Janet McConnaughey/Associated Press

Eight years into a U.S. program to control damage from feral pigs, the invasive animals are still a multibillion-dollar plague on farmers, wildlife and the environment. They've been wiped out in 11 of the 41 states where they were reported in 2014 or 2015. And there are fewer in parts of the other 30. But in spite of more than $100 million in federal money, officials estimate there are still 6 million to 9 million hogs gone wild nationwide and in three U.S. territories, doing at least $2.5 billion a year in U.S. damages. Estimates in 2014 were 5 million hogs and $1.5 billion in damages. Experts say the bigger figures are due to better estimates, not increases.

The U.S. government has begun scattering millions of packets of oral rabies vaccine from helicopters and planes in 13 states from Maine to Alabama. The major aim is to help keep raccoons from spreading their variant of the deadly virus to states where it's less prevalent. The government is also continuing tests of a vaccine that Canada has approved to immunize skunks as well as raccoons. Rabies is generally spread through an infected animal's saliva. In the U.S., pet vaccination laws mean it's mostly spread by wildlife.

A stinky but handsome and widely popular landscape tree has become an aggressive invader. Callery pears create dense thickets that overwhelm native plants and bear four-inch spikes that can flatten tractor tires. Bradford pears and other ornamental Callery pears were cultivated from an import that saved pear orchards from a deadly fungus. And for decades, the decorative trees seemed near perfect, aside from a tendency to fall apart after about 15 years — and their stench. But they cross pollinated with other ornamental varieties. Invasive stands now have been reported in more than 30 states. Fourteen states have formally listed the trees as invasive.