The plants contain strong poisons, which likely limit natural enemies. Deer and cattle do not eat them. Researchers also are finding that pale swallow-wort may be replacing milkweeds in open fields in New York state and across the Northeast coast, within the migratory range of monarch butterflies. The monarchs lay their eggs on milkweed, and their larvae eat it as a primary food. Studies have shown that when the monarchs lay their eggs on pale or black swallow-wort, the larvae die within three days.
DiTommaso and his graduate students are investigating whether the plants release root chemicals that alter soil conditions. Preliminary findings suggest that communities of mycorrhizae -- soil fungi that help provide more nutrients to plant roots -- differ in species and abundance in areas surrounding swallow-worts.
"It could be that swallow-worts release chemicals that make an optimal environment for certain mycorrhizal fungi," said DiTommaso.
Through the altered soil, or possibly because of chemicals directly released by the swallow-worts, other plant species have a hard time establishing themselves wherever swallow-worts grow, DiTommaso said.
The pink-flowered pale swallow-wort grows rapidly in both forest understories and in open fields of undisturbed soil throughout central and upstate New York, around the Great Lakes and in Canada. The purple-flowered black swallow-wort prefers open areas and have a foothold in the Hudson Valley, Long Island, southern New York and the New England coast.
Like the common milkweed, swallow-worts release lightweight seeds with featherlike tails that are dispersed by wind and passing deer. Interestingly, between two to eight plants can germin
Source:Cornell University News Service