|Miller, Tom -|
|Legaspi, Benjamin -|
Submitted to: Biological Invasions
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 29, 2009
Publication Date: April 10, 2010
Citation: Miller, T., Legaspi, J.C., Legaspi, B. 2010. Experimental test of biotic resistance to an invasive herbivore provided by potential plant mutualists. Biological Invasions. 12(10):3563-3577. Interpretive Summary: When two different species interact so that each receives a benefit, the relationship is called “mutualism”. In the case of ants and cactus, the ants feed on nectar and other plant exudates, whereas the ant defends the cactus by eating insects attempting to feed on it. Ant-cactus mutualism may be an important factor in hindering the spread of the cactus moth, especially into new areas it has not yet colonized. Scientists at the USDA, ARS Center for Medical, Agricultural and Veterinary Entomology, Gainesville, FL and and colleagues at Rice University studied the interactions between two species of cactus and the naturally-occurring ant populations in two locations in Florida, and their effects on the cactus moth. The effects of removing ants from the plants was weak and variable. On some sampling occasions, the presence of ants appeared to benefit the cactus plant by eating the cactus moth. At other times, the reverse was found. In conclusion, we did not find strong and consistent evidence that the ant-cactus mutualism affects the cactus moth. This may not be the case in other locations haboring different ant species.
Technical Abstract: Understanding the influence of resident species on the success of invaders is a core objective in the study and management of biological invasions. We asked whether facultative, food-for-protection mutualism between resident, nectar-feeding ants and extra floral nectar-bearing plants confers biotic resistance to invasion by a specialist herbivore. Our research focused on the South American cactus-feeding moth Cactoblastis cactorum (Lepidopetra: Pyralidae) Berg in the panhandle region of Florida. This species has been widely and intentionally redistributed as a biological control agent against weedy cacti (Opuntia spp.) but arrived unintentionally in the southeast U.S., where it attacks native, non-target cacti and is a considered a noxious invader. The acquired host-plants of C. cactorum in Florida secrete extrafloral nectar, especially on young, vegetative structures, and this attracts ants. We conducted ant-exclusion experiments over two years (2008 and 2009) at two sites using potted plants of two vulnerable host species (O. stricta and O. ficus-indica) to evaluate the influence of cactus-visiting ants (total of eight species) at multiple points in the moth life cycle (oviposition, egg survival, and larval survival). We found that the presence of ants often increased the mortality of lab-reared C. cactorum eggsticks (stacks of cohered eggs) and larvae that we introduced onto plants in the field, although these effects were variable across sites, years, host-plant species, ant species, and / or between old and young plant structures. In contrast to these “staged” encounters, we found little evidence that ants influenced the survival of or plant damage caused by cactus moths that occurred naturally at our field sites. In total, our experimental results suggest that the influence of cactus-visiting ants on C. cactorum invasion dynamics is weak and highly variable. Our results also highlight the distinction between effects that can occur under experimental conditions and those that do occur in nature.