This mass of small shiny red and black beetle-like insects are nymphs of the white-margined burrower bug (Sehirus cinctus). They belong to the order Hemiptera or True Bugs and Superfamily Pentatomoidea (Shield Bugs). White-margined burrower bugs are common insects in damp meadows and fields wherever plants in the mint family (Lamiaceae) grow because the nymphs eat the seeds of mints (Mentha) and their relatives and sometimes seeds of stinging nettles (Urtica dioica) which are in the family Urticaceae.
I used to see these bugs in the late summer and early fall on ditch banks in the fens near Hibbing, Minnesota but did not know what they were beyond “stinkbug”. Now it makes sense that there were hundreds of these bugs in the fens since there are many Lamiaceae species growing there. These include bugleweed (Lycopus), wild mint (Mentha), skullcap (Scutellaria), and horse nettle (Galeopsis) which would provide plenty of food for the nymphs to eat. Adults feed on a variety of plants and plant parts including seeds.
Adult female white-margined burrower bugs lay their eggs in the soil with a stash of seeds and protect the eggs while they incubate. After hatching the nymphs live in small colonies for about two weeks. For the first few days the mother will bring seeds to the nymphs until they are large enough to forage on their own. This is a form of insect social behavior that loosely parallels ants, wasps, and bees.
White-margined burrower bugs and other Pentatomoidea feed not by chewing but by piercing the food item with a sharp beak, injecting digestive enzymes, and then sucking in the partially digested food. The process is known as “extra-oral digestion”.
Adults are broadly ovate about 6 mm long, shining, pitch-black or blue-black, head, thorax and wing covers punctate. Along the border of the thorax and abdomen is a thin white line. Usually there are two white dots on the wing tips but sometimes males may not have these. There are short white segments on the outer sides of the tibia.
The nymphs are also oval and shiny but are bright red with three or four black bars across the top of the abdomen and black dots along its margin. The head, thorax and developing wings are black. Nymphs live in groups and tend to move together. After each molt the nymphs will be white or yellow but soon become red and after the fifth molt become adults.
Class: Insecta (Insects)
Order: Hemiptera (Bugs)
Suborder: Heteroptera (True Bugs)
Infraorder: Pentatomomorpha (Stink Bugs, Flat Bugs, Seed Bugs)
Superfamily: Pentatomoidea (Shield Bugs, Chust Bugs, and Stink Bugs)
Family: Cydnidae (Burrower Bugs)
Subfamily: Sehirinae (A Subfamily of Burrower Bugs)
Genus and species: Sehirus cinctus (White-margined Burrower Bug)
Range and distribution
White-margined burrower bug occurs over much of Canada, the United States and Mexico.
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