Wednesday, October 26, 2016

Phoresy and the Carrion Beetles

Gold-necked Carrion Beetle
After emerging from the miasmic swamp of news about the toxic Presidential campaign I felt the need to study something that was relatively pleasant and clean by comparison.  I immediately thought of carrion beetles and the mites that infest them.  I had been clearing my mind of political news by collecting some Gold-necked Carrion Beetles crawling around a long dead rat and noticed the orange mites clinging on them like campaign advisers.  (Arrrgh..... must get out in nature more often!)

Tomentose hairs on thorax, mites clinging to the neck - Click to enlarge
Gold-necked carrion beetle is named because of the patches of golden yellow hairs (setae) on the pronotum.  Its formal name is Nicrophorus ("Death carrier" L.) tomentosus ("cushion stuffing" L.), the latter referring to the flattened matted hairs on the pronotum.

Phoresy in the title refers not to the lead singer of a hard rock band but the act of hitching a ride on a larger animal of a different species as transport from place to place.  This  relationship excludes parasitism where the transporting animal  is harmed.  In general there are three factors in common in phoresy.  (1) The mite actively seeks the host.  (2) It doesn't feed or develop while on the host.  (3) It departs at a food source to mature and reproduce.  Obviously carrying around a load of mites is expensive in energy so why has the relationship evolved?   Jim McClarin explained their mutualistic relationship this way in Bugguide.
"The mites benefit the beetle. They eat the eggs and freshly-hatched maggots of carrion flies that would both compete for food and poison the food with their high-ammonia waste products. The mites help the beetle's larvae to survive, giving their own young a new generation of beetles to ride to the next carcass. It's a beautiful relationship that stinks to high heaven :-)"
Mites lack the wings to seek out dead animals which are their source of food.  Fortunately they have become good neighbors with carrion beetles, in this case Nicrophorus tomentosus.  Different species of mites tend to ride on beetles in the same genus and some species seem to prefer either N. tomentosus or the closely related N. orbicollis.*  Nicophorus sp. support at least 14 species of mites in 4 different families.  One of the most studied and largest at 1mm is Poecilochirus carabi.*

Poecilochirus nymphs - Steve Nanz
There are at least 14 Poecilochirus species of mites.  They generally have a symbiotic relationship with the beetles, protecting the beetle larvae and their food supply from fly larvae.  As you might guess, they are not a very popular topic on the Internet and most available sources come from previous books.

Mites on the elytra - Tom Murray CC
Jim McClarin again explains.  "Their position atop the beetle's wingcovers (elytra) signifies that this beetle has just flown and/or is about to fly. Normally, unless the beetle is super-loaded with mites, they cling to the beetle's underside. However, when the beetle is about to fly, the mites climb up on top, all facing forward. They are protected in this position because the beetle rotates its elytra up and toward the center, forming a tent-like enclosure with the mites inside:" 

"All aboard!" - REK
When the Nicrophorus beetle arrives at a new carcass, the mite nymph climbs off and molts into an adult within 24-48 hours.  They mate and their next generation of nymphs may (1) attach to an adult Nicrphorus to head to the next carcass when this one is consumed, (2) follow the beetle offspring into a pupal chamber or (3) attach to another vector leaving the carcass.  Most chose #1 and are off to another carcass.  Messy yes, but hey, its a living! 

Mites: Ecological and Evolutionary Analyses of Life-history Patterns
More on mite phoresy at