In the genus Hemileuca, the "half-white" moths that are so easy for humans to hate, the species oliviae may be the most hated. The caterpillars eat grass. When inadvertently ingested by grazing animals they may make cows and sheep sick. On contact with human skin, which they don't particularly try to avoid, they raise an unpleasant rash. And the moths aren't even pretty.
Photo by Arturoc.
Google has a lot of material about this moth. Unfortunately for those who like the pretty photo essays, the majority of what Google is willing to show is about the history of humans' efforts to kill it. This is the Hemileuca that was officially ruled a pest. Its population surge about a hundred years ago may have been a direct consequence of early efforts to spray it to death; spraying poisons that eliminate most of the target species, the first year, always breeds more resistant individuals in the target species faster than it breeds resistant predators. This moth has taught us a lot about the importance of working with natural predators rather than trying to poison lifeforms we don't like. Nature and public spirit may still tell us to apply a stout stick to any specimens of this species we meet, like good natural predators, fully reintegrating the body into the soil, but we do owe them thanks for the lesson they have taught.
In fact, the irruption of this species in the early twentieth century may have been caused by a temporary decline in populations of the microscopic parasite species, Anastatus semiflavidus:
A full-grown Anastatus semiflavidus looks, under a microscope, like a sort of four-winged wasp, except that the largest known individual reached a length of a quarter of a centimeter. It lays its eggs inside the eggs of H. oliviae and ensures that most of them will not hatch. Humans aren't likely to see these little animals. Other tiny wasps and flies parasitize caterpillars and pupae. Some larger animals were also observed to eat these caterpillars when they were abundant, with skunks, which normally pig out on ground-nesting wasps, ingesting heroic numbers of caterpillars. All we need to do is avoid spraying any kind of "pesticides" anywhere ("herbicides" do indeed kill insects), and allow Anastatus to follow its natural food source....
...Well...the other natural predators also help. So, in addition to the stick method outlined above, does catching caterpillars in a narrow, relatively high-walled container and pouring in boiling water or alcohol to cover. Then there's always collecting the moths: If you find a young female and enclose her in a box covered in fine mesh, you may be able to catch a dozen or more male moths. Each one pinned to a board soon means fifty or a hundred fewer in June...
How many fewer, exactly? Nobody seems to be rearing the moths to count how many eggs each female lays, but one study determined that, of 100 eggs, fewer than 40 caterpillars would live long enough to shed their third caterpillar skins. Most wild animals die young. Though stingingworms seem underpredated to normal humans whose ideal population count would be zero, the odds are against any individual egg becoming a moth, just as they are for luna moths and monarch butterflies.
Why oliviae? It means "of the olives," but olives aren't its food plant, nor are they a major crop in New Mexico, the species' home base. Many of the Hemileucas were named after goddesses, but Olivia seems to have been the name of real women, but not of a major goddess, in ancient Rome. This could be because olives were important enough that prayers and offerings for their success were offered, not at a special temple for Juno of the Olives, but at the main temples of Juno and Jupiter. Anyway Hemileuca oliviae was probably named in honor of some nineteenth century American.
They are one of the few species that have English names other than "stingingworm," though their names also describe only the caterpillars. They are Range Caterpillars or Grassworms in much of their habitat (New Mexico, contiguous States in the US and Mexico, and they have been found in western Oklahoma).
The problem the Range Caterpillars were presenting was vividly stated, indeed overstated, by V.L. Wildermuth in a 1916 US Government pamphlet on The New Mexico Range Caterpillar and Its Control. Wildermuth and his young assistant, D.J. Caffrey, who went on to write more scientific studies of this species and described some things he and Widlermuth said in the pamphlet as "harmful" errors, estimated that in much of New Mexico's grassland the overpopulated Range Caterpillars reached a population density of thirty million to a square mile. The caterpillars were obviously overpopulating their food supply and may actually have tried eating "wheat, oats, barley...corn and alfalfa," none of which they normally eat. During this period when they became a positive plague, they may also have left behind enough cast-off skins to "poison the plants" they didn't eat. Pastures may well have looked like mown lawns, in which the caterpillars then died of starvation after eating all the grass. The caterpillars may have been active from June through September, and the caterpillars may well have been "greedy" and "wasteful eaters," gnawing on plants they could not digest and leaving the rest of the plants on the ground, excreting great quantities of undigested wasted grass. Dusting with arsenic, as Wildermuth recommended, may have seemed necessary to panicky ranchers who had already given up trying to feed cattle. Obviously, dusting any usable crop or pasture land with arsenic would have bene a Very Bad Idea.
Over time, as the ecology returned to a more nearly normal balance, the caterpillars have become a less serious problem, though like all stingingworms they are still a considerable nuisance. By 1987 a population count recorded a maximum of 24 or 25 caterpillars to 100 square meters, or 250/km, or 400 to a square mile. They reduce the amount of grass available for cows and sheep. They can eat lawn grass, and may infest lawns, where contact with humans can make lawn work a real pain.
Some people are determined to make lemonade out of every "lemon" they encounter. Range Caterpillar eggs on grass may upset cows' digestion because they contain protease inhibitors, which interfere with enzymes breaking down proteins. Protease inhibitors can also interfere with the activity of disease virus. Extracts of biochemicals from these moths' eggs have been tested, but so far not used, as possible ingredients for a cure for cancer. I am not making this up.
In the same spirit, some authors note that native woodland mice will eat the caterpillars. Apparently mice in the genus Peromyscus are immune to the toxic biochemicals the caterpillars seem to be made of.
Others observe that the moths lay their eggs on stems of grasses the caterpillars can eat. They normally lay eggs about eight or ten inches above the ground. If there's no long grass in your neighborhood and you keep your grass well mown, you might be able to have a grassy lawn without attracting these vermin. If there is any possibility that a grass stem within a mile of you will be allowed to grow more than four or five inches above the ground, maintaining a grass-free desert garden might be a good idea within this species' range. Other stingingworms can and do eat other native plants but oliviae must have grass.
Are there people who might find them pretty? Well, they are pale, uninteresting Hemileucas. Like the rest of the genus they have very thick thoraxes, in which they store most of the body fat on which they live, and little flat button heads; they can look headless. They do have heads but, since all their heads have room for are eyes and antennae, no mouths, their heads are very small. The fur on the thorax is longer and shaggier than the fur on most Hemileucas, hanging over the little black eyes. The most obvious difference between oliviae and hualapai is that hualapai have faces while oliviae have antennae sticking out of a mop of hair.
Photo by Jcook91. In the shadows behind all that fur, it does have eyes.
Photo by Smellyturkey. This is one of the species most likely to have wings so pale that it takes a second look to see the standard Hemileuca wing markings, although they are there. (Tilt the screen if you don't see it.) The off-white scales can fray off the wings, too; the wings can become translucent, with patches that can be transparent.
It's not hard to entice a silk moth to climb onto your hand, or a more suitable object for moving it, if you want to move one. Place the target object against its foot while it's resting, and the moth will probably climb sleepily onto the object and wait to find out what the object was when it feels livelier. Silk moths don't eat so they need to save their energy for their one purpose in life: mating. However, while people say it's "safe" to handle other Hemileuca moths, who look "cuddly" to some eyes, there are complaints that even the shed hairs and scales of the adult oliviae can be irritating!
Photo by Deeanne.
Freshly eclosed moths have thick capes of fur covering the thorax; older moths tend to lose hair and may show bare brown chitin.
Photo from BugGuide.net, showing a faded and worn individual.
The pluminess of the antennae, on the other hand, indicates gender. Males have feathery antennae all their lives, and females have simple flat antennae.The pluminess of the antennae is thought to give the male moth a fantastic ability to recognize and track scents. The scent of greatest interet to these moths is the odor released by newly eclosed female moths, who are full of eggs and eager to get those eggs fertilized and placed on twigs.
People have tried to identify how the caterpillars choose the grass they eat. Their findings were not conclusive. One problem for Westerners is that stingingworms don't spend all their time on their food plants, anyway. They like blue grama grass, but when the ground gets hot, stingingworms crawl up whatever plants they find, and may be found anywhere. They need fields of grass to live on, but if mice and other woodland animals wouldn't eat them they'd take their siestas in the shade of trees.
While most Hemileucas fly in the daytime and some fly at night, oliviae is most active toward the end of the day. They don't fly a great deal. Male moths are a bit more active, but one study found that most female moths lay their eggs within ten yards from where they pupated,
Richard S. Peigler compared the behavior of Agapema dyari, which is another big silk moth that doesn't look much like Hemileuca dyari, with the behavior of Hemileuca oliviae, which does look similar to H. dyari. He observed that oliviae caterpillars eat relatively short grass in the morning. As the sun rises higher and the ground gets hotter, the caterpillars climb up higher plants. They seem to feel cooler when hanging out on higher plants with their heads downward, as if preparing to crawl back down to the plants they actually eat.
Photo by Gracefli. Later in the day, the caterpillars do indeed crawl back down to eat more grass.
This species normally flies at dusk and mates after dark, which partly explains the absence of photos of couples online. Another part of the explanation is that silk moths like to enjoy mating, taking it slowly with lots of cuddle time, and humans who know what these moths are know what that means an opportunity to do.\
Nobody seems to have bothered photographing eggs, either. According to drawings, they are placed around a grass stem in rings like other Hemileuca eggs, except that the smaller size of the host plant produces more smaller rings in a longer, thinner cluster.
Hatchling caterpillars are about a quarter-inch long, already prickly, and black or sable in color. Though they hatch from eggs laid on grass stems rather than tree branches, and the egg clusters form unusually thick ring shapes, the caterpillars manage to live in clusters on grass stems at first. Their color gradually lightens with more whitish and yellow spots and hairs replacing brown or black on each new skin. Like other Hemileucas, the caterpillars typically grow about two inches long but can get closer to three inches.
The caterpillar has some branching bristles, but more of its venomous bristles grow in a flat rosette shape that puts more bristle tips in contact with your skin if it happens to touch you, delivering more venom for a more painful sting.
Photo by Meganbunker.H. oliviae is different from other stingingworms in ways that are not yet fully understood. Athough our stingingworms and their venom are obviously similar ("related"!) to Brazil's Lonomia genus, whose venom is known to have killed human adults, ours usually cause only "minor" stinging rashes; venom does continue to seep slowly out of any bristle tips that remain in skin for several days, and does have anticoagulant properties preventing these tiny surface wounds healng until the bristle tips are removed. Repeated exposure to other stingingworms' venom tends to build a sort of resistance to the venom, but repeated exposure to oliviae and to brown-tail moth venom tends to aggravate sensitivity, making each reaction nastier than the one before. Nevertheless, hospitalization from anaphylactic reactions to Hemileucas are rare.
Photo by Willjaremkowright.The caterpillars are very vulnerable to fungus infections if the weather is not very dry, and they can also be killed by hailstorms.
Normally there are five caterpillar skins. Reportedly oliviae is one of the species that can go through seven caterpillar skins, and some say more, but nobody has confirmed more than seven molts or identified factors that cause extra molts.
When pupating, these caterpillars look for a few leaves and stems that are close together and use silk to bind them together into a lttle tent in which to morph. As with other Hemileucas, the pupa manages to wriggle out of the caterpillar skin, then lies down and tries to look like a pebble. The moth probably emerges in autumn, but nobody seems positive about whether this species can pupate for a full year and eclose next year.
Photo by Miguel1958.
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