative humidity and wind have been reported as important factors that significantly affect thrips numbers (Ananthakrishnan, 1993; Kirk, 1997; Legutowska, 1997). Relatively high temperature and lack of rainfall have been associated with increase in thrips population, while high relative humidity and rainfall reduce thrips population (Hamdy and Salem, 1994). Development of thrips is strongly related to environmental conditions, especially temperature (Edelson and Magaro, 1988). Photoperiod does not induce diapause in the development of thrips, but the adults exhibit temperature-induced reproductive quiescence (Jenser & Szénási, 2004).
Weather Parameters and Thrips Population Dynamics
Temperature: Optimal temperatures for chilli thrips population growth generally range between 26.2°C and 36.2°C. Maximum temperatures around 35°C favor population build-up, while extreme heat can lead to declines in thrips numbers due to increased mortality rates (Mandal & Mondal, 2023). Specifically, studies have shown that high temperatures can allow thrips to reproduce faster (Waiganjo et al., 2008; Garrick et al., 2017).
Humidity: Relative humidity plays a critical role in thrips population dynamics. High humidity levels (65% or higher) tend to negatively impact thrips populations, while lower humidity can promote their survival and reproduction (Mandal & Mondal, 2023; Waiganjo et al., 2008). A significant negative correlation has been observed between relative humidity and thrips density, indicating that as humidity increases, the population tends to decrease (Mandal & Mondal, 2023).
Rainfall: Rainfall also affects thrips populations; excessive rainfall can wash thrips off plants, leading to sharp declines in their numbers. Dry conditions are associated with increased thrips density (Waiganjo et al., 2008; Garrick & Liburd, 2017). Studies have shown that during dry seasons with moderate temperatures, thrips populations can peak significantly (Waiganjo et al., 2008).
Seasonal Variations
The dynamics of chilli thrips populations vary seasonally. For instance, during the pre-kharif season (a planting season), maximum and minimum temperatures along with sunshine hours is found to correlate positively with thrips populations (Mandal & Mondal, 2023). In contrast, during periods of high rainfall and humidity, a decline in their numbers is noted (Waiganjo et al., 2008).
First Season vs. Second Season: Weather parameters contributed differently to population fluctuations across seasons. For example, during the first season, weather factors accounted for approximately 55.4% of the variation in thrips populations, while this figure dropped to 18.9% in the second season (Mandal & Mondal, 2023).
This suggests that environmental conditions may have a more pronounced effect during certain times of the year.
Conclusion
The interaction between climate and the population dynamics of chilli thrips is complex and influenced by multiple weather parameters such as temperature, humidity, and rainfall. As climate change continues to alter these factors, it is imperative for agricultural practices to adapt accordingly. Continuous monitoring and research will be vital in predicting future trends in pest populations and ensuring sustainable crop production.
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