Finding a Ridiculous Amount of Pipevine Swallowtail Eggs

Pipevine swallowtail (Battus philenor hirsuta) eggs are now pouring in in huge numbers from the spring flight.

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Yesterday we had an exciting moment when we found our first pipevine swallowtail (Battus philenor hirsuta) eggs of the year at Albany Hill. Today we were blown away when we went to Tilden Regional Park. 

Immediately stepping onto the scene. we were warmly welcomed by a whirl of black figures fluttering back and forth across nectar flowers like sage and in the background. On our last two trips two and four weeks ago, we had also seen a number of pipevine swallowtails flying around but by the time we got the the pipevine (Aristolochia californica) we knew things were quite different. There were an absurd amount of butterflies--at least a dozen or two--and we soon became aware that there were a number of distinctive females flying about the pipevine.  They behaved by constantly flying about the pipevine, landing and coming in contact with the new shoots, investigate by crawling around and drumming their feet (where chemical receptors are in highest concentrations) and then fling off to test another spot. 

​At first we were slow to realize just how many eggs must have been laid or were being laid when we were there. It took us a while before we found our first two egg clusters which were located on the same small vine growing on the ground. We even found an empty chrysalis directly on one of the vines. But soon, we were finding clusters left and right, almost all of which were located on the relatively healthy stems near the ground of near the tops of the climbing vines. The eggs were of all different developmental stages.

In total, we counted a whopping 172 eggs over 10 clusters (27,24, 20, 20, 17, 16, 14, 14, 9, 5 with 6 that fell off that were most likely from the cluster of 17). We were quite surprised at how large some of these clutches were, considering that we never found any that even had up to 20 eggs last year and most sources report that they typically lay between 10-20 eggs together. The northern California species hirsuta supposedly does lay in greater numbers than the nominate species but 27 is really a lot considering the gravid females probably carry less than a hundred eggs at a given time! Here are photos of each cluster:

Even though we did find a ridiculous amount of eggs already thanks to the fact that they were conveniently laid in such large clusters, we are certain that we didn't get nearly all of them. In fact, in one of the pictures that was taken of a female in flight you can see a leaf containing a small cluster of eggs on it if zoomed in. We didn't even realize this at the time but the cluster seemed unfinished (there are only 7 eggs visible in the picture) and the female was probably in the process of laying these eggs!  ​
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Certainly if we were to go back another day we would find these eggs that we missed. We would probably also find that an incredible amount more would have been laid by then, if even that day is tomorrow. The place is now swarmed with butterflies as the spring flight has truly begun. We are 100% sure that the last time we came here there had not yet been eggs. Given that Albany Hill also started to receive eggs right now, the time that they decided to begin is probably no coincidence and is actually according to schedule and that our previous trips were most likely actually too early despite observing the butterflies.

This schedule corresponds with what we were able to observe last tear which were (only) mature caterpillars and fresh chrysalises in mid June. Assuming they take about a month or so to grow, this would align perfectly to when the eggs could have been laid (late April to mid May). With this information, it can be concluded with a certain level of confidence that these pipevines swallowtails are bivoltine with two distinct broods (the second one may be partial because first brood chrysalises can still go into diapause) with the first starting in early May and the second in late June. ​

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​Authors
Brian Liang

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