While browsing the website Parasite of the Day, I came across an interesting article about dodder and a reference about how they key on odors or chemical signals of some plants to find a host. Here is a detailed entry about it in the Why Files Blog. This species is considered uncommon in New York and there are four other species that are endangered and threatened in the state. To find out more about two of them, you can go to the NY Natural Heritage Program Plant Conservation Guides.
The flowers of Cuscuta gronovii, a common species of dodder, in Schenectady. Photo Steve Young