SILVEIRAS, Brazil  Jorge da Silva plucked a giant ant from the muddy ground, pulled off its wings, legs and mandible, and tossed it into his mouth like popcorn.

“Tastes like mint,” said Mr. da Silva, 58, with an audible crunch of his teeth.

In his rubber boots, Mr. da Silva roamed the hills above this town of 6,000 people with a stick and a plastic bucket. He was on the hunt for Silveiras’s obsession and a rare gastronomic delicacy in Brazil: içás, or queen ants.

The thunderous spring rains in October and November drive the ants out of the ground, and for a few short weeks Silveiras becomes a frenzy of ant hunting. Residents stock up, cleaning the içás and freezing them in one and two-liter bottles to get through until the next season.

But this year the ant haul was smaller than usual, residents said, and the number of ants has been dwindling. The principal culprits are pesticides used on eucalyptus trees that are planted to produce cellulose for paper and other products, residents and local officials said.