Novel Insecticides for the Management of Pod Borer Complex in Pigeonpea Crop

Pigeonpea (Cajanus cajan L.) is the second most important grain legume crop after chickpea that occupies second largest area among the various pulse crops grown in India.  The crop yields are generally hampered by a variety of insect that result in avoidable yield losses extending up to the tune of 78 per cent in India (Lateef and Reed 1983). More than 250 insect pests are known to attack pigeonpea (Sharma et al., 2008).  Pod borer complex comprising the spotted pod borer, Maruca vitrata (Fabricius) and gram pod borer, Helicoverpa armigera (Hübner) caused 60 to 90 per cent loss in the grain yield in pigeonpea (Lal et al., 1992). Sahoo and Senapati (2001) reported that the pod borers together damaged 57.07, 54.09 and 40.08 per cent pods and 34.79, 30.90 and 20.20 per cent seeds incurring the yield losses of 28.07, 21.01 and 15.02 per cent in early, medium and late maturing cultivars of pigeonpea, respectively.
 
The ravages of lepidopteran pod borers during flowering and pod formation stage and pod fly during pod formation stage are the major bottleneck in attainment of desired productivity levels of pigeonpea (Wadaskar et al., 2013). Insect infestation on the reproductive parts leads to maximum reduction in grain yield. The main thrust of farmers has been towards application of insecticides for the management of insect pests because of convenience and easy availability of chemicals. Application of insecticides against the insect pests is still the widely followed means of pest control. Quite often, it forms the only solution to curb the outbreak of insects. Farmers, use chemical pesticides indiscriminately, which leads to increased cost of plant protection resulting in lower profitability. On these grounds, newer insecticides with novel mode of action are needed to be evaluated to find out an effective and economical insecticide for the management of pigeonpea pod borer complex. It is, therefore, imperative to test the efficacy of new insecticides and work out economically feasible insecticide having novel mode of action against the lepidopteron pod borer complex in pigeonpea crop.

The research trials were conducted at two locations in Punjab (Ludhiana and Gurdaspur) to evaluate the bioefficacy of new insecticides for the management of pod borer complex, M. vitrata and H. armigera for three consecutive years of kharif 2016 to 2018. The early maturing pigeonpea variety, PAU 881 was sown following the recommended agronomic practices (Anonymous 2018). The experiments were laid out in randomized block design with eight treatments (including untreated control) and each treatment was replicated thrice. Since the spotted pod borer appears at the flowering stage of the crop, the first foliar spray of test insecticides was given at the bud-initiation stage of the crop, followed by second application at pod formation stage (against H. armigera).  Observations were recorded on the number of Maruca webs per plant before and 7 days after spray (DAS), number of H. armigera larvae per plant before, 3 and 7 DAS pod damage per plant from three randomly tagged plants from each treatment. Assessment of pod damage inflicted by M. vitrata as well as for H. armigera was done at separately at maturity wherein random samples of 200 pods per plot per replication were collected to observe the number of damaged pods. The data were converted into per cent damaged pods and cumulative pod damage was calculated. The grain yield data were also recorded from each plot after harvest. The data were pooled and analyzed statistically using CPCS1 software. The economics and net returns for each of the treatments were also worked out.

Effect of various treatments on M. vitrata and H. armigera
 
The data pertaining to the relative incidence of spotted pod borer, M. vitrata in different insecticidal treatments before and 7 days after spray across different locations have been presented in Table 1. The data showed that the mean number of Maruca webs before spray ranged from 9.05-9.82 per plant in all the treatments and there were non-significant differences in the pest incidence. However, after 7 DAS, all the insecticidal treatments were significantly better than untreated control in their efficacy against the spotted pod borer. There were significant differences was observed in the mean number of webs after 7 days of spray and the number of webs ranged from 10.22-13.61 per plant in different insecticidal treatments. Significantly lower number of mean Maruca webs of 10.22 per plant was recorded in the treatment chlorantraniliprole 18.5 SC and followed by 11.66 webs per plant in the treatment flubendiamide 480 SC @ 150 and 125 ml per hectare, respectively. The untreated control recorded significantly higher mean number of Maruca webs (19.60 webs/plant).
 

       
As far as H. armigera incidence was concerned, it varied non-significantly among different treatments before spray (Table 2). However, after 3 and 7 DAS, all the insecticidal treatments were significantly better than untreated control in their efficacy against H. armigera larvae. There were significant differences in the mean number of larvae after 3 days of spray and the number of larvae ranged from 0.48-1.44 per plant in different insecticidal treatments. Significantly lower number of mean H. armigera larvae of 0.48 per plant was recorded in the treatments comprising chlorantraniliprole 18.5 SC and flubendiamide480 SC@ 150 and 125 ml per hectare respectively. The untreated control recorded significantly higher mean number of H. armigera larvae (1.92 larvae/plant) at 3 DAS. Almost similar trend was observed in the effectiveness of the insecticides towards H. armigera at 7 DAS. There were significant differences in the mean number of larvae after 7 days of spray and the number of larvae ranged from 0.22-1.40 per plant in different insecticidal treatments. Significantly, lower number of mean H. armigera 0.22 larvae per plant was recorded in the treatment comprising chlorantraniliprole 18.5 SC @ 150 ml followed by 0.37 larvae per plant in the treatment flubendiamide480 SC @ 125 ml hectare. The untreated control recorded significantly higher mean number of H. armigera larvae (2.66 larvae/plant) at 7 DAS.
 

       
Results revealed that all the insecticidal treatments were found to be significantly superior over untreated control by recording lower population of pod borer complex per plant.
 
Effect of various treatments on pod damage by pod borer complex
 
The data pertaining to per cent pod damage due to M. vitrata in different insecticidal treatments have been presented in Table 3. All the insecticidal treatments were found to be superior over untreated control with respect to per cent pod damage. The data revealed that the mean pod damage due to M. vitrata in different insecticidal treatments ranged from 3.95-9.76 per cent. The treatment comprising chlorantraniliprole 18.5 SC @ 150 ml registered significantly the lowest (3.95%) pod damage followed by 4.89 per cent pod damage in the treatment flubendiamide480 SC @ 125 ml per hectare. Significantly highest mean pod damage was observed in untreated control, which recorded 19.99 per cent pod damage due to M. vitrata.  The mean pod damage due to H. armigera ranged from 1.09-3.19 per cent in different insecticidal treatments as compared to 8.62 per cent in untreated control. The treatment comprising chlorantraniliprole 18.5 SC @ 150 ml registered significantly the lowest (1.09%) pod damage followed by flubendiamideSC 480 @ 125 ml (1.40%) per hectare due to H. armigera (Table 3).
 

       
The cumulative pod damage due to both the pod borers in different treatments has been presented in Table 4. The data revealed that the mean cumulative pod damage due to pod borer complex in pigeonpea crop with different insecticidal treatments ranged from 4.77-12.79 per cent. The treatment comprising chlorantraniliprole 18.5 SC @ 150 ml registered significantly the lowest (4.77%) pod damage followed by 5.92% damage in the treatment flubendiamide 480 SC @ 125 ml per hectare. However, other insecticidal treatments comprising indoxacarb 14.5 SC and flubendiamide 480 SC @ 500 and 100 ml per hectare, respectively were also statistically at par with the best treatments in terms of cumulative pod damage. Significantly highest mean cumulative pod damage (26.46%) was observed in untreated control to pod borer complex. Based on severity and extent of pod damage at different locations, M. vitrata emerged to be a more serious pod borer species in pigeonpea crop as compared to H. armigera.
 

 
Effect of various treatments on seed yield and economics of pigeonpea cultivation
 
The insecticidal treatments showed significant differences in the mean seed yield of pigeonpea and the results have been presented in Table 4. The mean grain yield per hectare in different insecticidal treatments ranged from 1047-1176 kg as compared to 639 kg in untreated control, thereby proving that all the insecticidal treatments were superior over untreated control. Significantly higher mean grain yield (1176 kg) was recorded in the treatment chlorantraniliprole 18.5 SC @ 150 ml ha^-1 followed by 1156 kg per hectare in the treatment flubendiamide 480 SC @ 125 ml ha^-1. However, other insecticidal treatments comprising indoxacarb 14.5 SC @ 500 ml ha^-1 (1147 kg) and flubendiamide 480 SC @ 100 ml ha^-1 (1117 kg) were also statistically at par with the best treatments in terms of seed yield of pigeonpea. The comparative economics of different insecticidal treatments evaluated against pod borer complex over untreated control have been depicted in Table 5. The net returns over control of different insecticidal treatments ranged from Rs. 17680-22450 ha^-1. The highest net returns of Rs. 22450 ha^-1 were recorded in the treatment chlorantraniliprole 18.5 SC @ 150 ml ha^-1 followed by Rs. 21700 and Rs 21650 ha^-1 in the treatments indoxacarb 14.5 SC @ 500 ml ha^-1 and flubendiamide 480 SC @ 125 ml ha^-1, respectively.
 

       
The overall data on efficacy of different insecticides against pod borer complex in pigeonpea at all the locations revealed that the treatment chlorantraniliprole 18.5 SC @ 150 ml ha^-1 registered lowest pest population, cumulative pod damage and highest grain yield along with highest net returns over untreated control, thus, indicating superiority of the treatment over rest of the treatments. The next best treatments were flubendiamide 480 SC @ 125 ml ha^-1, indoxacarb 14.5 SC @ 500 ml ha^-1 and flubendiamide 480 SC @ 100 ml ha^-1.
       
The present findings are in conformity with the findings of Mahalle and Taggar (2018) who reported that chlorantraniliprole 18.5 SC @ 30 g a.i. ha^-1 was significantly superior to all other treatments in controlling M. vitrata in pigeonpea as this treatment recorded significantly low pest infestation of 9.55 webs per plant as well as lowest pod damage (7.22%). This treatment registered the highest seed yield (1278 kg ha^-1) and maximum net monetary returns over control (Rs. 22,020 ha^-1). The next in order were indoxacarb 14.5 SC @ 73 g a.i. ha^-1 and flubendamide 480 SC @ 48 g a.i. ha^-1. Das et al., (2009) reported that two sprayings, initiating at 50 per cent flowering and repeated at 10 days interval of chlorantraniliprole (rynaxypyr) 20 SC @ 30-40 g a.i. ha^-1 was quite effective in controlling pigeonpea pod borer complex. Satpute and Barkhade (2012) also reported chlorantraniliprole (rynaxypyr) 20 SC @ 40 g a.i. ha^-1 as the most promising insecticide against pod borer complex of pigeonpea and to give highest yield of 17.52 q ha^-1. The present findings were also in accordance with the earlier reports of (Nishantha et al., 2009 and Chowdary et al., 2010) who reported chlorantraniliprole (rynaxypyr) 20 SC @ 30 g a.i. ha^-1 as superior molecule in recording lower pod damage and higher grain yield in pigeonpea against pod borer complex.
       
Studies conducted by Patange and Chiranjeevi (2017) revealed that the treatment application of chlorantraniliprole (rynaxypyr 18.5 SP) @ 30 g a.i. ha^-1 shown the lowest pod damage (5.59%) due to pigeonpea pod borers and recorded highest gain yield of pigeonpea (7.60 q ha^-1). The results also indicated that rynaxypyr 18.5 SP was more effective against pigeonpea pod borers and reducing the pod damage with its novel properties which ultimately leads to increase in yield and highest cost benefit ratio. The results obtained in the present investigation in relation to economics of different treatments are in accordance with the earlier work of Singh (2014) where in highest cost benefit ratio reward (1: 4.24) was obtained from chlorantraniliprole. Sreekanth et al., (2014) documented highest grain yield and ICBR in chlorantraniliprole (686.1 kg ha^-1 and 1:4.64), followed by flubendiamide (595.8 kg ha^-1) and spinosad (589.0 kg ha^-1 and 1:4.50). Similarly, Wadaskar et al., (2013) reported that highest yield and ICBR was obtained from flubendiamide 20 WDG (13.30 q ha-1 and 1:6.8) followed by emamectin benzoate 5 SG at the rate of 0.3 g per l (1:5.0) and spinosad 45 SC at the rate of 0.3 ml per l (1:4.6).
       
Similar studies conducted by Sreekanth et al., (2015) revealed that the per cent inflorescence and pod damage due to M. vitrata in pigeonpea was lowest in chlorantraniliprole 18.5 SC and flubendiamide 39.35 SC, followed by spinosad 45 SC. Highest grain yield was recorded in chlorantraniliprole treated plots (686.1 kg ha^-1) with 127.5 per cent increase over control, followed by flubendiamide (595.8 kg ha^-1) and spinosad (589.0 kg ha^-1) with 97.6 and 95.3 per cent increase over control (301.6 kg ha^-1), respectively. The cost effectiveness of chlorantraniliprole and flubendiamide was highly favourable with incremental cost benefit ratios of 1:4.6 and 1:4.5, respectively.
       
The results obtained in the present investigation are also in agreement with the findings of Haritha (2008), who reported that chlorantraniliprole (0.009%) recorded least pod damage (1.6 %) due to M. vitrata on pigeonpea than control (5.3%). Similarly, Patel et al., (2015) observed that among the various insecticides, chlorantraniliprole @ 30 g a.i. ha^-1 was the most effective insecticide against pod borer complex in pigeonpea as it registered the lowest pod damage and also recorded the highest yield. The present findings are also closely related with the work of Nishantha et al., (2009) who reported that chlorantraniliprole (rynaxypyr) @ 40 g a.i. ha^-1 proved most effective against gram pod borer and tur plume moth in pigeonpea. However, Chaitanya et al., (2013) reported that thiodicarb recorded not only the lowest pod damage due to M. vitrata, but also resulted in the highest pod yield of pigeonpea.

Flubendiamide is a new benzenedicarboxamide insecticide developed for Lepidoptera pest control. It is known to act on insect ryanodine receptors. Tang (2008) reported that flubendiamide was a diamide insecticide have a unique chemical structure and a novel mode of action and show excellent efficacy, a broad insecticidal spectrum against lepidopteran insect pests, excellent safety against various beneficial arthropods and natural enemies and no cross-resistance to existing insecticides and very suitable for insecticide resistance management and IPM programmes.  Flubendiamide is a new chemical option for control of multi-resistant noctuid pests and an excellent choice in resistant management strategies for lepidopteran pests in general (Nauen et al., 2007). Dodia et al., (2009) reported that flubendiamide 20 WDG at 50 g a.i. ha^-1 when sprayed against H. armigera infesting pigeonpea showed most effective results with 5.98 per cent damage. Deshmukh et al., (2010) determined that flubendiamide 0.007 per cent in pigeonpea was found the most effective in reducing the H. armigera population and pod damage and showed the highest yield of 1850 kg ha^-1 and cost benefit ratio of 1 : 6.10.
       
According to Neharkar et al., (2018), flubendiamide emerged out as the most superior one against pigeonpea pod borer complex in reducing per cent fruiting body damage (on green pod) per cent pod damage at harvest, per cent grain damage at harvest followed by chlorantraniliprole and emamectin benzoate. Similar results reported by Meena et al., (2006) suggested that the bioefficacy of some newer insecticides such as flubendamide against gram pod borer was found to be most effective and recorded minimum grain damage in pigeonpea. Tohnishi et al., (2010) concluded that flubendiamide was the first example of 1, 2-benzenedicarboxamide insecticides but also the first practical synthetic insecticide with a mode of action as an activator of ryanodine receptors. It shows high selective activity against lepidopteran insect pests, which leads to excellent efficacy in the field and excellent safety against non-target organisms, including various beneficial arthropods and natural enemies.
       
The safer chemical control methods reduce the pest population, pod and grain damage with higher yield; therefore, chemical management popularizes as an effective, practical alternative and makes lucrative cultivation of pigeonpea crop. These properties suggest the suitability of chlorantraniliprole and flubendiamide for integrated pest management (IPM) programs. Keeping in view the pooled data of all the parameters, viz. number of webs and larvae per plant, cumulative pod damage and grain yield in different research trials, the new generation novel insecticides like chlorantraniliproleand flubendiamide were found effective against the pod borer complex in pigeonpea coupled with higher net returns. Hence, it is suggested that the effective insecticides may be alternated in order to avoid the development of resistance to pod borers in pigeonpea.

The pod borer complex, comprising M. vitrata and H. armigera, causes considerable losses in yield of early maturing varieties of pigeonpea crop. The present study clearly revealed better response of the application of chlorantraniliprole 18.5 SC @ 150 ml ha^-1 followed by flubendiamide 480 SC @ 125 ml ha^-1 resulting in minimum larval population, lowest pod damage by pod borer complex ultimately leading to higher grain yield and net returns. These new generation green labeled insecticides with novel modes of action offer safer chemical control options for use in IPM programmes to manage pod borer complex infestation in pigeonpea crop at farmers fields.