Borax and sugar kills the ants so incredibly well, it’s delightful! Except that the big brown ants don’t like it, won’t touch it, and so it doesn’t kill them. But it’s killed every other variety of ants we have here. 
Yep, I’m delighted.
I did a 1:1 mix and it worked just fine. You should know how it works… The ants pick up the sugar/borax and take it back to the nest, where it’s redistributed among the colony. So it kills the entire family – and it can take 2-3 days.
The pitfalls:
Here, where the humidity levels are so high, the sugar/borax will melt into a glassy slab of the stuff in a couple or three hours rather than stay as individual crystals. So, when I put it out, if it’s inside, I put it on a yoghurt lid or some such for easier cleanup. Outside, I don’t particularly care. Rain’ll wash it 
away eventually. :p
With those tens of thousands of ants, I put out a few piles of sugar/borax that were each a couple of teaspoons and it did the trick. I probably didn’t need to use anywhere near as much as that.
As for Oberon, he doesn’t go near the stuff. Well, he smells it, then sometimes he sneezes, and then he goes away.
Will the stuff kill the monitor lizards or geckos? I doubt that monitor lizards would be affected – they’re so much bigger than ants that it would probably take a half pound or more of the stuff. Geckos, I don’t know.
Thing is, I don’t know that I really want to kill the monitor lizards. By all accounts, they’re why the squirrel and rat population is under control and why we don’t have more bugs in the house, so I think we’ll just leave them alone.
