The Cycnia collaris larvae have disappeared, presumably to
complete the next phase of their life cycle.
It was a lot of fun to have the opportunity to observe their behavior on
a daily basis, even though some of the observations were rather disturbing.
The 70 or 80 larvae that began their lives crowded in a
small clump of Common Milkweed plants, soon became mobile and began to
disperse. Larvae of a size approximating
half of their potential full grown dimensions were found wandering as far as 50
feet from their birth plants. In all
cases there were milkweed plants in the direction of their travels. It wasn’t long before all larvae were absent
from the original plants.
Their dispersal pattern spread the larvae out, but instead
of having a couple of larvae on each plant in the vicinity, they clustered into
small groups.
Once they got settled into their new locations, I began
seeing some ominous ornamentation on many of the larvae. On closer examination, each white spot was
identified as the egg of a predator.
This is the culprit, a Tachinid Fly, a species that lays its
eggs on the bodies of other insects.
Observers of Monarch butterfly caterpillars are familiar with this
species and its ability to decimate caterpillar populations on milkweed
plants. Fly larvae hatch from the eggs and immediately enter the body of the caterpillar, where they will feed until
they are mature enough to pupate. Caterpillars
do not survive the experience.
Most affected larvae displayed one or two eggs.
Rarely, I found larvae carrying three eggs. About half of the larvae I inspected were
carrying fly eggs.
The video shows a fly searching the leaf for larvae. Although the fly investigates two of the
three larvae found beneath the leaf, it laid no eggs. When I later checked, these three larvae were
already carrying eggs. Perhaps the fly
senses larvae that are already infested, and passes without leaving additional
eggs. In an earlier encounter I watched
a fly chase a Cycnia larvae that was running across the surface of a leaf. The fly was approaching from the right and
the larva kept making quick directional changes to the left. When the fly was
within range it leapt on the Cycnia larva’s head. Almost immediately, the Cycnia larva snapped
into a C shape and popped off the leaf.
It ended up in the water, so I scooped it out and gave it a close
examination. Two fly eggs were attached
just behind the head. I am assuming that
one or both of these eggs had just been attached.
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