NEW ORLEANS (AP) — Every Labor Day weekend, St. Mary Parish celebrates two industries at a tourist event with a seemingly improbable title: the Louisiana Shrimp and Petroleum Festival.

With a shrimp in a hard hat clinging to an oil derrick as its logo, the festival may the best example of how diverse economic interests mesh in south Louisiana — and how attempts to curb the spread of COVID-19 have visited a kind of triple economic whammy on the state.

A worldwide oil glut was pushing down prices even before the pandemic fight lowered energy demand, contributing to layoffs. Festival-driven touri...