Effect of Different Insecticides on Yield, Yield Attributes and Pollinator Behaviour of Different Pigeon Pea Varieties

Pigeon pea [Cajanus cajan (L.) Millspaugh] is one of the pulses of prime importance, which belong to family Fabaceae. In India, it is consumed as decorticated split seeds, popularly called as ‘Dal’. It was widely adopted to provide a list of priority of issues regarding the research. Inputs and their efficient management was the main factors revealed from different studies enquiring for causes of lower productivity of pulses (Singh et al., 2015).

Pre-anthesis cleistogamy is one of the important characters, found in flowers of pigeon pea (Sexena, 2006). According to Onim et al., (1979), in bud stage pigeon pea anthers dehisce although pollen germination starts after starting the withering of the flower that is 24-28 hours after dehiscence. Receptivity of stigma occurred 48 hours before the opening of the flower and continued afterward up to 4 days of flower opening (Luo et al., 2009). For this character, there is a chance of insect pollination because of delay in pollen germination and stigma receptivity lasts even after the opening of the flower. Pollination, when took place by the help of pollen of the same flower it requires 54 hours to reach to the ovary and also pollen tube is reluctantly grown within the style (Datta and Deb, 1970). Long stamens of the flower are encouraged to insect outcrossing during the intermittent visit of the pollinators (Saxena, 2006). Being an often cross pollinated crop, pigeon pea registers 3-26 per cent cross pollination (Reddy et al., 2004). In terms of yield increment due to cross pollination, 110 to 600 per cent yield increment was reported in often cross pollinated legumes (Abrol and Shankar, 2015).

Ivey et al., (2003) found that insects, those who visit the flower need not be an essential pollinator. Some of the insect visitors only visit to gather nectar, thus they do not play any role in pollination and perform as nectar thieve (Castro et al., 2008). Those who come in contact with anther and stigma of the flower will carry a various amount of pollen (Adler and Irwin, 2005) and their visiting frequency also will be different while foraging (Monzon et al., 2004). Among the different types of pollinators in pigeon pea, bees are the essential pollinators due to their intermittent visit to the field to collect more and more food (nectar and pollen) for raising their young ones due to their eusocial nature.

Very few studies have been conducted especially on the role of flower visitors/pollinators of pigeon pea and how they are productive in increasing yield especially in Pantnagar region. Detailed systematic studies are needed to understand the pollination requirements of pigeon pea. Hence, the current study was conducted with objectives to record diversity of flower visitors and abundance of insect pollinators, to work out pollination efficiency of major pollinators and to evaluate the effect of different varieties and insecticidal treatments on the pollinator behaviour, yield and yield attributes of pigeon pea to reduce the ecological impact of chemicals as well as increase the productivity of pigeon pea.

The experiment was conducted in Norman E. Borlaug Crop Research Centre of G.B. Pant University of Agriculture and Technology, Pantnagar, Uttarakhand, India (29°N, 79.3°E, 248.8 m) during kharif season of 2016 and 2017. The experiment was designed in a Split plot arrangement replicated thrice. The varieties were assigned in main plots and had four levels such as UPAS-120, PA-406, PA-421 and PA-441. Types of insecticides applied were taken in the sub plots and it also had four levels viz. Quinalphos 25 EC (325 g a.i./ha), Lufenuron 5.4 EC (30 g a.i./ha), Deltamethrin 2.8 EC (12.5 g a.i./ha) and Indoxacarb 15.8 EC (50 g a.i./ha).

Observations on pollinator visits were recorded at 10 per cent flowering, peak flowering and cessation of flowering for 3 times a day. With a view to determine the volume of nectar present in of pigeon pea flower, 30 flowers were sampled from each plot and then by using the forceps the petals of the flower were removed. The flower was placed under microscope and with the help of graduated micro-capillary tube nectar was collected from the nectar gland. The observations were recorded twice a day at 06:30 a.m. and 02:30 p.m. Pollination efficiency of different insect pollinators was calculated on the basis of their relative abundance, foraging rate and the number of loose pollen grains sticking to their bodies. The pollination index was calculated by following the method suggested by Kumar et al. (2012). The insecticides were applied at onset of flowering stage. Pollinator abundance of insect pollinators was recorded before and after spray of insecticides for side by side comparisons. At the time of spraying insecticides, pest populations were below economic threshold level so the effects of insecticides on yield due to control of pest is ignored for current study.

For the yield estimation, yield from net plot area was harvested and expressed as yield in kg/ha. For yield attributes such as pods/plant, seeds/pod, 5 representative plants from each plot were selected and observed. For 100 seed weight, a representative sample was drawn from the net plot yield and counted for 1000 seeds and weighed, expressed as seed index in grams.

All the statistical analysis were done using IBM SPSS 24.0 software package developed by IBM Corp. (2016). Data from two years were subjected to combined analysis as the year to treatment variance was found to be in the acceptable limits (Gomez and Gomez, 1984). Duncan’s multiple range test at 5% probability level was used for the purpose of mean comparison (Duncan, 1955).

Total 25 species of flower visiting insects from 14 different families and 6 orders were observed during the experiment. They belonged to order Hymenoptera (15 spp.), Lepidoptera (5 spp.), Hemiptera (1 sp.), Diptera (2 spp.), Coleoptera (1 sp.), Thysanoptera (1 sp.), respectively (Table 1). Among the order Hymenoptera, the families Apidae (6 spp.), Megachilidae (7 spp.), Halictidae (1 sp.) and Scoliidae (1 sp.) were recorded. Among the 25 insect species recorded on the flowers of the pigeon pea, 8 species belonging to orders Lepidoptera, Coleoptera, Thysanoptera and Hemiptera were harmful and were not considered for further studies and study was mainly concentrated on the insect pollinators of pigeon pea (Table 1). Pollination efficiency ranking was based on the overall pollination rate, foraging behaviour and abundance. Highest pollination efficiency was observed for Megachile disjuncta which was followed by Xylocopa latipes and Megachile bicolor. Pollinators of Apis sp. were not found to be true pollinator on the investigation of body adhered pollens. Flower morphology had been observed to play a greater role in the pollination effectiveness of the insects. This results were in accordance to (Singh, 2016).



Among the varieties, PA-441 was most preferred by three species such as Apis mellifera, Xylocopa latipes, Megachile bicolor whereas UPAS-120 and PA-421 were preferred by two species each (Table 1). Keeping the efficiency rating in the mind, UPAS-120 was preferred by the most efficient pollinator and PA-441 was also preferred by two efficient pollinators. Thus this two varieties were largely benefitted by the insect pollinations (Table 2). There was also highest nectar volume recorded with these two varieties which were significantly higher than the rest of the varieties. Nectar volume is also known to be a factor for the attraction of the insect pollinators. It was also observed in the study that mean abundance followed the same trend of the nectar volume present in the flower. Higher abundance of pollinators resulted in significantly higher percentage of cross pollination and correspondingly higher fertilization rate in the varieties. However, effect of insecticides on the nectar volume was found non-significant. On the other hand, insecticides had affected the pollinator abundance and consequently affected the percent cross pollination and fertilization. Mean abundance of pollinators was found highest with Deltamethrin 2.8 EC followed by Quinalphos 25 EC and Indoxacarb 15.8 EC (Table 2).



In order to evaluate the effect of insecticides on the abundance of pollinators, species wise pre-treatment and post treatment abundance readings were compared side by side (Table 3). Deltamethrin 2.8 EC was found to have significantly negative impact on A. florea and X. latipes; other pollinators were not affected or positively affected. Lufenuron 5.4 EC was having net negative impact on the abundance of A. dorsata, A. florea, M. bicolor and M. disjuncta. Indoxacarb 15.8 EC was found to reduce the abundance of A. dorsata, X. latipes and M. disjuncta significantly. Quinalphos 25 EC was found to have significantly negative impact on the abundance of A. dorsata, A. mellifera, A. florea, X. latipes and M. disjuncta (Table 3). These results well justify the trends in Table 2.



Economic yield and yield attributes such as pods/plant, seeds/pod were found to be significantly affected by both the factors. However, effect on the 100 seed weight was found to be unaffected by the levels of insecticide as it was varietal character. Highest yield was recorded from UPAS-120 (1181.3 kg/ha) followed by PA-441 (1160.1 kg/ha). Among insecticidal treatments highest yield was recorded from Deltamethrin 2.8EC (1219.8 kg/ha). Number of pods as well as number of seeds per pod was determined by the fertilization percentage and hence followed the same trend. UPAS-120 and PA-441 were observed to have highest pods/plant and seeds/pod. Among insecticides, Deltamethrin 2.8 EC with the least negative impact on pollinators had highest pods/plant and seeds/pod. On the contrary, treatments having lower pods/plant and seeds/pod were having higher 100 seed weight due to the more availability of photo synthates to the seeds.

Outcomes emanated from the current study revealed that Megachile disjuncta, Xylocopa latipes and Megachile bicolor were the three most important pollinator bee species on kharif pigeon pea in the tarai region of Uttarakhand. Among the varieties, UPAS-120 and PA-441 had better cross pollination potential over the PA-406 and PA-421. Insecticides had overall negative impact on the insect pollinators. However, Deltamethrin 2.8 EC at recommended dose was found to have least impact on the insect pollinators. Quinalphos 25 EC and Lufenuron 5.4 EC were found to be more deleterious on pollinators. The ultimate impact on the pollinator insects were profound on the yield and yield attributes of pigeon pea. Not only the yield and economic damage to the farmers, these practices can cause more harm to the agro-ecosystem. This study is expected to provide a guidance to the farmers for increasing the pigeon pea yield in the area through selection of varieties and more ecologically feasible pest management options for a sustainable future.

The research is a part of the M.Sc. Thesis submitted by Mr. Supratik Banik to G.B. Pant University of Agriculture and Technology, Pantnagar in 2018. The financial assistance received from ICAR and GBPUAT are duly acknowledged.