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ARS Home » Southeast Area » Tifton, Georgia » Crop Protection and Management Research » Research » Publications at this Location » Publication #259549
Title: Ant predation on an invasive herbivore: Can an extrafloral nectar-producing plant provide associational resistance to Opuntia individuals?

Author
item JEZOREK, HEATHER - University Of South Florida
item STILING, PETER - University Of South Florida
item Carpenter, James

Submitted to: Biological Invasions
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/10/2011
Publication Date: 10/1/2011
Citation: Jezorek, H., Stiling, P., Carpenter, J.E. 2011. Ant predation on an invasive herbivore: Can an extrafloral nectar-producing plant provide associational resistance to Opuntia individuals? Biological Invasions. 13:2261-2273.

Interpretive Summary: Cactoblastis cactorum, the South American cactus moth, is a well-known biological control agent for cactus species of the genus Opuntia. However, the arrival of the moth in Florida and its subsequent spread through the southeastern United States poses a threat to the diversity of these native cacti in North America. Of particular concern are the ecological and economic impacts the moth could have in the cacti-rich southwestern United States and Mexico, where both native and cultivated Opuntia species are important resources. A USDA abatement program against this invasive pest currently employs sanitation and the sterile insect technique, and efforts are being made to incorporate additional tactics such as biological controls. Although C. cactorum is attacked by predators in some parts of its world range, little is known about the impact of predators on C. cactorum in the southeastern United States or the type of plant assemblages that may encourage predation. 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 C. cactorum, are often found growing in close association with C. fasciculata. We tested the hypothesis 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. 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 determined that ants do prey upon C. cactorum eggsticks and pupae; overall predation rates were 15.9% for eggsticks and 17.6% for pupae. From August to October of 2008, ant predation of eggsticks and pupae was significantly higher when they were placed on Opuntia individuals near C. fasciculata, but this effect was not seen in November of 2008, probably due to decreased extrafloral nectar production by that time. In summary, Opuntia plants close to C. fasciculata benefit from reduced herbivory by C. cactorum, and this effect is driven by ants that spillover from C. fasciculata to Opuntia.

Technical Abstract: 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 hypothesis 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 determined that ants do prey upon C. cactorum eggsticks and pupae; overall predation rates were 15.9% for eggsticks and 17.6% for pupae. From August to October of 2008, ant predation of eggsticks and pupae was significantly higher when they were placed on Opuntia individuals near C. fasciculata, but this effect was not seen in November of 2008, probably due to decreased extrafloral nectar production by that time. In summary, Opuntia plants close to C. fasciculata benefit from reduced herbivory by C. cactorum, and this effect is driven by ants that spillover from C. fasciculata to Opuntia