Seasonal population dynamics of major thrips species associated with soybean and groundnut in the Chhattisgarh Plain Zone

Introduction

Thrips (Thysanoptera: Thripidae) comprise an important group of minute insects associated with a wide range of agricultural crops. Several phytophagous species cause direct feeding injury to leaves, flowers and developing fruits, while some species also act as vectors of economically important plant viruses (Tyagi et al., 2017) [1] . Their widespread distribution and ability to damage crops and transmit plant viruses make them an important group of pests in agricultural production, particularly in tropical and subtropical regions. Soybean and groundnut are important oilseed crops that are affected by several economically important insect pests, including thrips. In groundnut, thrips feeding can cause silvering, necrosis and distortion of young leaflets. Some species like Thrips palmi is also a vector of Groundnut bud necrosis virus (GBNV; Orthotospovirus arachinecrosis) (Lakshmi et al., 1995; Ghosh et al., 2019) [2,3] . In soybean, thrips feeding can cause silvering, bronzing and distortion of young foliage, while Neohydatothrips variabilis is recognized as the primary vector of Soybean vein necrosis virus (SVNV), with Frankliniella fusca and F. tritici also capable of transmitting the virus (Han et al., 2019) [4] . Seasonal studies on groundnut have reported distinct periods of thrips occurrence and peak abundance, with substantial variation among seasons and locations (Wankhede et al., 2020) [5] . Such seasonal variation highlights the importance of understanding the timing and magnitude of thrips populations for effective monitoring and timely management. However, information on the species-specific seasonal dynamics of thrips associated with soybean and groundnut remains limited in the Chhattisgarh Plain Zone, particularly in relation to crop phenology and weather conditions. Temperature, relative humidity and rainfall may influence thrips development, activity and population growth, although the nature and magnitude of these relationships can vary among species, seasons and locations (Priyanka et al., 2023) [6] . Species-specific studies have also demonstrated differences in the timing and magnitude of seasonal activity in soybean, emphasizing the importance of region-specific information on population peaks and crop-stage associations (Keough et al., 2018) [7] . Such information is essential for improving pest monitoring and developing timely management strategies. Therefore, the present study was undertaken to investigate the seasonal population dynamics of major thrips species associated with soybean and groundnut and to determine their relationship with crop growth stages and prevailing weather parameters in the Chhattisgarh Plain Zone.

Materials and methods

The field experiment was conducted at the Instructional-cum-Research Farm, Indira Gandhi Krishi Vishwavidyalaya, Raipur, Chhattisgarh, during kharif 2025-2026. Soybean (GC Soybean-11-15) and groundnut (CG Mungfali) were grown in plots of 5 × 4 m following recommended agronomic practices, except for insecticide application. Thrips populations were monitored at weekly intervals from 20 days after sowing until harvest. Five plants were randomly selected per plot, and observations were recorded from the upper, middle and lower portions of the plants. Thrips were counted by gently jerking leaves and flowers over a white paper sheet, and populations were expressed as mean number of thrips per leaf and per flower. Observations were recorded in terms of standard meteorological weeks (SMW) and days after sowing (DAS). Leaf populations were monitored from 20 DAS, while flower observations were recorded from the onset of flowering. Weather data, including maximum and minimum temperature, rainfall, morning and evening relative humidity, wind velocity and bright sunshine hours, were obtained from the Agricultural Meteorological Observatory, IGKV, Raipur. Microclimatic observations were also recorded near the crop canopy using a temperature-humidity meter (FD-HYGRO, Forensics Detectors). The association between thrips population and weather parameters was determined using correlation analysis (Steel et al., 1980) [8] . Population abundance on leaves and flowers was compared statistically, with significance assessed at the 5% level.

Results and discussions

Seasonal population dynamics of thrips

The population dynamics of Megalurothrips usitatus, Haplothrips ceylonicus and Thrips parvispinus are presented in Table 1 and table 2. All three species exhibited a gradual increase in population from the vegetative stage, followed by a decline towards crop maturity, although the timing and magnitude of peak abundance varied among species and crops.

Megalurothrips usitatus

Among the thrips species monitored, M. usitatus exhibited the highest population levels on both soybean and groundnut. It was first recorded during the 27 th standard meteorological week (SMW) with 1.10 thrips/leaf on both soybean and groundnut. Its population on soybean increased progressively and reached a maximum of 7.52 thrips/leaf during the 33 rd SMW (61 DAS), after which it declined and became absent after the 39 th SMW. On groundnut, the population reached a maximum of 5.70 thrips/leaf during the 32 nd SMW (55 DAS) and subsequently declined towards crop maturity. The seasonal mean population was 3.26 thrips/leaf on soybean and 2.27 thrips/leaf on groundnut. Thus, the maximum population of M. usitatus occurred during 32 nd -33 rd SMW, corresponding to the flowering to early poddevelopment period. This seasonal pattern agrees with previous observations of M. usitatus on soybean (Chang, 1987) [9] where its populations on soybean increased during vegetative growth, reached maximum abundance during flowering and declined as the crop matured. The higher seasonal mean on soybean further suggests that crop characteristics and the availability of suitable feeding sites may influence host preference. The concentration of the population during flowering indicates that this stage represents an important window for surveillance and management of M. usitatus.

The population on flowers showed a similar increasing trend but reached considerably higher levels than on leaves. On soybean, M. usitatus was first observed during the 29 th SMW (34 DAS) with 2.40 thrips/flower and increased to a maximum of 9.52 thrips/flower during the 33 rd SMW (61 DAS). On groundnut, it appeared during the 31 st SMW (48 DAS), increased to 5.70 thrips/flower during the 32 nd SMW and attained a maximum of 7.95 thrips/flower during the 33 rd SMW (61 DAS). The seasonal mean populations were 5.48 and 5.12 thrips/flower on soybean and groundnut, respectively. M. usitatus has been reported to show a clear preference for reproductive plant parts (Chang, 1988) [10] , while flowering legumes provide suitable conditions for feeding, oviposition and population growth (Tang et al., 2023) [11] . Haplothrips ceylonicus H. ceylonicus was first observed during the 27 th SMW at 1.10 thrips/leaf on both crops. On soybean, the population increased gradually and attained a maximum of 4.95 thrips/leaf during the 34 th SMW (68 DAS). On groundnut, the maximum population of 4.50 thrips/leaf was recorded during the 36 th SMW (81 DAS). The seasonal mean populations were 2.67 thrips/leaf on soybean and 2.33 thrips/leaf on groundnut. The species persisted for a longer period on groundnut than on soybean. The gradual increase in population from the vegetative stage to flowering is consistent with the general ecology of Haplothrips species, which are commonly associated with flowers and pollen (Lewis, 1997) [12] . H. ceylonicus appeared on soybean flowers during the 29 th SMW and attained a maximum of 5.95 thrips/flower during the 34 th SMW (68 DAS). On groundnut, it was first recorded during the 31 st SMW and reached a maximum of 5.85 thrips/flower during the 35 th SMW ( 74Thrips abundance differed between plant parts, with flowers supporting higher populations than leaves across both crops. The mean popu-lation of all the three species occurred at higher densities on flowers than leaves in both crops. The consistently greater abundance on flowers indicates that reproductive structures provided more favourable feeding and breeding conditions than foliage. The greater abundance of H. ceylonicus and T. parvispinus on groundnut flowers may be associated with the availability of fresh flowers over a relatively extended reproductive period. Floral resources, particularly pollen, can provide important nutritional resources for flower-associated thrips and contribute to population de-velopment and reproductive success (Kirk, 1997; Reitz et al., 2020) [13, 16] . Therefore, the present results indicate that flowering is a critical phenological stage for the build-up of the thrips complex on soybean and groundnut. Consequently, monitoring programs based exclusively on leaf sampling may underestimate thrips abundance during the flowering period.

Relationship with weather parameters

The correlation coefficients between major microclimatic factors and the population of the predominant thrips species on soybean and groundnut during the Kharif season of 2025-2026 are presented in Table 4. Under microclimatic conditions, M. usitatus on groundnut showed a significant positive association with morning relative humidity (𝑟 = 0.470 *

), this suggests that relatively humid morning con-ditions may favour its activity and survival. Similar observations were reported where the development and abundance of M. usitatus were influenced by environmental factors, particularly temperature, humidity and host phenology (Tang et al., 2023) [11] , whereas T. parvispinus on soybean exhibited significant negative associations with maximum temperature (𝑟 = -0.589 *

) and minimum temperature (𝑟 = -0.575 *

), but a positive association with morning relative humidity ( 𝑟 = 0.619 *

). Associations between T. parvispinus population dynam

Conclusion

The population dynamics study revealed distinct seasonal fluctuations in the major thrips species associated with soybean and groundnut during kharif 2025-2026, with population buildup coinciding broadly with the flowering and early reproductive stages. Megalurothrips usitatus was the predominant species and attained its maximum population during the 33 rd SMW, reaching 7.52 thrips/leaf and 9.52 thrips/flower on soybean. Haplothrips ceylonicus reached maximum abundance during late flowering to early pod development, whereas T. parvispinus attained its highest population around 68 DAS. Populations were consistently higher on flowers than leaves, highlighting the importance of reproductive plant parts for monitoring. Weather variables influenced thrips abundance differently among species and host crops, with temperature and relative humidity showing the most notable associations. These findings indicate that crop phenology, together with prevailing weather conditions, should be considered when developing monitoring and forecasting strategies for thrips in soybean and groundnut.

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