Approximately half the ant genera in the world (41%) form facultative mutualisms with 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 (FF) 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 FF than mesquite lacking AHM. Indeed, mesquite that hosted AHM produced significantly more FF than mesquite not hosting AHM (F2,117= 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. Removal of AHM was maintained through the year as a long-term press experiment. Prior to the removal of AHM (June 2013), for each tree I quantified FF abundances and vacuum sampled the arthropod community. Mesquite were resampled in June 2014 to determine the effect of AHM removal on FF production and the arthropod community.
Results/Conclusions
In June 2013, prior to the AHM removal treatment, FF abundances did not differ for trees assigned to the AHM removal treatment and controls (F1,94 = 0.9914; p = 0.3219). Control trees with AHM produced 20% more FF 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 FF than did those in the AHM removed treatment (747.48 ± 71.5 versus 535.23 ± 41.2; F1,94 = 4.19, p = 0.0434). Hosting AHM appears to provide a reproductive benefit to mesquite. In addition, mesquite trees in which AHM were removed had significantly higher abundance of chewing (Z = -2.422, p = 0.0154), sap-sucking (Z = -2.181, p = 0.0292) and predatory (Z = -2.086, p = 0.0370) insects. The current study provides evidence in the role AHM play in structuring the arthropod community as well the indirect positive affect AHM can have on their host plant through the removal of harmful herbivores.