The Wonderful World of Parasitoids

 

 

Most people who have ever tried to grow tomatoes have certainly run into hornworms. These caterpillars are the larvae of hawk moths.  Strangely enough, the hornworm most commonly found on tomatoes is not the tomato hornworm, Manduca quinquemaculatus (Haworth), but the tobacco hornworm, M. sexta (L.).manduca_larva

As common and large as this pest is, many people never see (or never know they’ve seen) the adult moths.  They are nocturnal and do their very best to look like tree bark when resting during the day.manduca_sexta_adult

Tobacco hornworm larvae can get rather large: the size of an adult’s finger is not unusual.  Owing to their great camouflage, people ofter don’t even see these gigantic critters on their plants until they notice a lot of missing leaves, or they see the suddenly obvious caterpillars looking like this:

Parasitic-wasp-eggs-on-hornworm1

So what are those cute little white tufts?  No, they are not decorations the caterpillar has chosen, they are cocoons.  Each cocoon has been spun by a tiny Cotesia congregata wasp larva which hatched from an  egg laid inside this rather unfortunate hornworm.  The hundreds of eggs in each caterpillar hatch in a few days and the larvae then feed on the caterpillar from the inside for a couple of weeks, causing its ultimate demise.

This killing of the host, BTW, is the difference between a parasite and a parasitoid: a parasite feeds on another organism but does not usually threaten the host’s existence (think fleas or lice–annoying, but not inherently deadly).  Conversely, a parasitoid feeds on its host, and in so doing, kills it–this is the case with these tiny wasps.

In a move straight from Alien, the mature larvae chew their way out of the caterpillar, through its integument (“skin”), which brings us to the photo and video I took on 15 Sep 2015 when I found this caterpillar on our tomatoes.  In the photo, we can see:

  • The heads of several larvae just having chewed their way through the integument
  • Several larvae about 1/4 the way out
  • At least one larva ca. 1/2 way out
  • Fully emerged larvae
  • Larvae spinning silken cocoons
  • Cocoons already thick enough that we can no longer see the larvae within

manduca_contesia

Click on photo to enlarge it & sharpen its text

And, just for fun, here’s a video I made using my son’s microscope.  You can see a larva’s head just emerging through the integument, and next to that one, a larva busily making its silken cocoon.

When all is said and done, we shall have a very dead hornworm and a whole bunch of adult Cotesia wasps:

cotesia_adult