Sunday, July 15, 2018

Pustulated Carrion Beetle

Pustulated Carrion Beetles (Nicrophorus pustulatus) in the family Silphidae are frequent visitors to our farm. I encounter them at the white sheet and MV light sometimes in large numbers. Living on a farm means there is often carrion around. This might be in the form of wild animals like raccoons, or occasionally livestock like a cow or chicken. Carrion beetles, blow flies, rove beetles and an assortment of other carrion loving insects all manage to sniff out the unsavory smell and converge on the carcass in short order. However crowded the dining conditions may be on these carcasses I rarely encounter the Pustulated Carrion Beetle when inspecting the insect life feeding there. I know they are present on the farm, so why aren't they feeding on these resources? Some research revealed that carrion beetles in the genus Nicrophorus  typically locate small, recently deceased carcasses such as mice or birds. They will measure the size of the carcass using their antennae. Once they determine it is of suitable size for their needs, and that they are able to move it, the pair works together to drag it to a nearby location. A hole is dug in the ground from underneath the carcass and it will be slowly buried over the course of a day or two.
 In fact the name Nicrophorus  is a variation of the word Necrophorus, and translates into "Carrier of the dead." Which is an apt description for beetles in this genus.
Hair or feathers are removed before the beetles coat the carcass in protective anal secretions they produce in order to discourage the growth of fungus. Once mated the female will lay eggs within the chamber. The adult pair will feed on the carcass and when the eggs hatch they will coax the tiny grubs to the carcass by using sounds made through stridulation (rubbing their legs together). Larvae are fed masticated bits of anal secretion coated carrion by the parents. Sounds like the stuff of a vomit inducing nightmare, but apparently if you are a carrion beetle this is pretty tasty. They are one of the few beetle genera in the World to exhibit parental care. Litter size is controlled by the parents as well; if they misjudged the size of the carcass and the food source runs low they will begin cannibalizing on the larvae to reduce their numbers so the food does not run out before the offspring can finish their development.

An accidental discovery made by a couple of researchers over a decade ago found these beetles utilizing another resource for rearing their young.....snake eggs. Most specifically black snake (Pantherophis obsoletus) eggs. These researchers were studying communal nesting sites of black snakes when they discovered many, if not most of the nesting sites overrun with pustulated carrion beetles rearing young on the eggs. They published their findings which was picked up on by some individuals who studied beetles within this genus. It helped solve a long held mystery of why this species was no longer found on mouse-sized carcasses in the wild, even though many other species within this genus are. This discovery has encouraged many researchers to study this phenomena within laboratory conditions. Published studies have found that N. pustulatus nearly always chooses snakes eggs over other proffered food sources, like mice carcasses. When offered both snake eggs and mice they will often drag the mouse carcass to the snake eggs, and utilize both food sources. Males produce larger levels of pheromones in response to snakes eggs versus mice or other carrion.
Black rat snake eggs ready to hatch
communal nesting
It is widely known within the herpetology community (at least among those who study snakes) that black snakes in northern climates nest communally and nests may be shared by as many as a dozen females and contain as many as 110 eggs. This phenomena has played out in my own back yard, where I discovered 7 black snakes using an old decaying stump for an egg laying location. I dug up 105 eggs in June that I reburied and later discovered hatching in August. Of the 105 eggs I know at least 76 eggs hatched. I did not find any beetle damage within the nesting site, but since these beetles reside in fairly large numbers on our farm it is only a matter of time before they find this often used communal black snake nesting location.

I would assume these beetles have found other such snake nesting sites on our farm and have used them for a food source. How the beetles locate the snakes eggs is still a mystery and up for much guessing and hypothesis. Do they follow the scent of damaged or decaying eggs? Do they follow the scent of the snakes themselves? Is it happenstance? Research may reveal the secret of how these beetles locate and utilize black snake eggs, but there is an equally good chance it may remain an unsolved mystery of nature.

Newly emerged adult (L)
Recently I found several dozen of these beetles at a MV light I put out at a white sheet to collect insects. There was something unusual about them.....they were brown! I could not decide if they were an odd color form of this beetle or if it is was an entirely different species. I posted pictures to a FB page I belong to and soon had the answer. The brown varieties were newly emerged adults that had not developed their adult color of black with red spots. So much for thinking I had discovered something unique. Why I did not figure this out for myself I still do not know, as I am well aware that insects are lighter in color when they first emerge from their pupal chamber.....cicadas, beetles and cockroaches all are white, light green or very pale. It became apparent we had a mass emergence of new adult Pustulated Carrion Beetles that night.

 These beetles reach lengths up to 20 mm (1 inch) and are black with bright orange or orange-red spots at the edge of the wings near the tip of the abdomen and a spot on either side of the wings. Like all carrion beetles they smell absolutely terrible when handled......after all a life among dead things is not exactly an advertisement for Chanel #5. It takes numerous hand washings to remove that odor, trust me!

N. pustulatus with mites
Many carrion beetles offer transportation for mites from carcass to carcass. This is a mutually beneficial service. When a beetle lands on a carcass.... mites will jump aboard and eat any dead skin cells, fly eggs or other unsavory debris that may be residing there. The beetle buses the mites to the next carcass where they offload and hitch a ride with the next beetle that can offer a buffet of their favorite foods. Sometimes this free ride back fires on the beetle when the mite decides to dine on the eggs of the beetles themselves within the nesting chambers. N. pustulatus is not known to harbor many mites, since they rarely visit the larger carcasses to pick them up. Even small carcasses like mice can be found to have other carrion beetles visiting it and mites can potentially jump ship to other beetles. However since N.pustulatus has virtually stopped using mice as a resource in the wild you will rarely find one with mites. I did however find one feeding at a dead fish several years ago,  it goes to show that there are exceptions to every rule of nature. It was covered in mites!

Carrion beetles, natures little decomposers, are one of the most important components to make up the natural world. They recycle carcasses, removing potentially diseased animals from the environment by feeding on them or using them as a food source for their offspring. This makes the environment healthier for other animals, including humans. The leftover bits and pieces of carrion that the beetles don't use provide beneficial nutrients for soil health. This helps plants thrive.
They may not be the most attractive of insects, and definitely do not hold the same appeal as butterflies, but they have a charm all their own and provide a much needed environmental service that should be respected and appreciated.

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