First Pipevine Swallowtail to Eclose

Today our first Pipevine Swallowtail (Battus philenor) of the year eclosed!

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The Pipevine Swallowtail (Battus philenor) is a species of tropical origin and there is evidence that suggests that its arrival here in California is relatively recent. Most of its relatives in the Aristolochia Swallowtail tribe Triodini live in the tropics, a biome characterized by indistinct seasons and more or less constant temperatures throughout the year. Whether or not this is responsible for their irregular diapausing schedules is unclear, but it probably is one reason. After all, the pupae do not seem to be very responsive to photoperiod like other species. 

Some of the Pipevine Swallowtails that we caught as mature caterpillars in June that pupated have mostly stayed in diapause until now. One of the thirteen healthy chrysalises that pupated on 6/21, the only one that did not diapause, was going to emerge on 7/14 but the chrysalis was badly deformed and the misshapen butterfly died while attempting to break out of the cuticle. Another two chrysalises that became quite pharate around 8/1 never eclosed; after several days we broke them open and found very dead butterflies. Forgiving our misfortune, this data is probably consistent with their irregular diapausing schedules in which some eclose in the normal amount of time (two to three weeks) without undergoing any sort of diapause, some diapause but then eclose later in the season of the same year (allowing them to fly as late as October!), and the rest diapause and eclose in the following spring. Basically, it is mixed bag, even with individuals of the same clutch as these probably were. 

Whether or not the cues for diapause are affected at all by typical regulators as photoperiod and temperature is unclear; it seems more or less random for the most part. Probably another important factor to consider, though, is host plant. In California, the Pipevine Swallowtail utilizes only the native California pipevine (Aristolochia californica, another recent arrival of tropical origin) which stops proliferating by summertime unless forced to regenerate, which is unfortunate because the swallowtails lay exclusively on the new shoot tips. Perhaps there is an element of "bet-hedging" to the split flights in which the species depends on at least some individuals eclosing at a favorable time. Plus, it has been suggested that diapause may be somehow related to the water content of the plant that the caterpillar fed on.  

Anyways, it has been observed that the "estival" diapausers tend to break sometime in August (at least, this is when the numbers spike). This is probably true. Today, in the middle of August, one of our June chrysalises finally eclosed about two months after pupation. He was a beautiful prismatic blue male, which is a much more attractive look than the mostly drab gray female we caught in June. Like in the treasured birdwing butterflies (Ornithoptera) and many other Triodini relatives, it is always the male that is the very showy one and the female that is plain. The wing shape is also more typical of Triodini members than either of our two fluted swallowtails (Papilionini), the Anise Swallowtail (Papilio zelicaon) or Western Tiger Swallowtail (Papilio rutulus); the forewings are relatively long and angular. Not to mention the rather short tails, which would make sense as they are a highly aposematic species (birdwings don't even have tails!) and the tails normally serve as a form of defense (fake antennae to confuse an attacking bird into biting the wing instead of the body). The same lack of traditional defense is seen in the caterpillar too, which are also aposematic; they rarely employ their stink horns (osmetarium) which rely on both retraction speed to ward off motion-sensitive predators (or parisitoids) and chemical assault (butyric acid). They feed openly, often gregariously at least when they are young, and as a result they are routinely parasitized by chalcid wasps.

​Sadly, of course, no other chrysalis shows any sign at all of emerging at this time so we released it. Otherwise we could have attempted to hand-pair.

- Brian