How do plants cope with herbivores? It’s an obvious topic for study, and there’s a whole library of papers on plant-herbivore interactions. A new article by Soper Gorden and Adler looks at this problem but adds more context. They note that plants don’t just interact with herbivores. They also interact with pollinators at the same time and nectar robbers. Soper Gorden and Adler say: “Although many studies have examined the effects of single floral antagonisms on subsequent pollination and plant reproduction, we know very little about the combined, potentially non-additive effects of multiple flower–insect interactions.”
The authors provide plenty of examples of how florivores can impact pollination. Some examples are obvious, for example, removing flowers reduces the emissions of volatiles to attract pollinators. Others are more surprising. They cite Ye et al. (2017), who found that florivory can cause a pollinator to become a nectar robber. Not only do they comment on antagonists affecting pollinators, Soper Gorden and Adler also ask if pollinators can affect antagonists, like nectar robbers or florivores.
There is good reason to think they might. Pollination can cause changes in flowers to direct pollinators to where they are needed. For example, there can be changes in flower colour or scent. Soper Gorden and Adler say that since antagonists are often attracted to flowers the same way as pollinators, differences that repel pollinators could repel antagonists too. To test their ideas, Soper Gorden and Adler examined the effects of florivory, nectar robbing and pollination on Impatiens capensis, also known as orange jewelweed in its native North America.
They found that artificial florivory increased natural florivory, compared to a control population in I. capensis, but reduced floral visits from pollinators and nectar robbers. One explanation is that the change of display reduces the attractiveness of a floral display, but why would it increase florivory? One possible reason the authors propose is that floral damage releases its own volatiles, which can act as a signal to florivores.
Contrary to expectations they found that pollination, in I. capensis, did not reduce visits from nectar robbers nor florivores. The authors say: “It is possible that the addition of pollen might alter floral scent…, potentially changing attractiveness cues. However, we could find no record of supplemental pollination making flowers more attractive to other nectar consumers. Since pollinators had no effect on any floral attractive or defense traits we measured…, it is unclear why plants with hand-pollinated flowers were consistently more attractive to subsequent insect visitors, including leaf herbivores that are, presumably, not as tied to floral traits as floral visitors.”
It’s certainly a different tack to many floral interaction papers, which tend to pare the interaction down to one or two variables. Soper Gorden and Adler make a strong argument that while this is good, when there are non-additive effects to interactions, then that complexity needs to be studied. Reading through the paper, a few experiments occur to me looking at the interaction between antagonists and pollinators. It would, for example, be interesting to see if pollination has an effect on florivory in other species.
The complexity the authors are chasing is difficult to catch without losing any signal in a morass of factors, but here the authors show that while studying the relationship between plants, pollinators and antagonists is complicated, it’s also rewarding.