Local authorities said preliminary investigations had showed the dead woman and her children, aged 22 and 28, had travelled to the area from La Paz to recover a debt.
The tree they were tied to was a palo santo, the favourite haunt of colonies of the Brazilian fire ant known for their extremely painful bites.
Police Chief Gunter Agudo said: "We managed to rescue all three people but one of them, the 52-year-old woman, was in a bad way and had to be taken to hospital.
"She died at 3.30pm on New Year's Eve.
"Initially the investigation was opened as a probe into an attempted car theft but now it has been changed to a murder and serious assault investigation."