Spotted beebalm, a.k.a horsemint, is one of my favorite flowering plants by virtue of the vast number of insects it attracts. Over the last week or so members of the digger wasp clan, including the great black wasp, swarmed spotted beebalm in my garden. In addition to its pollinator magnetism, it has one of most clever pollination systems of any flower in my garden. Unlike open-faced flowers like sun flowers, horsemint provides a curious array of petals, anthers, and stigmas. Nectar fiends like the great black wasp land on petals and as they probe deeper into the flower, pollen-laden anthers dip down and discharge their yellow pollen grains onto the back of the insect. As the wasp moves to the next floret, the stigma, which is also displayed above the insect, bends down and touches the pollen on the back of the wasp, thereby completing the transfer of gametes.
When the wasp probes the blossom for nectar, anthers dip down and release their pollen onto its back.
The great black wasp was one of the first species of digger wasps to be carefully observed and described in North America way back in 1749. These very large but gentle giants are relatives of sphecid wasps we met in previous episodes, including “White grubs beware”, “The killing fields”, “Window wasps", and “Diggers and daubers”. As a member of the digger clan, great black wasps excavate galleries a foot or more deep in the soil. This crypt will serve as the nursery and larder for the developing wasp larvae.
Female great black wasps search the treetops for those beautiful and melodious nighttime troubadours, the katydids. When she locates a katydid, she stings and captures her prize and takes it back to the subterranean burrow. Inside the burrow, she provisions each brood cell with two to six victims and lays an egg on the underside of one of the katydids. Here is where this macabre tale gives me the willies. The katydids in the crypt are not really dead. They are just mostly dead, like the tortured Dread Pirate Roberts in the Princess Bride movie. Ah, but there will be no Miracle Max to rescue these unfortunate creatures. You see, the venom of the great black wasp does not kill its prey, it merely paralyzes the victim. The moribund katydids are alive but cannot escape the jaws of the wasp larva as it proceeds to consume the hapless prey one by one over the span of about ten days. Whew, makes me glad I am not a katydid!
Fully developed larvae spin cocoons in autumn and remain underground throughout winter, awaiting the return of summer when they will emerge from the earth to sip nectar and hunt. In addition to hunting and capturing katydids, the great black wasp will also use grasshoppers as food for its young, thereby providing a modicum of biological control for these often pestiferous leaf munchers. So, plant some horsemint and watch great black wasps turn yellow.
The fascinating article “The life-history and habits of the digger-wasp Ammobia pennsylvanica (Linn.)”, by J. A. Frisch, was used as a reference for this episode. Bug of the Week thanks Dr. Shrewsbury for providing the inspiration for this story.