Anise Swallowtail Isolated Plant Technique

Anise Swallowtails (Papilio zelicaon) are super common here, and with the mid-June heat rolling in they have certainly begun to peak for the season. In this post, we go over a recent egg hunting trip and the isolated plant technique. 

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​This Wednesday, we scheduled another one of many trips to El Cerrito's Ohlone Greenway from Brighton Ave. to Fairmount Ave to forage for Anise Swallowtail (Papilio zelicaon) eggs and caterpillars. It had actually been over two weeks since the last time we had come searching which is relatively long considering will typically go every weekend so we hoped that they would have accumulated by now.

The Ohlone Greenway is an isolated walkway stretching from city to city along the BART tracks with a decent amount of native plants and trees growing wildly. However, in a lot of places of the trail (including the section we went to today) the city must have decided to cut down on the plants directly under the BART tracks so they could screw up the soil and plant other things like grass and flowers in what we suppose is an attempt to make the trail look more friendly. While this is pretty artificial and destructive of the native habitat, it is good for one thing -- wild fennel (Foeniculum vulgare).

Originally introduced from Europe, the little intruder grows incredibly well here in Northern California. You see, F. vulgare is extremely opportunistic and will readily take advantage of any "disturbed areas" where the topsoil has been removed in a ridiculous case of secondary ecological succession. Needless to say, "disturbed areas" are very common in the urban setting and the Ohlone Greenway is no exception. Wild fennel grows to be much large than the stuff you find in grocery stores and once established is impossible to get rid of due to its huge taproot (it is in the carrot family after all!). 

Anyway, the Anise Swallowtail's number one most preferred host plant by a very generous margin is none other than the slimy F. vulgare. Not too long ago considered a relatively "rare" species, the Anise Swallowtail population is now booming due to the recent arrival and rapid spread of  F. vulgare and they seemed to have all but forgotten about the hardy little native umbelifers that they normally would have utilized. In fact, so many native plants are becoming increasingly rare here that it would be very difficult to find any native host plants such as Water Parsley (Oenanthe sarmentosa) and Yampah (Perideridia spp.) around anyhow, unless you were to search up into the mountains. But, after all, P. zelicaon's ancestral species most likely utilized F. vulgare back in Europe anyway before even arriving to North America so its not a surprise that they would so readily revert back to it. 

Based on our own experience, the move to wild fennel in the city was a very quick one. The first time we found Anise Swallowtails eggs three years ago in 2013, they were on parsley from a local vegetable garden. From there, we consistently found the eggs in nurseries on their parsley and dill that year. At the time, wild fennel was relatively uncommon here in Albany and was not nearly as rampant as it is now. There certainly wasn't very much of this big weed on the Ohlone Greenway or in anybody's front yards. For a long time we were not even aware that the butterflies would lay their eggs on fennel since we could never find any there. Ironically, that year was the last and only year that we have ever found Anise eggs on anything other than the sly F. vulgare.

Since 2013, fennel has tightened it grip in our region with incredible speed and the Anise Swallowtails have followed suite, rewarding it with the status of being it's most preferred host plant here on the coast of California. Yet, even with what look like fields of wild fennel growing everywhere under the BART tracks on the Ohlone Greenway, we failed to collect even a single egg or caterpillar from there on today's trip. Why? Simply because there's too much there in that particular area. . . .

When looking for most butterfly eggs or caterpillars, following the "isolated plant technique" or "edge effect" is your golden rule of thumb to maximize efficiency. We have found that it works especially well with Anise Swallowtails. In essence, it is strategic to search on isolated host plants in noticeable open areas rather than where the plant is very dense. This technique is based on a few simple principles:
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1. Most butterflies that lay eggs singly on host plants (non-gregarious caterpillars) are programmed to only lay a few per area (an "area" consists of all host plants close together in that particular location), most likely to avoid overcrowding or overfeeding of the plant as well as to make it so the caterpillars are less susceptible to predators (whenever we have tried raising a huge number of Anise caterpillars on a host plant outside, the population has been all of a sudden completely decimated by paper wasps one day because they work on an all-or-nothing agenda where once they spot one caterpillar, they spot them all). Therefore, the ratio of eggs to plant mass goes down drastically when you have a ton of host plants (or just a very large one) and it starts become like finding a needle in a haystack.                                                                                             
2. Butterflies act differently under sub-optimal or stressful conditions. Despite the first principle, most of the times there really are more eggs on isolated plants than on a group of them, which seems rather counter-intuitive since you would think that the number of eggs is proportional to the size of the plant. However, it is important to understand that when there is a host plant that is isolated the butterfly becomes very stressed and at the same time doesn't have the luxury of being choosy with which host plant it lays its eggs on since there is only one plant and is therefore extremely likely to lie on this particular plant. Also, from an evolutionary standpoint it does make sense for a butterfly to go all-out when the host plant is limited since it has absolutely nothing to lose no matter how undesirable the plant may be (not to mention running the risk of facing the issues named in principal one); if it doesn't lay the eggs on that plant, then the chance of passing on its genes is absolutely zero since there are no other plants around but if it does lay the eggs then the chance on its genes is obviously higher. This is probably the same principal with Giant Silkmoths which will lay fertile eggs even when no host is present.                                                
3. The butterfly needs to see the plant to lay eggs on. An isolated host plant is not only away from other host plants but also not concealed by other non-host plants. In addition, it is critical that the butterfly actually has the space it needs to physically move in, bend its abdomen and lay the eggs while in mid-flight. You would never hope to find anything on a plant that is densely surrounded on all sides by other plants because they create a physical barrier against the butterfly. That is why the next best thing to an isolated plant is a plant that is somehow more exposed (semi-isolated) by being, for example, around the outer perimeter of the area or significantly taller than the other plants. 

At the same time, the fennel fields growing under the BART tracks are almost always under the shade of the tracks. In order to increase the chances of finding eggs or caterpillars, the plants must be in an open, exposed area that receives direct sunlight because the sun is directly responsible for warming up the Anise butterfly's body and inducing oviposition. . . . 

Keeping all of these points in mind, we knew we had hit it hard when we came to the very end of this section of the trail to Fairmount Ave. and found a single medium-sized, healthy fennel plant sitting in the sun. From this single plant we reaped the majority of 92 Anise eggs and 19 caterpillars we collected today. 

- Brian