It’s probably a good idea, but you don’t have to. If your lawn has a lot of thatch and needs to be dethatched, you should do so before applying pre-emergent. Dethatching stirs up the soil, and pre-emergents like prodiamine need to bond with the soil to protect your lawn from weeds and invasive species. Dethatching after applying pre-emergent can reduce the pre-emergent’s ability to protect your lawn or possibly even make it not work at all.