Thursday, July 23, 2015

Paying attention, again

Again?
No, it was not a good day for wasp-watching at Houghton's Pond baseball field #2.  Conscientious groundskeepers had dragged some sort of machinery over the ball fields, burying almost a hundred mama wasps in their burrows, and earning their undying enmity.*  Most mamas had not finished digging out.  I found only two beetles on the ground, and caught only two wasps carrying beetles in the air--and this in well over two hours' work. 

I have now spent over ten hours total divided between contemplating little holes in the ground, and dancing madly, lunging and swinging at flying critters.  (Actually, chasing wasps is time wasted: realizing they're danger, they begin evasive maneuvers.)

Besides doing a public service (and working on my tan lines), and without necessarily meaning to, I've become a bit of a knowledgeable observer of bare-ground ecology in general and these wasps in particular.  

 Called "velvet ants" because of their shape and covering of soft hairs, 
the many species of these wasps sport a variety of bright colors.  
In general, it's best to avoid animals with bright colors: they warn of venom.

Velvet ants are now old acquaintances.  These brightly colored insects running about the dusty ground aren't ants at all, but a very large family of wingless wasps masquerading as ants.  They aren't interested in me, and I leave them alone: the sting has earned some the nickname "cow killer."  (Best not to sit down on bare ground.)

Several other flying, running and hopping insects have caught my notice as typical residents of this habitat, though I mainly register them as "non-target species," as I go about my job. 

 You've heard of the Right Whale, so called because it was the right one for whalers to chase?
Well, this is the Wrong Wasp.  Notice the burrow is angled, and the hill asymmetrical.  (Cerceris fumipennis makes neat, vertical round holes surrounded by symmetrical hills.)

I've noticed the wasps vary widely in size, with the largest the equal of big hornets.  Why?  These beetles are, I believe, all about the same age.  The few wasps I've netted carrying beetles seem to show that big wasps catch big beetles, while smaller wasps catch smaller beetles.  (The eleven beetles I discovered abandoned outside a burrow on Monday were remarkable for their uniform size.)


I'm guessing these beetles were all captured by a rather small wasp.

Finally, the wasps themselves, Cerceris fumipennis aka "smoky-winged beetle bandits" are a trip.  Especially today.  They apparently navigate to their burrows at least partly by sight.  Today, emerging into a changed landscape after the groundskeepers left, the wasps spent a lot of time flying around their burrows, trying to fix their location without familiar landmarks.  They reminded me of my own maneuvers when I cannot remember where in the crowded mall lot I left my car.

 "I could have sworn I left the front door open!"

 "Yikes!  That wasn't MY front door at all!"

 "--and don't come back!"

Sometimes they get their navigation wrong.  I saw one wasp land on a burrow blocked by newly-excavated dirt.  Since these wasps push the dirt out from beneath, the blockage could only conceal a wasp already in that burrow.  The new wasp dug industriously through the blockage and entered the burrow.  Inside of three seconds she had re-emerged and taken wing, and then circled confusedly for a second or two before flying off to check her luck elsewhere.  The earlier inhabitant emerged partway, looking thoroughly put out.

The second time this happened, in a different place with other participants, I thought to make a video.  In this case, the digging wasp worked furiously without finding the hole she was looking for.  She would take a break, fly around a bit, and resume work.  I don't know what happened to her; I don't know if a wasp can start over and make a new burrow so late in the season. 

video

You can feel her frustration at not being able to open what she has identified as her burrow.  
She practically does head-stands trying to force an entrance, 
and has to take a flight break once in awhile.

Twice more I saw wasps try to enter burrows already occupied; I'd never seen the like before.  One of the disappointed wasps then flew down and crawled under my shoe.  When I carefully lifted it up, leaving the wasp in bright sunlight, she headed under my other shoe.  When that didn't work she crawled briefly into the shadow of a blade of grass.  From this I gather that they are drawn to dark places; and in fact the burrows are very visible when an overhead sun lights the ground and the holes stand out by contrast.

I put on my evolutionist thinking cap while leaving the field in late afternoon.

What is the "adaptive landscape" for this animal?  The wasp must dig out in spring, mate, make and maintain a burrow to protect its young, locate beetles, capture and paralyze them, lay an egg on each, and do all this while dealing with potential predators, groundskeepers, and nut-cases wielding nets.  The wasp that does any of these more successfully than its neighbors will have more successful offspring and be favored in the "struggle for existence."

Wasp size probably has big consequences.  A big wasp catches big beetles, and has offspring that have a bigger food supply, and those young probably grow into large adults.  (The adults eat, but only flower nectar, which has no protein.)  I wonder whether bigger wasps lay more eggs than smaller ones?  I wonder what the mortality rate among the young is by size class?  I wonder how small a wasp is too small to catch beetles that are big enough so wasp young don't starve?

I wonder how the peculiar hunting method of the wasp developed.  The wasp stings the beetle in  a particular place: a leg joint where the beetle's shell is thin--a chink in their  beetley armor.  I can imagine an ancestral wasp population that tried to sting beetles in a variety of places, but those that passed down in their genes a tendency to hit that exact spot had more success, produced more offspring, and spread those genes more widely in the population over time.


*Kidding about this last part.  These wasps are not vindictive: I have captured dozens, and when I release them they just go about their business, though with slightly dazed expressions. 

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