Post by catrpillrenthusiast on Oct 17, 2021 19:36:47 GMT -8
For the past month I have been observing an army of late-season Black Swallowtail caterpillars. When they ran out of parsley a few weeks in, and faced a choice of pupation or starvation, I hypothesized about which ones would successfully pupate and which ones would not. But my rather straightforward prediction (I thought, anyway) wound up being totally wrong! Now I'm trying to figure out why.
Some context: In late September, I found a huge population of baby Black swallowtail caterpillars on a potted parsley plant on my back porch. There were over 50 of them, the most I've personally seen at once (in my admittedly limited experience.) I assumed that as time went on, the population would decline to a more reasonable number, because in the past, whenever I would find a group of these caterpillars in their later instars, there would be only 2-10 at a time. That was my first incorrect prediction.
Instead of declining, almost all of them survived. I came back from a trip to find around 30 third-instar caterpillars (and many more smaller ones) on a picked-clean plant. I enjoy watching caterpillars and find it relaxing, so I splurged and bought them about $30 USD worth of new parsley plants in order to sustain my caterpillar-watching hobby. I must admit I was shocked when the little things razed all the rest of the plants to the ground in less than a week! What a hardy batch of caterpillars. Unfortunately, my penny-pinching nature precluded me from purchasing any more parsley plants for them (and I was unable to find wild species such as Queen Anne's Lace despite looking around for them), so I said to them, "My friends, now is the time. You either pupate, or you die."
I knew Black Swallowtail caterpillars are notorious for traveling a long way away from the host plant to pupate, so when they began to vacate their host plant, I let them wander into the lawn for several meters before transferring only the fourth instars to a "pupation station". This consisted of a large container, filled with some of the dirt from their host plants, and with dozens of sticks stuck into the soil of various lengths and angles. Additionally, I set some of the caterpillars up in individual large ventilated jars with sticks. My sample size of caterpillars was around 20 (since I let the second/third instars wander off as a lost cause).
My prediction: The fattest, biggest, healthiest looking fourth-instar caterpillars will pupate since they are naturally closer to being ready to pupate. The smaller, skinnier, newly turned fourth-instar caterpillars would not, and would starve and die.
What actually happened: The exact opposite.
To my utter surprise, all of the largest, most developed caterpillars (~16) refused to pupate, instead choosing to wander in circles around the enclosures for several days before ultimately dying. This was the case for those in the individual jars as well as those in the pupation station. The smaller ones on the other hand (~5), despite being barely fourth-instar, nearly all climbed up a stick, made their silk harness, did the "J" pose, and transformed into a chrysalis within a day and a half (again, half were in jars, other half in the pupation station). I am left with four successful, but humorously tiny chrysalises.... and many many formerly large and healthy caterpillars, dead due to refusal to pupate.
So my question is: why, in an environment with no food, do the more mature fourth-instars refuse to pupate, while their smaller, skinnier siblings get the memo and pupate immediately? Is this just a strange coincidence, or is there an explanation for this behavior?
Thanks!
Some context: In late September, I found a huge population of baby Black swallowtail caterpillars on a potted parsley plant on my back porch. There were over 50 of them, the most I've personally seen at once (in my admittedly limited experience.) I assumed that as time went on, the population would decline to a more reasonable number, because in the past, whenever I would find a group of these caterpillars in their later instars, there would be only 2-10 at a time. That was my first incorrect prediction.
Instead of declining, almost all of them survived. I came back from a trip to find around 30 third-instar caterpillars (and many more smaller ones) on a picked-clean plant. I enjoy watching caterpillars and find it relaxing, so I splurged and bought them about $30 USD worth of new parsley plants in order to sustain my caterpillar-watching hobby. I must admit I was shocked when the little things razed all the rest of the plants to the ground in less than a week! What a hardy batch of caterpillars. Unfortunately, my penny-pinching nature precluded me from purchasing any more parsley plants for them (and I was unable to find wild species such as Queen Anne's Lace despite looking around for them), so I said to them, "My friends, now is the time. You either pupate, or you die."
I knew Black Swallowtail caterpillars are notorious for traveling a long way away from the host plant to pupate, so when they began to vacate their host plant, I let them wander into the lawn for several meters before transferring only the fourth instars to a "pupation station". This consisted of a large container, filled with some of the dirt from their host plants, and with dozens of sticks stuck into the soil of various lengths and angles. Additionally, I set some of the caterpillars up in individual large ventilated jars with sticks. My sample size of caterpillars was around 20 (since I let the second/third instars wander off as a lost cause).
My prediction: The fattest, biggest, healthiest looking fourth-instar caterpillars will pupate since they are naturally closer to being ready to pupate. The smaller, skinnier, newly turned fourth-instar caterpillars would not, and would starve and die.
What actually happened: The exact opposite.
To my utter surprise, all of the largest, most developed caterpillars (~16) refused to pupate, instead choosing to wander in circles around the enclosures for several days before ultimately dying. This was the case for those in the individual jars as well as those in the pupation station. The smaller ones on the other hand (~5), despite being barely fourth-instar, nearly all climbed up a stick, made their silk harness, did the "J" pose, and transformed into a chrysalis within a day and a half (again, half were in jars, other half in the pupation station). I am left with four successful, but humorously tiny chrysalises.... and many many formerly large and healthy caterpillars, dead due to refusal to pupate.
So my question is: why, in an environment with no food, do the more mature fourth-instars refuse to pupate, while their smaller, skinnier siblings get the memo and pupate immediately? Is this just a strange coincidence, or is there an explanation for this behavior?
Thanks!
