The fate of Vetrazin® (cyromazine) during woolscouring and its effects on the aquatic environment

Abstract

A number of ectoparasiticides are used on sheep to protect the animals from ill health associated with infestations of lice and the effects of fly-strike. Most of the compounds currently in use are organophosphate or pyrethroid-based and have been used for 15-20 years, or more. In more recent times, as with other pest control strategies, there has been a tendency to introduce 'newer' pesticides, principally in the form of insect growth regulators (IGRs). Vetrazin® was the first IGR-based compound to be registered for use as an ectoparasiticide in Australia and New Zealand. The active ingredient of Vetrazin® is cyromazine, a triazine-based compound. Prior to the introduction of Vetrazin®, and indeed more recently with other 'new' pesticides, little was, or is known regarding the fate of these compounds during woolscouring. Additionally, given the present practice for woolscours to discharge their aqueous effluents to the aquatic environment, little was, or is known regarding their effects on these receiving waters.

In addition, cyromazine is a chemically 'unique' compound, apparently possessing chemical and environmental stability, as well as being highly polar. This uniqueness required the development of specialised techniques for its analysis. Its environmental stability and polarity also suggested that it could become a significant aquatic contaminant, certainly in localised situations.

This thesis outlines the development of a clean-up method for the analysis of cyromazine residues in woolscouring waste waters and on greasy (shorn) and scoured wool. A clean-up method using Extract-Clean™ ion-exchange cartridges and analysis by both gas chromatography with nitorgenphosphorus detector (GC/NPD) and gas chromatography/mass spectrometry (GC/MS) is described. The fate of cyromazine residues during the scouring of greasy wool was investigated by both laboratory and industrial pilot plant trials. Cyromazine was shown to be readily removed in the first three scouring bowls of a woolscour. A simple model is described that can be used to predict cyromazine effluent concentrations based on a known content for greasy wool. These effluent concentrations can then be used to assess compliance with a proposed environmental quality standard (EQS) established for cyromazine, based on aquatic bioassays.

Aquatic bioassays were conducted with both Chironomus zealandicus (Diptera: Chironomidae) larvae and eggs and Deleatidium sp. (Ephemeroptera: Leptophlebiidae) nymphs. Cyromazine possessed negligible acute toxicity to both organisms, with the concentration required for 50% mortality (LC50) being greater than 100 mg/l. However, significant chronic toxicity to C. zealandicus was observed, with a lowest observable effect concentration (LOEC) of 25 µg/l being recorded. A proposed environmental quality standard (EQS) value of 1.0 µg/l has been suggested, a figure supported by limited aquatic toxicity studies performed elsewhere.

Cyromazine was shown to dominate approximately 30% of the fly-strike control market in New Zealand. This high usage, plus a moderate EQS value, suggests that a minimum seven week withholding period be instigated between dipping for fly-strike control and shearing of treated sheep.

Limited toxicity testing was carried out on another IGR-based ectoparasiticide, Zenith®, containing diflubenzuron. This testing, combined with published aquatic toxicity data, suggests than an EQS value for this compound be set at 0.01 µg/l and a withholding period for dipped sheep be set at 18 weeks. Such a long withholding period is unlikely to be attainable in practice such that, if highly aquatically toxic compounds are going to be used as ectoparasiticides on sheep, then woolscours are going to have to invest in technology to remove pesticides from their effluents.