What is DE?
DE is diatomaceous earth, a natural product often used in insecticides. From Wikipedia:What is DE?

A sample of food grade diatomaceous earth
Diatomaceous earth ( /ˌdaɪ.ətəˌmeɪʃəs ˈɜːrθ/) – also known as D.E., diatomite, or kieselgur/kieselguhr – is a naturally occurring, soft, siliceous sedimentary rock that is easily crumbled into a fine white to off-white powder.