Black or purple wasps are either soldier flies which are the adults of the commonly-seen compost maggot or mud daubers. Neither sting and both are highly beneficial. Soldier fly maggots help break down compost and mud daubers kill spiders. Mud daubers most common victims are the black widow spiders. Soldier flies can’t sting and the mud dauber females only sting if grabbed. So don’t do that!
Common name: Soldier Fly
Scientific name: Order Diptera, family Stratiomyidae, several species
Size: Adult - 1/2"
Identification: Adult flies are bare of bristles and not strong fliers. Antennae have three segments, often carried in the form of a Y. Colors range from dark metallic to brightly banded with yellow and pale green. Wings are large and lie over each other. Unlike syrphid flies, they do not hover. Larvae are tough worms with small heads and pointed ends.
Biology and life cycle: Eggs are laid in decaying organic matter, wood, mud, and dung. Larvae are maggot-like scavengers or predators. All are thick-skinned and somewhat flattened. Pupation occurs in the last larval skin. Adults are dark, sometimes banded flies. All life forms are found in the same habitat.
Black soldier fly larvae feed on decomposing matter, including kitchen scraps that should not go into a compost bin.
Habitat: Wet, low-lying areas, sappy wood, dung, and wet soil.
Feeding habits: Adults feed on flower nectar; larvae feed on decaying organic matter.
Economic importance: Help with the breakdown of organic matter, especially dung. People raising earthworms find the manure-feeding larvae to be very helpful.
Natural control: None needed.
Organic control: None needed.
Insight: Gardeners are sometimes concerned when they buy earthworm castings or other organic fertilizers and find these maggot-like worms. No reason for concern - they hurt nothing and are quite beneficial.