Pollination is the process of transferring pollen from the male part of a flower to the female part, enabling the plant to produce seeds and reproduce. This transfer is facilitated by wind, water, or animals. The specialized relationship between flowering plants and butterflies is known as psychophily, identifying flowers adapted to attract these specific daytime flyers. Butterflies play a significant role in the plant life cycle, especially for species that have evolved floral structures matching the insect’s feeding style. This symbiotic interaction has shaped the evolution of numerous plant and butterfly species.
The Mechanics of Butterfly Pollination
Butterfly pollination relies on accidental contact rather than specialized pollen-collecting structures like the pollen baskets found on bees. As the butterfly lands on a flower to drink nectar, pollen grains stick incidentally to the insect’s long legs, antennae, and the fine scales covering its body. The butterfly’s feeding apparatus is a coiled, straw-like tube called a proboscis, which it unrolls to reach nectar deep within the flower.
The search for nectar requires the butterfly to move around the flower’s reproductive organs, ensuring that pollen brushes off onto its body. When the butterfly visits a second flower of the same species, the carried pollen is deposited onto the new flower’s stigma, completing the transfer. Because the butterfly does not actively collect pollen, this method is considered less efficient than bee pollination, but the butterfly’s long proboscis allows it to access nectar in deep, narrow floral tubes that other insects cannot reach.
Flower Characteristics Attracting Butterflies
Flowers that rely on butterflies have evolved distinct characteristics to attract their pollinators. Butterflies have excellent daytime vision and are sensitive to bright colors, often preferring shades of pink, purple, red, and yellow. Many butterfly-pollinated flowers possess patterns on their petals, known as nectar guides, which are visible under ultraviolet (UV) light and direct the insect toward the nectar source.
While some pollinators are drawn by strong aromas, butterfly-attracting flowers emit a mild, sweet scent that is pleasant but not overpowering. The flower’s physical structure is specialized, featuring a flat, broad surface that acts as a secure landing platform for the butterfly while it feeds. The nectar is stored in a narrow, tubular corolla or spur, which perfectly accommodates the butterfly’s long proboscis.
Specific Examples of Butterfly Host Plants
The Zinnia flower is a classic example of a butterfly magnet, offering a reliable nectar reward and a suitable structure. Butterflies are primarily attracted to single-flowered varieties because the exposed center, or disk floret, provides an easily accessible landing platform. These broad, daisy-like blooms are a long-lasting nectar source throughout the summer and fall.
Milkweed (Asclepias) demonstrates a complex, co-evolved pollination mechanism. Its pollen is not loose grains but is packaged into waxy masses called pollinia, hidden within a specialized structure called the gynostegium. For pollination to occur, a butterfly’s leg must slip into a stigmatic slit and pull out a sticky pair of pollinia, which is then transferred to another flower. The invasive Butterfly Bush (Buddleja davidii) attracts butterflies with copious nectar and strong olfactory cues, but it does not serve as a host plant for the caterpillars of many native butterfly species.
The Ecological Significance of This Relationship
The partnership between plants and butterflies drives genetic diversity and evolutionary change within ecosystems. By visiting multiple individual plants of the same species, butterflies facilitate cross-pollination, introducing genetic material from different parents. This genetic mixing enhances the plant population’s ability to adapt to environmental stresses, such as disease or climate shifts.
This long-standing interaction has led to an evolutionary “arms race” between certain groups, such as the Pieridae butterflies and the Brassicales plants (including cabbage and mustard). Plants in this group developed toxic chemical defenses, such as glucosinolates, to deter herbivores. In response, the butterflies evolved molecular counter-adaptations to detoxify these compounds, spurring the plants to develop more complex defenses. This resulted in the diversification of both plant and insect species.