warty leaf beetles

Exema is a genus of very small case-bearing leaf beetles. There are 14 Exema species worldwide, 9 species in North America north of Mexico, and at least 2 species in Minnesota.

Exema larvae live on the leaf surface. Each larva creates a case out of its own excrement in which it can be completely concealed. When active, the front half of its body protrudes from the case, and it carries the case about. This is the feature that gives the subfamily its common name. Mature larvae attach the case to a twig, stem, or branch, seal the opening, and pupate within the case.

Exema adults are active from April to October in other parts of their range, but in Minnesota all sightings are from May to September. They are found in fields and roadsides.

Most North American Exema beetles feed on several species of plants, and most of those plants are in the Aster (Asteraceae) family.

Exema beetles are black to dark brown and very small. They are among the smallest case-bearing leaf beetles in North America. They are difficult to detect because they very closely resemble certain caterpillar droppings. Birds avoid them and humans overlook them unless they are specifically looking for these beetles.

Adults are 1⁄16″ to ⅛″ (2.1 to 3.5 mm) in length. The body is compact, robust, oblong, and somewhat square or pentagonal in cross section. The color is dark brown to black, nonmetallic, and usually dull. Some species have many small coppery or yellow spots intermixed.

The head is dark brown to black with coppery or yellow markings. It is inserted into the thorax up to the eyes. The antennae are short, thread-like, and widely separated at the base. They are broadened and sawtoothed (serrate) from the sixth segment to the end. They fit into grooves on the thorax. The eyes are large.

The thorax and wing covers (elytra) are covered with wart-like bumps (tubercles). The inner (sutural) margin of each elytron is serrate, with each tooth fitting into a groove on the other elytron.

Sources

27, 29, 30, 82, 83.

This genus has no common name. The common name of the tribe Fulcidacini is warty leaf beetles, and it is used here for convenience.

Exema warty leaf beetle

Exema, warty leaf beetle, found at Barnes Prairie Remnant, Racine Co., WI; photos taken on June 3, 2023.

Location: Barnes Prairie Remnant, Racine Co., WI

Exema, warty leaf beetle, found at Barnes Prairie Remnant, Racine Co., WI; photos taken on June 3, 2023.