The sight of ants scrambling across the stem, leaves, or buds of a sunflower is a common observation. Their presence indicates a direct and intentional attraction to the plant. The primary reason for this close association is food: a sugary reward provided by the sunflower that initiates a complex biological interaction. This relationship benefits both the plant and the ant, creating a dynamic system of feeding and defense.
Extrafloral Nectaries The Sunflower’s Sweet Secret
The sunflower attracts ants using specialized nectar-producing glands located outside of the flower head, known as extrafloral nectaries (EFNs). These small structures are found on the vegetative parts of the plant, including the leaves, the base of the leaf stems, and the involucre (the green bracts that cup the flower bud). This placement is distinct from the floral nectaries inside the main flower head, which are designed to attract bees for pollination.
The nectar secreted by EFNs is a valuable resource for ants, primarily consisting of sugars such as fructose, glucose, and sucrose. It also contains trace amounts of amino acids, which provide necessary protein building blocks. This sugary liquid is often more concentrated than the nectar produced inside the flower meant for pollinators.
The sunflower expends energy to produce this external nectar, effectively paying a fee to attract a specific visitor. The strategic location of these nectaries ensures that ants are drawn to the plant’s vulnerable growing points and leaves, where herbivore damage is most likely to occur. The nectar is secreted over an extended period, providing a continuous food source. This sustained reward encourages the ants to establish a constant presence on the plant and remain on patrol.
The Protective Role of Ants
The sunflower’s investment in extrafloral nectar is a form of biological self-defense. By offering a readily available food source on its stems and leaves, the plant recruits ants to act as bodyguards. This ecological partnership, where both species benefit, is an example of a mutualistic relationship.
Ants are naturally aggressive predators drawn to the plant by the sugary reward. Once on the sunflower, they actively patrol the leaves and developing buds. The mere presence of ants can deter smaller, soft-bodied herbivores, such as caterpillars or beetles, that might otherwise eat the foliage.
If an herbivore attempts to feed on the sunflower, the patrolling ants quickly confront the intruder. Ants aggressively pursue, bite, and drive away or kill various pests that threaten the plant tissue. This defense mechanism helps the sunflower minimize damage to its photosynthetic surfaces and reproductive structures, justifying the energy cost of producing the nectar.
Honeydew and Secondary Food Sources
While extrafloral nectaries are the direct reward, ants are also frequently found on sunflowers due to a secondary, indirect food source called honeydew. Honeydew is a sugary, sticky liquid excreted by sap-sucking insects like aphids, scale insects, and mealybugs. These pests feed on the phloem sap flowing inside the sunflower’s tissues.
Ants actively protect and “farm” these sap-suckers to harvest the honeydew, which serves as a reliable sugar source. This behavior benefits the aphids, as the ants protect them from natural predators like ladybugs and parasitic wasps.
This relationship introduces a conflict for the sunflower: it benefits from the ants’ protection against chewing herbivores but suffers damage from the sap-suckers the ants are farming. The honeydew itself can also lead to the growth of sooty mold, a black fungus that covers leaves and inhibits photosynthesis. The ant’s presence is thus driven by two distinct sugar sources: the plant’s intentional extrafloral nectar, and the pests’ accidental byproduct of honeydew.