Parasites on Parasites...

Catalpa Tree

Catalpa trees were plagued by the catalpa sphinx caterpillars this year. I watched the beautiful heart shaped leaves disappear nibble by nibble. The sound of chewing and crunching leaves was not drowned out by traffic. On the ground all around the tree lay tiny pellets, apparently caterpillar poo!

Catalpa Sphinx Moths

Catalpa sphinx caterpillars are the larva of grey-brown sphinx moths. Their only food source is catalpa leaves. To keep these parasites from reaching their carrying capacity, are other parasites. Primarily braconid wasp larva feeds on the caterpillars. Adult wasps deposit their eggs inside the body of sphinx caterpillars. Because the caterpillar’s natural defense mechanisms would normally kill the wasp larva, the adult wasp also injects particles that scientist have identified as viral in nature. Therefore, the wasps seem to be actually raising viruses to use against the caterpillar which protects the survival of their larva. The wasp larva grows inside its host, feeding off its blood, and then forms tiny white cocoons on the caterpillar’s skin.

A caterpillar with parasitic wasp larva

While examining this relationship between the catalpa tree, the catalpa worms, and the parasitic wasps, I noticed one or two worms covered in wasp cocoons being attacked by ants. Upon further investigation it seemed that the ants were carrying off and apparently eating the wasp larva off the caterpillar.

Ants eating wasp larva off the caterpillar that is defoliating the tree

In my excitement over this unique series of relationships, I told John all about the science happening outside the backdoor. John became excited as well, but for an entirely different reason. Catalpa worms, as he calls them, are the best fish bait in the world. He retrieved a ladder and spent a good part of the day collecting fat caterpillars in a Ziploc bag which he put in the freezer for a good fishing day.

And the parasitic cycle grew another layer of symbiosis: the catalpa worms eat catalpa leaves, the parasitic wasps eat the caterpillars, then the ants eat the parasitic worms off the caterpillars, and finally the fisherman plucks the entire brood off the leaf to feed to a fish that will in turn feed the fisherman. 

In the end, many wasp cocoons died and many survived, the some catalpa worms died but most made their brown cocoons and will fly away as moths before the year ends.

The cocoon of a catalpa sphinx caterpillar

The tree will survive, but many of its branches are bare. The cycles and layers of symbiotic relationships go on and on. Who is to say which creature should live or which creature should go hungry?

The skeleton of a leaf eaten by catalpa worms