NEW ORLEANS — The tallow tree, a “super invader” with toxic leaves and no natural enemies in North America, is conquering the South. Overtaking forests from Texas to Florida, tallows grow three times faster than most native hardwoods, and each one casts off 100,000 seeds a year. Controlled burns haven’t stopped their spread, nor have herbicide sprays from helicopters. Cutting them down works only when each stump is immediately doused with chemicals. Harvesting them for biofuel remains more a promise than a practical solution. Some scientists say introducing a flea beetle from the tallow’s native habitat in eastern China may be the best alternative. Yes, they’re aware of “nightmare scenarios” with other non-native plants and bugs, environmental scientist Michael Massimi said.