Eucheira socialis, commonly known as the Madrone butterfly is a lepidopteran that belongs to the family Pieridae. It was first described by Westwood in 1834. Locally known as Mariposa del madroño or tzauhquiocuilin, it is endemic to the highlands of Mexico, and exclusively relies on the Madrone (Arbutus spp.) as a host-plant. The species is of considerable interest to lepidopterists due to gregarious nest-building in the larval stages, and heavily male biased sex ratio. It takes an entire year for this adult butterfly to develop from an egg. The eggs are laid in the month of June and the adults emerge the following May–June. The adults have a black and white pattern on their wings, and the males are generally much smaller and paler than the females. The larvae do not undergo diapause and continue to feed and grow communally in the coldest months of the year. There are two subspecies of E. socialis, named E. socialis socialis and E. socialis westwoodi.
Distribution
The distribution of E. socialis is restricted to the highlands of Mexico at elevations of 1800 meters in madrone habitats. They are generally found in pine-oak and arid tropical scrub ecosystems. They are distributed from northern Sonora to Jalisco. E. socialis socialis are found in central Mexico, while E. socialis westwoodi is found in Sierra Madre Occidental in the north. The population distribution is patchy due to the poor dispersal ability of the adults, and the patchy distribution of the host-plant, madrone.
The range of E. socialis is dependent on the availability of their preferred host plant, the madrone. In recent times, the madrone trees have been cut and used as firewood and for making furniture and other crafts. The destruction of the madrone habitat threatens the relationship between the plant and the butterfly. However, the inaccessibility of these habitats due to their confinement in high elevations possibly decreases the risk.
Life cycle
Characteristic of all lepidopterans, E. socialis are holometabolous and go through four distinct developmental stages namely egg, larva, pupa and adult. There are six larval instars and all instars are known to be gregarious. It takes nearly an entire year for the adult to emerge from an egg. The eggs are laid in July, and the adults eventually eclose in May–June. Consequently, there is only one generation of eggs laid each year.
The eggs are bluish-white and are laid in clumps on the underside of the host plant Madrone in the month of June–July. The larvae hatch out of the eggs after approximately 3–4 weeks, sometimes after up to 60 days, in August. Upon hatching, they communally feed on the leaves and terminal branches of the plant. They consume the epidermis and mesophyll and leave the venous skeleton of the leaf intact. The larvae then build their first communal nest by folding these consumed leaves and securing them with silken strands. The larvae are a bright green and mildly fuzzy when they hatch, but turn brown and less pubescent as they grow. There are in total six larval instars. Larval mortality is disproportionately high in males than in females. Despite extremely low temperatures in the winter, the larvae do not undergo diapause, and continue to feed and grow throughout the year. A fully-grown larva is generally between 25 and 30 millimeters long, and pupates by April.
The pupae are initially light-green on pupation and later turn yellow. They pupate head down in the silk nests, and lack silken girdle unlike other Pieridae. The black and white adult wing markings are visible through the pupal case. There exists sexual dimorphism in the size of pupa. The male pupae are generally much smaller (18–20 mm) than female pupae (21–23 mm). Pupal mortality is much higher in females than in males. The pupal stage lasts for about a month and the adults finally emerge in May–June.
Adult butterflies emerge from their nests through the small exits while they are still teneral. Their escape is facilitated by soft, pliable wings. The wings also have an atypical shape and venation patterns adapted to escape the nest. The adults have black and white coloration on the wings, and the pattern is more prominent on the males. The males also are usually much paler and smaller than the females. The males have larger eyes and higher wing venation as compared to the females. The proboscis does not anneal properly post-eclosion and is non-functional.
Reproduction
Mating
The adults are weak fliers, and display simple sexual behavior. Mating takes place near the communal nests right after the adults emerge from the nests. The females mate only once in their lifetime, and most males fail to find mates.