Floral Deterrent

There is no mistaking the allure of certain floral scents. Entire industries have developed around capturing the essence of plant reproduction. Millions of years before we humans adopted floral scents for our own bidding, plants were producing them to attract pollinators. Even plants that produce sickening smells meant to mimic rotting flesh are trying to get the attention of something. It seems so obvious that these perfumes evolved as attractants but there are some plant species whose floral scents do the opposite. 

When I first caught a whiff of the blooms of the fragrant olive (Osmanthus fragrans) it didn't take long to track down its source. In my opinion it is one of the most enticing fragrances I have ever encountered. Native throughout much of Asia, this evergreen shrub produces sprays of tiny, pale, 4 lobed blossoms. Its attractive form and heavenly scent have led to its popularity in horticultural circles the world over. Surely its intense fragrance also makes it quite popular with pollinators. The truth is actually quite surprising.

In its native range, few insects seem to show it any attention. Aside from some hover flies, the list of floral visitors is surprisingly quite depauperate. What is going on? Why would this shrub go through the trouble of producing such volatile compounds? Has its intended pollinator gone extinct? Though it can be difficult to answer such inquiries, research done in Japan suggests that these scents may actually function as a deterrent rather than a lure. 

Researchers looked at a common butterfly species known to pollinate other flowering species in the area. The results of their experiments were quite shocking to say the least. Even when food was withheld for a period of time, the butterflies never visited the flowers of the fragrant olive. Only when the team replaced the flowers with replicas infused with a neutral scent did the butterflies pay them any attention. 

The fact that hover flies do in fact visit the flowers suggests that the scent compounds serve a dual function - attracting the intended pollinators while at the same time deterring potential nectar thieves. More work is needed before we can be completely confident in these conclusions but the idea nonetheless highlights the fact that more is going on with flowers than we realize. 

Photo Credit: hto2008 (http://bit.ly/2hPOhnO

Further Reading: [1]