Introduction

Pigeonpea (Cajanus cajan (L) Millspaugh) is a tropical grain legume mainly grown in India and ranks second in area and production and contribute about 90% in the world’s pulse production. In India pigeonpea was grown in 3.96 mha with a production of 2.56 mt and productivity of 646 kg  ha^-1, whereas, in Andhra Pradesh, India the area, production, productivity of pigeonpea was 0.22 mha, 0.13 mt and 600 kg ha^-1, respectively during 2015-16 (Anonymous, 2017). Though the area under redgram is increasing, the yields have remained stagnant (500-700 kg ha^-1) for the past 3-4 decades, largely due to insect pest damage (Sharma and Pampapathy, 2004). More than 300 species of insect pests were reported infesting the crop (Lal and Singh, 1998) of which pod borers viz., gram pod borer, Helicoverpa armigera and spotted pod borer, Maruca vitrata are very important.  Both the pests prefer to feed on flowers and fruiting bodies, thereby causing heavy yield loss. The yield loss due to H. armigera and M. vitrata was up to more than 60 and 84% (Vishakantaiah and Jagadeesh Babu, 1980), respectively. The annual monitory loss due to H. armigera and M. vitrata was estimated globally as US $ 400 million (ICRISAT, 2007) and US $ 30 million (Saxena et al., 2002), respectively. Spotted pod borer, is basically a hidden pest and completes its larval period inside the web formed by rolling and tying together leaves, flowers, buds and pods. This typical concealed feeding protects the larvae from natural enemies, human interventions and other adverse factors including insecticides (Sharma, 1998). Spodoptera will not cause any economic loss to farmers as it feeds mainly on leaves and the plant has the capacity to compensate the vegetative loss. Management of pod borers relies heavily on insecticides (Sreekanth, 2018; Sai et al., 2018; Mahalakshmi et al., 2016; Sunita Devi et al., 2014). However, indiscriminate use of insecticides has resulted in the development of resistance and resurgence. In order to optimize the application of insecticides, studies on  monitoring and influence of  various weather parameters on the population build up and seasonal incidence of the pest are very much required for planning an effective pest management strategy that will help farmers benefit financially without the risk of long term problems including resurgence (Rao et al., 2013; Imosanen and Singh, 2005; Sivaramakrishna et al., 2004; Kumar et al., 2003; Srivastava and Vaish, 2000). Hence, an attempt was made to monitor the pod borer population along with studies on influence of weather parameters on the population buildup.

Materials and Methods

2.1.  Experimental details

The population buildup and seasonal abundance of pod borers on pigeonpea (cv. ICPL 85063) was ascertained by raising the crop in 1000 m² area during Kharif, 2015-16 at Regional Agricultural Research Station, Lam farm, Guntur, Andhra Pradesh, India by following all the package of practices recommended for the crop in the region and season except the insecticidal contamination. In order to monitor the population of Helicoverpa and Spodoptera, pheromone traps @ 10 ha ^-1 were erected 60 cm above the crop canopy.  The male moth catches were recorded once in each standard meteorological week (SMW) starting from flower bud initiation to pod maturity stage of the crop and expressed as number of months trap^-1 week^-1. The lures were changed at 30 days interval. The larval population of Helicoverpa, Spodoptera and Maruca was also recorded at weekly intervals on 10 randomly selected tagged plants from three locations in the plot and expressed as number of larvae plant^-1.  Abiotic factors such as temperature (maximum, minimum, mean), relative humidity (morning and evening), sunshine hours and rainfall were also recorded from meteorological observatory, RARS, Lam.  The meteorological data thus collected was subjected to simple correlation analysis with larval population and male moth catches to know the influence of abiotic factors on the occurrence of pod borers(Gomez and Gomez, 1984). 

Results and Discussion

The results during 2015-16 showed that male moth catches of H. armigera were observed (2.0 moths trap^-1 week^-1) in 44^th standard meteorological week (SMW) (Oct. 29-Nov.4) and reached peak (8.0 moths trap^-1 week^-1) during 47^th SMW (Nov. 19–25). Whereas, the male moth catches of S. litura were observed in 43^rd SMW (Oct. 22-28) with a peak twice at 45^th SMW (Nov. 5-11) and 50^th SMW (Dec. 10-16), which recorded 99.3 and 78.0 moths trap^-1 week^-1, respectively. The larval population of H. armigera was observed in 44^th SMW and reached peak (5.2 larvae plant^-1 week^-1) during 48^th SMW (Nov. 26-Dec. 2), which coincides with peak flowering stage of the crop. Similarly, the larval population of M. vitrata was observed in 43^rd SMW and reached peak (15.6 larvae^-1 plant^-1 week) during 48^th SMW (Nov. 26-Dec. 2), which coincides with peak flowering stage of the crop (Table 1 and Figure 1, 2 and 3).

Table 1: Monitoring of Helicoverpa, Maruca and Spodoptera on pigeonpea

Figure 1: Incidence of H. armigera on pigeonpea during kharif, 2015 at RARS, Lam, Guntur

Figure 2: Incidence of M. vitrata on pigeonpea during kharif, 2015 at RARS, Lam, Guntur

Figure 3: Incidence of Spodoptera litura on pigeonpea at RARS, Lam, Guntur

The results were in conformity with the findings of Sreekanth and Ramana (2017), who found that peak larval population of M. vitrata was observed during 50^th SMW. Similarly, Sreekanth et al. (2016) reported that larval and moth catches of H. armigera were more during 48^th SMW during 2012-13. Sreekanth and Ratnam (2016) in their studies during 2013-14 found that male moth catches of H. armigera were more during 48^th and 2^nd SMW which coincides with 50% flowering and pod development stage, respectively. Similarly, two larval peaks were observed during 51^st and 5^th SMW which coincides with full bloom to pod initiation stage. Sreekanth et al. (2015) during 2013-14 found that peak larval population of M. vitrata was recorded during 51^st SMW. Srivastava and Vaish (2000), who observed that peak male moth catches of H. armigera were noticed from 43 to 45^th SMW in pigeonpea at Sriganganagar (Rajasthan). The results obtained were also  in concurrence with the reports of  Sharma and Franzamann (2000), who found that incidence of M. vitrata on pigeonpea was bimodal where early infestation starts from September reaching its first peak during middle October and second peak during December. The incidence of M. vitrata increased with the initiation of flowering, having the highest population at full podding stage of pigeonpea (Imosanen and Singh, 2005).

Highly significant correlation was obtained between pheromone trap catches of H. armigera and rainfall, rainy days, sunshine and wind speed with correlation coefficient (r) being 0.576, 0.649, -0.528 and 0.685, respectively. Similarly, moderately significant correlation was observed between pheromone trap catches of S. litura and wind speed with correlation coefficient (r) being 0.475.Further, highly significant correlation was observed between RH I and RH II with larval population of H. armigera with correlation coefficients (r) being 0.503 and 0.777, respectively. Similarly, highly significant correlation was observed between RH II and larval population of M. vitrata with correlation coefficient (r) being 0.919 (Table 2).  

Table 2: Correlation coefficient between weather parameters and pest incidence

The present findings were in conformity with findings of Sreekanth et al. (2015) and Kumar et al. (2003) who reported that maximum, minimum and mean temperatures and relative humidity recorded at morning, evening and mean were found to be highly correlated with that of larval population of M. testulalis. However, Arulmozhi (1990), Lakshmi (2001) and Sivaramakrishna et al. (2004) reported that highly significant correlation was obtained between M. vitrata and minimum temperature and wind speed. The larval population of M. vitrata was significantly influenced by average temperature and relative humidity at Hisar (Naresh and Singh, 1984). Akhauri (1992) reported that population buildup of M. vitrata varied remarkably in different parts of the country probably due to differences in agro climatic conditions and crop types. Similarly, Vishwa Dhar et al. (2008) reported that minimum and maximum temperature and relative humidity greatly influence the moth population of H. armigera at Kanpur. Yadav et al. (2009) also found that relatively cooler pre-monsoon period, lower amount of monsoon rainfall, rain free post monsoon period with high evening relative humidity have been found to be congenial for build-up of higher population and subsequently resulting higher moth catches of H. armigera in pheromone traps during rainy season on pigeonpea. The population buildup of H. armigera varied remarkably in different parts of the country probably due to difference in agro climatic conditions and crop types (Akhauri, 1992). The rain free weeks after rainy weeks were found to be congenial for population buildup. Similarly, during post monsoon period, host plants including cotton and pigeonpea were available in abundance resulting build up of population.  However, the gaps in knowledge remain to be filled by concentrating on migration, survival and carryover of this dreaded pest in different agro-eco-regions of the country. Rao et al. (2013) reported that morning and evening relative humidity showed significant positive correlation and minimum temperature showed significant negative correlation with the larval population of M. vitrata in rice fallow blackgram.  However, the findings were in contrary to the observations of Kumar et al. (2003), who reported that larval population of H. armigera remained unaffected with weather parameters. Similarly, Sreekanth and Ratnam (2016) in  their studies during 2013-14 found that a strong negative correlation was observed between H. armigera larvae and mean temperature.

Conclusion

There was only single peak without any multiple peaks or overlapping broods of H. armigera and M. vitrata, whereas S. litura has two peaks. Highly significant correlation was observed between pheromone trap catch of H. armigera and rainfall, rainy days, sunshine and wind speed. Similarly, highly significant correlation was observed between RH I and RH II with larval population of H. armigera and M. vitrata. Thus, farmers can optimize the application of insecticides.

Acknowledgment

The authors are highly thankful to Associate Director of Research, RARS, Lam, Guntur and Project Coordinator, All India Coordinated Research Project (AICRP) on Pigeonpea, IIPR, Kanpur for providing necessary facilities to carry out the work.

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