Monarch Monitoring Project: Week 4

I am a proud mother! Four of my Monarchs have emerged from their chrysalids at the Northwood Center. All were large, healthy boys. Hooray! And they are the start of my small subset of data examining whether Monarchs that are reared in captivity are larger than wild Monarchs. I am hypothesizing that...hmm.....what am I hypothesizing? Well, my original thought was that Monarchs that were raised in captivity with an abundant source of milkweed would be larger than wild Monarchs with a limited milkweed supply. Monarchs vary in size a lot with no size difference between males and females. But, after measuring my four monarchs (woohoo what a HUGE sample size?! just kidding) I am having second thoughts. Maybe some Monarchs just get confused and decide to form a chrysalis earlier than others?

Natural variation in size between net-caught Monarchs

One of my chrysalids did not make it. Rather, the Monarch emerged but it was not at all healthy looking: the wings remained crumpled and he was unable to extend them. Poor guy. Which leads us to our discussion of parasites that attack Monarchs! Yep, I did mention that monarchs are poisonous. But, they also provide a perfect host for the tachinid fly. Monarch parisitoids include 12 different species of tachinid flies and at least one species of brachonid wasp. Parasitism rates are higher in large patches of milkweed than on isolated plants. Interestingly, non-migratory populations of Monarchs, like in the Southern tip of Florida, Hawaii, and Australia, have VERY high parasitism rates (nearing 100%) whereas the migration east of the Rockies (aka, the migration we see pass through Cape May) experiences much lower rates of parasitism, ranging from 1-12%. If your caterpillar has been parasitized, this means that a tachinid fly laid an egg in the caterpillar. The caterpillar will often survive until it starts to form the pre-chrysalis “J”. At this point, the Monarch will go limp, hang straight, and not form a chrysalis. Sad day.

Caterpillar parasitized by a Tachinid fly

Ophryocystis elektroscirrh, also known as OE, is a protozoan parasite that also affects Monarchs. OE can only reproduce within the Monarch body. Only 8% of the Eastern population is impacted. OE spores cannot be seen with the naked eye, but often are found between the scales of a Monarch’s wing and abdomen. Once butterflies are infected, they do not recover. Adults infected with OE may be weak and have difficulty emerging from the chrysalis. Often, they are smaller than healthy Monarchs. Many infected Monarchs look perfectly healthy, though!

OE spores (tiny dots) with scales

And….week 4 update: 42.75 Monarchs/hour. Wow. Quite low compared to our previous week average of 511.63 Monarchs/hour. But hey, if every week were like week 3, it wouldn’t be as special! I was hopeful for an increase in Monarchs/hour during week 5, but with only two days left, it looks like our average will not be super high. Saturday was a beautiful day with many Monarchs filtering through the point and we even had some small roosts forming last night and some movement across the dunes this morning. We are still hopeful that the season has not yet peaked! Stay tuned!