Pollinator harassment by invasive ants alters floral utilization by honey bees

Background/Question/Methods

The popular managed honey bee (Apis mellifera) provides vital pollination services in agricultural systems. However, when crops are not in bloom, these bees must sustain themselves and supplement their diet with floral resources from the natural and semi-natural environment. In Southern California, floral resources are typically limited in the early winter (January-early February), and presently, these floral resources are further limited due to the severe drought the western U.S. currently faces (2014). Drought adapted succulents, such as aloe (Aloe spp.) can provide critical early season floral resources to bees during this time. However, other organisms may compete with and restrict floral access by bee pollinators.

Here we asked if the invasive Argentine ant (Linepithema humile) restricts honey bee access to aloe flowers and by which mechanisms. We conducted a two-part study that examined: 1) bee visitation rate and bee visitation duration to aloe flowers with and without ants, and 2) bee visitation to aloe flowers without the presence of ants, but with and without artificial ant trail pheromone. 

Results/Conclusions

In our first study, we found that bees visit all flowers equally (where total visitation is the sum of all approaches and landings), but that bees preferentially landed on flowers without ants present. When bees do land on flowers with ants, the visitation duration is shorter than on flowers without ants. The bee is only able to acquire nectar and pollen resources when it fully lands on the flower, and is likely to acquire more resources during a longer visit. These results suggest that bees may be harassed by ants, they prefer not to concurrently occupy flowers with ants present, and that the presence of the invasive ants limits resource acquisition by the bees.

In our second study, we found that bee landing rate is reduced on flowers with ant trail pheromone compared to non-pheromone flowers, even without the presence of ants. This suggests that bees avoid flower with ants by using the ant pheromone as a cue and may therefore avoid future ant harassment. However, the implications are that the invasive Argentine ant may limit bee access to a vital resource during a critical early-season period. Future work will examine if there is a threshold the bees will tolerate in order to acquire resources despite ant harassment, or if ants can severely impact floral resource acquisition by bees.