Monday, July 8, 2013

Miniature "pots" contain beneficial wasps


At first, I wasn’t sure what they were:  Four tiny round structures firmly attached to a thin willow twig in shallow water.

Four marble-size structures attached to a thin willow twig in the shallow lake water
Nests, I presumed, belonging to some sort of insect.  The dwellings – if that’s what they were – appeared to be made of mud, bleached white and dried by the sun.  Adding to their mystery and allure, each marble-shaped building had a single entry hole.  I was immediately intrigued.  Although I saw no insect entering or leaving, my gut said the nests belonged to some sort of wasp. 

Curious, I began researching as soon as I returned home.  What I discovered reinforced my instinct – a wasp did indeed make the nests.  More importantly, the information I uncovered introduced me to a world of wonder and fascinating facts about one of nature’s often unnoticed but important creatures.

The tiny structures I observed were built by Eumenes fraternus, commonly known as ‘potter wasp’ or ‘mason wasp’ because its small round nest looks so much like a miniature hand-thrown clay pot.  A potter wasp is just under an inch long, predominantly black with thin ivory bands along its thorax and abdomen and a noticeably elongated, narrow waist.

Potter wasps have very narrow waists and distinctive colors that make them easy to identify.  Photo credit:

Unlike social wasps that live in groups, potter wasps are solitary insects.  Adults feed on flower nectar and are not aggressive toward people.  They rarely sting, even when inadvertently touched and are considered beneficial because they control caterpillars that harm garden plants. 

I found little of note about the male wasp since his role centers on the act of procreation alone.  However, the female, who is larger than her male counterpart, has multiple jobs.  After mating, she must find a nest site, gather materials needed for the laborious job of building a structure and then procure enough food to secure her future offspring’s survival.  Only then does she deposit eggs – one egg per domed cell – before sealing it within the cell with more mud over the entry hole.  When all these tasks are complete, she flies off to begin the process again in the next structure.

Although the initial nests I found were on a willow twig growing in the lake’s shallow water, I’ve since discovered other potter wasp nests on window screens, attached to shrubbery and irrigation pipes.

Potter wasp building nest on our irrigation pipe

Apparently, the main factor the female wasp seeks in a location is to be somewhat close to a source of mud, since mud is the main component of the soon-to-be-built nest.

A potter wasp nest is an architectural marvel.   The female wasp begins by finding a wet patch of sandy soil.  Using her mandibles, she rolls a portion of the muddy soil into a ball, which she carries back to deposit on the nest site, spread out and mix with saliva to increase its hardness.  This tedious procedure involves repeated mud-gathering trips until an adobe-like round brood cell takes shape.  When the structure is an appropriate size (big enough to accommodate one egg and enough food to sustain its growth) the female flies off to stock the larder with caterpillars. 

This is when things get interesting. 

When the wasp finds a caterpillar, she stings it just enough venom to cause paralysis but not death.  She then lugs the inert bug back to the cell to stuff inside the small round opening she left in the jug-like structure.  After much effort, the hole is filled with anywhere from one to twelve caterpillars.

The sex of the future wasp depends upon the number of the caterpillars upon which it will feast.  In nests containing more than five caterpillars, a female wasp will emerge.  If the nest contains fewer caterpillars, the wasp will be male.

Once she has secured an adequate food supply, the female lays a single egg suspended above the caterpillar mass by a strong thread, backs out of the hole and covers the opening with more balls of mud moistened and smoothed out with saliva.  At this point, her responsibilities to that particular egg are over and she is ready to repeat the procedure for her next future offspring.

Meanwhile, inside its mud incubator, the developing wasp larva feeds on the fresh meat of the unfortunate caterpillars until the food is gone and the wasp is ready to leave the nest.  At that point, the emerging potter wasp drills though the side of its adobe abode to begin the cycle anew.

I had no idea my discovery of four white, round domed shells attached to a willow twig would trigger such a wealth of new information.  

Every time I observe some previously unfamiliar object like the potter wasp, I can’t help but wonder how many other unknown marvels I miss even though they are right there in plain sight. 

The discovery of the potter wasp nest was my latest reminder that nature has so much to offer if I only take the time to look. 

A "Simply" Extra
I found a fascinating 8-minute video online documenting how a potter wasp builds its nest, stuffs numerous caterpillars into it and then seals it up tight.  You can watch it by clicking on the following link:  Potter Wasp or Mason Wasp - Master Architect.

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