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Nature Notebook – Plant Protectors

Gardeners that notice ants on their plants may worry the ants are damaging the plants. In some cases that may be true. Many of the ants, however, may actually be protecting the plants from ravaging herbivores.

One of the many weapons available in a plant’s arsenal is the ability to convince other animals to provide defensive services. The plants “pay” their protectors with sugar- and amino acid-rich nectar. However, the guardians collect their pay from sites that differ from their pollinator counterparts. Extrafloral nectaries (EFN), nectar-producing glands, have been found in 4,000 plants on every part of the aboveground plant parts that a crawling or flying insect can easily access. The most common places are on the outside of flower buds and the vulnerable areas of leaves (the base and underside). If a plant is attacked by an herbivore the EFN produce more nectar to entice insects.

The nectar is rich in sugar and contains some amino acids but it is not a complete food. Therefore, the visiting insects still need protein. The plant hopes that the protein consumed will come from the attacking herbivores. Ants are the predominate consumer and predator attracted to EFNs. Mites, ladybird beetles, some wasps, lacewing larvae and spiders are also attracted to the nectar.