FROGS AND CHEMICALS - DO THEY MIX ?

Written by Reinier Mann of the School of Environmental Biology at Curtin University of Technology (Bentley, W.A.) and published in early 1998 in "In the Spotlight" (Vol 4, No. 1).

If I were a gardener, I may have discovered sooner that our unkempt garden beds were harbouring frogs. Instead it was several months after moving into our Yarraville property in Melbourne, and on an extremely wet evening that I was pleasantly surprised to find frogs crawling out of the garden bed to frolic on the warm wet driveway. Our over excited cats were promptly locked inside. We lived about 200 meters from Stoney Creek. I knew there were frogs in the creek but I had no idea they were in the garden.

Now if I were a gardener (or a reserve manager) who relied upon insecticides, herbicides and other products available to make the job easier, would I still have frogs in my garden? How tolerant are frogs to this kind of chemical assault? In the context of the global decline in amphibian populations, this may be an important question.

One such chemical has been in the spotlight recently. The chemical is a herbicide called glyphosate and it is considered to be one of the most non-toxic pesticides available. Consequently, glyphosate has a reputation as an environmentally friendly pesticide. This reputation extends to the numerous products which incorporate this chemical, but these products are formulations which incorporate various additives which may not be as deserving of glyphosate's benign reputation.

In response to numerous anecdotal reports of call cessation or mortality of frogs following herbicide application, the Western Australian Department of Environmental Protection (DEP) commissioned the Curtin University Ecotoxicology Unit to investigate the toxicity of commonly used glyphosate-based herbicides to several species of Western Australian frogs.

The subsequent report (Bidwell and Gorrie, 1995), highlighted the importance of formulation additives as toxicological agents rather than the active ingredient glyphosate. The additive in question was a surfactant which has been used for many years in such formulations. The toxicological properties of the surfactants employed in these formulations with regard to some aquatic species have been known for a long time (Folmar, et al., 1979). In general, such chemicals have not been considered a threat to aquatic organisms for two reasons. Firstly they do not persist in the environment and second, the dilution factor in open water bodies is considered great enough to reduce the concentrations of these chemicals below toxic thresholds. Contrary to this conventional wisdom, the DEP report concluded that the herbicide in question may present a toxic risk to tadpoles and frogs where the water is shallow and the dilution factor is low.

As a consequence of this report the National Registration Authority released the NRA Special Review of Glyphosate (NRA, 1996). The review recommended that the majority of glyphosate based products (i.e. all those that utilise these particular surfactants) be re-labeled (before 30 June 1997) such that they are no longer recommended for aquatic use. They can however still be used in "Dry drains and channels, dry margins of dams, lakes and streams."

Frogs do not live in open water bodies and they don't necessarily live along waterways. They often live and breed in and beside puddles along the road or under tree roots or in burrows in dry drains and beside dams etc. The question I would like to raise is: What is the consequence of herbicide application for adult frogs which live in the soil, or those frogs and tadpoles restricted to small or temporary ponds in areas which are not designated as 'aquatic'. While I am reluctant to endorse any chemical, where a glyphosate based product is deemed necessary, it may be prudent to employ one of the newer products which have been registered for aquatic situations, in preference to any of the older formulations, irrespective of the location.

While glyphosate formulations are presented here as a case in point, my question is just as relevant to a broad suite of chemicals. What are the implications of the use of other 'weed-killers', 'bug-killers' and soil wetting agents? If I were to follow this argument further I would also ask: What are the implications of the use of fertilisers and even animal manure which may radically change the pH and chemical profile of the soil? We don't know the answers to these questions. As a PhD student at the Curtin University of Technology I am looking at some of these chemicals and their potential as toxic agents to frogs.

If you are a gardener, a golf course green keeper, a reserve manager, a municipal environmental officer or an employee of a state conservation department, you may want to keep these questions in mind. After all, you may have frogs in your garden, and like myself, you may not even be aware that they are there.

Author: Reinier Mann

REFERENCES

Bidwell, J. R. and Gorrie, J. R. (1995) Acute toxicity of a herbicide to selected frog species., Technical Series:79, Department of Environmental Protection, Perth.

Folmar, L. C., et al. (1979) Toxicity of the herbicide glyphosate and several of its formulations to fish and aquatic invertebrates., Archives of Environmental Contamination and Toxicology. 8: 269-278.

NRA (1996) NRA Special Review of Glyphosate., NRA Special Review Series:96.1, Chemical Review Section, National Registration Authority for Agricultural & Veterinary Chemicals, Canberra.

EDITORIAL NOTE

As far as we are aware the only chemical mix that is currently thought to be OK for use near wetlands is a mix being sold by Monsanto Chemicals called Biactive. We have tried to source it but it seems it is only available in large quantities. Perhaps some pressure from gardeners and small users of such herbicides may cause Monsanto to make this mix more readily available.