S. dorsalis was first recorded on castor shoots and chillies in Coimbatore (India) in 1916 and was described as a new species by Hood (1919) (Duraimurugan and Jagadish, 2011). It is apolyphagous insect infesting about 20 hosts in India (Ananthakrishnan, 1971; Kumaresan et al., 1998). Recently it has become a major pest on tea plants of north-eastern India including Assam (Varatharajan et al., 2019). Present investigation was undertaken to study development, survival, fecundity and demographic parameters of S. dorsalis on tea cultivars, TV1, TV17 and TV23.
The average incubation period of 3.41 ± 0.51 d (TV1), 3.99 ± 0.67 d (TV17), 4.8 ± 0.62 d (TV23) in the present study was in close agreement with the range reported by Dev (1964), Raizada (1965), Patnaik et al. (1986), Duraimurugan and Jagadish (2011) and Rahman et al. (2014). Tatara (1994) of Japan and Seal et al. (2010) of Florida observed that the mean incubation time of S. dorsalis were 10.5 d at 25°C and 6.8 ± 0.2 d – 7.6 ± 0.3 d at 26 ± 1°C, respectively. The longer period reported by them may be due to the difference in host plants and other ecological factors. The total larval durations observed in the present study overlaps with the reports of Ayyar et al. (1935), Dev (1964), Patnaik et al. (1986), Tatatra (1994), Duraimurugan and Jagadish (2011) and Rahman et al. (2014). Raizada (1965) and Seal et al. (2010) reported 7–8 d and 8.4 ± 0.2 d − 10.0 ± 0.4 d of larval duration, respectively, which was slightly longer than the present investigation. This may be due to the difference in host plant and other environmental and climatic factors of those study sites compared to Assam. The pupal periods which ranged from 5-6.26 d are in agreement with the reports of Ayyar et al. (1935), Raizada (1965) and Duraimurugan and Jagadish (2011). Shorter periods of 2.6 ± 0.2 d – 3.3 ± 0.3 d of pupal development was reported by Seal et al. (2010) which may be due to difference in host plants. The total development period from egg to adult emergence on the three tea cultivars (TV1, TV17, TV23) overlaps the findings of Ayyar et al. (1935), Dev (1964), Patnaik et al. (1986) and Duraimurugan and Jagadish (2011) on chilli, tea, chilli and rose respectively. Adult longevity of 5.24 ± 0.43 d (TV1), 4.93 ± 0.62 d (TV17), 4.01 ± 0.56 d (TV23) was also comparable with reports of Dev (1964), Patnaik et al. (1986), Duraimurugan and Jagadish (2011) and Rahman et al. (2014). Sufficiently longer adult longevity ranged from 13.80 ± 1.20 d – 19.00 ± 0.86 d was observerved by Seal et al. (2010) on different host plants in Florida. Undoubtedly, to determine the mechanisms by which different host plants shorten or prolong the developmental period of each stadium of S. dorsalis, additional research in required.
The demograohic parameters R0, rm, λ, WMR, T and DT are used to measure the effect of host plant on the development, survival and reproduction of insect arthropods (Silveria Neto et al., 1976). The values obtained for R0, rm, λ and WMR are higher (Price, 1997) and T and DT are lower (Van Lenteren and Noldus, 1990) in suitable hosts. Among the life table parameters, R0 is elementary to assess the quality of food since it represents an innate characteristic of the population (Bernardi et al., 2012). The thrips population tended to increase on all the three tea cultivars as R0 was greater than 1.0 in each case (Birch, 1948; Souza et al., 2022). However, the comparison of two or more populations by means of their reproductive rates may be misleading to a certain extent unless the mean length of generations is the same (Birch, 1948; Garad et al., 1983). Indeed, it is evident that on the basis of the net reproductive rate (R0) TV17 occupied first position, but, it ranked second on the basis of innate capacity for increase in numbers (rm). This positional shift of TV17 is due to a relatively higher value of generation time which reduced the rm value. Life tables providing the statistics on the innate capacity for increase in numbers of a particular species give insight into the characteristic life patterns of different species (Garad et al., 1983). Innate capacity for increase in numbers (rm) is a relative measure of host quality and indicates the biotic potential of arthropods in a given environment (Castro-Guedes et al., 2016; Souza et al., 2022). From the point of view of pest multiplication, TV1 with high rm value would be the most suitable for the insect pest in the present study. Greater innate capacity for increase in numbers also measure the host’s susceptibility to insect attacks and vice-versa (Musa and Ren, 2005; Ahmed et al., 2021). Unlike rm, which is the continuous form of rates of increase for a population, finite rate of increase in numbers (λ) is multiplication rate for a discrete unit of time (Birch, 1948; Nanthagopal and Uthamasamy, 1989), i.e., it represents the number of individuals added to the population per individual per unit of time (Rabinovich, 1978; Souza et al., 2022). λ values of 1.09 (TV1), 1.08 (TV17), 1.06 (TV23) indicate that the females reared more than one individual daily. Weekly multiplication rate of the thrips population was highest on TV1 and expectedly the time interval between each generation (T) and population doubling time (DT) was lowest on TV1. The rm, λ and WMR were maximum on TV1. All these statistics were minimum on TV23. These results indicate that the tea thrips, S. dorsalis prefer TV1 cultivar for feeding and oviposition compared to the other two tea cultivars. The results of the present investigation were in close conformity with works on Scirtothrips bispinosus (Bagnall) on tea by Mahendran (2011). Similar observations have also been recorded by Seal et al. (2010) on S. dorsalis feeding on Knockout rose and Jalapeno pepper. The values of innate capacity for increase in numbers (rm) and finite rate of increase in numbers (λ) were slightly greater in Ceratothripoides claratris feeding on tomatoes (Premachandra et al., 2004). The life table parameters of insect pests differ with changes in biological factors (Huang and Chi, 2012; Chen et al., 2017; Ahmed et al., 2021). Among the biological factors, a host plant can have an important impact on development, fecundity and reproduction (Ahmed et al., 2021).
S. dorsalis presented a high biotic potential on tea cultivars. These information on survival and reproduction provide insight into the interaction between S. dorsalis and tea plants, which can help develop a management program for the pest species. Among the three cultivars, TV1 has the highest preference from S. dorsalis as the multiplication rate was slightly higher on it than the other two cultivars. Therefore, TV1 is more prone to thrips attack than TV17 and TV23.