Pipevine Swallowtails Really Love Those New Shoots

As we were looking for Pipevine Swallowtails (Battus Philenor) at Tilden Regional Park, Berkeley, CA, we realized how critical it was for the species to have new pipevine (Aristolochia) shoots available to lay their eggs on and for the newborns to eat.

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It is probably a well-known fact that healthy, tender foliage is more attractive to most adult Lepidoptera, particularly butterflies, and is more appealing for them to lay eggs on. Anybody who is willing to take the time to search for eggs or caterpillars will soon realize that one of the best strategies to maximize efficiency is to first look for them on younger plants or on the new growth. 

However, this preference for new growth varies from species to species and is slightly more complex then it seems when considering that most plants only exhibit new growth during a certain time of the year. In particular, Pipevine Swallowtail (Battus philenor) females are notoriously picky with the egg-laying sites, probably because it lays them in relatively large clusters as opposed to singly, and therefore must put extra care in ensuring that it does not jeopardize the future of a very large number of its offspring with one wrong move. Even more so with our "hairy" Californian subspecies hirsuta which is known to lay larger clusters of eggs, on average, than the species. The females may spend hours circling a potential egg-laying site before deciding on the best location to deposit her eggs. And the worst part is that she will only ever lay eggs of new shoots tips or leaves of their beloved Pipevine no matter how large or robust the vine itself may be or how risky it may be to the future of the species (We can testify to this as all 18 egg clusters we have encountered this year have been laid on new growth.). The dilemma here is that Pipevine, at least the single variety utilized in our region Aristolochia californica, only proliferates during the spring to early summer. Now, one would think that a simple solution to this would for the Pipevine Swallowtails to simply enter pupal diapause once new growth ceases in the summer months. But that is clearly not the case as they can actually be seen in flight well into the fall and continue to have many reproductive events months after new growth should have dissapeared. How is this possible?
 
Well, to tell the truth, not all spring and early-summer generation pupa emerge to continue breeding; many of them do, in fact, enter diapause until the following spring where there is a massive flight. The choice to diapause and diapause length is seemingly random (not controlled by photoperiod as it is in many species) with some pupa emerging normally in two to three weeks after pupation, some emerging later in the season of the same year in a rather staggered fashion, and the rest diaupausing through the entire winter until the spring flight mentioned before. Through this strange regulation of the life cycle, the Pipevine Swallowtails can ensure that even if there happens not to be any new shoots on which to breed later in the season, the entire population doesn't simply collapse and that there will be at least some survivors of the generation that will breed next spring. Yes, it is pretty unstable, but it must work to some extent otherwise they would never have evolved to be like this and would probably be extinct by now.

In addition, new Pipevine growth after early summer is not impossible. Like most plants, especially fast growing vines, Pipevine will regenerate itself rapidly should it be cut down or burned. Because this is so rare, any new growth during this time after early summer is likely to be utterly clobbered in eggs. In fact, unlike most species, Pipevine Swallowtails actually seemed to be encouraged to lay eggs where there are already rather then being deterred by the competition.

So when we went down to Tilden Regional Park today, we brought a pair of scissors so that we could cut the old shoots off the Pipevines in order to stimulate regeneration. The pipevines there actually already have a reasonable amount of relatively new shoots because it is not especially late in the season (hence the fact that virtually of the egg clusters we have collected have been from here), but a little extra help for the butterflies couldn't hurt. On the other hand, the pipevine growing at Canyon Trail Park has not a trace of new growth; despite capturing a female butterfly there, we have yet to find a single cluster of eggs. The next time we visit, we are sure to also bring scissors and cut down every shoot we see. 

- Brian