Crest and San Diego County west of the Laguna Mountains averages somewhere between twenty and thirteen inches of rain a year, so technically little Crest here is not desert, speaking as a hard-core Desert Rat, who only wishes he could live in Wikieup, Arizona. Temperature-wise, we shoot way past a hundred degrees, Fahrenheit, and worms are supposed to do their best between 60 and 80 degrees. My original worm bins were inside an old barn, where the temp could easily be kept in comfortable range. They'd still be there if the roof over the bins had not collapsed and ruined them.
What I ended up doing was building a free-standing bin, about waist-high on legs, four foot-by eight, the outside walls one foot tall, with a canted roof of plywood, tar paper and sheet rock. Center-line, eight foot length, is roughly two feet tall. Both sides of the roof and the eight-foot long walls are hinged. Down the center is chicken wire that divides the bin into two sides. With the first bins, I found that four feet deep is just too hard to work a shovel into. Two feet is manageable, also working toward my plan to see if I can supply my chickens with a supplemental food source of protein, but that's way off.
I placed the bin under what I thought would be enough shade, but as it turned out, the sun still spends way too much time beating down on it. Best laid plans, right? Now I'm thinking I have to build a shade source over it. Unlike the desert, the air does cool off at night, generally, during the summer, so with enough insulation, the temps should be stable inside the bin itself. The design offers some insulation, but I'm going to have to do more. I'm even kicking around trying to build a straw bale structure around it, that could also serve as a root cellar, if I do it well. Heat, I think, contributed to my recent die-off, but I think the zinger was the horse manure I used. Despite some efforts to reduce the issue, I'm afraid the wormer the horse owners uses comes out intact enough in the manure to kill earthworms. Since then, I've made a point to compost the manure. Even if I don't have enough brown material and lose some nitrogen, its better than killing off the worms in the garden.
The reason I put the bin on legs is, I can paint the legs with something like used motor oil, and ants and bugs can't get in. I've also ended up running my chicken run's electric fence system around the legs to keep out mice, rats and squirrels.
I'll keep you up-dated, as this is a work in progress.