Harvester ants in the genus Pogonomyrmex have the most toxic venom documented in any insects based on mice LD50 values, with P. maricopa venom being the most toxic. The LD50 value for this species is 0.12 mg/kg injected intravenously in mice, equivalent to 12 stings killing a 2 kg (4.4 lb) rat. A Pogonomyrmex sp. sting produces intense pain in humans that lasts up to 4 hours.
See more: Most toxic insect venom - part II
Source: University of Florida, Wikipedia