Influence of Weather Parameters on the Seasonal Abundance of Legume Pod Borer Maruca vitrata (Crambidae: Lepidoptera) on Dolichos Bean (Lablab purpureus var. typicus)

Dolichos bean or Lablab (Lablab purpureus var. typicus) is an important and the most diverse domesticated vegetable legume grown in many parts of Tamil Nadu and Andhra Pradesh, Karnataka, Madhya Pradesh and Maharashtra. It is also used as livestock feed, green manure, ornamental or medicinal herb (Pengelly and Maass, 2001; Maass, 2016). It is rich in protein, minerals and vitamins and is a significant source of protein for the South Indian diet. Insect pests, especially lepidopterans pod borers, account for the production constraint by causing significant yield losses globally in the Dolichos bean (Rouf and Sardar, 2011). Amid this, the genetically complex M. vitrata (Lepidoptera: Crambidae) is considered as one of the severe legume pests having high damage potential and a wider host range (Jackai 1995; Sharma et al., 1999; Margam et al., 2011; Periasamy et al., 2015), with a higher preference to Dolichos bean over the other legumes (Rekha and Mallapur, 2007; Mallikarjuna et al., 2012).
       
Further, it is a pest of regional significance in Tamil Nadu and Karnataka (Anonymous, 2014). The larva causes economic damage to the Dolichos bean by feeding on the flower buds, peduncle, stem, tender and matured pod (Jackai, 1990). Though it is a severe pest, there was a dearth of research related to the dynamics of the key pest, M. vitrata. The information on the seasonal abundance and distribution of the insect pest concerning the weather parameters forms the essential tool for forecasting and developing integrated pest management strategies. Hence, the present study was framed to understand the influence of weather parameters on the abundance and population build-up of M. vitrata on the Dolichos bean.

Seasonal abundance of M. vitrata on Dolichos bean was recorded on three consecutive cropping seasons during 2019 (May-December) at Narasipuram village of Coimbatore district (11.0152°N, 76.9326°E). M. vitrata larval population and number of active larval webbings were used to find the seasonal abundance. The larval population were recorded from ten samples representing flowers and pods respectively on five locations in the field at weekly intervals starting from 40 (5 to 6 weeks after sowing) to 90 (13 weeks after sowing) days after sowing and per cent larval abundance was estimated. The number of active larval webbings from five racemes per plant was made on ten randomly selected plants at weekly intervals starting from 40 to 90 days after sowing. The data on the larval population and the number of active webbings recorded throughout the cropping season were correlated with weather parameters viz., maximum temperature (T_max), minimum temperature (T_min), relative humidity (RH) and rainfall data obtained from the Agro Climate Research Centre, Tamil Nadu Agricultural University, Coimbatore. The multiple linear regression analysis was carried out to assess the degree and level of influence of the various weather parameters on the larval abundance and the number of webbings of M. vitrata. The correlation and multiple linear regression analyses were done using SPSS ver.17.0 software to work out the correlation coefficient and regression, with the formula suggested by Regupathy and Dhamu (1990).
 

Even though there was ample information available in India on the M. vitrata seasonal abundance, infestation and impact of weather parameters on major pulses, pigeon pea (Sharma et al., 1999; Sreekanth et al., 2015; Sreekanth et al., 2019; Taggar et al., 2019; Sujayanand et al., 2021) and black gram (Swamy and Devaki, 2015; Kapoor, 2019), the data is inadequate concerning Dolichos bean (Lablab purpureus var. typicus). The results on the seasonal larval abundance of M. vitrata in the Dolichos bean have been depicted in Table 1. The results have shown that the larval abundance of M. vitrata was noticed from six weeks after sowing (19^th, 32^nd and 45^th - SMW) during the entire study period and it remained to persist in varying populations up to thirteen weeks after sowing (26^th, 39^th and 52^nd - SMW).
 

       
During Summer 2019, the larval abundance in flowers and pods (Fig.1 and 2) ranged from 1.80-11.80 and 1.20-8.80 per 50 samples, respectively, whereas the number of active webbings (Fig.3) ranged from 1.27 to 9.28. The initial larval abundance in flowers (2.60 larvae per 50 samples), pods (1.40 larvae per 50 samples) and webbings (1.27 active webbings per 10 plants) were minimum. The larval abundance reached its peak in flowers (11.80 larvae per 50 samples), pods (8.80 larvae per 50 samples) and webbings (9.28 active webbings per 10 plants) during 24^th SMW, which coincided with the peak bud initiation, flowering and pod formation stage of the crop. The results on the larval abundance in the Dolichos bean are similar to the previous reports by Sampathkumar and Durairaj (2015), who observed that the relative abundance of M. vitrata was the maximum at the time of flowering. Subsequently, the larval abundance started declining from the 26^th SMW (13 WAS) since the crop was approaching physiological maturity.
 

 

 

       
The larval population during Kharif 2019 was witnessed one week earlier than the Summer season during the 31^st SMW with 1.40 and 0.00 larvae per 50 samples in flower buds and pods, respectively. There was a fluctuation in the larval incidence from 31^st to 37^th SMW, whereas the Summer season had a gradual spike followed by the population decline at the final stage of the crop. The highest peak of the pest was attained during 35^th SMW and 38^th SMW with 13.20 and 11.40 larvae per 50 samples in flower buds and pods, respectively. The number of active webbings was higher during the 36^th SMW, with 15.74 webbings in five racemes per ten plants.
       
During the Rabi 2019, the larval abundance in flowers and pods (Fig.1 and 2) ranged from 3.20-14.60 and 3.20-13.40 per 50 samples, respectively, whereas the number of active larval webbings (Fig.3) ranged from 4.92 to 18.76. During Rabi 2019, the larval abundance in flowers (9.40 larvae per 50 samples), pods (3.20 larvae per 50 samples) and webbings (5.23 active webbings per 10 plants) were found to be higher even in the initial stage of flowering (6 WAS) when compared to other two seasons. The larval abundance reached its peak in flowers (14.60 larvae per 50 samples), pods (13.40 larvae per 50 samples) and webbings (18.76 active webbings per 10 plants) during 48^th, 50^th and 49^th SMW, respectively, which overlapped with the peak bud initiation, flowering and pod formation stage of the crop. The present results are corroborating with the results of Mallikarjuna et al., (2012), who reported that the population density of M. vitrata in Lablab purpureus (L.) Sweet reached its peak from the first week of December (48^th SMW). Similarly, in pigeon pea [Cajanus cajan (L.)], Sreekanth et al., (2015) and Sreekanth et al., (2019) reported that the larval population per plant increased from 47^th SMW and reached its peak during the end of December (12.6 larvae/plant at 51^st SMW). Like the Summer season, the larval abundance gradually declined and reached 3.20 larvae in flowers and 4.60 larvae in pods.
       
Irrespective of the cropping seasons, the larval abundance in pods increased gradually from 12^th WAS and 13^th WAS compared to flowers. The highest percentage of larval abundance was observed in flowers and pods (Fig.4) with 20.85% and 15.90% during Rabi season; similarly, the lowest rate of larval abundance was recorded during Summer 2019 with 9.75% 7.75% in flowers and pods, respectively. The present results are in line with the findings of Ali et al., (2004), who reported that the M. vitrata larva preferred flowers of Country bean. Also, our findings are in accordance with the reports of Sharma et al., (1999) and Jayasinghe et al., (2015) in yard long bean, who reported that the M. vitrata larva prefers flower buds rather than pods resulted in higher abundance and infestation in flower buds. During the study period, the fluctuation in M. vitrata larval population in Dolichos bean was based on the availability of other host plants like pigeon pea, cowpea, black gram and green gram in the vicinity. However, the larval population tends to remain in varying proportions year-round.
 

       
The data relating to the correlation studies between M. vitrata larval abundance in flowers and pods, mean number of active webbings and different weather parameters are presented in Table 2. The results revealed that maximum temperature significantly influenced the larval abundance in flowers and pods among the weather parameters. A significant negative relationship was observed between maximum temperature and the larval abundance in flowers (r= -0.570) and pods (r= -0.523). The results of Sreekanth et al., (2015) are reliable with our findings, who observed that the minimum and mean temperature had a significant negative correlation (r= -0.759 and -0.815, respectively) with the larval population. Most of the previous reports in other host plants depicted a contrasting result that, M. vitrata larval population had a positive correlation with maximum temperature (Kuldeep and Ram (2007) in pigeon pea, Sravani et al., (2015) in green gram, Dumala (2015) in cowpea, Swamy and Devaki (2015) in black gram.
 

       
Similarly, a significant negative correlation was shown between maximum temperature and number of active webbings (r= -0.477). At the same time, the number of active webbings and larval abundances in flowers and pods showed a non-significant relationship with the minimum temperature, relative humidity and rainfall. These findings are in concurrence with the report of Taggar et al., (2019), who documented a similar non-significant correlation between the larval population and rainfall. In contrast, Dumala (2015) reported the significant influence of the rainfall (r= -0.226) on the M. vitrata larval population in Green gram.
       
The correlation analysis showed that an increase in maximum temperature by 1^oC decreased M. vitrata larval population in flowers and pods by 0.88 and 0.69 units (Y= -0.889X + 35.32 and Y= -0.069X + 37.29), respectively. In the same way, the larval population in pigeon pea tends to reduce by 1.25 units when there was an increase in temperature by 1^oC (Taggar et al., 2019). Correspondingly, with an increase in minimum temperature by 1^oC, there was a reduction in M. vitrata larval population in flowers and pods by 1.27 and 1.19 (Y= -1.274X + 37.29 and Y= -1.198X + 33.81), respectively. However, for every 1 mm increase in rainfall, the larval population in flowers and pods and the number of active webbings decrease by 0.07, 0.17 and 0.12 units (Y= -0.072X + 7.78, Y= -0.173X + 6.39 and Y= -0.127X + 7.83), respectively.
       
The multiple linear regression analysis (Table 3) was carried out with M. vitrata larval abundance in flowers, pods and the number of active webbings as dependent variables against the weather parameters. The results revealed that the coefficient of determination (R²) was significant with the weather parameters viz., maximum temperature, minimum temperature, relative humidity and rainfall contributed directly towards the larval abundance in flowers and pods to the extent of 39.20 and 44.30 per cent, respectively. Moreover, our findings are consistent with the previous reports of Sujayanand et al., (2021). They reported that the weather parameters influenced over M. vitrata larval population by 41% (R²= 0.41). The regression coefficient (B) values indicate that the maximum temperature significantly influences the larval abundance in flowers and the number of active webbings (B= -1.26 and -1.25, respectively).
 

       
Similarly, rainfall showed a significant influence on the larval abundance in pods (B= -0.26). The results are in contrast with the findings of Sujayanand et al., (2021). They conveyed that the correlation coefficient (B= -0.12) observed for relative humidity significantly influenced M. vitrata larval population. The fluctuation in the larval abundance in the present study was mainly due to the prevailing weather condition in a particular location and the availability of the most preferred host plant.
       
The present study revealed that the M. vitrata larval abundance in flowers, pods and the number of active webbings on Dolichos bean was highest during 48^th to 50^th SMW of 2019. Also, the larval abundance initiated from the peak flower setting and pod formation stage with the varying population during three consecutive cropping seasons of 2019. Concerning crop phenology, the larval population tends to migrate from flowers (8 to 10 WAS) to pods (after 10 WAS) primarily due to the source-sink relationship. These results insist on the importance of timely insecticide application for better management of M. vitrata during the early stage of infestation. The correlation and multiple regression analysis reveal the importance of weather parameters on larval abundance. Furthermore, the data on the seasonal larval abundance in Dolichos bean will help develop better integrated ecological engineering and eco-friendly management tactics for this devastating pest.

Forecasting and developing integrated pest control strategies require information on the seasonal abundance and distribution of insect pests in relation to weather parameters. As a result, the abundance of M. vitrata larvae in flowers, pods and the number of active webbings on Dolichos bean was highest during the 48th to 50th SMW of 2019. The necessity of timely insecticide application for improved management of M. vitrata during the early stages of infestation is emphasized by these findings.

The authors are highly thankful to the Head, Department of Agricultural Entomology, Tamil Nadu Agricultural University, for providing the research facilities required to complete this study.