Nectar is an effective concoction for establishment and maintenance of plant-animal interactions. Using nectar, plants can build a community of allies for defense and even manipulate the behavior of pollinators. We are accustomed to nectaries found in flowers. Plants however, have evolved nectaries in other locations in their above-ground parts including the base of the flowers and petioles. These are called extrafloral nectaries. The sugary reward coming from the extrafloral nectaries invites ants to defend them. Here we can see an example in the purple passion flower vine (Passiflora incarnata) endemic to the southeastern United States. Extrafloral nectaries have evolved to solve/mitigate at least two biological problems.
Firstly, ants are everywhere, therefore once recruited they are quite effective in keeping potential herbivores away. Sometimes sheer presence of ants is enough to keep most intruders away. For instance, ants aggressively chase the picture wing flies away whenever they encounter them on the plant. Extrafloral nectaries establishes and reinforces a spatial memory on ants so the workers of a colony marks the plant as their territory. At the same time, these nectaries distract ants away from the flowers so that pollinators can have better access.
Second, extrafloral nectaries of the passionflower resemble butterfly eggs. Butterflies avoid laying their eggs on host plants if they see other eggs. This is particularly true for two species of butterflies Gulf fritillary (Agraulis vanillae) and Variegated fritillary (Euptoieta claudia). Caterpillars of these butterflies are cannibalistic and early emerging larvae can consume late emerging ones. Passionflower therefore protects itself by visual trickery. Fooling herbivores by egg mimicry know as Gilbertian mimicry.
Here you can see how the purple passionflower vine (Passiflora incarnata) forms alliance with ants through its extrafloral nectaries. A green-orange ground beetle (Calleida punctata) is forced to go away by ants using formic acid as a deterrent. Filmed on location at the Georgia State Botanical Garden in Athens, GA, USA.
This trick does not work all the time though. For instance cone-headed planthoppers can evade the protection ants provide and tap into the sap inside the stem of the purple passionflower.
(*) Gulf fritillary (Agraulis vanillae)
butterfliesandmoths.org/species/Agraulis-vanillae
(*) Variegated fritillary (Euptoieta claudia)
butterfliesandmoths.org/species/Euptoieta-claudia
(*) The green-orange beetle is Calleida punctata:
bugguide.net/node/view/74734
You can watch the next episode of this series at this link: http://naturedocumentaries.org/114/the-passionate-wait/
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