Intruders in a pollination mutualism: exploring the impacts of florivores and honeybees on the floral biology of Datura wrightii

Background/Question/Methods

Researchers know little about how insects that feed on flowers, termed florivores, affect the stability of mutualisms involving pollinators and plants.  Damage to flowers may negatively affect pollinator service by altering visual or odor cues needed for identification of suitable flowers. Here we examined the impact of florivory on the interaction between Datura wright (Solanaceae), and its main pollinator, the hawkmoth Manduca sexta. Earlier observations showed that two of the most abundant visitors are honeybees (Apis mellifera) that collect pollen, and masked chafers (Cyclocephala melanocephala) that eat the corolla.  In this study we thus asked if 1) If masked chafers cause a significant amount of damage to D. wrightii flowers, and 2) If honeybees have the potential to pollinate D. wrightii flowers.

We mapped approximately fifty D. wrightii plants in the Santa Rita Experimental Range near Tucson, Arizona in 2011. We recorded the number of one-day-old flowers with corolla damage and quantified the amount of damage using a 4-point scale.  We also quantified anther damage on 5-point scale.  We observed honeybee visits over a period of several evenings, recording the numbers of approaching honeybees that gathered pollen and those contacting the stigma.

Results/Conclusions

96% of all opened flowers received corolla damage during the duration of flowering.  Approximately 40% of the flowers had less than 10% corolla damage, 25% had between 10-20% damage, and 28% had greater than 20% corolla damage.  Nearly 46% of all flowers had at least one anther damaged or removed during flowering.  Of the honeybees that approached flowers, approximately 36% collected pollen. Of those, 54% also brushed the stigma, such that approximately 20% of all bees contacted both anthers and stigmas.

The extent of damage we observed was surprising and substantial, and may affect M. sexta’s ability to find flowers if the damage alters the scent or appearance of the blossoms. Because honeybees contacted both the anthers (while collecting pollen) as well as the stigma at relatively high frequencies, it is possible that they may cause autogamous selfing in D. wrightii.  Honeybee visitation may also limit the amount of pollen available to more efficient pollinators, like M. sexta.  Taken altogether, these results suggest that floral interlopers may have significant impacts on the tight mutualism seen in this system.