Research has some interesting answers to help us understand the relationship between ants and peonies.
It is commonly known that nectar is produced in association with plant flowers and that it encourages pollinators. The flower nectar provides food for insects, birds and even bats. Many plants also have extrafloral nectaries.
These special nectar-producing glands are located physically apart from the flower. Studies have shown that more than 2,000 plant species in more than 64 families contain extrafloral nectaries.
These plant species frequently are perennial and woody. Nectary locations vary by plant: passion flower on leaves, trumpet vine on petioles, partridge pea on petioles, hairy vetch on stipules, willow on leaves, smilax on leaves, elderberry on leaves and stems, viburnum on leaves, morning glory on leaves and petiole, sunflower on bracts, peach on leaves, cotton on lower leaf mid-vein.
Russell C. Mizell, an entomology professor at the University of Florida in Gainesville, reports that peonies contain these extrafloral nectaries. He has gained knowledge that is useful in understanding the benefits of ants on plants (and peonies).
Studies indicate that various insects use extrafloral nectaries. They receive nectar that is about 95 percent sugar and 5 percent nutrients, protein and amino acids - different from floral nectar and by plant species.
It is easy to observe beneficial insects, such as ladybird beetles, feeding from the nectaries. Many species of ants are found in association with these plants, and the ants are known to use the nectaries.
Research shows there are only a few plants containing extrafloral nectaries in areas that do not have ants. Ants especially seem to benefit from extrafloral nectaries.
Studies of the plant species with extrafloral nectaries indicate that the plant uses these to attract beneficials for plant defense.Possibly, the continuous presence alone of ants may be adequate protection. The ants' aggressive behavior detours pests of prey. Extrafloral nectaries may distract the ants from insects that produce honeydew after feeding on the plant.
Ants may also prune competing neighboring plants, distribute seed and fruit, provide pollination and feed the plant essential nutrients.
Scientists continue to gain understanding of plants' evolution of nectar rewards, and study the correlation between ants and plants. Peonies use ants to defend them from pests and provide food to attract them to the plants.
To assist the natural defense process taking place in your yard, simply let the army of ants continue to march on your peonies.
That's right, growing peonies and encouraging beneficial insects like the spring tiphia seen preying on the grubs above can help decrease the numbers of those pretty, but ever so destructive Japanese beetles pictured below.
As we have so much further to go in our understanding of the interactions between plants and insects, I prefer not to use pesticides in my yard. My plantings are constructed with an eye towards supporting biological insect diversity. Support the predators and they will reduce the prey population.