What are these beetles doing on an apple tree?

 

The beetles in the above photos are American Carrion Beetles (Necrophila americana, Superfamily Staphylinoidea, Family Silphidae) and typically feed on the flesh of dead and decaying animals. But here are three carrion beetles feeding on or getting ready to feed on the sugary, slightly fermented sap oozing from a wound in an apple tree. There isn’t a scrap of meat in there but their feeding on sap is not as unusual as it may seem.

American Carrion Beetles beetles are part of a larger family of beetles known as Silphidae and not all are scavengers or even strictly carnivorous. One species, Aclypea bituberosa, eats spinach and other crop plants. Adult American Carrion Beetles, while principally feeding on carrion and the fly larvae that eat carrion, will feed on sweets. Cultivated food crops like sugar beet and pumpkin are particular favorites and it can become a pest insect on these. I have seen them on rotting mushrooms where they were probably eating fly maggots and on rotting soybean meal which smelled like decaying meat. Feeding on sap may be a way to acquire carbohydrates before winter hibernation.

Description
The length of the American Carrion Beetle is between 13.8-20 mm. It is readily distinguished by its bright finely to roughly pitted (punctate), yellow thoracic shield (pronotum) that is usually marked by a large roundish black spot. The wing covers (elytra) are black (rarely bluish) although in females the apical tips are yellow. There are three raised veins (costae) running down the elytra. There may also irregularly shaped tubercles usually connected to the costae. The last few segments of the abdomen protrude from beneath the elytra.

Life Cycle
Larva are elongate, black and armored with wide scale-like coverings on each body segment. Reproduction is from late May to July. Eggs of American Carrion Beetles are laid on dead carcasses shortly after flies arrive. Upon hatching the larva eat fly eggs and maggots, decaying flesh, dried skins, and each other. Development for egg to adult takes about 10 to 12 weeks. There is only one generation per year. Adults over winter and in the spring emerge to begin the reproductive cycle anew.

A type of mutualism (symbiosis beneficial to both organisms) exists between American Carrion Beetles and certain mites (often Poecilochirus spp.) that reduces competition for food resources. The mites shelter on the bodies of adult American Carrion Beetles leaving when the beetles arrive at a carcass. Once there the mites begin eating the eggs of flies whose larva would compete for food resources with the beetle larva. The adult beetles also eat fly eggs and maggots as well as the not too decayed flesh. The mites will lay eggs of their. After the eggs hatch the immature mites feed on fly eggs. When mature they will attach to (but not harm) the new adult beetles.

Taxonomy
Class: Insecta
Order: Coleoptera
Suborder: Polyphaga (Scarab, Lady, Rove, Lightning Beetles and several others)
Infraorder: Staphyliniformia (Rove, Scavenger and Water Scavenger Beetles)
Superfamily: Staphylinoidea (Rove and Carrion Beetles)
Family: Silphidae (Carrion Beetles)
Genus and Species: Necrophila americana (American Carrion Beetle)

Habitat
American Carrion Beetles prefer open habitats like fields and low marshy ground but can also be found in forests.

Range
American Carrion Beetles occur east of the Rocky Mountains from Manitoba to New Brunswick and south to Florida and Texas.

References consulted
Anderson, R. S. (1981). Resource partitioning in the carrion beetle (Co1eoptera: Silphidae) fauna of southern Ontario: ecological and evolutionary considerations. Canadian Journal of Zoology 60: 1314-1325.

Bug Guide Series Staphyliniformia

Fitcher, G. S. (1949). Necrophily vs. Necrophagy. The Ohio Journal of Science 49(5): 201-204.

Majka, C. G. (2011). Journal of the Acadian Entomological Society. 7: 83-101.

Knox, T. T. and Scott, M. P. (2006). Size, operational sex ratio, and mate-guarding success of the carrion beetle, Necrophila americana. Behavioral Ecology 17 (1): 88-96.

Ratcliffe, B. C. (1996) The Carrion Beetles (Coleoptera: Silphidae) of Nebraska. Bulletin
of the University of Nebraska State Museum Volume 13. Published by University of Nebraska State Museum Lincoln, Nebraska.

Steele, B. F. (1927). Notes on the Feeding Habits of Carrion Beetles. Journal of the New York Entomological Society Vol. 35 (1): 77-81.

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