Will Monarch butterflies do damage to plants if they are released in a garden at their adult stage?
The adult Monarch butterfly eats flower nectar, not leaves (the caterpillar form eats the leaves of one plant, milkweed–Asclepias), so that should not be damaging in any way.
I would question the practice of purchasing and releasing butterflies, though. It is not a good environmental practice, and can be harmful to the released butterflies and native butterfly populations. The North American Butterfly Association has information on this issue, excerpted below.
“Most fifth graders can tell you how the magnificent Monarch butterflies migrate thousands of miles every autumn from the United States and Canada to a few small mountain tops in Mexico. There they find the right environmental conditions that allow them to survive the winter. With the advent of spring, they begin their return journey. This migratory phenomenon is truly a wonder of nature that sparks the imagination.
Now imagine tens of thousands of mixed-up Monarchs unable to find the way to their overwintering grounds. This depressing image may become a reality if the rapidly-growing fad of releasing butterflies, including Monarch butterflies, at weddings, state fairs, and other public events continues to spread. Because the released Monarchs may have come from California, for instance, where they do not migrate to Mexico, their offspring may not be able to orient properly,. Because the Monarchs were raised inside under unnatural conditions, it is possible that their delicate migratory physiology may not have been turned on.
Public interest in butterflies is increasing dramatically. We hope and expect this greater involvement with butterflies will eventually lead to much-needed support for butterfly conservation and studies, but the release of live butterflies is the dark side of this increase in popularity. Although this practice is understandable to naive newlyweds-to-be (what could be more beautiful than adding butterflies to the environment?) it is really a particularly long-lasting form of environmental pollution.
Butterflies raised by unregulated commercial interests may spread diseases and parasites to wild populations, with devastating results. Often, butterflies are released great distances from their points of origin, resulting in inappropriate genetic mixing of different populations when the same species is locally present. When it is not, a non-native species is being introduced in the area of release. At best, this confuses studies of butterfly distribution and migration; at worst, it may result in deleterious changes to the local ecology. The Hollywood Jurassic park message, “Don’t fool with Mother Nature,” has scientific foundations. Recently a high profile report in Science magazine found that even the careful introduction of species for biological control often causes unexpected negative results.
In addition, these releases create a commercial market for live butterflies (currently about $10/apiece), with the result that, for example, the Monarch overwintering sites in Mexico and on the California coast are now targets for poachers.
Currently, the interstate shipment of live butterflies requires a permit from the U.S. Department of Agriculture but this law is not usually enforced. In general, the Dept. of Agriculture may issue a permit for shipping any of the following species: Monarch, Painted Lady, American Lady, Red Admiral, Giant Swallowtail, Gulf Fritillary, Zebra (Heliconian), and Mourning Cloak. Shipping Red Admirals, Giant Swallowtails, Gulf Fritillaries and Zebra (Heliconians) is particularly inappropriate because they are not naturally found over much of the United States.
A solution that better serves the public interest with less regulatory burden is to ban the environmental release of commercially-obtained butterflies (we would exempt education institutions, although even here we would encourage schools to keep commercially-obtained butterflies within the confines of the school). The intentional release of native birds was outlawed in 1947. The time has come to do the same with butterflies.”