Mallee Frog Blog

It's raining again! 10.5mm so far. We probably won't get much more but it will be good for the crops and frogs. They are still quiet - and I have been amazed at how little water the pond has been losing to evaporation over the last month, despite the warm (20C days) weather we've had.

Last night, I discovered that the comment-email function on this blog wasn't working, and read through some comments that I'd missed. An interesting comment was by Michael Angel on End of Season, where I'd been thinking about whether my spraying glyphosate might have affected the frogs in the dam. Michael reckoned the surfactant in the glyphosate could be a bigger problem than the glyphosate itself. I've had a brief look around and found this online thesis by RM Mann:

Toxicological Impact of Agricultural Surfactants on Australian Frogs

Much of it has been published elsewhere, including the reference I'd used on glyphosate toxicity in the earlier post. It may be a useful link for people who don't have access to the journal articles.

Looking at it very briefly, surfactants seem to affect tadpoles at 1.1 mg/L and higher, depending on type. I'd estimated 250mg surfactant (50ml Glyphosate 450 containing 10% surfactant, 5% landing on water in the dam) added to 150L or more of water, so there could have been a mild effect. What interests me more though, is how the surfactants in our shampoo, washing detergent and soap might have affected the frogs. They were living and breeding in our greywater, after some treatment in a septic tank and a bit of 'biofiltration' in the dam. But that's a calculation for another day!

I went for a walk last night with the spotlight and was startled by the number of wrigglers in the dam. It's easy to see a few during the day but at night the water is thick with them. The pond is fine. I think the mosquito numbers have been building up and they are now going crazy with the warm weather.

That means the season has to end for the dam. We live so close to it that we just can't risk having so many mosquitoes over summer. It has been a bit of a stretch sending all of the grey water there with no rain too - I'd been hoping we might have had a crop-saving 25mm or so that might also have filled the dam and given us a chance to use the greywater on the garden. The last decent rain was 2mm in early October and there was very little in September, so if we are going to garden we need to start using that water.

So I've started pumping the greywater (the dam will still get the occasional overflow) and this morning I've sprayed out the grass in the dam (glyphosate). I'm hoping the dead grass will act as a bit of a mulch until we get some vegies started on what's left of the water - there'll be heaps of water stored in the clay under the dam. There will be plenty of seed left to get the grasses going next year.

Hopefully the glyphosate won't have too much impact on the frogs. For people who are interested in these things, this is one paper that reports a tests of glyphosate on frogs at various stages:

Mann and Bidwell(1999)

The species they tested included Limnodynastes dorsalis, the WA version of the Pobblebonk, (which we have in the dam as an adult).  

I estimate that I put 50ml of 450g/l glyphosate on - 23g of active ingredient. Most of that was over the dry part of the dam. If 5% ended up in the water, that would be 1.2g. Mann and Bidwell's lowest threshold for tadpoles was 8 mg/l, which I would have exceeded if the 1.2g was dissolved in less than 150l of water. That's probably about as much water as is there, but I doubt that 5% of the total glyphosate applied made it into the water. The threshold for adults and metamorphs is much higher - if there are tadpoles there I hope that's the stage they're at by now.