Ant predation on an invasive herbivore: can an extrafloral nectar-producing plant provide associational resistance to <Emphasis Type="Italic">Opuntia</Emphasis> individuals?

The legume Chamaecrista fasciculata attracts ants to its extrafloral nectar (EFN) which can lead to reduced herbivory and increased fecundity for the plant. In Florida, Opuntia stricta and O. humifusa, hosts of the invasive moth Cactoblastis cactorum, are often found growing in close association with C. fasciculata. We tested the hypotheses that O. stricta and O. humifusa individuals have higher ant abundance, lower levels of herbivore damage, and increased growth when growing in close association with C. fasciculata compared with individuals not growing near the plant. We also experimentally placed C. cactorum eggsticks and pupae on Opuntia individuals to see if ant predation of these stages occurred, and if so, whether predation rates were higher on individuals growing close to C. fasciculata. Opuntia plants near C. fasciculata were less likely to be attacked by C. cactorum and had higher ant abundance than plants far from C. fasciculata. Field surveys showed that Opuntia plants near C. fasciculata had a lower proportion of cladodes with C. cactorum damage of any type. Proportions of cladodes with damage from five native herbivores were not significantly different between treatments. In addition, Opuntia individuals growing near C. fasciculata added proportionately more pads during the growing season. We found evidence of ant predation on 15.9% of C. cactorum eggsticks and 17.6% of pupae. In August and October of 2008, there was significantly more evidence of predation on eggs and pupae placed on Opuntia individuals near C. fasciculata. No effect of distance to C. fasciculata was seen in November of 2008, potentially because plants were no longer producing EFN at this time. Our finding that Opuntia plants close to C. fasciculata show reduced herbivory from invasive C. cactorum, but not from the native herbivores examined, suggests that patterns of associational resistance may be influenced by the co-evolutionary history of the organisms in question.