I assume you already have it in full shade? Make sure your bin has lots of air flow. You could try placing a wet towel over your bin - as the water evaporates from the towel it will cool the bin, especially in your dry climate. If you have a wooden bin you could even wet down the wood itself, that would help it to stay cool. I've even heard of of people putting a frozen ice pack or water bottle inside on top of the castings or partially buried.

Give some of those ideas a try and I'm sure your worms will be fine. Hope this has helped.

Harry

I've never had to deal with that issue (yet?)... but if it were me, I think I'd dig a hole 2 or even 3 feet down into the soil and put the bin in there with a shade sail or even just a tarp over it and angled to promote rain run-off. The earth is basically a large heat sink of sorts and as such tends to resist atmospheric temperature fluctuations thus it will appear to be warmer in the winter and cooler in the summer. Its just a thought...

Paint the bin white or silver so it absorbs less heat from the sun.

When using frozen ice packs/water bottles, make sure you wrap them in newspaper so worms will not stick to them and die. It will also keep them clean for re-use.

An old refrigerator picked up free off CraigsList is as good a system as I've found. Turn t-stat to warmest setting and keep unit in shade. 75% of the time refrigerators quit because the $10 fan inside burnt out,many times oiling bearings start's fan working again. If fan is out and you are really frugal,just remove most devider and insulation between freezer section and refrieration section. If box is visiable in garden,paint bugs,flowers,small animals, leaves and such on box. I realize rewireing fridge is out of reason for many but sometimes you an have a handy freind do it for you. For winter months a switch connected to defrost heater will keep box warm unless weather stay's below teens for days on end. I am experimenting with two boxes where I turned compressor 90 degrees so box will run laying on it's back. Fan inside box is disconnected, if box has one( not all do) the exterior fan must remain operational and provision made to alow free flow. T-stat is disconnected and compressor run's 10 minutes once at daybreak,once mid-day once mid-afternoon and once at dusk on timer. I salvaged defrost timers off commercial freezers but timers are cheap at big box store. I removed bottom cover in freezer compartment to drill several holes between coils into refergerator section. Through the holes I ran bare scrap aluminum & copper wire and pipe. Wire and pipe are coiled in freezer amoung other scrap bits of Copper/aluminum as ballast,through holes and 25% to 75% of way through refrigerator section. Conduction carry's cooling to refrigerator which is filled with bedding. If you lay a box on it's back,cover door or rain will go into the box. I screwed roofing metal to door overhanging all sides 2".