OBJECT OF THE MONTH - August, 2008

The One-spotted Tortoise Beetle (Physonota unipunctata)

An adult one-spotted tortoise beetle (Physonota unipunctata).

Tortoise beetles are herbivores, meaning that they eat plants. Herbivores may be generalists, eating many different kinds of plants, or they may be specialists, eating only one or a few closely related plant species.

Plants, however, have ways to reduce feeding by herbivores. One such form of protection is the production of chemical compounds that are distasteful or harmful if eaten. Specialist insects have evolved the ability to feed on plants high in these chemical compounds and to use these compounds for their own protection against predators.
Many species of leaf beetles (family Chrysomelidae) are specialists on plants high in defensive compounds and often recycle these compounds to use for their own defense. The tortoise beetles (subfamily Cassidinae) are noted for their use of host plant chemical compounds for protective purposes.
Adults and larvae both feed upon the host plant. Larvae usually feed in groups and typically accumulate a fecal shield on a caudal fork, which consists primarily of their frass (= poop). This frass is very rich in host plant chemical compounds and is waved or thrust at potential predators and is a very effective means of defense.
Ken Keefover-Ring, a recent graduate from the Department of Ecology and Evolutionary Biology, studied the one-spotted tortoise beetle (also called the horsemint tortoise beetle or bee balm tortoise beetle), Physonota unipunctata, and its host plant in the mint family (Lamiaceae), Monarda fistulosa, called horsemint, bee balm or wild bergamot.
Like most mints, horsemint produces terpene compounds in trichomes on leaves and reproductive parts, which defend the plant against most herbivores, but not the tortoise beetle. These compounds are eliminated in the insect's frass and are the primary defense components of the fecal shield. Whether the specialist tortoise beetle has played a role in shaping the chemistry of horsemint and whether there are reciprocal effects of host chemistry on the herbivore are unknown.