Wildlife Wednesday Hornworm

What you see here is a hornworm or sphinx moth caterpillar that has been compromised by a tiny parasitic wasp (Cotesia congregata), from the family Braconidae. Because the larvae of several species of sphinx caterpillar feed on plants from the family Solanaceae (nightshades), they are considered agricultural pests. Some of the plants people grow that are eaten by these caterpillars are tomatoes, tobacco, potatoes, eggplant, and peppers. So, from a human and nightshade plant perspective, “hornworms” are a pest; therefore, the tiny parasitoid wasp is considered “beneficial.” Though I’m sure the caterpillars would very much disagree.

First, a female parasitic wasp locates a hornworm caterpillar, host appropriate for her species. She then uses her ovipositor to insert 4 different things into the caterpillar. These are her fertilized eggs, teratocytes, a virus, and venom. These fertilized eggs are each paired with teratocytes , cells that activate suppression hormones that arrest development in the caterpillar when the eggs hatch. This is achieved with the help of the venom.

The immunosuppressant virus introduced to the hornworm caterpillar with the venom, eggs, and teratocytes is called bracovirus. It is named thus because it is only and always associated with braconid parasitoid wasps. The teratocytes and hatching eggs are invaders in the caterpillar’s body. Without the virus suppressing the caterpillar’s immune system, the aforementioned would be attacked and killed. The virus is a strange one and is symbiotic (mutually beneficial) with the wasp. This virus exists ONLY inside the wasp, and further, ONLY in the ovaries of the female wasp. It isn’t contracted environmentally, or passed down from mother to female offspring. It is actually created in the wasp’s ovaries from her own DNA. For the reproduction of this wasp to be possible, all 4 elements mentioned here are required.

One female chooses one caterpillar. Multiple females do not utilize a single caterpillar. Upon initial hatching and for most of their development, wasp larvae will only consume nonfatal tissues inside of the caterpillar. Prior to pupation, though, they start to eat everything. Once ready to pupate, the larvae eat through the skin of their caterpillar host and pupate on the outside. They surround themselves in the white cotton like cocoons you see in the picture.

Finally, female adults emerge from the cocoons on a dying or dead caterpillar. Yes, just female. There are no males of this species. Female C. congregata are parthenogenic. This means they can produce fertilized eggs without genetic contributions from males. This adaptation allows the process to occur more quickly, eliminating the need to seek and find another mate. What you see in the picture I shared shows the host caterpillar who was still alive, opened and closed cocoons and a recently emerged adult wasp fully equipped and ready to terrorize other caterpillars.

Because the horror movie rearing of the wasp’s young takes place in an agricultural pest, the wasp is deemed “beneficial.” It is a natural and organic form of pest control. The dead caterpillar becomes compost.

However, if the adult moth had survived its larval stage it would become a pollinator, also a beneficial role in agriculture.

So, who is and is not beneficial in this situation depends on who you are. I always find it intriguing that humans determine the value of another species based on how its existence impacts us. Remove humans from a scenario, then who is who?

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