Unexpected Cycnia and Tachinid Flies

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.