A MIGHTY MITE!

By Arthur V. Evans

In the deserts of Africa, Asia, Europe, and North America large velvety red mites appear suddenly after heavy rains. Southwestern United States has at least two species of these amazing mites.

This past July, I came across a lone individual of a giant red velvet mite, Dinothrombium magnificum (LeConte) emerging from its burrow just east of the Patagonia Mountains in southeastern Arizona where it inhabits the Sonoran Desert and adjacent uplands.

Giant red velvet mites are spectacular for several reasons. First, the largest individuals measure in at a whopping one centimeter in length, which makes them the largest mites in the world. They are covered with a thick coat of scarlet hair-like setae. The mite’s bright red color is apparently aposematic in function and serves to warn predators of their bad taste. Entomophagous animals offered giant red velvet mites either rejected the arachnids outright or quickly spit them out.

Although often difficult to find, they are sometimes extremely abundant locally, if only for a few hours at time. For example, after a brief yet intense thunderstorm, a massive emergence of giant red velvet mites was sighted from the air at an altitude of 1500 feet just north of  Tucson. An estimated 3-5 million mites had emerged in an area roughly two acres in size!

The annual emergence of the giant mites is apparently timed to coincide with that of their primary prey, termites. However, their opportunity to gorge themselves on abundant termite reproductives is quite limited. After mating, the termites quickly shed their wings and bury themselves so that they are out of reach of the mite’s predatory embrace. Adult giant red velvet mites spend most of their lives in subterranean burrows in a diapause-like state waiting for a specific set of ecological conditions triggered by summer monsoons.

Resources:

Evans, A.V. 2007. National Wildlife Federation Field Guide to Insects and Spiders of North America. NY: Sterling. 497 pp.

Lighton, J.R.B. and F.D. Duncan. 1995. Standard and exercise metabolism and the dynamics of gas exchange in the giant red velvet mite, Dinothrombium magnificum. Journal of Insect Physiology 41(10): 877-884.

Newell, I.M. and L. Tevis, Jr. 1960. Angelothrombium pandorae n.g., n. sp. (Acari, Trombidiidae), and notes on the biology of the giant red velvet mites. Annals of the Entomological Society of America 53: 293-304.

Tevis, L., Jr. and I.M. Newell. 1962. Studies on the biology and seasonal cycle of the giant red velvet mite, Dinothrombium pandorae (Acari, Trombidiidae). Ecology 43(3): 497-505.

Zhang, Z.-Q. 1998. Biology and ecology of trombidiid mites (Acari, Trombidioidea). Experimental and Applied Acarology 22: 139-155.

© 2010, A.V. Evans

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COW KILLERS LACK THE VELVET TOUCH

By Arthur V. Evans

Velvet ants, some of which are also known as cow killers, are actually solitary wasps. The females are wingless and sting, while the stingless males are fully winged. Although incredibly painful, the sting is seldom dangerous. Velvet ants are rarely abundant enough to need any sort of control and are best left alone to go about their business.

Velvet ant diversity is greater in southwestern United States, less so in the Southeast. Although there are more than 40 species of velvet ants found in the Southeast, only one species in the region, Dasymutilla occidentalis, stands out. It is the largest species of velvet ant in North America and occurs from Connecticut to Florida, west to South Dakota and Texas.

In spite of its nickname “cow killer,” the stings of the female D. occidentalis are not fatal to cattle. The bold and contrasting colors of this velvet ant serves to warn predators that they are quite capable of defending themselves. They also make a squeaking sound by rubbing two abdominal plates across one another as an additional warning. The stingless male is automatically defended by its close resemblance to the female.

Lone females are often seen wandering about on the ground in open habitats from spring through late summer. Winged males patrol these same habitats for mates. Both males and females drink nectar for their nourishment. After mating, females begin searching for the ground nests of bumble bees. Upon finding a nest, the female velvet ant lays a single egg at the entrance of a bumble bee nest. The larva develops inside the nest as an external parasitoid on a bee grub; pupation occurs in the bumble bee’s nest.

Resource: Evans, A.V. 2007. National Wildlife Federation Field Guide to Insects and Spiders of North America. NY: Sterling. 497 pp.

© 2010, A.V. Evans

TIPPING THE SCALES

By Arthur V. Evans

Wax scales that is. Indian wax scales to be precise.

While trimming our nandina hedge this afternoon, I noticed a couple of small, white, barnacle-looking lumps on a stem. They were female Indian wax scales, Ceroplastes ceriferus (Fabricius). Sexing Indian wax scales is easy since males are not known in any wild population in Virginia. Adults are covered with a thick, white waxy layer that not only protects them from predators, parasitoids, and pesticides, but also helps them to survive freezing temperatures during the winter.

Reproduction is by parthenogenesis. One generation is produced annually in Virginia, but two or more appear in warmer climates. The first instars, or crawlers, hatch in spring and early summer and feed on leaves. They are not covered with a protective wax layer and are very susceptible to dehydration, parasites, and pesticides.

Adult Indian wax scales are conspicuous in late summer and early fall and suck sap from at least 122 plant species in 46 families. Prolific breeders, they quickly cover ornamental plants. Burgeoning wax scale populations not only ruin the plant’s appearance, but also cover them with sooty mold that develops on the prodigious amount of sticky waste (honeydew) that they produce.

Carefully tipping or lifting the scale to one side to detach the it from the plant stem reveals the orange and segmented body underneath. In the adjacent photo, the anterior of the body is on the lower right, while posterior is on the upper left. The mouthparts are visible and appear as a dark central spot at about the anterior third of the body.

Resource: Kosztarab, M. 1996. Scale Insects of Northeastern North America. Identification, Biology, and Distribution. Virginia Museum of Natural History, Special Publication No. 3. Martinsville, VA. 650 pp.

© 2010, A.V. Evans

SHARPSHOOTERS AND BROCHOSOMES

By Arthur V. Evans

Broad-headed sharpshooter, Oncometopia orbona.

Sharpshooters (Oncometopia species) measure 11-13 mm in length and are among the largest of North America’s leafhoppers. They feed on a wide variety of plants growing in gardens, parks, meadows, and woodland edges during summer and fall. Their sap feeding activities may spread plant pathogens. Females use their knifelike ovipositors to insert eggs into soft stems. The eggs are covered with a chalky substance (egg brochosomes) that make them more resistant to excess moisture and protect them from fungal infections and possibly attacks by parasitoids.

Broad-headed sharpshooter with brochosomes.

Brochosomes are intricately shaped proteinaceous particles that are produced by kidney-like structures called Malpighian tubules and excreted as a solution. After the sharpshooter molts, the solution is spread over the exoskeleton as a water-proof coating. Female sharpshooters store brochosomes as a single white dot on each forewing to be used later as a protective coating for their eggs.

© 2010, A.V. Evans

CAN YOU SAY OSMETERIUM?

By Arthur V. Evans

This summer a cadre of dedicated parents and volunteers joined forces at a nearby elementary school to create an outdoor classroom. The Holton Learning Project Garden includes a vegetable and butterfly garden that will introduce Holton Elementary School students, their families, and the residents of Belleview and beyond to the pleasures and benefits of urban gardening.

Compared to the dreary, sterile plantings of exotic trees, shrubs, and groundcovers found throughout much of the neighborhood, the vegetable and nascent butterfly garden has rapidly become a local hot spot for insects and spiders. As such, it provides an excellent site for macro photgraphy. Since August, I have endeavored to photograph as many of its multi-legged denizens as possible as part of an ongoing effort to document the arthropod diversity of my neighborhood in Richmond, Virginia.

While walking through the garden yesterday afternoon, I noticed several clumps of green spikes rising sadly from the straw-covered beds. I soon confirmed my initial suspicions as to the identity of the culprits that laid these once fat bunches of parsley to waste. At the very base of one of the clumps were two brightly banded larvae of the black swallowtail, Papilio polyxenes, polishing off the last few leaves.

When I knelt down to photograph the ravenous caterpillars, I accidentally brushed up against their food plant. Both caterpillars reacted immediately by assuming defensive postures. Bent over backwards, they spit up green fluid and produced a pair of long tentacles (osmeterium), that resembled bright orange horns. Soon my nostrils were filled with a strong, disagreeable odor that is best described as “spicy vomit.”

The osmeterium consists of two soft, finger-like tubes that are everted from inside the body through a slit in the prothorax just behind the head as a result of  increased blood pressure. This defensive gland is found in the caterpillars of swallowtail butterflies and is coated with highly noxious chemical compounds (2-methylbutyric acid and isobutyric acid) that deter predators, especially ants.

© 2010, A.V. Evans

TARANTULA VS. TARANTULA HAWK

By Arthur V. Evans

One of nature’s classic battles is that of the lopsided struggle between a tarantula and its arch nemesis, the tarantula hawk (Pepsis, Hemipepsis). I say lopsided because the odds are usually stacked against the tarantula. The arachnid, paralyzed by the wasp’s sting, is destined to be dragged off and stuffed down a previously dug burrow to become an egg-laying site and eventual fodder for a ravenous wasp grub.

In August, I photographed a tarantula hawk as it dragged a paralyzed female desert blonde tarantula, Aphonopelma chalcodes, across a coarsely gravelled driveway at the foot of the Huachuca Mountains in Sierra Vista, Arizona.

I first became aware of this saga in Walt Disney’s Academy Award winning documentary The Living Desert (1953 and later re-released in 1971) that depicted a day in the life of desert flora and fauna and the struggles of the latter to simultaneously find food and avoid being eaten themselves. It was 10 minutes of film footage featuring a tarantula hawk grappling with a tarantula shot by N. Paul Kenworthy, then a doctoral student at UCLA, that inspired Disney to produce his first documentary. Kenworthy later became one of the two macro cinematographers on the project. I met The Living Desert’s other macro cinematographer, entomologist Bob Crandall, while I was in high school. But that is another story for another time.

© 2010, A.V. Evans

GRASSHOPPER LOVE

By Arthur V. Evans

Arguably the most spectacular looking and certainly among the most distinctive of all the grasshoppers in North America, painted grasshoppers, Dactylotum bicolor (24-32 mm) are a riot of color. These boldly marked orthopterans are also known as rainbow or barber-pole grasshoppers. Studies have shown that diurnal predators, especially birds, will avoid eating them presumably because of their aposematic coloration. Females tend to be significantly larger than the males.

Painted grasshoppers make their living along the western edge of the Great Plains from southern Saskatchewan south to western Texas and northern Mexico, and west to Arizona. Active from mid- to late summer, painted grasshoppers feed on a wide variety of desert plants, especially grasses and low broadleaf plants.

© 2010, A.V. Evans