Wasp Wednesday: Myzinum

Sexual dimorphism, the graphic physical and morphological differences between genders, can be extreme in the wasp world. One example of this is in the genus Myzinum, members of the family Tiphiidae (see "Update" below). They should be common right now in most parts of North America, at least east of the Rockies and in the Southwest where they visit autumn wildflowers like goldenrod (Solidago) and thoroughwort (Eupatorium).

There are currently ten recognized species in the genus north of Mexico, but they have been very difficult to separate, even for experts. There is no such issue when it comes to telling males from females, however. Well, the only problem for non-experts is recognizing that the genders don’t constitute different species, if not different genera or different families.

Male Myzinum species are seemingly more abundant than the females for a number of reasons. They spend more time on flowers and so are more conspicuous. They can also gather in “slumber parties,” bedding down in the early evening in large groups on vegetation in fields and meadows.

The uninitiated assume that they are female wasps because the males sport an intimidating “pseudostinger” at the tip of the abdomen. The curled spine, part of the external genitalia, looks menacing to be sure. The body of the male is very slender, and he has long, straight antennae.

It may sound stereotypical and sexist to describe the female Myzinum as being larger-bodied, but there is no getting around that fact.

Her abdomen is very robust, her legs stouter for digging up the host organism (more on that in a minute), and she has short, coiled antennae. She is built for her lifestyle to be sure. Meanwhile, the male is merely a missile-shaped sperm-delivery animal. (Hoping that gets me off the male chauvinist pig hook).

Myzinum species are parasitoids of scarab beetle grubs, especially the “white grubs” of the May beetle genus Phyllophaga. Parasitoids are parasites that invariably kill their hosts. Female wasps somehow divine the presence of a grub below ground and dig up the beetle larva. The wasp then stings it into a brief paralysis and lays a single egg on it. The beetle grub regains control of its faculties shortly, and quickly buries itself once more, but the damage is done. The larval wasp that hatches from the egg bores into the beetle grub and begins slowly consuming it. The grub still feeds, creating more tissue that its internal wasp parasite will eventually eat. This host-parasite treadmill continues for some time, but eventually the wasp larva kills the beetle grub. The larval wasp then pupates and emerges as an adult wasp the following summer.

Pat yourself on the back if you simply recognize that the male and female Myzinum are two halves of the same organism. You are already ahead of the game. Remember that even entomologists who study this genus are continually boggled by them when it comes to sorting out the different species. Special thanks should go to Dr. Lynn Kimsey for correcting the mistakes of her predecessors and providing revised descriptions and a key for our nearctic fauna, not a simple task!

Update: This genus has now been placed in the family Thynnidae.

Sources: Kimsey, Lynn. 2009. “Taxonomic purgatory: Sorting out the wasp genus Myzinum Latreille in North America (Hymenoptera, Tiphiidae, Myzininae).” Zootaxa 2224: 30–50.