Wasp Parasitoids

Every now and then, I devote a column to a different group of flying animals, the insects.  We’ll recognize the insects as honorary birds today.

This column was precipitated by an interaction I saw last weekend while I was mowing the lawn.  I saw a black wasp interacting in the grass with what I thought was perhaps a second wasp.  Going closer, I realized the wasp was attacking a large wolf spider.  This interaction was the beginning of a fascinating but macabre relationship.

SpiderThe spider, at least five times the size of the spider wasp (family Pompilidae), tried to escape as the wasp injected a neurotoxin into the spider.  The toxin quickly took effect and the spider was paralyzed.  The wasp then quickly dragged the spider to the side of our house.  It walked up a granite foundation stone and underneath the lowest cedar shingles.

The rest of this tale will take place sight unseen.  The wasp will lay one inside the paralyzed spider.  Though paralyzed, the spider will live.  The wasp egg will hatch inside the spider and the larva will eat the internal organs of the spider.  The wasp larva will eventually pupate, later emerging as an adult.  Maybe I should have saved this story for a Halloween column!

Entomologists classify wasps like the one I saw as a parasitoid. Unlike a true parasite, a parasitoid either kills or sterilizes its host.  The wasp I saw is hardly unique.  Many wasps, in  dozens of families, are parasitoids.  Some lay eggs on other insects but wolf spiders are commonly used.  Wolf spiders typically do not build webs.  A spider’s web is used primarily to capture prey but offers a secondary benefit of protection from wasps.  The free-ranging wolf spiders are therefore at risk to parasitoid wasps.  I may have unwittingly caused the demise of the spider by cutting the grass and making the spider easier to see.


Bird migration is near its peak now.  We can predict the order of departure in the fall: first the swallows and flycatchers, then the warblers, then the sparrows and hawks.  Migration implies a predictable, seasonal movement.  Animals may engage in nomadic wanderings are not predictable enough to warrant as migration.  For instance, White-winged Crossbills wander widely to find bumper crops of conifer cones.

Although birds are the best migrants, they do not have a monopoly on migration.  You have no doubt seen videos of wildebeest, zebras and other mammals migrating to and from the Serengeti Desert.

Some insects migrate as well.  A widely distributed dragonfly, the Wandering Glider, occurs on six continents.  Populations in Africa migrate to India after the monsoon season starts.  The abundant rains provide ample opportunities for the females to lay eggs in aquatic habitats.

Some butterflies migrate as well.  In North America, the best known migrant is, of course, the Monarch.  Any Monarchs you see this fall will attempt to fly south to a small pine forest in mountains in northwestern Mexico.  Even they can make it to the wintering grounds, they will not return.  The complete migration from Mexico back to Mexico the following year requires five or six generations!

Originally published on September 14, 2014]