Pilgrimage of the Roaches

From the publishers blurb:

A group of Roachling pilgrims from the chasms below the earth have emerged and are fortifying the abandoned chrysalis of a death gnat. They prepare to wage a profane war of infection against those who refuse to live in squalor. The human village of Peldor has already been ravaged by the emergence of death gnat; it is not equipped to handle the insect threat.