The present invention relates to synergistic pesticidal composition for the control of plant pests. More particularly, the present invention relates to novel synergistic pesticidal composition comprising (A) Pymetrozine or its agrochemically acceptable salts; (B) Isoprothiolane or its agrochemically acceptable salts; (C) at least one strobilurin fungicide selected from Azoxystrobin, Pyraclostrobin, Fluoxastrobin, Trifloxystrobin, Kresoxim-methyl and Picoxystrobin or its agrochemically acceptable salts; (D) one or more agrochemically acceptable excipients. The present invention further relates to process of preparing said synergistic pesticidal composition. BACKGROUND OF THE INVENTION Pesticides are substances used to control pests (Insects, Diseases or weeds). The Food and Agriculture Organization (FAO) has defined pesticide as: any substance or mixture of substances intended for preventing, curating, controlling or destroying any pest, including vectors of human or animal disease, unwanted species of plants or animals, causing harm during or otherwise interfering with the production, processing, storage, transport, or marketing of food, agricultural commodities, wood and wood products or animal feedstuffs, or substances that may be administered to animals for the control of insects, arachnids, or other pests in or on their bodies. The term includes substances intended for use as a plant growth regulator, defoliant, desiccant, or agent for thinning fruit or preventing the premature fall of fruit. Also used as substances applied to crops either before or after harvest to protect the commodity from deterioration during storage and transport. Combination of insecticides and fungicides are used to broaden the control spectrum of insect and fungal pests, reduce dosage, thereby reducing environmental impact, and decrease of the incidence of resistance development. The combination of insecticides and fungicides results in improved pest control. Seed damage caused by insect and fungal pests is another major concern for stakeholders. Various diseases such as seed rot, reduce germination rates and cause a significant decrease in yield. Using insecticidal and fungicidal combinations on crops helps reduce damage from soil pests. Another advantage of treating crops or specific plant propagation material is the increased germination rates, yield, and plant health. When fungicide resistance results from modification of a single major gene, pathogen subpopulations are either sensitive or highly resistant to the pesticide. In this case, resistance is defined as total loss of disease control that cannot be regained by using higher rates or more frequent fungicide applications. This type of resistance is known as "qualitative resistance." Controlling phytopathogenic fungi is economically important because fungal growth on plants or plant parts inhibits the production of foliage, fruit or seed, as well as overall quality of a cultivated crop. It is known in the art to use several fungicides and insecticides individually or in binary combinations. However, as resistance develops over time, greater dosages are required, which has a detrimental impact on plant health and yield. Therefore, a combination is required that not only lessens the likelihood of resistance development but also enhances the spectrum of disease, pest management, crop yield, and health. Pymetrozine was first disclosed in US4931439 and US4996325. Pymetrozine is a member of the class of 1,2,4-triazines [from a chemical class (pyridine azomethines)] and is chemically known as (E)-4,5-dihydro-6-methyl-4-(3-pyridylmethyleneamino)- l,2,4-triazin-3(2H)-one and having chemical structure as below: o ^A^N X HN TM V Pymetrozine Pymetrozine is the only representative of the pyridine azomethines, highly active and specific against sucking pests such as whiteflies, hoppers and aphids, Brown plant hopper, white Backed Plant Hopper, Green Leaf Hopper, Rice Ear Head Bug. It has a unique mode of action and can be applied both foliar and soil. It has no direct toxicity against insects, but it prevents sucking pest's stylet penetration, which results in immediate and irreversible cessation of feeding within few hours of pesticide application, followed starvation and death. The mode of action of Pymetrozine in insects involve effects on neuroregulation or nerve-muscle interaction. Physiologically, it appears to act by preventing these insects from inserting their stylus in to the plant tissue. Pymetrozine is selective against Homoptera, causing them to stop feeding. Pymetrozine is used in control of aphids and whitefly in vegetables, potatoes, ornamentals, cotton, deciduous and citrus fruit, tobacco, hops; both juvenile and adult stages are susceptible and also control planthoppers in rice. Isoprothiolane, is a systemic fungicide with curative and protective effects. Isoprothiolane is a chemical compound from the group of Dithiolanes. Isoprothiolane has a conspicuous ketene-di thioacetal structure. It is chemically known as Diisopropyl l,3-dithiolan-2-ylidenemalonate and having chemical structure formula represented as below: CH3 O O CH3 AJULA O' CH3 H3 1, synergism observed Experiment for Synergistic activity and efficacy of composition of present invention: Experiment 1- Details of experiment on Paddy crop: Field experiment for synergistic activity of Isoprothiolane, Trifloxystrobin and Pymetrozine for the control of different pests in Paddy crop and its cumulative impact on grain yield was conducted by the Research & Development Department of the inventors at farmer field, Sirisi (Karnataka). To evaluate the synergistic effect, seeds for Paddy [Variety: Jaya] crop sown in raised seed bed. Field for experiment was ploughed and divided in plot size of 6 m x 5 m keeping 0.5 m buffer between plots to avoid drift of chemicals. About one month old seedlings were transplanted in the main field with 20 cm x 10 cm distance between plants and rows on 22.12.2021 with three replications and in Randomized Block Design (RBD). The treatments were applied as single directed foliar spray @ 1000 g/ml/ha at pest incidence over economic threshold level (ETL) on 22.02.2022. The active components were applied in different ternary and binary combinations. Solo components were also taken for comparison. The treatment details are as under: Table 11 S.No. Treatment Formulation Tl Isoprothiolane + Trifloxystrobin + Pymetrozine 10%+2%+60% T2 Isoprothiolane + Trifloxystrobin + Pymetrozine 30%+5%+15% T3 Isoprothiolane + Trifloxystrobin + Pymetrozine 50%+10%+10% T4 Isoprothiolane + Trifloxystrobin + Pymetrozine 20%+50% +5% T5 Isoprothiolane + Trifloxystrobin 10%+2% T6 Isoprothiolane + Trifloxystrobin 30%+5% T7 Isoprothiolane + Trifloxystrobin 50%+10% T8 Isoprothiolane + Trifloxystrobin 20%+50% T9 Trifloxystrobin + Pymetrozine 2% +60% T 10 Trifloxystrobin + Pymetrozine 5%+15% T 11 Trifloxystrobin + Pymetrozine 10%+10% T 12 Trifloxystrobin + Pymetrozine 50% +5% T 13 Isoprothiolane + Pymetrozine 10%+60% T 14 Isoprothiolane + Pymetrozine 30%+15% T 15 Isoprothiolane + Pymetrozine 50%+10% T 16 Isoprothiolane + Pymetrozine 20%+ 5% T 17 Isoprothiolane 10% T 18 Isoprothiolane 30% T 19 Isoprothiolane 50% T20 Isoprothiolane 20% T21 Trifloxystrobin 2% T22 Trifloxystrobin 5% T23 Trifloxystrobin 10% T24 Trifloxystrobin 50% T25 Pymetrozine 60% T26 Pymetrozine 15% T27 Pymetrozine 10% T28 Pymetrozine 5.00% T29 Untreated control - The treatments Tl to T28 were applied as foliar spray and in T29 control treatment only water was sprayed. Grain yield was recorded for all the plots and converted into per hectare and presented as kg/ha. From yield percent increase against control was calculated. Based on percent yield increase against control in solo treatments the Expected control was calculated and compared with Observed control. The results of the trial have been presented here under in Table 12. Table 12: Synergistic effect of pesticidal composition of the present invention on grain yield in Paddy crop s. No. Treatment Formulation Yield Expected % increase against control Colby Ratio (Difference) Kg/ha % increase against control 1 Isoprothiolane + Trifloxystrobin + Pymetrozine 10%+2% +60% 4140 47.33 46.22 1.11 2 Isoprothiolane + Tnfloxystrobin + Pymetrozine 30%+5% +15% 4461 58.75 56.04 2.71 3 Isoprothiolane + Tnfloxystrobin + Pymetrozine 50%+10% +10% 4219 50.14 47.96 2.18 4 Isoprothiolane + Tnfloxystrobin + Pymetrozine 20%+50% +5% 3940 40.21 38.74 1.47 5 Isoprothiolane + Tnfloxystrobin 10%+2% 3420 21.71 24.82 -3.11 6 Isoprothiolane + Tnfloxystrobin 30%+5% 3674 30.75 40.01 -9.26 7 Isoprothiolane + Tnfloxystrobin 50%+10% 3691 31.35 43.28 -11.93 8 Isoprothiolane + Tnfloxystrobin 20%+50% 3709 31.99 37.47 -5.48 9 Tnfloxystrobin + Pymetrozine 2% +60% 3738 33.02 37.81 -4.79 10 Tnfloxystrobin + Pymetrozine 5%+15% 3930 39.86 43.10 -3.24 11 Tnfloxystrobin + Pymetrozine 10%+10% 3621 28.86 30.03 -1.17 12 Tnfloxystrobin + Pymetrozine 50% +5% 3524 25.41 26.43 -1.02 13 Isoprothiolane + Pymetrozine 10%+60% 3778 34.45 38.14 -3.69 14 Isoprothiolane + Pymetrozine 30%+15% 3800 35.23 43.39 -8.16 15 Isoprothiolane + Pymetrozine 50%+10% 3628 29.11 31.76 -2.65 16 Isoprothiolane + Pymetrozine 20%+ 5% 3274 16.51 18.42 -1.90 17 Isoprothiolane 10% 3190 13.52 — — 18 Isoprothiolane 30% 3449 22.74 — — 19 Isoprothiolane 50% 3530 25.62 — — 20 Isoprothiolane 20% 3280 16.73 — — 21 Tnfloxystrobin 2% 3177 13.06 — — 22 Tnfloxystrobin 5% 3438 22.35 — — 23 Tnfloxystrobin 10% 3477 23.74 — — 24 Tnfloxystrobin 50% 3510 24.91 — — 25 Pymetrozine 60% 3610 28.47 — — 26 Pymetrozine 15% 3561 26.73 — — 27 Pymetrozine 10% 3042 8.26 — — 28 Pymetrozine 5.00% 2867 2.03 — — 29 Untreated control - 2810 - — — It is clearly evident from the data shown in above Table for yield increase against control in Paddy crop that the ternary combination composition of the present invention comprising Isoprothiolane, Tnfloxystrobin and Pymetrozine Tl to T4 are highly synergistic with positive Colby's Ratio. The binary combination of two fungicides Isoprothiolane + Tnfloxystrobin (T5 to T8) or one fungicide and one insecticide Trifloxystrobin + Pymetrozine (T9 to T12) and Isoprothiolane + Pymetrozine (T13 to T16) are compatible but non-synergistic for the yield increase as compared to ternary combination of Isoprothiolane, Trifloxystrobin and Pymetrozine. The binary combinations of two pesticides are thus less effective in yield increase as compared to ternary combination of three pesticides Isoprothiolane + Trifloxystrobin + Pymetrozine (Tl to T4). Experiment for Bio-efficacy evaluation of composition of present invention: For the ternary composition based on Isoprothiolane, Trifloxystrobin and Pymetrozine evaluation under field conditions, the basic test composition as Isoprothiolane 40% EC, Trifloxystrobin 10% WG and Pymetrozine 50% WDG. For trinary composition, Isoprothiolane 30%) + Trifloxystrobin 5% + Pymetrozine 15% WG was used at different concentrations of 700, 800 and 900 g/ha. Example 9: Evaluation of the composition of the present invention on Paddy The synergistic composition of the present invention Isoprothiolane 30% + Trifloxystrobin 5% + Pymetrozine 15% WG @ 700, 800 and 900 g/ha along with different solo formulation were evaluated on Paddy crop variety Abhilash. The experiment was conducted at a farmer field, farmer field, Sirisi (Karnataka) during July to November, 2022 following Randomized Block Design (RBD) with three replications and maintaining a distance of 20 cm x 10 cm between plants and rows. Two applications of pesticidal treatments were done with 10 days interval starting from on 25.09.2022 on the crop in experimental plots. The observations were recorded for the following objectives. Objectives: 1. To determine the efficacy of synergistic ternary composition of present invention against Panicle Blast {Pyricularia oryzae) and brown plant hopper (BPH)in Paddy crop and its impact on grain yield increase. 2. To observe phytotoxicity, if any on Paddy. 5% + Pymetrozine 15% WG @ 700 g/ha 5% + Pymetrozine 15% WG @ 800 g/ha 5% + Pymetrozine 15% WG @ 900/ha 5% + Pymetrozine 15% WG @ 700 g/ha 5% + Pymetrozine 15% WG @ 800 g/ha 5% + Pymetrozine 15% WG @ 900/ha 5% + Pymetrozine 15% WG @ 1200 g/ha 5% + Pymetrozine 15% WG @ 1800 g/ha 3. Economics of treatments based on Cost: Benefit Ratio Treatment details: For Bio-efficacy evaluation Tl - Isoprothiolane 30% + Trifloxystrobin T2 - Isoprothiolane 30% + Trifloxystrobin T3 - Isoprothiolane 30% + Trifloxystrobin T4 - Isoprothiolane 40% EC @ 750 ml/ha T5 - Trifloxystrobin 10% WG @ 500 g/ha T6 - Pymetrozine 50 % WG @ 300 g/ha T7 - Untreated control For phyto-toxicity evaluation Tl - Isoprothiolane 30% + Trifloxystrobin T2 - Isoprothiolane 30% + Trifloxystrobin T3 - Isoprothiolane 30% + Trifloxystrobin T4 - Isoprothiolane 30% + Trifloxystrobin T5 - Isoprothiolane 30% + Trifloxystrobin T6 - Untreated control Methodology: All the treatments were imposed by high volume knapsack sprayer fitted with hollow cone nozzle on pest incidence on Paddy plants. During the study period, two applications was given. Observation on Panicle Blast (Pyricularia oryzae) and brown plant hopper (BPH) were taken at 10 days after second application on twenty hills per treatment per replication. The crop yield was recorded at harvest and grain yield has been expressed as kg/ha. The observations for phyto-toxicity symptoms on Paddy crop at 1, 3, 7 and 10 days after second application were also recorded. Based on treatment application cost, market price of produce and net profit, the Cost: Benefit Ratio was calculated for the economics of ternary combination treatments. The data were subjected to statistically analysis of variance. Results are presented in Tables below: Results: The results for Panicle Blast (Pyricularia oryzae) and Brown Plant Hopper (BPH) incidence on Paddy crop recorded during the experimental period and per cent reduction over untreated control are summarized in Table 13. The data on yield is presented in Table 14. The observations recorded for phytotoxicity symptom is presented in Table 15. Table 13: Field bio-efficacy evaluation of synergistic composition against pest incidence in Paddy crop Dose Blast BPH Treatment g a.i./h a Formulati on (g/ml/ha) PDI* %Reducti on against Control no./hill ** %Reduct ion against Control Tl Isoprothiolane 30% + Trifloxystrobin 5% + Pymetrozine 15% WG 210 + 35 + 105 700 3.90 (11.3 3) 71.32 6.30 (2.70) 70.42 T2 Isoprothiolane 30% + Trifloxystrobin 5% + Pymetrozine 15% WG 240 + 40 + 120 800 2.80 (9.58) 79.41 4.70 (2.38) 77.93 5% + Pymetrozine 15% WG T2 Isoprothiolane 30% + Trifloxystrobin 5% + Pymetrozine 15% WG 240 + 40 + 120 800 4530 52.99 1:2.0900 T3 Isoprothiolane 30% + Trifloxystrobin 5% + Pymetrozine 15% WG 270 + 45 + 135 900 4568 54.27 1:2.0752 T4 Isoprothiolane 40% EC 300 750 3590 21.24 1:1.5678 T5 Trifloxystrobin 10% WG 50 500 3548 19.82 1:1.5556 T6 Pymetrozine 50 % WG 150 300 3682 24.35 1:1.5385 T7 Control — — 2961 - 1:1.2591 CD. ~ — 253.49 9 — — SE(m) ~ — 81.369 — — CV ~ — 3.617 — — Table 15: Phytotoxicity evaluation of synergistic composition on Paddy plants Treatment A.I. dose(g/ ha) Formu-lation dose (ml or g/ha) Phytotoxicity parameters observed* (mean data recorded at 1, 3, 7 and 10 days after second spray) L w N V E H Isoprothiolane 30% + Trifloxystrobin 5% + Pymetrozine 15% WG 210 + 35 + 105 700 0 0 0 0 0 0 r2 Isoprothiolane 30% + Trifloxystrobin 5% + Pymetrozine 15% WG 240 + 40 + 120 800 0 0 0 0 0 0 n Isoprothiolane 30% + Trifloxystrobin 5% + Pymetrozine 15% WG 270 + 45 + 135 900 0 0 0 0 0 0 r4 Isoprothiolane 30% + Trifloxystrobin 5% + Pymetrozine 15% WG 360 + 60 + 180 1200 0 0 0 0 0 0 rs Isoprothiolane 30% + Trifloxystrobin 5% + Pymetrozine 15% WG 540 + 90 + 270 1800 0 0 0 0 0 0 r6 Untreated control - 0 0 0 0 0 0 u - Leaf injury on tips/ surface, W - Wilting, N - Necrosis, V - Vein clearing, E - Epinasty, H - lyponasty Based on 0-10 scale where: 0=0%, 1=1-10%, 2=11-20%, 3=21-30%, 4=31-40%, 5=41-50%, 6=51- 0%, 7=61-70%, 8=71-80%, 9=81-90%, 10=91-100% From the experiments conducted during kharif, 2022, it is apparent that the composition of Isoprothiolane with Trifloxystrobin and Pymetrozine described in the present invention is superior in protecting Paddy crop from pest incidence in comparison to registered solo formulations comprising Isoprothiolane 40% EC, Trifloxystrobin 10% WG and Pymetrozine 50%) WDG at registered dosage levels. The grain yield of Paddy also improved in the ternary combination of Isoprothiolane 30% + Trifloxystrobin 5% + Pymetrozine 15% WG as compared to binary and solo components. One of the reasons of higher grain crop yield is due to control of panicle blast and BPH causing damage to crop before harvest and therefore causing maximum economical damage. The Cost: Benefit Ratio revealed the same wherein all the doses of ternary combination of Isoprothiolane 30% + Trifloxystrobin 5% + Pymetrozine 15% WG were most economical with higher Cost: Benefit Ratio as compared to other binary and solo treatments. We Claim: 1. A synergistic pesticidal composition comprising: a. Pymetrozine or its agrochemically acceptable salts; b. Isoprothiolane or its agrochemically acceptable salts; c. at least one strobilurin fungicide; and d. agrochemically acceptable excipients. 2. The pesticidal composition as claimed in claim 1, wherein at least one strobilurin fungicides is selected from Azoxystrobin, Pyraclostrobin, Fluoxastrobin, Trifloxystrobin, Kresoxim-methyl and Picoxystrobin or its agrochemically acceptable salts. 3. The pesticidal composition as claimed in claim 1, wherein Pymetrozine or its agrochemically acceptable salts is present in an amount ranging from 5 to 75% w/w of the total weight of the composition. 4. The pesticidal composition as claimed in claim 1, wherein Isoprothiolane or its agrochemically acceptable salts is present in an amount ranging from 10 to 50% w/w of the total weight of the composition. 5. The pesticidal composition as claimed in claim 1, wherein the strobilurin fungicide or its agrochemically acceptable salts, is present in an amount ranging from 2 to 50% w/w of the total weight of the composition. 6. The pesticidal composition as claimed in claim 1, wherein the strobilurin fungicide is Trifloxystrobin. 7. The pesticidal composition as claimed in claim 1, wherein the agrochemically acceptable excipients are selected from the group comprising dispersants, anti-freezing agent, anti-foaming agent, wetting agent, suspension aid, anti-microbial/anti-bacterial agent, thickener, quick coating agent or sticking agents, spreading agents, solvents, buffering agent, binders, adjuvants, fillers, emulsifiers, colouring pigments, dyes, preservatives and combinations thereof. 8. The pesticidal composition as claimed in claim 1, wherein the formulation is selected from Capsule suspension (CS), Dispersible concentrate (DC), Dustable powder (DP), Powder for dry seed treatment (DS), Emulsifiable concentrate (EC), Emulsifiable granule (EG), Emulsion water-in-oil (EO), Emulsifiable powder (EP), Emulsion for seed treatment (ES), Emulsion oil-in-water (EW), Flowable concentrate for seed treatment (FS), Granules (GR), Micro-emulsion (ME), Oil-dispersion (OD), Oil miscible flowable concentrate (OF), Oil miscible liquid (OL), Oil dispersible powder (OP), Suspension concentrate (SC), Suspension concentrate for direct application (SD), Suspo-emulsion (SE), Water soluble granule (SG), Soluble concentrate (SL), Spreading oil (SO), Water soluble powder (SP), Water soluble tablet (ST), Ultra-low volume (ULV) suspension, Tablet (TB), Ultra-low volume (ULV) liquid, Water dispersible granules (WG), Wettable powder (WP), Water dispersible powder for slurry seed treatment (WS), Water dispersible tablet (WT), a mixed formulation of CS and SC (ZC) or a mixed formulation of CS and SE (ZE) and a mixed formulation of CS and EW (ZW). 9. The pesticidal composition as claimed in claim 1, wherein the composition further comprises of silicone ethoxylated oils. 10. The pesticidal composition as claimed in claim 1, wherein the composition further comprises of blend of polyterpene resins. 11. The pesticidal composition as claimed in claim 1, wherein the composition comprises of: a. Pymetrozine or its agrochemically acceptable salts in an amount in the range from 5% to 75% by weight of the composition; b. Isoprothiolane or its agrochemically acceptable salts in an amount in the range from 10% to 50%) by weight of the composition; c. at least one strobilurin fungicide in an amount in the range from 2%> to 50%> by weight of the composition; and d. one or more agrochemically acceptable excipients. 12. The pesticidal composition as claimed in claim 1, wherein the composition comprises of: a. Pymetrozine or its agrochemically acceptable salts in an amount in the range from 5% to 75% by weight of the composition; b. Isoprothiolane or its agrochemically acceptable salts in an amount in the range from 10% to 50%) by weight of the composition; c. at least one strobilurin fungicide in an amount in the range from 2% to 50% by weight of the composition; d. silicone ethoxylated oils; e. blend of polyterpene resin; and f one or more agrochemically acceptable excipients. 13. A method for pest control comprising applying to the plant or portion thereof or soil, a pesticidal effective amount of the composition as claimed in any one of the claims 1- 12.