Rationale behind the use of insecticide mixtures for the management of insecticide resistance in India

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1 WFL Publisher Science and Technology Food, Agriculture & Environment Vol.2 (2) : Rationale behind the use of insecticide mixtures for the management of insecticide resistance in India A. Regupathy, T. Ramasubramanian* and R. Ayyasamy Department of Entomology, Tamil Nadu Agricultural University, Coimbatore , Tamil Nadu, India. * tramasubbu@ rediffmail.com Received 15 December 2003, accepted 11 April Abstract The development of resistance has limited the application of single insecticides and resort to tank mixtures. Mixing of two or more insecticides is very common among the Indian farmers though not recommended. Pest populations to be controlled should be susceptible or have low levels of resistance to each insecticide used in the mixture. In India more than 85% of the combination products have synthetic pyrethroids as one of the components, to which the cotton bollworm (Helicoverpa armigera) has developed very high level of resistance. Hence, sustaining the efficacy of the combination products is questionable in the near future. Combination products if permitted will be used as a matter of routine and there may be development of multiple resistant individuals. If there is any need arises, then mixtures can be considered for one time remedy. Testing of the combination products can be taken for generating data and may be recommended in India after evaluating the development of resistance. Scope and considerations for proper use of insecticide mixtures with special reference to insecticide resistance management in India have been discussed in this review. Key words: Insecticide mixtures, insecticide resistance, pyrethroids. Introduction The pest management strategy in India is mainly relying on chemical pesticides. The quick and effective control of pests by insecticides convinces the farmers easily as against the non-chemical methods of pest management. More often insecticides form the only solution to sudden outbreak of pests. About 183 pesticides and formulations have been registered for use in India under the Insecticides Act, The knowledge regarding the most susceptible stage of the pest and actual impact of pest populations on yield loss is crucial for economic and successful pest management. The lack of knowledge in these aspects led the Indian farmers to use insecticides indiscriminately, which resulted in the development of resistance in major agricultural pests. The development of resistance and resurgence has limited the application of single insecticides and resort to tank mixtures. Survey of insecticide usage pattern in South Indian cotton ecosystem revealed that 60 75% of the farmers applied the insecticides as mixtures of 3 6 in an interval of 2 3 days during critical periods especially against Helicoverpa armigera Hubner 2. The mixtures of two or more insecticides are used in agriculture for various reasons. A mixture may give best control of a complex of pests with varying susceptibilities to the different components of the mixture 3. Insects that are resistant to one or more insecticides may be susceptible to a combination of toxicants 4 or synergism may be exhibited by the components 3, 5. Mixtures of insecticides are also used because of cost efficacy 6. How best these mixtures can be utilized to overcome the problem of resistance, and how can such mixtures be screened for synergy? - These aspects have been discussed in this paper. Tank Mixture vs. Ready Mix Formulations Problems with tank mixtures: The insecticide mixtures can be classified into two major groups, the tank mixtures and prepacked mixtures (ready mix formulations). The tank mixtures are prepared 278 in the field directly by farmers little time before spraying. This type of mixtures implies several problems 7. 1) It is common that farmers lack the knowledge about the compatibility of the components. 2) Aside from the degree of physical, chemical and biological compatibility the farmers have no idea about the most adequate proportion of the components in the mixtures. Consequently the used proportions arise from intuition and ignorance and this action is far from the best option. 3) The amount of diluents in the tank mixture increases when the two formulations mixed in the field. In consequence, the crop is receiving an overdose of diluents without having any idea about its consequences on produce quality or any negative interaction with the toxicants when there is no compatibility. These problems can be overcome by promoting ready mix formulations. Government regulations usually obligate chemical companies to prepare mixtures in a compatible manner. The only combination product of insecticides registered a couple of decades back was gamma HCH 4% + carbaryl 4% for the management of pests of rice and sugarcane. In the recent years with the problem of resistance to insecticides in important agricultural pests, there are as many as 17 ready mix formulations which have been registered in India 1 (Table 1). The ready mix formulations can be classified in to two broad categories based on the risk of developing resistance 7 : low risk and high risk insecticide mixtures. High risk insecticide mixtures are used to control same type of insect species and biological stage. This type of mixture is justified only when the target population is fully susceptible to all the toxicants in the mixture; otherwise a useless compound is being sprayed and the biological efficacy may be masked by the effective one. Low risk mixtures are used against two different insect pests such as chewing and sucking individuals. One of the compounds in the low risk mixtures is generally effective against chewing insects and also interferes

2 with the normal physiology of sucking pests making them more susceptible to the other compound which is specific against sucking pests. Thus benefits to farmers increase when low risk formulations are made by reputable chemical companies 7. There are few such formulations (Confidor ultra, which contains betacyfluthrin 50% + imidacloprid 50%) available in the Indian market. Problems with ready mix formulations: 1) The biological interaction among the components in mixture may change according to the relative proportion used. It is possible to observe potentiation, similar action or antagonism at different proportions of the same components in the mixture 8, 9. The proportion of the same components varies widely among the ready mix formulations available in India. For example, Chlorguard Plus (Gharda Chemicals Limited) and Duet (Cyanamid Agro Limited) both are having alphamethrin and chlorpyrifos as components; however, the ratio was 1: 20 and 1:16 respectively. Similarly the ratio of cypermethrin and chlorpyrifos was 1:10 in Nurelle D 505 (De Nocil Crop Protection Limited) and Cyphos 202 EC (De Nocil Crop Protection Limited), whereas it was 1:9 in Lethal Super. This needs further verification. 2) Specificity is an important problem in case of low risk insecticide mixtures. If two chemicals which are specifically toxic to two different pests are combined in a prepackaged product, the effect of one of the insecticides may be wasted if the target pests are not present at the same time. 3) As with tank mixtures, compatibility of the ingredients in the ready mix formulations is also a problem. Reactions may take place between the components which result in the loss of desired biological efficacy before the product proceeds through the distribution channel and is ultimately used by the consumer. Sometimes the stability of such formulations can be increased sufficiently with the addition of stabilizers; often it is economically impractical to achieve proper stabilization. These problems can be overcome by promoting tank mix combinations. Chemicals which are incompatible in long-term storage can be combined successfully because they are exposed to each other in the spray tank for a short period of time. Screening of Insecticide Mixtures For Synergism / Joint Action Analysis of Insecticides Synergism or antagonism in insecticide interaction can be assessed using various methods Sun and Johnson 10 proposed a simple method for the calculation of the joint toxicity of various insecticide mixtures to differentiate among synergism, antagonism, similar and independent actions. The joint action pattern of insecticides is judged on the basis of co-toxicity co efficient (CTC) values. The pyrethroid and non-pyrethroid combinations prepared on the basis of LD 50 of each insecticide revealed that pyrethroids when mixed with OP compounds (profenofos and chlorpyrifos) exhibited synergistic action (CTC>100), but in combination with endosulfan showed antagonism (CTC<100) 13 (Table 2). Sun and Johnson 10 method is not applicable when the toxicity of only a single concentration of combination products is tested, because the log-dose or log-concentration probit lines cannot be obtained with single concentration. The methodology adopted by Mansour et al. 11 can be followed to evaluate the joint toxicity of the mixtures on the basis of co-toxicity factor when single concentration is applied. Ready mix formulations in pest management - Indian scenario The ready mix formulations were reported to be very effective in managing the pests in different crop ecosystems (Tables 3 and 4). They were found to be better than the efficacy of the component chemicals when applied alone 46. (35% of triazophos + 1% of 1.25 lit ha -1 was significantly better than deltamethrin and triazophos in reducing the bollworm damage and increasing the seed cotton yield. Polytrin C (40% of profenofos + 4% of 1.50 lit ha -1 was superior to all other treatments against H. armigera however; both the component chemicals (cypermethrin and profenofos) gave poor control of the bollworm when applied individually 46. Considerations for proper use of insecticide mixtures in IRM 1) Pest populations to be controlled should be susceptible or have low levels of resistance to each insecticide used in the mixture. Under such conditions, the frequency of multiple resistant genotypes in the population may be maintained at lower levels 47. Among the ready mix formulations registered in India, more than 85% are having synthetic pyrethroids as one of the components in the mixture (Tables 1 and 5). Survey on insecticide resistance in India clearly indicated that the resistance to synthetic pyrethroids was very high especially, in the cotton bollworm, H. armigera Hence, sustaining the efficacy of the ready mix formulations is questionable in the future. 2) Each insecticide of a mixture should not have cross-resistance to populations of resistant pest concerned 47. Since, the components of the available ready mix formulations in India have different modes of action and detoxification routes, the possibility of development of cross-resistance may not arise. However, we cannot stick to this point firmly, because the deltamethrin resistant strain of H. armigera had developed positive cross-resistance to chlorpyrifos 53 even though mode of action of these two chemistries are entirely different. Table 1. Approved formulations of combination pesticides 1. S. No. Combination Product 1. Carbaryl 4% + Gamma BHC 4% Gr. 2. Deltamethrin 1% + Triazophos 35% EC 3. Cypermethrin 4% + Profenofos 40% EC 4. Cypermethrin 5% + Chlorpyrifos 50 % EC 5. Cypermethrin 3% + Quinalphos 20% EC 6. Alphamethrin 1% + Chlorpyrifos 16% EC 7. Fenvalerate 3% + Acephate 25% EC 8. Cypermethrin 5% + Phosalone 24% EC 9. Cypermethrin 5% + Ethion 40% EC 10. Deltamethrin 0.75% + Endosulfan 29.75% 11. Cymoxanil 8% + Mancozeb 64% WP 12. Methyl bromide 98% + Chlorpicrin 2% 13. Propoxur 0.25% + Cyfluthrin 0.025% Aerosol 14. Cyfluthrin 0.025% + Tranfluthrin 0.04% 15. Imiprothrin 0.1% + Cyphendthrin 0.15% 16. Propoxur 0.5% + Cyfluthrin 0.025% Spray 17 Propoxur 0.5% + Cyfluthrin 0.015% Spray 279

3 Table 2. Co-toxicity co-efficient (CTC) for insecticide mixtures against H. armigera 13. Insecticide LD 50 CTC Profenofos + Fenvalerate Profenofos + Cypermethrin Profenofos + Lambda cyhalothrin Chlorpyrifos+ Fenvalerate Chlorpyrifos + Cypermethrin Chlorpyrifos + Lambda cyhalothrin Chlorpyrifos + Deltamethrin Endosulfan + Fenvalerate Endosulfan + Cypermethrin Endosulfan + Lambda cyhalothrin OPs may be competitive substrates for the same oxidase. Consequently, in the resistant strain, binding of OP to monooxygenases firstly activates the molecule and secondly may prevent the binding and subsequent degradation of pyrethroid by monooxygenases 53. Hence, it may be suggested to go for pyrethroid- phosphorothionate combinations rather than pyrethroid-phosphate mixtures. The OP + pyrethroid mixtures were found to be synergistic as per the co-toxicity coefficient values against the pyrethroid resistant population of H. armigera in India. However, the level of synergism was not much encouraging to go for field recommendation 13. Nurelle D 505,, Polytrin C 44 EC, Chlorguard plus 50.4 EC, Decidan 32.8EC, Cyphos 202 EC and Duet 17 EC were identified as potentiating mixtures against bollworms on cotton 46. However the implications of continuous Table 3. Combination products in cotton pest management. Combination product Dose (per ha) Target pest 1.25 lit H.armigera, Bemisia tabaci Gennadius Earias vittella F., Pectinophora gossypiella Saunders 14 Nurelle D lit H..armigera 14 Deltaphos 36 EC 1.0 lit P.gossypiella, E.vittella, H.armigera 14 Etofenprox 10 EC 150 ml H.armigera 17 + Cypermethrin 25 EC ml Alphacypermethrin 4% ml P.gossypiella, E.vittella, H.armigera 14 Chlorpyrifos 48% lit Polytrin C 0.06% H.armigera 18 Polytrin C 1.0 lit Bollworm complex 19 Lufenuran 400 ml Bollworm complex and sucking pest complex 22 + Diafenthiuron Cypermethrin 2.5% + Chlorpyrifos 22.5% ml 2 lit ( g a.i) E.vittella,E.insulana, P.gossypiella Sylepta derogata Fb. 20 Lufenuran 3% g a.i Bollworm complex 27 + Profenofos 30% Deltamethrin 0.8% 1.25 and 1.5 lit Bollworm complex 32 + Endosulfan 32% Cypermethrin 5% 1.0 and 1.25 lit + Triazophos 35% Endosulfan 25% 2.0 and 2.5 lit + Triazophos 25% Deltamethrin 1% + Triazophos 35% 1.25 lit Chlorguard Plus 1.0 and 1.2 lits Bollworm complex 37 Chlorguard Plus 50.4 EC Nurelle D lit Bollworm complex 16 3) The mixtures selected should have significant synergism to reduce the selection pressure of pesticide to pest 47. The cypermethrin-profenofos combination was reported to be the best mixture to manage the pyrethroid susceptible H. armigera. This mixture however, had lost efficacy with the occurrence of pyrethroid resistance. Consequently, the pyrethroid-triazophos association was found to be the best to revert the resistance in H. armigera 53. The synergistic effect observed between the deltamethrin-triazophos mixture (1:1) against the pyrethroid resistant population might be due the competitive inhibition of monooxygenases. The monooxygenases, which are overproduced in the resistant strain activate the phosphorothionate OPs through oxidative desulfuration (the activation of P=S to P=O) to toxic oxon analogues 54. Thus, the pyrethroids and phosphorothionate use of the mixture on the development of resistance need to be looked into. 4) The insecticide mixtures should delay the development of resistance 47. The rate of development of resistance in H. armigera was reported to be low while using the mixture as against the selection by single insecticide. The resistance increased very slowly when H. armigera was exposed to a mixture of three insecticides (cyhalothrin + phoxim + parathion methyl or cyfluthrin + endosulfan + quinalphos) for twenty generations. In contrast, it was very rapid when the selection pressure was created by single insecticide; intermediate increase in resistance occurred to twoinsecticide mixtures 55. The mixtures were also found to delay the resistance development in sucking pests also. After 14 generations 280

4 Table 4. Combination products in pest management in different crop eco systems. Combination product Dose (per ha) Target pest Rice Cypermethrin 2.5% + Chlorpyrifos 22.5% EC 2 lit. Scirpophaga incertulus W., Cnaphalocrocis medinalis Guenee, Hieroglyphus banian Fb. 21 Bull dock star EC Upacy 50 DF Confidor ultra 100EC Chick pea Koranda 28 EC Nagata 45 EC Nurocombi 55 EC Chlorguard Plus 50.4 EC 392 g a.i 500 g a.i 30 g a.i 2.0 lit. 1.0 lit lit. 1.0 lit lit lit. 440 g a.i 450 g a.i C. medinalis, S. incertulus 42 H. armigera 43 H. armigera 29 Pigeonpea 440 g.a.i H. armigera 28 Bhendi Polytrin C 44EC 2 lit. Earias.spp., Aphis gossypii Glover, Tetranchus cinnabarinus Boisdual, Amrasca devastans (Dist.) 44 Cypermethrin g a.i E.vittella and H. armigera 26 + Chlorpyrifos Brinjal 0.044% B.tabaci lit. Leucinodes orbonalis Guenee 34 Carbosulfan + Quinalphos 1 ml each/lit L.orbonalis % L. orbanalis 38 Tomato Lethal super 1 lit. 1 lit. 2 lit. H. armigera 30 Mango Nurelle D ml/lit Amritodes atkinsoni L. 35 Sugar beet Nagata 45 EC 675 g a.i Spodoptera litura F. 45 Cabbage 1.25 lit 31, 33 Plutella xylostella (L.) Table 5. Ready mix formulations under evaluation and/ or commercially available in the Indian market. Ready mix formulation Components Synthetic pyrethroid + OP Compounds Duet 17 EC Alphamethrin 1% + Chlorpyrifos 20% Chlorguard Plus 50.4EC Alphamethrin 24% + Chlorpyrifos 48% Bulldock star Betacyfluthrin 1.25% + Chlorpyrifos 25% Nurelle D 505 Cypermethrin 5% + Chlorpyrifos 50 % Cyphos 202 EC Cypermethrin 2% + Chlorpyrifos 20% Lethal Super Cypermethrin 2.5% + Chlorpyrifos 22.5% Cypermethrin 4% + Profenofos 40% Cyperphos Cypermethrin 5% + Triazophos 35 % Nagata Cypermethrin 5% + Ethion 40% Upacy Cypermethrin 5% + Acephate 45% Deltamethrin 1% + Triazophos 35% Koranda Fenvalerate 3% + Acephate 25% Synthetic pyrethroid + OC Compound Decidan 32.8 EC Deltamethrin 0.8% + Endosulfan 32.0% Synthetic pyrethroid + Neo nicotinoids Confidor ultra Betacyflutrhin 50% + Imidacloprid 50 % OC + OP Compound Endophos Endosulfan 25% + Triazophos 25% 281

5 of selection the level of resistance in aphids, Myzus persicae Sulz. was 52.6 fold to fenvalerate and 11.1 fold to sumithion while it was only 3.5 fold to the mixture of fenvalerate and sumithion (3:7) 56. 5) It has been estimated that only 1.0% 57 or 0.1% 58 of the applied insecticides reaching the target and the remaining enters the environment as pollutant. Since, the effect of pesticide mixtures is considered more toxic than their individual components, extra care should be taken to reduce the health hazards. In formulating a mixture, each candidate should have lower mammalian toxicity in order to decrease overall mammalian toxicity of mixture 47. Scope For Insecticide Mixtures in IRM in India 1.The insecticide mixtures so far evaluated had synthetic pyrethroids as one of the components. The resistance to synthetic pyrethroids in H. armigera and resurgence to A.gossypii and B. tabaci have been well documented The possibility of combination products of similar action hastening the development of resistance in insects cannot be ruled out. 3. Chances for the development cross resistance may increase. 4. In insecticide resistance management, one of the strategies is to use the different chemicals alternately. The combination products will remove the scope of using insecticides alternately. 5. The recommendation of insecticide mixtures may make the farmers who do not use pesticide combination, will also do the same. 6. Combination product if permitted will be used as a matter of routine. There is no effective enforcement mechanism to counsel its one time or need based use. If there is any need arises, tank mix can be considered for one time remedy under technical supervision. 7.The combination products if permitted may be extended to botanicals and other insect growth regulators, defeating the very purpose of developing neem and biorational compounds. 8. The insecticide mixtures should be considered as one of the tactics for resistance management strategy. If any pesticide mixture is used extensively there will be a danger of selection for resistance in pest insects. 9. New combination of insecticides (potentiating mixtures) effective against resistant insect pests need to be identified, this will guide the pesticide industry for further development of proper ready mix formulations. 10. Pest-specific ratio of components in the mixture need to be fixed on the basis of LD 50 values which can be served as a ready reckoner to the farmers for scientific mixing of insecticides against target pests. 11. Testing of the combination products can be taken for generating data and such combinations can be recommended after evaluating the development of resistance. 282 Guidelines for The Evaluation of Pesticide Mixtures Fast development of pesticide technology, more appropriate use of pesticides and improved IPM methods, render safer use of pesticide mixtures. The strengths and weaknesses of pesticide mixtures, such as the opportunity for overcoming resistance, improving biological spectrum and reducing the overall dose of chemicals applied should be analysed. The following guidelines are being followed in Tamil Nadu Agricultural University, India for the evaluation of pre-mix or tank mix formulation. 1.Permission may be given for evaluating mixture of pesticides belonging to different categories (insecticide + fungicide and so on). 2.Combination of two insecticides having independent mode of action may be tested (e.g. insect growth regulator + neurotoxicant). 3.Pest populations to be controlled should be susceptible or have low level of resistance to each pesticide used in the mixture. This will prevent development of multiple resistance genotypes in the population. 4.Target pest concerned should exhibit no cross resistance to pesticide molecules in the mixture concerned. Cross resistance problem is serious and extends to other pesticides, when there is a common de-toxication system or target site insensitivity. 5.The mixtures should have significant potentiation effect so as to reduce the dosage and thereby reduce the selection pressure and to delay the development of resistance. 6.Each candidate pesticide to be used in the mixtures should have lower mammalian toxicity in order to decrease the overall mammalian toxicity of mixture. 7.The combination products even though allowed for testing, will not be recommended for adoption till definite data are generated on their side effects namely resurgence, resistance, secondary pest outbreak and other similar effects in addition to bioefficacy, phytotoxicity, effect on non-target organisms, residues and waiting periods. 8. A panel of experts scrutinize the individual cases of pesticide combinations based on the above guidelines and they may allow or reject the combinations. These guidelines may be used as a model for the evaluation of pesticide mixtures by other research institutions also. Conclusions Considering the positive cross resistance among the synthetic pyrethroids 60, lack of significant synergism, low rate of literacy of Indian farmers and their small land holdings, the blanket use of combination products may not be advisable in the present day intensive Indian farming. References 1 Society of Pesticide Science India Pesticide statistics. Pesticide Research Jour Fakrudin, B., Patil, B. V., Badariprasad, P. R. and Prakash, S. H Insecticide usage pattern in South Indian cotton ecosystem to control cotton bollworm Helicoverpa armigera. Resistant Pest Manage Newsl.

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