The parent page for this one is at https://beatingbedbugs.wordpress.com/dustspowders/ and contains additional information.
In regard to humidity and diatomaceous earth, the following has been found according to the article at http://www.pctonline.com/pct1213-Diatomaceous-earth-study.aspx,
“Laboratory studies have further shown that diatomaceous earth is less effective against bed bugs and other pests at higher humidity (Subramanyam and Roesli 2000, Doggett et al. 2008). Such conditions may occur in the tight spaces where aggregating bed bugs tend to dwell. Moreover, many dusts, including DE, have repellent effects on insects. We’re currently investigating this in respect to bed bugs and the industry’s use of dust.”
Studies were done to assess the efficacy of Cimexa in dry dust form vs an aqueous (mixed with water and sprayed) form. According to the article at http://www.pctonline.com/Author.aspx?AuthorID=9828,
“Lab and field tests further indicate that silica gel is more effective applied as a dust than an aqueous suspension.”
What does this mean? That, when utilizing dusts/powders to control and exterminate bed bugs, an environment with a low level of humidity is more effective. One way to achieve a lower level of humidity in an environment is to utilize a dehumidifier.