Ant-hemipteran mutualism facilitates flower and fruit production of an invasive hardwood, honey mesquite (Prosopis glandulosa), in south Texas

Background/Question/Methods

Approximately half the ant genera in the world (41%) form facultative mutualisms with a variety of insects, particularly hemipterans. Hemipterans feed on phloem and excrete honeydew, a renewable and reliable food source harvested by ants. In turn, the ants indiscriminately defend hemipterans from predators, parasitoids, and competing herbivores. The removal of competing herbivores has the potential to benefit the host plant. However, our understanding of the beneficial role of ant-hemipteran mutualisms (AHM) on long-lived plant hosts is lacking. I hypothesized that AHM will positively affect flower and fruit production of an invasive hardwood, honey mesquite (Prosopis glandulosa). In the summer of 2012, I censused 120 mesquite trees at the Welder Wildlife Foundation Refuge in south Texas to determine if mesquite that naturally hosted AMH produced more flower and fruits than mesquite lacking AHM. Indeed, mesquite that hosted AHM produced significantly more flowers and fruits than mesquite not hosting AHM (F_2,117 = 25.63, p = <0.0001). The following summer (2013), I haphazardly selected 100 mesquite trees hosting AHM and randomly assigned 50 trees from which I removed AHM, and 50 to serve as unmanipulated controls. Flower and fruit abundances were then quantified for each tree.

Results/Conclusions

In June 2013, prior to the AHM removal treatment, flower and fruit abundances did not differ for trees assigned to the AHM removal treatment and controls (F_1,94 = 0.9914; p = 0.3219). AHM were then removed as a press experiment maintained throughout the year. Control trees with AHM produced 20% more flowers and fruits than they had in the previous year, compared to an 8% increase for AHM removal trees from 2013 to 2014. Mesquite with AHM present produced significantly more flowers and fruits than did those in the AHM removed treatment (711.67 ± 69.18 versus 542.04 ± 53.71; F_1,94 = 4.19, p = 0.0434). Hosting AHM appears to provide a reproductive benefit to mesquite. The enhancement of reproduction by AHM could have significant impacts on the viability, spread and successful establishment of invasive trees.