What are Mexican Jumping Beans?

Mexican jumping beans are seeds which have been colonized by moth larvae. The larvae move around inside the seeds, causing them to roll or tumble, although they rarely literally jump. The seeds have long been sold as novelty items in Mexico, where they are called brincadores, and their popularity has spread to other parts of the world as travelers bring them home. Although Mexican jumping beans are certainly fun, they also illustrate a very interesting relationship between plants and insects, and it is only one of many such relationships in the natural world.

The name “bean” is a bit of a misnomer, as Mexican jumping beans are actually individual carpels or sections of the seeds of shrubs from the genus Sebastiana. These plants have dark green leathery leaves which turn a rich red in the winter, and they are native to Mexico and parts of the American Southwest. In the spring, when Sebastiana plants flower, small gray moths lay their eggs in some of the flowers, and when the flowers develop into seeds, the eggs are trapped inside.

Once the eggs mature into larvae, the larvae eat out the inside of the seed to make a cozy home for themselves. They also spin a series of threads, creating a complex internal web inside the seed. For unknown reasons, the larvae tumble around inside the seeds, causing them to move. These movements may encourage dispersal of larvae across a wide region, and they appear to be stimulated by exposure to warmth.

As the larva grows inside the seed, it ultimately turns into a moth, and drills a small trapdoor into the seed so that it can escape. Once it emerges, the moth has only a few days to live; it generally seeks out a mate so that the life cycle of the Laspeyresia saltitans moth can begin all over again. Fortunately for the host plants, the moths do not colonize all of the seeds on a shrub, ensuring that the plants will be able to perpetuate themselves, thus providing future homes for the moths.

The seeds do rather resemble small beans, which explains the name. A collection of Mexican jumping beans can be quite interesting to watch, as the seeds will rattle against each other and shake out of shallow containers if given the opportunity. The Mexican jumping beans will continue to move for weeks or months if cared for properly, although they may experience periods of dormancy. Since the moths only colonize plants in the Sebastiana genus, the cycle will stop unless the young moths have access to the right hosts after they emerge.

Mexican Jumping Beans are seed pods from the Mexican shrub Sebastiana pavoniana. They are inhabited by the larva of a small moth, Cydia deshaisiana. The "jumping" motion occurs when the larva inside the bean moves, causing the bean to react to the heat from a person's hand or sunlight, as it tries to move to a cooler environment.

The female moth lays her eggs on the flowers of the host plant. Once the eggs hatch, the larvae bore into the developing seed pods and eat the inside. The pod eventually hardens around the larva, creating a protective capsule. This unique relationship is a fascinating example of insect-plant interaction.

Mexican Jumping Beans are not harmful to humans or pets. They are a natural curiosity rather than a pest or danger. However, it's important to remember that they are living organisms, so they should be treated with respect and not ingested or given to small pets that might swallow them.

While Mexican Jumping Beans can be kept temporarily for observation, they are not traditional pets. They have a short lifespan, with the larva eventually metamorphosing into a moth and leaving the bean. To keep them active, they should be stored at room temperature and away from direct sunlight to prevent drying out or overheating.

The larva inside a Mexican Jumping Bean will eventually pupate after several months, depending on environmental conditions. Once metamorphosis is complete, the adult moth will emerge from the bean to begin its life cycle anew. The empty bean husks are left behind as evidence of this natural marvel.

Mexican Jumping Beans are native to the arid regions of Mexico, particularly in the states of Sonora and Chihuahua. They grow on the slopes of mountains in the shrub's natural habitat. During the right season, typically May through August, they can be found in local markets or souvenir shops in the region.