Sunday, May 30, 2010

Sexton Beetle and Friends

The Creator, if He exists, has “an inordinate fondness for beetles

It is not every day that I can find a new beetle and identify it.  As Haldane alluded to above, there are more varieties of beetles than any other insect, with over 24,000 species in the United States alone.  Of all living species including plants, one out of five is a beetle.  Therefore, only the most distinctive species can be identified by an amateur naturalist. 
A ride across the field just at dark with an open butterfly net yielded an unexpected find.  This distinctive beetle had orange spots on its hard outer wings and short antennas with orange tipped knobs on the end.  Its abdomen came to a point, unusual for most beetles. Also, he stinks like rotting meat when you sniff him.
Click on pictures to enlarge.
Nicrophorus orbicollis
After letting him chill in the refrigerator, I inspected him with a magnifier and was startled to find eight bright orange mites crawling over the ventral thorax between its legs.  The next morning they were in the same location, showing no signs of wandering off.
Thumbing through insect field guides, I hit upon the identical creature, a carrion beetle in the genus  Nicrophorus.  It is also called a sexton or burying beetle.  While they are usually found around dead animals, my friend had apparently been on a crepuscular cruise, looking for love in one of the wrong places.
The male and female of this genus typically dig under a dead animal. until it sinks into the ground, thus burying for larval food.  They remove the skin and hair, leaving a tasty meatball for the kids to come.  When the larva hatch they are fed pieces by the adults.
German researchers have noted that beetles will fight to decide which one gets the carcass.  The loser frequently will lay its eggs anyway and some of the time the winner will raise some of those larva as its own.  This is called brood parasitism and its best known example in the USA is the cowbird.
Searching by "Necrophorus mites" I came up with the answer for its orange friends.  Many of these beetles carry mites with them to the next dead animal.  As they compete with fly maggots for their tasty treat, taking along mites that will feed on the maggots helps cut down on competition.  The mites sometimes feed on the beetle's larva as well.  So what is the over-all advantage in carrying the mites?  The rest of the story is told in this paper by Karen Wilson of Colorado State University.
"The benefits to the mite are obvious. They feed on the carcass, fly eggs, and in some cases, eggs of the beetle that carry it. They also benefit in dispersal. The benefits to the beetle may be less apparent initially and some costs are involved. In the presence of flies, reproduction of the beetle was shown to be dependent on the presence of certain mites, especially when repeated infestation by flies was a problem. they decrease brood size and increase larval weight. The mites also reduce colonization of the carcass by soil microbes. It is unclear as to whether or not the increase in individual larval size is a true advantage over large brood size. However, larger individuals are more successful in competitive situations and therefore may have a long-term competitive edge."
Remember that in most species that fight over sexual favors or breeding sites, the bigger bug wins.  Carrying a few mites on your chest seems to be a small price to pay for raising the biggest kids on the block
I believe that this is Nicrophorus orbicollis, as described at Insects of West Virginia and Wikipedia.

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