Before Christian Puchi set off for work in the rainforest, he made sure his machete was fastened to his hip and his fellow forest rangers were doused in mosquito repellent. They jumped in their boat and navigated through throngs of tourists already on the water.

The tourists clutched binoculars, hoping to catch a glimpse of Costa Rica’s famous turtles. Mr. Puchi and his men just hoped to come back unscathed.

They can handle the poisonous frogs, venomous snakes and crocodiles. But with too few staff and inadequate gear, they’re no match for the most dangerous threat now lurking in the national parks, violent drug cartels.

“We used to focus on conservation, finding jaguar tracks, turtle nests, normal stuff. Now, protected areas like this one have become drug warehouses,” said Mr. Puchi, 49, a forest ranger for over 20 years.

Costa Rica, often considered one the region’s most idyllic destinations, long escaped the scourge of cartels that has pervaded the region. Its national motto, “pura vida” or pure life, has for decades attracted honeymooners, yoga retreat goers and bird-watching enthusiasts.

But now, the lush forests blanketing a quarter of Costa Rica are being infiltrated by drug cartels seeking new trafficking routes to evade the authorities.