PARENT SESSION
Oral Session #20: Invasions: Effects of invaders -- terrestrial systems. Presiding: R. Callaway.
Tuesday, August 7, 2001. 8:00 AM to 12:00 PM. Madison Ballroom C.

Exotic ant meets native mutualism: Solenopsis wagneri and the insectary - plant Catalpa bignonioides (Bignoniaceae).

Ness, Josh¹, ¹

ABSTRACT- Many ants participate in "food for protection" mutualisms with plants. Plants offer food for protection from herbivores. The imported fire ant, Solenopsis wagneri Santschi (Hymenoptera: Formicidae) excludes native arthropods in invaded habitats, and changes in faunal composition may influence these mutualisms. Truly mutualistic interactions, in which both participants benefit, require 1) visitors capable of protecting plants, 2) attacked plants, and 3) visitors collecting food. Additionally, an ideal mutualist does not disrupt other, co-occurring mutualisms. We contrasted the potential of S. wagneri and a native ant,Forelius pruinosus (Roger), to act as mutualists with Catalpa bignonioides (Bignoniaceae), a generalist insectary plant. S. wagneri rarely collected extrafloral nectar, whereas F. pruinosus was frequently observed at plant nectaries. Ant exclusion experiments demonstrated that both species defend the plant against caterpillars, and that S. wagneri is a predator of Cotesia congregata (Say) (Hymenoptera: Braconidae), another Catalpa mutualist. Mortality rates of herbivore pupae were extremely high in S. wagneri-dominated sites. Because F. pruinosus collects extrafloral nectar, defends vulnerable plants, and does not disrupt the plant-parasitoid mutualism, it acts as a Catalpa mutualist. S. wagneri defends plants, although its indifference towards extrafloral nectaries suggests little reciprocal benefit. The competitive exclusion of Catalpa's traditional nectivorous mutualists, combined with the rarity of herbivore recruitment and plant attack in invaded habitats, indicates a defended plant and locally defunct mutualism.

KEY WORDS: mutualism , catalpa , solenopsis, nectary