Study of Population Dynamics of Insect-pests in Different Growing Environments and their Relationship with Microclimate of Pigeonpea Cultivars

Pigeonpea (Cajanus cajan L. Milli sp.) commonly known as red gram, is a low input, rainfed crop with characteristics that provide economic returns from each and every part of the plant. The Pigeon pea is grown for grains, green manuring, fodder and forage in all cropping systems i.e. as sole crop, intercrop, mixed crop and sequential crop mostly in tropical and sub tropical countries all over the world. Some of the countries with notable pigeonpea production are India, Nepal, Myanmar, Malawi and Uganda along with some other countries in eastern Africa and the Dominican Republic in the Americas (Ahlawat and Shivakumar, 2006). India accounts for 90 per cent of world’s pigeonpea growing area and 85 per cent of world’s production (Dahariya et al., 2018). In world pigeonpea is cultivated on 6.93 mha with annual production of 6.67 MT. In India pigeonpea is cultivated on 5.38 m ha with a total production of 4.87 MT (FAO, 2017). In Haryana, it is cultivated on 15.1 thousand hectares with production and productivity of 16.4 thousand tonnes and 1086 kg/ha respectively (Anonymous, 2014).
       
It is grown as Kharif crop sown in month of June and July and is attacked by a large number of insects at all stages of growth i.e. seedling to harvest stage and as per a conservative estimate, losses due to these insect pests may vary from 27 per cent to even 100 per cent. Among the various constraints for low productivity in pigeonpea crop, the infestation of insect pests is the main contributor. Insect-pests are the major biotic constraints limiting the productivity of this crop. More than 250 insect species have been recorded feeding on pigeonpea and among these only few cause significant and consistent damage (Keval et al., 2017). Information on pest complex in particular agro-climatic condition is a prerequisite, which helps in designing a successful pest management strategy. However, no systematic efforts have been made to observe the diversity of insect pests and their seasonal occurrence with relation to crop phenological stages. Pest menace on pigeonpea (Cajanus cajan) has assumed serious proportions, even to the extent of suicidal deaths of farmers. Information related to diversity and seasonal occurrence of pests on this crop is very much important and is of great significance in effective pest management practices. Among the several insect pests attacking different parts of pigeonpea plants, pod borers are most injurious; they attack both the flowers as well as pods and cause major losses, often threatening the cultivation of this crop (Prasana and Bhalani, 1994 and Mittal and Ujagir, 2005). In pigeonpea, insect pests are one of the major hamper infesting the crop from seedling to harvest of crop and even the storage are highly vulnerable. Damage inflicted by Helicoverpa armigera larvae, is bound to flowers, seeds and pods and a single larva can destroy 30-40 pods, whereas Maruca vitrata larvae destroy the plants by causing bore into the pods and webbing to flowers (Sreekanth et al., 2015). M. obtusa and Clavigralla gibbosa attack the crop from pod filling to pod maturity stage. Reddy et al., (2001) during their research work observed the impact of various abiotic factors on population built of pigeonpea pest viz., Empoasca keri, Megalurothrips usitatus, Mylabris pustulata, H. armigera and M. testulalis. They found that biotic and abiotic factor plays crucial role in the population build up of insect pests and predators. Keeping all these factors in mind, the present experiment was conducted during 2017 to study the population dynamics of major insect-pests in different growing environments and their relationship with microclimate of pigeonpea cultivars.

This experiment was conducted in Kharif season 2017 at research farm, Department of Agricultural Meteorology, CCSHAU Hisar, Haryana. The field area was adjacent to Agrometeorological observatory at 29°10' N latitude, 75° 46' E longitude and altitude of 215.2 m. The experiment was laid out in split-plot design, the main-plots treatments consisted of three dates of sowing i.e. D₁ (First fortnight of May), D₂ (First fortnight of June), D₃ (Second fortnight of June). The sub-plots treatment consisted of three cultivars Manak, Paras and UPAS-120. Population dynamics of major insect-pests infesting pigeonpea viz., M. vitrata and H. armigera were observed on pigeonpea crop. Five randomly selected plants were tagged from each variety per replication. Insect-pests population was recorded from the tagged plants at weekly interval starting from 38^th standard meteorological week (SMW) till harvesting of the crop. Ground sheet method was used to record the population of larvae of M. vitrata and H. armigera. A cloth sheet of size 60 cm × 60 cm was laid near plant stem. The plant was tilted toward the sheet and jerked gently to make the insects fall down on the sheet. Larvae of M. vitrata and H. armigera falling on cloth sheet were counted. Web formed by M. vitrata larvae were visually counted and then removed to count larvae feeding inside the webs. The seasonal incidence of test insect-pests was correlated with microclimatic parameters viz., temperature and relative humidity. Regression analysis was carried out to develop the relationship of insect-pests population of pigeonpea with microclimatic parameters.

Gram pod borer (Helicoverpa armigera)
 
The overall range of H. armigera larval population among different date of sowings was found 0.09 to 0.61 larvae per plant as shown in Fig 1. H. armigera infestation started from 38^th SMW (3^rd week of September) in D₁ and D₂ sown crops while in D₃ sown crop, it started in 40^th SMW (1^st week of October), which gradually increased till 44^th - 45^th SMW and reached its peak in 44^th SMW (1^st week of November) in D₁ sown crop while in D₂ and D₃ sown crops, reached its peak in 45^th SMW (2^nd week of November) and after this started declined up to 48^th SMW (last week of November). Mean larval population of H. armigera, among different date of sowings, was found highest (0.29 larvae/plant) in D₁ sown crop followed by D₂ (0.24 larvae/plant) and D₃ (0.19 larvae/plant) sown crops. These results are in accordance with Verma (2006) and Surana (2001) who reported that infestation of H. armigera started from flowering to pod maturity stages.
 

       
The overall range of H. armigera larval population among different varieties was found 0.05 to 0.58 larvae per plant as shown in Fig 2. Similar results were reported by earlier workers i.e. 0.07 to 0.48 larvae per plant (Bajya et al., 2010) and less than 1.10 larvae /plant (Sujithra and Chander, 2014).  H. armigera infestation started from 38^th SMW in all the varieties, which gradually increased till 44^th - 45^th SMW and reached its peak in 45^th SMW and then started decline up to 48^th SMW. On the basis of varietal mean of number of larvae of H. armigera, among different varieties, highest larval population (0.29 larvae/plant) were found on variety Manak followed by Paras (0.24 larvae/plant) and UPAS-120 (0.18 larvae/plant).
 

 
Spotted pod borer (Maruca vitrata)
 
The larval population of M. vitrata started from 38^th SMW (3^rd week of September), 39^th SMW (4^th week of September) and 40^th SMW (1^st week of October) in D₁, D₂ and D₃ sown crops, respectively (Fig 3). Larval population was maximum during 41^st SMW (2^nd week of October) and after this the population was declined up to 45^th SMW (2^nd week of November) in all date of sowings. After 45^th SMW (2^nd week of November), no larval population of M. vitrata was found. During the entire period larval population of M. vitrata ranged from 0.13 to 0.84 larvae per plant. Mean larval population of M. vitrata, was highest in D₁ (0.26 larvae/plant) sown crop followed by D₂ (0.21 larvae/plant) and D₃ (0.17 larvae/plant) sown crops. These findings are in accordance with the finding of Sujithra and Chander (2014) who reported that M. vitrata larval population started appearing from 36^th SMW to 44^th SMW and reached its peak during 39^th SMW. The larval population during the entire period ranged from 0.02 to 0.82 larvae per plant. Similarly, Verma (2006) and Surana (2001) reported 0.50 insect per plant during entire crop duration.
 

 
The larval population of M. vitrata started from 38^th SMW (3^rd week of September) in all varieties (Fig 4). Larval population was found maximum during 41^st SMW (2^nd week of October) and after this, the population was declined up to 45^th SMW (2^nd week of November) in all the varieties. After 45^th SMW (2^nd week of November) no larval population of M. vitrata was recorded. During the entire period, larval population of M. vitrata ranged from 0.02 to 0.82 larvae per plant. On the basis of varietal mean of number of larvae of M. vitrata, highest larval population were recorded on variety Manak (0.24 larvae/plant) followed by Paras (0.22 larvae/plant) and UPAS-120 (0.18 larvae/plant).
       

 
Web formed by M. vitrata larvae were visually counted from selected plants (Fig 5).  The formation of webs in D₁ sown crop started from 38^th SMW (3^rd week of September) while in D₂ and D₃ sown crops it was started in 39^th SMW (4^th week of September) and continued till 45^th SMW (2^nd week of November). After 45^th SMW (2^nd week of November) no web formation by M. vitrata was recorded. Maximum number of webs per plant was recorded in D₁ (3.36 webs/plant) sown crop followed by D₂ (3.18 webs/plant) and D₃ (3.09 webs/plant) sown crops during 41^st SMW (2^nd week of October). Among different date of sowings, highest number of webs per plant was found in D₁ (1.06 webs/plant) sown crop followed by D₂ (1.00 webs/plant) and D₃ (0.84 webs/plant) sown crops.
 

       
The formation of webs in all the varieties started from 38^th SMW (3^rd week of September) and continued till 45^th SMW (2^nd week of November) (Fig 6). After 45^th SMW (2^nd week of November) no web formation by M. vitrata was recorded. Maximum number of webs per plant was recorded on variety Manak (3.31 webs/plant) followed by UPAS-120 (3.11 webs/plant) and Paras (2.98 webs/plant) during 41^st SMW (2^nd week of October). Among different varieties, highest number of webs per plant was found on Manak (1.04 webs/plant) followed by Paras (0.95 webs/plant) and UPAS-120 (0.89 webs/plant).
 

 
Insect-pests relationship with crop microclimate
 
Correlation of major insect-pests population with microclimate of pigeonpea cultivars
 
Larval population of H. armigera showed non-significant positive correlation with temperature while significant positive correlation with relative humidity in all three varieties (Table 1). M. vitrata webs showed significant positive correlation with both temperature and relative humidity while larval population of M. vitrata was non-significantly positive correlated with both temperature and relative humidity in all the three varieties. M. vitrata webs showed significant positive correlation with both temperature and relative humidity while larval population of M. vitrata showed non-significant positive correlation with both temperature and relative humidity in all the three varieties. These findings are in accordance with the findings of Sahoo and Behera (2001).
 

 
Regression of major insect-pests population with crop microclimate
 
Linear regression
 
A linear direct relationship was found between temperature and larval population of H. armigera and M. vitrata webs on Manak variety explaining the variability up to 5 and 40 per cent, respectively. Relative humidity showed positive linear relationship with larval population of H. armigera and M. vitrata webs explaining the variability up to 50 and 36 per cent, respectively (Fig 7).
 

       
A linear direct relationship was found between temperature and larval population of H. armigera and M. vitrata webs on Paras variety explaining the variability up to 9 and 42 per cent, respectively. Relative humidity showed positive linear relationship with larval population of H. armigera and M. vitrata webs explaining the variability up to 41 and 34 per cent, respectively (Fig 8).
       

 
A linear direct relationship was found between temperature and larval population of H. armigera and M. vitrata webs on UPAS-120 variety explaining the variability up to 12 and 40 per cent, respectively. Relative humidity showed positive linear relationship with larval population of H. armigera and M. vitrata webs explaining the variability up to 44 and 39 per cent, respectively (Fig 9).
 

This experiment was conducted to study the population dynamics of major insect-pests in different growing environments and their relationship with microclimate of pigeonpea cultivars. According to the results obtained, H. armigera infestation started from 38^th SMW (3^rd week of September) in all the varieties and D₁ and D₂ sown crop while in D₃ sown crop it started in 40^th SMW (1^st week of October). On the other hand the larval population of M. vitrata started from 38^th SMW, 39^th SMW and 40^th SMW in all varieties and D₁, D₂ and D₃ sown crops, respectively where as the formation of webs in all the varieties and D₁ sown crop started from 38^th SMW while in D₂ and D₃ sown crops started from 39^th SMW and continued till 45^th SMW. Mean larval population of H. armigera, was found highest in D₁ sown crop followed by D₂ and D₃ sown crops. Mean larval population was maximum on variety Manak followed by Paras and UPAS-120. Same trend was followed in mean larval population of M. vitrata and number of webs per plant.
       
Larval population of H. armigera showed non-significant positive correlation with temperature while significant positive correlation with relative humidity in all three varieties. M. vitrata webs showed significant positive correlation with both temperature and relative humidity while larval population of M. vitrata showed non-significant positive correlation with both temperature and relative humidity in all the three varieties. Larval population of H. armigera and M. vitrata webs showed a linear direct relationship with both temperature and relative humidity in all the three varieties.

None.