Relationship Between Weather Factors and Sucking Pests of Okra in Mizoram

The population dynamics of Aphis gossypii (Glover), Dysdercus koenigii Fabr. and Amrasca biguttula biguttula (Ishida) with respect to meteorological parameters on okra was conducted at the experimental farm, ICAR Research Complex for NEH Region, Mizoram Centre, Kolasib, Mizoram. The results revealed that a decrease in minimum temperature and minimum relative humidity and maximum relative humidity and increase in maximum temperature by 30 to 40% favour the population buildup of A. gossypii . Whereas an increase in temperature coupled with dry weather and decreased relative humidity favoured the build-up of A. biguttula biguttula . Peak population of D. koenigii was recorded as soon as maximum and minimum temperature was below 24 and 32 0 C, respectively and minimum relative humidity and maximum relative humidity crossed 58 and 81%, respectively.

Okra, Abelmoschus esculentus (L.) Moench is an important vegetable crop grown throughout the year.It is a native crop of Africa, South East Asia and North Australia to the Pacific (Boswell and Reed, 1962).It is an annual crop of the old tropics and widely cultivated in the tropical and subtropical countries.Okra is a very common and widely consumed vegetable of North Eastern Hill Region of India.The crop is attacked by many insect pests (Ambegaonkar and Bilapate, 1984) and pest problem is the main limiting factor in production, causing more than 40% yield loss.
Climate conditions largely influence the pest number and activity as well as several parasitoids and predators either directly or indirectly (Chaudhari et al., 1999 andArif et al., 2006).For developing weatherbased pest forewarning system, information regarding population dynamics in relation to prevalent meteorological parameters (temperature, relative humidity, rainfall, etc.) is needed.Moreover, these meteorological parameters also influence the growth and development of crop.Therefore, a thorough understanding of interaction between crop growth stage and meteorological parameters/pest dynamics is pre-requisite for weather-based pest forecasting model.The present study was undertaken to develop a weather-based forewarning thumb rule model for sucking pests attacking okra.

Materials and Methods
The field experiments were conducted during rainy season of the year 2009 at the experimental farm, ICAR Research Complex for NEH Region, Mizoram Centre, Kolasib, Mizoram.Six okra cultivars namely, Crystel Seed, Green Challenger, Julie, Nisha, Parbhani Kranti and OH-597 were sown on 15 th April, 2009.The net plot size was 4x5 m with a spacing of 30x60 cm plant to plant and row to row, respectively in Randomized Block Design with four replications.All the recommended agricultural practices were followed while raising the crop.No active plant protection measure was taken throughout the crop season.The population count of nymphs and adults of A. gossypii, A. biguttula biguttula, and D. koenigii were made from 3 leaves in bottom, middle and upper canopy of 5 plants per plot selected at random at weekly intervals starting from second week of May to first week of August.The weather data with respect to maximum and minimum temperature, maximum and minimum relative humidity and rainfall were also recorded for the corresponding standard meteorological week.
Simple correlations were worked out between the number of sucking pests and meteorological parameters and sucking pests' population; multiple regressions of sucking pests with meteorological parameters were worked out using IRRISTAT software for predicting sucking pests of okra using their weekly mean incidence.

Correlation between meteorological parameters and sucking pests population
Aphis gossypii: The data on A. gossypii activity are furnished in  2) was negative with maximum relative humidity (r = -0.014),minimum relative humidity (r = -0.050)and rainfall (r = -0.276)but was positive with maximum temperature (r = 0.338) and minimum temperature (r = 0.404).
The above results are in conformity with the findings of Anitha and Nandihalli (2008).In order to study the combined effect, all the factors considered in the multiple linear regression models irrespective of their degree of contribution.All the abiotic factors jointly had the non-significant impact (F = <1) on A. gossypii.The coefficient of determination (R 2 ) was found to be 0.41 (Table 3).Y= -307.110+ 2.542 (X1) + 5.558 (X2) + 1.880 (X3) -0.229 (X4) -0.872 (X5) with all abiotic factors considered as full regression model.Y : number of sucking pests per 5 plants, minimum temperature (X1), maximum temperature (X2), minimum relative humidity (X3), maximum relative humidity (X4) and Rainfall (X5)

Table 3. Multiple regressions of sucking pests with weather parameters
Amrasca biguttula biguttula: The data on A. biguttula biguttula activity in okra is given in Table 1 for crop season 2009.The activity of A. biguttula biguttula commenced from 22 nd SMW.It is clear from Table 1 that with decreases in temperature favour the population buildup of A. biguttula biguttula.The correlation of A. biguttula biguttula population (Table 2) was positive with minimum temperature (r = 0.299), maximum temperature (r = 0.144) and maximum relative humidity (r = 0.082) but was negative with minimum relative humidity (r = -0.130)and rainfall (r = -0.418).Similar results were reported by Aheer et al. (2006) which indicates positive correlation between population of A. biguttula biguttula and temperature.
All the abiotic factors jointly had the non-significant impact (F = <1) on D. koenigii.The coefficient of determination (R 2 ) was found to be 0.20 (

Table 1 . Seasonal incidence of sucking pests on okra and weather parameters
Table 1 for crop season 2009.The activity of A. gossypii started from 23 rd standard SMW: standard meteorological week, Y : number of sucking pests per 5 plants meteorological week (SMW).The peak population was recorded during June and July.Satter Shah et al. (2009) reported that the peak population of A. gossypii was recorded during last week of July.It is clear from Fig 1 that with increase in temperature and decrease in relative humidity favours the population buildup of A. gossypii.The correlation of A. gossypii population (Table

Table 4 . Correlation between sucking pests of okra 135
Dysdercus koenigii: The data on D. koenigii population is given in Table 1 for crop season 2009.The population of D. koenigii appeared from 29 th SMW.It is clear from Table 1 that with decrease in relative humidity and temperature favours the population buildup of D. koenigii.The correlation of D. koenigii population (Table