Friday, July 17, 2015

Catalpa Sphinx Moth Caterpillars

Catalpa Sphinx Moth caterpillar or "hornworm" with frass (droppings) behind it.

     Catalpa Sphinx Moth caterpillars (Ceratomia catalpae) can only survive by feeding on leaves of their host plant trees of Catalpas, thus the inclusion of the tree's genus name even in its scientific name. Luckily for this species of hawk (aka sphinx or hornworm) moth, both North American species of trees (Northern and Southern Catalpas) have been planted as ornamentals well outside their natural range, including in the DC region. With it arrived the caterpillars to feed on it.
     Catalpa Hornworms or Catawba Worms, as they're sometimes called, are large, colorful (variable mix of black backs and yellow sides mostly) caterpillars with the spike typical of hornworms on the back end. They group together when young (the eggs are laid in rafts numbering several hundred) but become more solitary as they mature. They will commonly  thrash around and regurgitate green liquid if they're bothered.

Catalpa or Catawba Worm. It has already regurgitated some of its goo on my fingers behind it.

     Catalpa Worms are often used for fish bait and the tree itself is sometimes referred to as the "fish bait tree." The trees occasionally were even planted in small groves in the South for this purpose. Bass supposedly find the Catawba Hornworms irresistible. They've even been sold in bait shops in some parts of Florida.
     The caterpillars can break out in huge numbers, sometimes with two generations in a year, and are capable of defoliating the tree. For the most part though, the tree recovers with no long lasting harm. They usually do not have population explosions on the same tree in subsequent years. More over, the caterpillars have 15 species of parasitic wasps that feed on them and quickly reduce their numbers. With the spread of the Catalpa trees outside their native range and into our region, so have followed the Catalpa Sphinx Moth caterpillars, and the numerous parasites who need them have also followed.

Two Catalpa Hornworms. The bottom one is parasatized with Braconid Wasp cocoons.

     Many of these wasps are in the family Braconidae. Braconid wasps are very small and cannot sting people. They are however parasitoids and kill the host caterpillar. Parasites may harm and certainly do not help their hosts, but don't always kill the host. Parasitoids always kill or sterilize them. People often find caterpillars (including Catawba Worms) with little sacks attached to them that they assume are eggs. These are actually the pupae, or cocoons, of the wasps that have finished feeding and are metamorphosing into adults.
     The caterpillars, though often stunted and slow to react, look alive with their extra baggage of cocoons, but the damage has been done and their days are numbered. Removing the cocoons does nothing as the caterpillar is doomed. Its non-vital organs have been consumed and in essence only a zombie remains.

A Braconid Wasp (Apantles congregata) has just emerged from its cocoon near the spike on this Catalpa Sphinx Moth caterpillar.

     For a video of a Catalpa Sphinx Moth caterpillar covered in cocoons and with a Braconid Wasp (I believe it to be the most common one found on them, Apanteles congregata) emerging, check out the Capital Naturalist YouTube Channel link below. Copy and paste if the link does not work when clicked:
https://www.youtube.com/watch?v=h44tRF_WjJM

     This may seem cruel, but they keep the number of these caterpillars in check and thus keep everything in balance. Nature isn't always kind, but it is interconnected and fascinating.

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