NPV Virus Killing Our Monarchs

For the past several weeks, we have been witnessing almost all of the Monarch (Danaus plexippus) caterpillars on our milkweed dying from the Nuclear Polyhedron Virus (NPV)!

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A while ago, we had a large brood of Monarch (Danaus plexippus) caterpillars from a fertile female we had caught back in September. By the time most of the caterpillars had reach fourth and fifth instar, it was becoming a serious pain to keep supplying them with enough fresh milkweed every day. In fact, it was practically impossible. For their size, Monarchs are probably the fastest growing species we have ever witnessed, and the fast they grow the faster the rate that they need to eat. No matter how much milkweed leaves we supplied them, a group of fifty caterpillars or so could plow through it within hours, leaving them starving for the rest of the day while we were gone. By the time the first few caterpillars began to pupate, it had become a disaster. (It was so horrible, we had to end the rearing notes. . .) Simply put, the caterpillar were cannibalizing the prepupae and newly molted (teneral) chrysalises since they were starving and overcrowded. Not a single chrysalis was formed successfully without being eaten. 

Eventually, we decided that we would have to just release all of the caterpillars on our milkweed in our yard as the simplest solution if we didn't want to have to pick so much milkweed everyday. At first, it was okay. The caterpillars destroyed the plant, but it at least they were full. 

But a week or so in, the plant became almost completely stripped, the caterpillars were very overcrowded, and it started to rain a lot. Eventually we started seeing caterpillars crawl up to the top of the plant and die from NPV. They would become very limp and black. We're not sure what exactly could have caused the caterpillars to get sick (whether it was correlated to bad rearing conditions outside or if it was just bad luck) but needless to say, it was quite horrifying. Only a few survived. 

This year is the first time we have ever encountered the NPV virus. We would consider that lucky since it is quite common where winters are mild like in the Bay Area. We don't  plan on having to encounter it again, but whether or not we do is anyone's guess since it naturally occurs in the wild as we have seen here. In other words, we can do our best to try to rid of contaminated things right now, but there is no guarantee that the virus won't spring up again next year as it did this year. 

- Brian