By Cassandra Garrison TAPACHULA, Mexico (Reuters) -On a scorching afternoon in the rural heartland outside Mexico's southern city of Tapachula near the Guatemala border, rancher Julio Herrera calls his herd back from an afternoon of grazing.

Officials say the screwworm has the potential to devastate the cattle industry, send beef prices soaring and kill wildlife.

Mounted patrol officers, known as tick riders, who typically patrol the southern border to protect cattle from fever tick, will also now serve as the first line of defense against screwworm.