The showy and spectacular passion flower vine, or Passiflora, has its tendrils in religion, folklore and medicine. Some associate the blossom with the crucifixion of Christ. One version is that the fringed corona represents the crown of thorns and the five stamens represent the five wounds. Passiflora flowers last for three days, which is the time elapsed between the crucifixion and the resurrection. Medically, various cultures have used the passion flower for centuries as an anti-anxiety agent. With only a few exceptions, the passion flower vine has few natural enemies as the result of its unique defense mechanism--the plant produces deadly cyanide.
Evolution of Defenses
Not all of the chemicals produced by various varieties of passion flower vines are identical, but within the natural range of the plant--mostly Central and South America, parts of Florida and Hawaii--all produce some form of cyanogenetic glucocide toxic to insects.
How it Works
Cynanide or hydrocyanic acid is a poison that interferes with cell respiration. Thus even in the presence of oxygen, cells die. In most species, including humans, cyanide poisoning is often fatal. The presence of cyanide in the passion flower vine means that most sucking pests never have a chance to become established on the plant.
Because of its colorful blossoms, butterflies are attracted to the passion-flower vine and lay their eggs on the foliage. For most, this is a deadly mistake, but one genus of butterfly--the Heliconius--has developed an ability to metabolize the poison. Over time, different species of Heliconius butterflies have evolved in response to the special forms of cyanide produced by passion flower vines. These butterflies lay eggs which hatch usually as yellow worm-like caterpillars on the plant. These caterpillars feed on the plants, maintaining the poison in their systems. They then metamorphose into butterflies that also carry the toxin, protecting them, in turn, from other predators who recognize them by colors that mimic those of the flowers.
In the wild, passion flower plants have evolved along with their enemies, the Heliconius butterfly, by developing defenses specific to this predator. Some varieties produce yellow spots on their leaves that mimic the appearance of the Heliconius larvae. This may discourage new infestations. Better still, the yellow spots are actually nectaries which attract ants and other predators of the Heliconius larvae. Another species produces hook-like trichomes which mortally wound the caterpillars. Still another defense is a sticky substance on the bracts of some passion flower vines which prevent the butterflies from landing close to the flowers whose nectar they are seeking.
While small infestations will not hurt your passion flower vine, if caterpillar damage threatens to compromise the health of the plant, purchase and place commercially available caterpillar traps to reduce the population. You can also hand-pick and destroy the worm-like caterpillars. Chemical control is only recommended in extreme cases, especially if you enjoy the fact that your flowering vine is attracting butterflies to your garden.