The lady bug was adopted as the official state bug of Delaware in 1974 thanks to an intensive effort made by Mrs. Mollie Brown-Rust and her 2nd grade students of the Lulu M. Ross Elementary School in Milford, Delaware. Also called lady beetle, ladybird, or ladyfly, lady bugs help gardeners and farmers by eating tiny insect pests that damage plants. A ladybug can consume up to 60 aphids per day, but will also eat a variety of other insects and larvae (including scales, mealy bugs, leaf hoppers, mites, and other types of soft-bodied insects), and also pollen and nectar. Ohio, Massachusetts, New Hampshire, Tennessee, and New York also designate the ladybug as an official state symbol (see list of state insects for all 50 states).

According to John Losey, a Cornell University entomologist who leads the Lost Ladybug Project. (a project funded by a National Science Foundation grant recruiting citizen scientists, particularly children, to search for nine-spotted ladybug and other ladybug species and send photos of them to Cornell for identification and inclusion in a database) there are about 5,000 species of ladybugs, also known as ladybird beetles, with about 450 species in the United States.

Source:

Delaware Facts and Symbols: Delaware Official Website
Ladybug, Ladybug, Where Have You Gone? - msnbc

Links:

State Insects - 50 States List