Pollinator Post 10/9/23
What difference a day makes! We have gone from stifling heat to chilly overcast skies overnight. Not a good day to observe insects, I know, but the caterpillars would still be there! I decide to check on all the Pale Swallowtail caterpillars I have found recently, both at Inspiration Point and at Skyline Gardens.
At Inspiration Point, the mature caterpillar on the small Coffeeberry bush is no longer on its home leaf. I am not surprised, as it has probably gone off to pupate. These caterpillars usually do not pupate on their host plants. It has left a thick silk pad on its home leaf.
The Pale Swallowtail caterpillars have an endearing habit of weaving a silk pad on their “home” leaf where they usually rest when not feeding. They never eat their home leaves, but would wander further afield to find food. As the caterpillar gets older, its silk pad gets thicker. The caterpillars have crochets at the tips of their prolegs that they use to anchor themselves on the silk pad, sometimes in a vertical orientation.
Surprisingly, the little bird-poop caterpillar on a different Coffeeberry has disappeared from its home leaf, leaving a tiny silk pad. Has it been nabbed by a predator?
Scanning the foliage, I find a caterpillar on a different leaf on a lower branch close to the ground. I can’t be sure that it is the same caterpillar, but it is at an identical stage of development. I don’t know how far the young caterpillars are capable of traveling. Even if not the original caterpillar, it is probably a sibling, their eggs laid by the same mama butterfly on the same day. Note the characteristic eagle-shaped white marking across the caterpillar’s back, making it look very much like bird poop. It is thought that bird-poop mimicry is the caterpillar’s way of avoiding predators – who wants a mouthful of bird poop?
The caterpillar is acquiring a blue cast on its flanks, and it has spread a thin layer of silk on the leaf to anchor itself.
Without further ado, I head for Skyline Gardens, arriving at 10 am. The sky is heavily overcast, and the sun is not likely to appear today. Compared to the the temperatures we had over the past few days, today’s weather is bone-chilling.
I locate caterpillar B2’s home leaf quickly on the Coffeeberry. B2 is not on it and the silk pad is drenched in fog drip.
Neither is B2 on the leaf on which it sought shade yesterday. Faint traces of silk is left where the caterpillar has rested.
Then I spot B2 on the tip of another branch of the same Coffeeberry, about 12 inches below its original home leaf.
Covered in fog drip, B2 is not moving at all. It must’ve been feeding – there’s a small pile of poop at its rear end. Why didn’t B2 return to its home leaf?
According to Alan, the butterfly expert I have been consulting, B2 is already in its fifth (and final) instar. It must have molted around 10/5. At this final instar, it’s common for the caterpillars to wander from branch to branch. They don’t always bother reconstructing a full blown silk pad and just lay down a thin layer wherever they walk. It’s not necessary anymore since their grip is already quite strong at this stage. Without a full silk pad, the older caterpillars sometimes prefer to rest on the stem as it is the easiest part of the plant to grip. Alan added that he’s rather surprised that all the Pale Swallowtail caterpillars I have observed previously were rather sedentary, hardly moving from their home leaves even in their final instar.
B2’s development is proving to be quite different from Blue’s in other ways as well. Blue took a much longer time to develop from the bird-poop mimicking instar to the fifth instar. Alan agreed with me that the difference is probably due to temperature. Blue’s host plant was in the shade, while B2 is growing up in a much sunnier location. Our recently heatwave probably accelerated B2’s development even more.
View of B2 from the other side of the branch. Note the color difference depending on the light. Such are the challenges for the little critter – searing heat one day, then cold fog drip the next.
I return to check on B2 in the afternoon at around 4:30 pm. It hasn’t moved much, maybe a few millimeter forward on the same branch tip. It appears to be feeding, its reddish head visible, but I don’t see any movement at all. Note B2’s thoracic legs or true legs. They extend from under the three thoracic segments that form the enlarged “head” with the fake eye-spots. B2’s real head is actually the small reddish structure in front of the thorax. The rest of the body is B2’s abdomen. Abdominal segments 3 to 6 each have a pair of fleshy ventral prolegs. The last segment also has the similar prolegs known as anal prolegs. Each proleg has a crochet which helps in walking and gripping objects.