Can they been seen? I’ve heard they’re as large as an apple seed, so I would assume yes?
Correct but they didn't hatch that size. By the time they are the size of an apple seed, they have been breeding and the dark appleseed color means they are full of someone's blood.
We never tried the diatomacheous earth. Montag813 reports good results using diatomaceous earth for their infestations. I considered it for our infestation but due to the widespread nature of our infestation, I opted for "do it yourself heat treatment".
To me, the juvenile bugs are the greater problem than the mature, easier to detect appleseed sized ones. The juvenile bugs are tiny like a grain of sand and can be difficult to detect. They may get on you while you are sitting on the couch. Then, when you get up from the couch, go into the bedroom, sit on the bed to take your shoes off they may hop off onto the bed. In this scenario, any traps the bed posts are sitting in will have been of no effect as the bed became (re)infested via human carrier.
For local treatment of items like mattresses and box springs, 70 percent rubbing alcohol kills them on contact. Use the $5 "pro grade" spray bottle from Home Depot. The $3 standard grade spray bottles don't last long with the alcohol. Be careful about not getting the alcohol on varnish... we made that mistake.
But heat really gets them. They can survive the washing machine but the 130+ degree temps in the dryer kill them. Summertime temps inside a parked car sitting in the sun can also kill them.
For our house infestation, I got a few portable electric heaters at a local thrift store along with some box fans, ordered about 6 inexpensive wireless thermometers on Amazon. Turned the household HVAC off. Unplugged electronic devices in the room being heated. Put the thermometers between mattresses and box springs, inside drawers, inside jacket pockets, on the floor, etc. Turned the heaters and fans on and waited for all thermometers to hit 130F or higher then waited a couple of hours. From time-to-time I went into the room to visually make sure everything was ok, reposition fans and/or thermometers. Repeated the process room, by room. For the larger common areas, I added a kerosene and propane torpedo heater to help get the space up to 130F and higher.
Heat treatment sounds scary at first but 130F-160F temps are far from flammable.