Mexican jumping beans are mysterious objects that appear to move on their own. This leads to the misconception that they are actual beans or eggs that ‘hatch.’ However, their movement results from a biological interaction between a plant and a small insect. The ‘hatching’ is not from the bean itself, but the emergence of a creature that has been living inside.
What Are Mexican Jumping Beans?
Mexican jumping beans are not true beans, which are legumes, but rather seed pods from specific desert shrubs. The most common source is Sebastiana pavoniana, a plant belonging to the spurge family (Euphorbiaceae) native to mountainous regions of Mexico, including Sonora, Sinaloa, and Chihuahua. Another less common source is Sebastiana bilocularis, sometimes called the Arizona jumping bean. These pods are typically small, brown, and somewhat kidney-shaped, serving as a protective casing for the developing larva within.
The “bean” is merely a segment of the plant’s seed capsule that has been infested by the larva of a small moth, Cydia saltitans, also known as the jumping bean moth. This moth orchestrates the bean’s unique movements. The town of Álamos in Sonora, Mexico, is particularly known as the “Jumping Bean Capital of the World” due to the prevalence of the host shrub in the area.
The Secret Behind the Jump
The characteristic “jumping” motion of these beans is entirely due to the activity of the Cydia saltitans larva inside the seed pod. When stimulated, particularly by warmth such as from a hand or sunlight, the larva contracts and then strikes the inner wall of the pod with its body. This forceful movement causes the entire seed pod to twitch, roll, or ‘jump.’
The larva’s movements are not random; they serve a specific survival purpose. If the bean is in a hot, exposed location, the larva will actively cause it to move in an attempt to find a cooler, shaded environment. This behavior helps the larva avoid overheating and desiccation, which could be fatal in the harsh desert climate.
The Life Cycle of the Jumping Bean Moth
The life cycle of the Cydia saltitans moth is intricately tied to its host plant. In the spring, after the Sebastiana pavoniana shrub blooms, the adult female moth lays its tiny eggs on the immature, green seed pods. Once an egg hatches, the minuscule larva bores its way into the developing pod, which then seals around it.
Inside the pod, the larva spends several months feeding on the seed material, hollowing out the interior. As it grows, the pod ripens, hardens, and falls from the shrub. Before pupating, the larva chews a small, circular exit hole through the pod’s shell, but it leaves a thin, transparent “window” of skin intact. This pre-made exit is essential because the adult moth, which is jawless, cannot chew its way out. The larva then enters the pupal stage, transforming within the pod.
Caring for Jumping Beans and Their Eventual Transformation
For those who possess Mexican jumping beans, proper care can help prolong the larva’s activity. Keeping them in a cool, dry place away from direct sunlight is advisable, as extreme heat can quickly kill the larva. An occasional light misting with lukewarm water, about once a week, can provide essential moisture to the larva inside. Allowing them to rest in a dark, cool environment when not being observed helps conserve the larva’s energy.
After several months of activity, typically in late winter or early spring, the larva inside the bean will complete its pupation. A small, brown or silver-gray moth will then emerge from the pre-chewed exit hole, pushing through the thin ‘window’ and leaving its pupal casing behind. These adult moths are harmless, jawless, and generally live for only a few days, primarily to mate and lay eggs, restarting the life cycle.