Found a Papilio rutulus egg!

Today we were lucky enough to found a western tiger swallowtail (Papilio rutulus) egg on a willow tree at Cerrito Creek!

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Ever since we have gotten some caterpillar action going this month, we've been having to make several trips a week down to Cerrito Creek (just before Albany Hill) in order to pick willows (Salix), alder (Alnus rubra), and ceanothus (Ceanothus). The place is just packed full of life at this time of year and thus far we have never failed to find something interesting.
 
After encountering so many of these "random" caterpillars on these trips simply by chance and without any serious intention of doing so, we really started to question whether it was truly so difficult to find so-called "highly-cryptic" species like western tiger swallowtails (Papilio rutulus) or Smerinthus opthalmica. Surely, these can't be too much harder to overlook than speckled green fruitworm moth (Orthosia hibisci quenquefasciata) larva of which we have found twice in two weeks. We don't have the best eyes around (nearsighted) but we figured that unless they truly didn't exist on the trees of Cerrito Creek, we should be able to find them within time. 

With that said, it still came as a complete shock when we found a freshly laid Papilio rutulus egg lying around on the lower side of a willow tree (not more than 4-5 feet off the ground) in full sun, smack in the center of the last segment of trail just as we were about to head home. This was complete chance encounter. One moment you are walking along the trail and the next you are staring less than a foot away from a shiny object to the right! There are few other events where we have been as lucky as this one. ​

The egg is clearly at most a day old. More likely it was laid just today, which was one of the hottest days of this year so far. The perfect timing with the perfect whether. The only strange thing was that the exact location seemed more or less arbitrary. Not only is it hard to even imagine an adult rutulus flying so low--the rumor is that they typically lay on tree tops, the leaf was definitely not a particularly desirable one. 

​This egg is actually quite different then the Papilio glaucus egg we have to our surprise. To be precise, this rutulus egg is much larger (a little under 2 mm compared to the glaucus' 1.5 mm) and somewhat flat rather than perfectly spherical. This conforms with the pattern we have seen with many of our west coast species as compared to counterparts in other regions: the eggs tend to be proportionally larger whereas the adults average smaller. We have noted this in Smerinthus opthalmica and Hyalophora euryalus and have heard reports of this in Californian Antheraea polyphemus; and for some reason Battus philenor hirsuta tends to lay larger egg clusters than the nominate species. It also seems to be slightly lighter in color (yellower) than the glaucus egg when it was fresh.

We are now hoping for two things to happen: first, for the hybrid glaucus × rutulus egg to develop properly and hatch and second, for this new egg to hatch (into what we would prefer to be a female). It would really be an incredible opportunity to observe the two growing side by side. Below are the two eggs shot together for comparison.

Authors
Brian Liang

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