Looks like a soil moisture problem to me. Try repotting it in a glazed pot, because terra cotta pots can dry out pretty quickly in hot sunny locations. Some people recommend double-potting for this kind of situation.

Also, ferns don't always tolerate chemicals as well as other plants, and spraying with pyrethrin several times a week sounds to me like it would be too much. Is there a recommendation on the pyrethrin container about how often to spray? With ferns, you should figure that the recommended dosage should be cut at least in half, for treating them and also for feeding them. You might be loving it to death...

OK, I'll lay off the spray for a while and see about switching to a glazed pot. I hope I haven't doomed the plant, but if so I'll know better the next time. Thanks.

That is a water loving fern.One of the most water thirsty tree ferns you could keep.One hot day and it goes dry,it goes die.
Planted in ground in the right spot..it also is one of the fastest growing ferns..very nice when it flushs with a group of fronds.
You could save yourself an almost certain killing of it one day in a pot and free it by planting-but still water like heck!