Why do Anise Swallowtails Love Fennel?

In this post, we explore the feeding reaction to different parts of the sweet fennel plant (Foeniculum vulgare) as well as to citrus leaves in Anise Swallowtail caterpillars (Papilio zelicaon) and explain our theory on the phytochemical stimuli of their host plants. 

---

Most invasive species are unwelcomed and a pain to deal with because they have no natural predators or other ecological "checks" in the area they are introduced to and able to rapidly colonize it, as seen with wild or sweet fennel (Foeniculum vulgare) and disturbed city areas. But in addition to being so invasive here on the Coast of California, the sly old F. vulgare is intrinsically more attractive to the Anise Swallowtail (Papilio zelicaon) than any other host plant -- native or not -- in our region. Fennel in general is a winner among winners, topping a list of over 40 different genuses of known host plants. The butterflies absolutely love the stuff to lay their eggs on and the larvae accept it more readily than any other host despite it never even existing here less than two centuries ago. The inevitable result has been a complete population boom in P. zelicaon in areas (especially urban ones) that it probably would have otherwise been extremely scarce.  

But the big question is why. Why in the world would F. vulgare be so insanely popular with these swallowtails, even when native host plants still exist? 

Well, for a while we didn't really know. But over the years we have been testing around with food plants, including different parts of food plants, in order to determine any preferences. As a starter, we have found is that the caterpillars will readily switch from parsley and other Umbelliferae to F. vulgare but never the other way around. It is usually very rare for a caterpillar (of any species) to switch one host plant to another once it has already established itself on the first.

But where it gets tricky is that there are preferences within the fennel plant itself. For the longest time, we thought P. zelicaon were like a lot of insects in that the caterpillars would prefer eating the fresher and more tender new growths, or inflorescence, over older, tougher leaves. We would always try our best to feed these newer growths and it took us quite a while to finally realize that they didn't like it. The caterpillars would consistently gobble the older, seemingly less desirable leaves instead when given the choice. Later, when we tried feeding the caterpillars flowers we observed the same occurrence. The caterpillars would stear clear of the young flower buds and only eat the mature flowers and immature fruits (seeds). We hypothesized that some chemical produced as the plant's natural defense against herbivory discourages feeding even in P. zelicaon and is found in greater concentrations in the newer parts of the plant. 

At the same time, we realized that the caterpillars actually preferred the mature flowers over the normal leaves and preferred the fruits even more so than the flowers.  Like most monogamous feeding caterpillars, P. zelicaon caterpillars rely pretty heavily on phytochemical stimuli for feeding. After all, the little guys are virtually blind and would have absolutely no way of telling whether they are on the "correct" host plant without being able to detect chemicals specific to their host plants. It is therefore logical to assume that the higher the concentration of these chemicals, the greater preference the caterpillars and butterflies would have for certain plants and specific parts within the plant.

What is required to stimulate these caterpillars to eat in Apiaceae (umbels) are primarily two chemicals called anethole and anisic aldehyde, both of which are essential oils. Anethole is an extremely aromatic organic compound that is sometimes used in the flavoring of foods, especially as a sweetener, and is also found in some other plants like licorice (Fabaceae) and basil (Lamiaceae). Anisic aldehyde, like most aldehydes, also has a very strong aroma and is even extracted for use in some perfumes and soaps. Together, these chemicals are responsible for the sweet fennel-like smell in the oils of all umbels, but they, of course, are most present in fennel, hence the fact that fennel smells so strongly like, well, fennel.

​Then, if you take a look at the specific parts of the fennel plant we mentioned before, you can see the correlation there too. The fennel seeds, like most plant seeds or nuts for which oils can be extracted, are like big fat bombs that have, by far, the highest amount of essential oils than any other part of the fennel plant. At the same time, it is hard to confirm this because the flowering parts of the plant are often more proteinous than the leafy portions, which makes them more nutritious.To sum it up though, Anise Swallowtails probably prefer fennel over other Apiaceae due to its higher chemical content and prefer the fruits of the fennel because its higher chemical content is even higher then the rest of the plant. 

In addition, another Apiaceae essential oil called methyl chavicol that can also stimulate feeding in some Papilios also happens to be present in members of the Rutaceae family, or citrus, though its stimulatory effect is many times weaker than that of anethole and anistic aldehyde, at least in our particular race  of  of P. zelicaon (region specific subgroup or variation within the species; not necessarily a subspecies). Note that Apiaceae and Rutaceae are completely unrelated and the evolution of similar oils is completely coincidental (convergent evolution). Even so, we proved that it possible to feed Anise Swallowtails certain citrus (entirely chemical), by rearing a caterpillar all the way to sixth instar feeding only on lemon (Citrus limon) leaves. However, we noticed that this particular caterpillar's osmetarium, the pair of forked horns characteristic of all Swallowtails (Papillioninae), have a much milder odor than those of our fennel eaters and lacks the sweet fennel smell. The reason is these caterpillars extract chemicals such as the aforementioned anethole and anistic aldehyde from their food and store it in their body and osmetarium as a mild toxin to fight against predation where a bit of it is also converted into butyric acid in the osmetarium. If you are not aware, butyric acid is a fatty acid normally found in high concentrations in human vomit and gives milk fat a rancid smell when spoiled. It, of course, is also responsible for giving P. zelicaon osmetariums their noxious smell, with the intensity entirely dependent on the levels of the chemicals present. Logically, this would be the reason why we have observed a much milder odor (almost nonexistent) in the citrus eater. 

Interestingly enough though, when we attempted to feed the citrus eater a stalk of immature fennel seeds, it completely refused and went right on eating its beloved C. limon. Clearly, despite the high feeding chemical content in fennel, it was thoroughly ignored because of some kind of chemical imprinting mechanism that takes place in the first few days after birth of the caterpillar. In most species, once a caterpillar has established itself on one host plant, it is permanent and it will not accept any other host plants unless they are similar enough (for example, the citrus eater would probably accept some other citrus besides lemon and F. vulgare eaters would most likely accept other varieties of fennel). 

To conclude, we hope that our theories shed some light on the topic of host plant preferences in Anise Swallowtails. And perhaps there is a plant or other variety of fennel not yet popularized that is superior to F. vulgare and has an even higher chemical content that may be able to produce some monstrous sized caterpillars. But until we find it, F. vulgare is still their number one. 

- Brian