One of the things I'm working on down in the field in Southeastern AZ are a set of questions related to the parasitoids that attack the hawkmoth Manduca sexta , the primary insect we study in our lab. What's a parasitoid? Well, certainly you've seen the wildly popular movie Alien. That moment when the first chestburster encounter happens - absolutely gruesome and amazing. A terrible unknowing way to go. Do yourself a favor and remind yourself of it below.
Yeah. That is happening all the time to insects all around us (except insects don't really dig noodles). A parasitoid is simply a parasite that eventually sterilizes or kills its host. The kind that we study have a strategy that is not unlike the alien in the scene above. An adult form of the parasitoid (in our case a flying wasp or fly - more on that in a minute) cruises around the desert looking for an appropriate host. When she finds one, she'll either inject the host insect with eggs or lay them on the surface of their body. The eggs develop into larvae that begin to consume the host from the inside out, while it's still alive. Eventually, they'll kill the host, and emerge as adults, ready to mate and continue the cycle again.
In our system, the hosts (M. sexta ) have different life stages. Adult moths lay their eggs on plants, those eggs turn into caterpillars, they morph into pupae (pyoo-pay.. such a fun word to say) underground and eventually emerge as adult moths. Parasitoids nail two stages during this cycle - eggs and caterpillars. The egg parasitoids are incredibly small wasps, some no bigger than the head of a pin, up to 30 of them can emerge from a single moth egg that is only 1 mm in diameter. Somehow, amid the gusting winds, monsoon rains and brutal desert heat, these small insects can find and parasitize moth eggs on plants throughout the valley we work in. Though we know that parasitization is frequent, we have no idea how the small parasitoids find eggs - this is one of the things I hoping to answer in part this summer (more on this in a later post).
The caterpillars are also attacked by a slew of parasitoids. Down here in AZ, they appear to be in the most danger from Tachinid flies and a species of ichnuemonid wasp (Rhyncophion flammipennis - that's right, the species name is flaming penis. Amazing). Both species are incredibly amazing to watch in action, darting around a host plant searching for caterpillars and diving in to lay eggs on the body of the caterpillar, or oviposit (inject their eggs) directly into the caterpillar's body, all while dodging it's snapping mandibles and thrashing body and spraying caterpillar puke. Check out a M. sexta caterpillar battle a tachinid in the video below.
In general, we're interested both in how parasitoids find their hosts in complex environments (and the key may well be related to insects' spectacular sense of smell!), as well as the abilities of M. sexta to defend itself. Above, you can see the caterpillar thrashing and trying to bite the fly, but it's unclear how effective this. What's more, caterpillars often regurgitate on themselves after attack. Whether or not this is an effective defense remains to be seen.
The gist is that it's a dangerous life in the field for a large, juicy caterpillar. Oh, and chestbursters totally exist - except that caterpillars don't really have chests.