Interaction between ants, extrafloral nectaries and insect herbivores in Neotropical coastal sand dunes: herbivore deterrence by visiting ants increases fruit set inOpuntia stricta(Cactaceae)

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Abstract
1.This study examines the anti‐herbivore effect of ants visiting the extrafloral nectaries (EFNs) ofOpuntia stricta(Cactaceae) and its possible influence on the plant’s reproductive output in Mexican coastal sand dunes.Opuntia’s EFNs are located in the areoles of the developing tissue of emerging cladodes and flower buds.2.Ants visited the EFNs ofO.strictaon a round‐the‐clock basis. The associated ant assemblage was formed by nine species distributed in four subfamilies, and the species composition of the principal ant visitors changed markedly from day to night period.3.Cladodes of control (ants present) and treatment (ants excluded) plants ofOpuntiawere equally infested by sucking bugs and mining dipterans. Damage to buds by a pyralid moth, however, was significantly higher on treatment than on control plants. Ant visitation toOpuntia’s EFNs translated into a 50% increase in the plant’s reproductive output, as expressed by the number of fruits produced by experimental control and treatment branches. Moreover, fruit production by ant‐visited branches was positively and significantly associated with the mean monthly rate of ant visitation to EFNs.4.This is the first demonstration of ant protection leading to increased fruit set in the Cactaceae under natural conditions. Although the consequences of damage by sucking and mining insects remain unclear forOpuntia, the results show how the association of EFNs with vulnerable reproductive plant organs can result in a direct ant‐derived benefit to plant fitness.

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