Influence of weather parameters on population dynamics of thrips and mites on summer season cowpea in Eastern Plateau and Hill region of India MOANARO and JAIPAL SINGH CHOUDHARY*

Population dynamics of thrips, Megaleurothrips distalis (Karny) and mites, Tetranychus spp. were studied on summer season cowpea crop during 2013-2014 in Eastern Plateau and Hill region of India. Temperature, rainfall and host-plants were analyzed with respect to population fluctuation o f pests. The population of thrips and mites were observed from low to high throughout the entire season and the incidence were more or less in the same pattern of the population fluctuation. Infestation o f both thrips and mite initiated from 2nd week of March and continued up to the last week of June. The correlation analysis between thrips population and weather parameters indicated a strong significant positive correlation between the number of thrips and maximum temperature (p<0.01) and significant with the mean temperature (p<0.05). A significant negative correlation was observed between thrips population and rainfall, but a negative correlation with RH was found to be non-significant. Similarly, temperature showed positive and rainfall a negative correlation on mite population fluctuation. The linear regression model based on weather conditions as independent variable and thrips and mites population fluctuation as dependent variable, explained to 72 percent for thrips and 62 percent for mite population variability. Environmental factors played significant role in distribution and abundance of thrips and mites population.

in photosynthesis and transpiration activity leading to reduction in the amount of harvestable yield.
In India, M. distalis species of thrips and T. urticae species of mites are reported to be the most predominant (Chakraborty et al., 2014;Ghosh, 2013). The incidence and development of all the insect pests are much dependent upon the prevailing environmental conditions such as temperature, relative humidity and precipitation (Woiwod, 1997). Published informations on population dynamics and factors influencing population of thrips and mites on cowpea is scanty and incomplete especially in Eastern Plateau and Hill (EPH) region of India where it is totally lacking. Hence, the present study was carried out with the objectives as (i) to analyze the population dynamics of thrips and mites on cowpea in EPH region conditions and (ii) to analyze factors that affect the population dynamics. Keeping these views in mind, analysis on these issues in the region will provide essential information for understanding the population dynamics of thrips and mites in the whole region.  (2013 and 2014). Seeds were dibbled at a spacing of 40cm on raised beds in randomized block design, with three replicates with a plot size of 4m 2 under drip irrigation system in the first week of February. The distance between two laterals was maintained at 1m. All the post-sowing agronomic practices except plant protection were followed as per recommended package and practices for summer season cowpea crop.

MATERIALS AND METHODS
The species of thrips was determined as Megaleurothrips distalis Karny and that of red spider mites as Tetranychus spp. based on the morphological descriptions given by Diffie et al., (2008). The population of red spider mite from one leaf each from top, middle and bottom of the selected plants and that of thrips by tapping leaf/bud/ flower once from each of the selected plants in the morning hours were recorded at weekly interval. Five randomly selected plants in each replication of each variety were considered for population counting purpose. Data on population of both the pests were recorded from the first appearance to till maturity of the crop. The data on weather parameters viz., minimum and maximum temperature ( 0 C), relative humidity (RH) (%) and rainfall (mm) were collected from the agro-meteorological observatory of the Centre. The mean and simple correlation and regression were calculated by using SPSS V.16 in order to find out the effect of different weather parameters on the incidence and development of thrips and mites on the cowpea crop. The figures in the article were drawn using Microsoft office excel 2007.

Population dynamics
Mean pooled population of thrips, M. distalis and mites, Tetranychus spp. at various stages on all the three genotypes of cowpea recorded from the first incidence to till end of crop harvesting are summarized in Fig. 1 and 2. Both the thrips and mites population were observed low to high in the entire cropping season of cowpea. The incidences of both the pests were also more or less same in the pattern of population fluctuation on all the genotypes. The infestation of both the pests initiated from the second week of March and continued up to the last week of June. The pattern of pooled population of thrips was noticed from 0.11 to 3.57 per leaf in Swarna Sweta, 0.11 to 4.20 per leaf in Swarna Harita and 0.33 to 2.14 per leaf in Swarna Mukut and for mites, it was observed in the range from 0.11 to 4.50 per leaf in Swarna Sweta, 0.11 to 3.03 in Swarna Harita and 0.33 to 2.14 per leaf in Swarna Mukut. Patel et al. (2010) also noticed Megaleurothrips spp. in the month of March onward in cowpea. The peak of population was reached at flowering time of the crop on all the studied genotypes. Presence of flowers on the crop provides conducive environment for perpetuating thrips through quality feeding and breeding place (Buitenhuis and Shipp, 2007). Population build up of mites on cowpea in this region is in accordance with the buildup of mites population in Varanasi region where it also started from second week of March and the highest population buildup of mites were recorded on 18 th and 20 th standard weeks (Kumar et al., 2014).

Relationship between population and weather factors
Weather factors were one of the important factors that influenced the fluctuation of population of thrips and mites. The data on correlation between pooled incidence of thrips and mites population with weather parameters showed significant relationship as presented in Table 1. The correlation analysis indicated a strong significant positive correlation between the number of thrips per tap and maximum temperature (p<0.01) and significant with the mean temperature (p<0.05) ( Table 1). Earlier studies also explained that thrips population on different crops are positively influenced by temperature and population density increased with increasing temperature (Patel et al., 2010). A significant negative correlation was found with the thrips population and rainfall, even though a negative correlation was observed with RH, it was found to be non-significant.
Results are in line with earlier reports (Khan and Ullah, 1994). Overall, temperature had positive correlation with thrips incidence while RH and rainfall gave a negative correlation in this study. In extreme high temperature region, thrips population was found to be retarded on high temperature with low relative humidity (Dintenfass et al., 1987). Based on region specific findings, some are not supportive to the present findings (Panikar and Patel, 2001). Considering multiple factors influencing population of thrips as dependent variable and weather conditions as independent variable, linear regression analysis was carried out and presented in Table 2. The linear regression model based on weather variables could explain 60 to 68% variability on thrips population fluctuation on the studied varieties.
When it was combined on all varieties it reached to 72% explaining the thrips population variability.
Similarly, temperature showed positive and rainfall a negative correlation on mite population fluctuation (Table  1). Chinniah et al. (2007) in their study also revealed that the maximum temperature had significant positive correlation with two spotted spider mite population dynamics where as rainfall had a significant negative correlation with two spotted spider mite population. Most of the studies showed that temperature has positive and rainfall a negative relation with mite population in the majority of the conditions (Meena et al., 2013;Dhar et al., 2000). The present findings concord with the findings of Chavan et al., (2003) who reported a significant and positive correlation of mites population with temperature and a negative correlation with rainfall. Considering multiple factors influencing population of thrips as dependent variable and weather conditions as independent variable, linear regression analysis was carried out and presented in Table 2. The linear regression model based on weather conditions as independent variable and mites population fluctuation as dependent variable, which explained 41 to 59% of the variability in the mites population fluctuation on the studied varieties. When it was combined on all the varieties, it reached to 62% explanation of mite population variability. So, it is suggested that increase in population density of mites were associated with high temperature and low humidity and temperature appeared to be the regulatory factor for population buildup of mite pest.

CONCLUSION
It can be concluded that weather factors have regulatory roles for thrips and mites population build up in summer season grown cowpea in EPH region of India. These data also suggest that sowing dates can be adjusted in order to save cowpea crop from severe attack of insect pests reported in this study.

ACKNOWLEDGEMENT
Authors would like to thank Director of ICAR