The nasty sting of the velvet ant

By Jeff Mitton

A brilliant insect searched through the gravel of Cathedral Valley in Capitol Reef National Park, turning over smaller pebbles, thrusting its head into crevices and soft dirt. Its head and legs and ventral side were black, but the dorsal side of the thorax and abdomen were fiery orange.

This distinctive insect was a velvet ant, one of 3,000 velvet ants worldwide, most of them tropical; about 400 species are found in North America, mostly in the dry, sandy environments of the Southwest. But the common name is misleading, for these insects are not ants but wasps in the family Mutillidae. The one that I was watching was probably Dasymutilla californica, the California velvet ant.

This particular individual was female, for she had no wings; females of all species of velvet ants are wingless. Males have a pair of black wings, but in addition, males do not search and probe in gravel and dirt.

 Their sting is reported to be extremely painful and has earned them an alternate common name, cow killer. This name is simply an assessment of the magnitude of the pain, not an indication that velvet ants attack cows.But male and female velvet ants differ in other important ways. Like other wasps, females are diploid and males are haploid; that is, females develop from fertilized eggs and have two copies of their chromosomes, whereas males develop from unfertilized eggs and manage with a single copy of each chromosome. The sexes differ so much in color, pattern and sometimes size that it is difficult to identify them as the same species unless they are found mating.

Almost all species of velvet ants have brilliant colors, called aposematic coloration, that warn predators of a deadly defense. Males cannot sting, while females certainly do — the conspicuous and flightless females sting foolish predators that ignore the warning coloration. Their sting is reported to be extremely painful and has earned them an alternate common name, cow killer. This name is simply an assessment of the magnitude of the pain, not an indication that velvet ants attack cows. Naturalists report that they will sting if they are picked up and handled roughly.

The female velvet ant that I watched did not seem to be troubled by the fact that I was following it quite closely. She continued searching, digging, moving pebbles and thrusting her face into crevices and soft soil. Apparently the aposematic coloration and the sting were effective deterrents, and she did not need to flee looming animals. She was not hunting for food — adult velvet ants consume nectar. Furthermore, males do not exhibit this behavior, so why was she doing this — what was she searching for?

Velvet ants start off life as parasites. The female was searching for the nest of ground-nesting solitary bees, or bumblebees, or wasps, and she might also take advantage of larval flies and beetles. If the nest is sealed, she will chew her way in. Forcefully entering a bee or wasp nest seems imprudent, for stinging adults might be inside. However, the hide (formally, the exoskeleton) of velvet ants is thick and extraordinarily tough, very difficult to penetrate, so they are well defended against stinging insects protecting their nest.

When the velvet ant finds an egg, larva or pupa, she lays an egg next to it. When her egg hatches and becomes a larva, it will first eat the food stored inside the nest. But when the food is gone the larval velvet ant turns on the resident egg, larva or pupa. It begins to eat the larva or pupa, arresting its development, but stays with it until it is gone. The provisioned food plus a larva or pupa nourishes the velvet ant through its development.

Velvet ants parasitize many other species of wasps, some of them parasites of caterpillars or beetles. The parasite of a parasite is called a hyperparasite.

Jeff Mitton (mitton@colorado.edu) is a professor in the Department of Ecology and Evolutionary Biology at the University of Colorado. This column originally appeared in the Boulder Camera.

February 2014