Combination pesticide products: Do they serve the purpose?

                            Photo: Pesticide solution in preparation

                                Photo: drone spraying at Padegaon in Maharashtra

A case study for herbicides in sugarcane

Combination pesticide products, mixture of two or more or premix are used for control of pests similar to their components individually. These are supposed to have a broader range of biological activity and are more effective against the pests that have evolved or are likely to evolve resistance to one of components of the mixture. Furthermore, they help in economising cost of application as a single application is good enough rather than spraying two pesticides separately. As far as their ecotoxicity is concerned, these are as safe to the environment as they are used in the comparable doses as their approved components. In view of these merits, the regulatory agencies approve such products. Indirectly, this discourages tank mixture that the farmers do on their own. The latter may also risk the efficacy and safety by mixing two pesticides which may not be physically, chemically and biologically compatible. Over the years, the combination products are being increasingly used, as the farmers find them convenient to use.

But, question remains about their ability to serve the purposes. I have therefore decided to review approvals of the Central Insecticide Board [Directorate of Plant Protection, Quarantine and Storage, Ministry of Agriculture and Farmers’ welfare, the Government of India] with specific reference to the combination products for weed control in sugarcane. Of the herbicides registered under the Insecticide Act 1968, the following are approved specifically for control of weeds in the sugarcane updated to 31.05.2022 [http://ppqs.gov.in/divisions/cib-rc/major-uses-of-pesticides].

1.     Ametryn 80% WDG

2.     Atrazine 50% WP

3.     Clomazone 50% EC

4.     2, 4-D dimethylamine salt 58% SL

5.     2, 4-D sodium salt tech 80% w/w

6.     2, 4-D ethylester 38%EC

7.     Diuron 80% WP

8.     Halosulfuron methyl 75% WG

9.     Metribuzin 70% WP

10.Metsulfuron methyl 20% WP

11.Sulfentrazone 39.6% SC

Of these, only ametryn, atrazine, clomazone, diuron, metribuzin and sulfentrazone used in the combination products are approved, while others like hexazinone, mesotrione, topramezone and trifloxysulfuron are not to be found in the list of registered herbicides. Obviously, there is no information about the efficacy and safety of these standalone unregistered herbicides in the above website, except that they are a part of combination products.

Let us therefore look some aspects of their biological activities [Table].

 

Table. A list of combination products and their individual components

No	Name	Mode of action by inhibiting#	EPA/IARC class##	A.I. * gm/ha	No of weed species targeted	Waiting period [days]
1	Ametryn 73.1% w/w + Trifloxysulfuron sodium 1.8% w/w WG	both components have different mode of action		937.5-1125	Echinochloa colona, Cynodon dactylon, Cyperus rotundus, Amaranthus viridis, Trianthema spp., Digitaria sanguinalis [6]	221
	Ametryn 80% WDG	photosynthesis at PSII-serine 264 binders [Gr 5]	suggestive of being  ##	2000	Dactyloctenium aegyptium, D. sanguinalis, C. dactylon, Ageratum conyzoides, Trianthema monogyna, Parthenium hysterophorus [6]	311
	Trifloxysulfuron	acetolactate synthase [Gr 2]	not likely to be ##	no info	no info	no info
2	Clomazone 22.5% w/w + Metribuzin 21% w/w WP	both components have different mode of action		1080	Trianthema spp., A. viridis, Phyllanthus niruri, Bracharia spp., D. aegyptium, Echinochloa spp., Digitaria spp., C. dactylon [8]	307
	Clomazone 50%EC	deoxy-D-xylulose phosphate synthase [Gr 13]	not likely to be ##	750-1000	E. colonum, Brachiaria repens, D. aegyptium, Trianthema portulacastrum [4]	296
	Metribuzin 70% WP	photosynthesis at PSII-serine 264 binder [Gr 5]	not classi-fiable as ##;	1050-1400 after 25-30 DAP; 1050-2000 prior 3-5 DAP	C. rotundus, C. dactylon, Asphodelus fistulosus, Chenopodium album, Convolvulus arvensis, Portulaca oleracea, Anagallis arvensis, Cichorium intybus, E. colonum, D. aegyptium, P. hysterophorus, Commelina spp.[12]	60
3	Hexazinone 13.2% + Diuron 46.8 % WP	Both components have same mode of action		1200	E. colonum, D. aegyptium, T. monogyna, A. viridis, Ipomea spp.,  C. rotundus, Cyperus esculentus,  Setaria spp., P. hysterophorus, Euphorbia hirta [10]	282-306
	Hexazinone	photosynthesis at PSII-serine 264 binder [Gr 5]	not classifiable as ##	no info	no info	no info
	Diuron 80% WP	photosynthesis at PSII-serine 264 binder [Gr 5]	likely to be ##	1600-3200	Cyperus iria, Portulaca racea, Echinochloa crusgalli, Cynotis spp., Amaranthus spp., Convolvulus spp., Digitaria spp. [7]	no info
4	Mesotrione 2.27% w/w + Atrazine 22.7% w/w SC	both components have different mode of action		875	Trianthema spp., A. viridis, E. colona, D. sanguinalis, C. rotundus, D. aegyptium [6]	190
	Mesotrione	hydroxyphenyl pyruvate dioxygenase [Gr 27]	not likely to be ##	no info	no info	no info
	Atrazine 50% WP	photosynthesis at PSII-serine 264 binder [Gr 5]	not likely to be ##	500-2000	Portulaca oleracea, Digitaria spp., Boerhavia diffusa, Euphorbia spp., Tribulus terrestris [5]	no info
5	Sulfentrazone 28% + Clomazone 30% WP	both components have different mode of action		1400	A. viridis, Trianthema spp., Digera arvensis, Physalis spp., Brachiaria spp., C. dactylon, Echinochloa spp., D. aegyptium, C. rotundus [9]	302
	Sulfentrazone 39.6% w/w SC	protopor-phyrinogen oxidase [Gr 14]	no evidence of ##	720	Trianthema spp., Digera spp., Amaranthus spp., Phyllanthus spp., Euphorbia spp., Dacteloctenium spp., Digitaria spp., Brachiaria spp., Echinochloa spp., Cynodon spp., Cyperus spp.[11]	306
	Clomazone 50%EC	deoxy-D-xylulose phosphate synthase [Gr 13]	not likely to be ##	750-1000	E. colonum, B. reptans, D. aegyptium, T. portulacastrum [4]	296
6	Topramezone 10 g/l + Atrazine 300 g/l SC	Both components have different mode of action		930	A. viridis, C. dactylon, P. oleracea, Alternenthera sessilis [4]	268
	Topramezone	hydroxyphenyl pyruvate dioxygenase [Gr 27]	not likely to be ##,	no info	no info	no info
	Atrazine 50% WP	photosynthesis at PSII-serine 264 binders [Gr 5]	not likely to be ##	500-2000	P. oleracea, Digitaria spp., B. diffusa, Euphorbia spp., T. terrestris [5]	no info

*active ingredient; ##carcinogenic or carcinogen to humans; DAP, days after planting; Gr refers to separate classes that are distinct in their mode of action at the target sites developed by the Herbicide Resistance Action Committee; #https://hracglobal.com;

From the above table, it is clear that of six combinations, only two combinations are those whose individual components are registered, with information on their efficacy and safety as separate entities. The other four combination products have one component which is not registered and hence, there is no information on these unregistered products.

This raises the questions

1.     Do these combination products serve the purposes stated above in the absence of information on the unregistered component as a separate entity?

2.     Normally, components of the combination product have different modes of action, but not in case of hexazinone 13.2% + diuron 46.8 % WP which act by inhibiting photosynthesis at PSII-serine 264 binders [Gr 5]. How will such product have an ability to control development of herbicide resistance?

3.     Quite interestingly, the combination product, Sulfentrazone 28% + Clomazone 30% WP is recommended at twice the dose of its components, which does not offer any benefit of saving on quantity nor of extending bioactivity range or controlling evolution of resistance in the weeds. Furthermore, its two-fold more dose has the same waiting period as single-dose of its individual components.

4.     As most registered products have waiting period of more than 300 days, they are likely to contaminate products like cane juice, syrup, candy, jiggery as the canes are harvested over a range of duration. Is there any label precaution?

5.     What could explain low waiting period of metribuzin which has similar mode of action like that of ametryn and metabolism?

6.     The waiting period data for atrazine which is a component of two mixtures viz., topramezone 10 g/l + atrazine 300 g/l SC, mesotrione 2.27% w/w + atrazine 22.7% w/w SC is missing.

It is high time that the Government and the pesticide industry develop consensus on the combination products.