I think that Provado Ultimate Bug Killer should get it. Derris is no longer available; I'm not sure about malathion - on checking, yes, it looks like it's been banned too.
N. Yorks - wet, wet, wet....
Very similar to Stockton here in N. Yorks - dull and grey, but dry. But tomorrow we're back to lots of rain - "I'm dreaming of a flooded Christmas..."
The thing that I'm curious about is what the effect of chemicals caught in the filter might be. I have to assume that some of the chemicals "filtered out" actually remain bound to the granules.
The morning's rain has dried up now and, although overcast, it's mild at 7C.
It looks like it's related to a calathea to me.
In which case, it likes a moist atmosphere, indirect sunlight and a reasonably warm temperature, no lower than 15C. Keep moist but not soggy.
They can also come in via the holes left for pipes to go through - some spray expanding foam from the DIY store can solve that problem easily.
Similar here in North Yorks, but clouded over a bit towards evening, meaning we're due for a frost-free night.
The only thing you can do is cut it back to a section that is still reasonably vertical, put it in a light position in a warm room and hope. Since the plant is not frost hardy, your chances of it recovering are not very good, I'm afraid.
Absolutely throwing it down here most of the day, so the frost and ice is gone, if nothing else.