We have all had encounters with sticky plants. Outside of being an interesting sensory experience, the sticky nature of these floral entities would appear to have some evolutionary significance. Considering the cost of producing the glandular trichomes responsible for their stickiness, function is a reasonable question to ask about. For anyone who has taken the time to observe such plants, you will have undoubtedly noticed that insects tend to get stuck to them.
For carnivorous plants, the utility of these glands is readily obvious - trapped insects become food. Even non-carnivores like Roridula gain a nutrient benefit in the form of nutrient-rich feces deposited around the plant by specialized carnivorous bugs that consume trapped insects. However, there are many species of plants out there that fall under the category of "sticky" and a new paper explores this in a more general way.
The serpentine columbine (Aquilegia eximia) is endemic to the Coastal Range of California and it is indeed quite sticky. Its surfaces are covered in glandular hairs. Any given plant can be covered in insects unfortunate enough to come into contact with it. However, it is not a carnivore. As such, researchers wanted to see what benefits, if any, the columbine gained from producing these glands.
By manipulating the amount of insects that were stuck to each plant, researchers found that plants without "victims" actually received more insect damage. The key to this mystery were predators. Plants with lots of trapped victims had more predatory bugs hanging around. These predators, when present, reduced herbivory by deterring other insects that were too large to get stuck. What's more, most of the benefits were observed in the flower buds, which means predators increased the overall reproductive fitness of the serpentine columbine. If the columbine did not trap small insects, these predators would have no reason to hang around.
These predatory bugs were by no means specific to the columbine. In fact, observation of the surrounding plant community found that these predatory insects were present on other sticky genera such as Arctostaphylos, Hemizoni, Holocarpha, Calycidenia, Cordelanthus, Castilleja, Mimulus, Trichostema, and Grindelia. This suggests that the relationship between sticky plants and these generalist predators is more widespread than previously thought. It may also offer a unique window into one possible driver behind the evolution of carnivory in plants.
Photo Credit: David A. Hofmann (http://bit.ly/1l9OtwC)