5 males and 15 females are housed in a 19 gallon black Sterilite container. The house stays at 78 F constantly.
They may be breeding already - I saw the red egg sac things on the females' rears.
Either way, I bought a 10-20 heat pad, but I need to somehow attach a rheostat to it... the same type of heat pad heats my leopard gecko's hot side to 95-100 F. As I understand, that's too hot for dubias, so a rheostat must be necessary.
Even with the rheostat, will the heat pad melt the plastic container?
Is there any better or cheaper way to heat the container?
Also, do I need to be heating 3/8" dubias that are housed in a different container, or is the heat only needed for breeding?
78 isn't a bad temperature. You could leave them at that and they'll be fine. They just may not breed as quickly. I keep mine in the basement at around 65-70 and turn off the heat projector now and then to slow down the colony's growth. I don't think a heat pad would melt the plastic unless it malfunctioned (which is a legitimate concern). You could also try heating from above by using a ceramic heat emitter or a heat projector (cooler) like this one http://www.reptileuv.com/shop/index.php?main_page=product_info&products_id=196 The downside to heating this way is that it can cause the air to dry out if the food isn't replaced regularly (food being a source of moisture and humidity).