Cannibalistic Monarchs

After we went out collecting Monarchs (Danaus plexippus) today, we realized that the caterpillar we found had eaten three of the eggs! 

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As we went over in this June post, Monarch butterflies (Danaus plexippus) are freakishly abundant this year. We cannot remember a single day when we had not seen one if we just took a moment to step outside when it was sunny. We have witnessed them courting, mating, and laying eggs on multiple accounts, establishing an actual breeding colony here in Albany which is normally only a temporary migration destination during the winter. 

Today, we had casually been standing around in our yard for a few minutes in the afternoon when a female Monarch came to suck at our tropical milkweed (Asclepias curassavica) which was in full bloom. Of course, this is not a particularly uncommon sight this year. But this time we went ahead, grabbed our net, and caught it in one fell swoop just for the heck of it. We put it in a laundry bag covering a little potted tropical milkweed to try to get it to lay some eggs. 

While we were at it we noticed a few eggs on the milkweed, a fourth instar caterpillar, and a mature fifth instar. This entire year, our milkweeds had been covered over and over -- probably multiple times every week -- with Monarch eggs. Quite ridiculous, yes, and we probably could have collected hundreds of them by now if we had tried (this just goes to show how low the survival rate of these things, what with all the European paper wasps [Polistes dominula] and ladybugs [Coccinellidae] patrolling the plant all day since!). But for the most part we had left them alone, not having the motivation to rear this much overrated species once again with so many other projects going on; we have only reared them two times this year, in small batches, once in April and again in June.

We decided to go for a third since milkweed has become so abundant in Albany over the last year or so  and leaves wouldn't be a problem for potentially raising tons of caterpillars. For today, we collected five eggs and the two caterpillars from our milkweed and temporarily put them inside a small petri dish which was the only thing we hand on our hands at the time. 

A few hours later, after we were ready to deal with setting up housing and such, we realized that there seemed to be less eggs than we had collected. Upon closer examination, three of the eggs had been quite evidently chewed up. We should have been more careful! It is known fact that the caterpillars of some species of pierids (Pieridae) and nymphalids (Nymphalidae), the latter of which the Monarch is a member of, will cannibalize eggs when given the chance. We have had this problem with Cabbage Whites (Pieris rapae), Gulf Fritillaries (Augraulis vanillae) and Vanessas but not really Monarchs, a species we have raised for years, so we thought we were safe. But I supposed not! 

- Brian