More Hilltopping Insects

I finally got the full effect of a hilltopping experience on March 25 when I hiked to the summit of Sentinel Peak (“A” Mountain), just west of downtown Tucson, Arizona. At only 2, 897 ft. (883 meters) above sea level, and rising a mere 522 feet off the desert floor, it is more of a butte than a mountain. Still, its relative isolation makes it an effective magnet for hilltopping insects.

“A” Mountain is named for the immense concrete letter on its eastern slope, painted in the red, white, and blue colors of the University of Arizona. Most folks visit the top for the spectacular panoramic view of Tucson and its surrounding mountain ranges. It can be a romantic setting at dusk, too. Male insects of many species come there for romance as well, but usually have to defend territories or otherwise repel rival males in the process.

Among those insects are bumble bee-sized bot flies, Cuterebra austeni. The big, black and white adults are not often seen during their brief existence. They have no functional mouthparts and are fueled only by burning the fat reserves they accumulated in the larval stage. As larvae they are subdermal parasites of either rodents or rabbits, depending on the species. I’ll spare you the gory details.

The male flies conserve their energy and will allow you to approach extremely closely if you do so carefully. Once startled, or when they detect a rival or a female, they jet off at warp speed. Eventually they return to the same spot, or very close by.

A more typical, larger, and colorful hilltopping insect is the black swallowtail, Papilio polyxenes. These butterflies are in seemingly constant motion, flying swiftly across the summit and only pausing to perch for very brief periods.

They are incredibly aggressive, chasing each other mercilessly. You can actually hear their wings and bodies colliding, so violent are their aerial duels.

Perhaps the most unexpected and unusual insect I discovered was the pollen wasp Pseudomasaris maculifrons. Pollen wasps are solitary insects in the same family as potter wasps, mason wasps, and the social hornets, yellowjackets, and paper wasps. While her cousins feed their larval offspring with other insects, the female pollen wasp stores pollen and nectar in her mud nest.

Males are easily recognized by their peculiar antennae: long and clubbed. Females have much shorter, clubbed antennae. Males of this particular species are known to perch repeatedly in the same small area, day after day, for as long as 29 days (Alcock, John. 1985. “Hilltopping Behavior in the Wasp Pseudomasaris maculifrons(Fox),” Journal of the Kansas Entomological Society, 58(1), pp 162-166). Between February and May, different generations of males occupy the same locations in different years. They are very active insects, but do not appear to be nearly as belligerent as bot flies or black swallowtails.

It turns out the top of the peak also attracts other insect photographers. I crossed paths there with my friend Philip Kline, who I hadn’t seen in probably two years! We had a great time catching up, and helping each other identify the insects we were seeing.

May your own hilltop experiences be just as rewarding and enjoyable. You could very well discover something new (especially regarding bot flies), to yourself or the scientific world.

Sometimes you become the best perch on the peak!

Hilltopping

One of the more interesting types of insect behavior is something called “hilltopping.” While hiking along a ridge in Tucson Mountain Park on Friday, March 5, I witnessed this firsthand.

Hilltopping describes the mating strategy of male insects of some species that will fly to the highest point in their immediate landscape. This may be a mountain, a butte, or even a small hill, provided it is sufficiently elevated above the surrounding terrain.

Once at the summit, the insects pursue one of several courses of action. Males of some species will defend a small territory such as a perch on a boulder, shrub or tree that affords the best view for spotting approaching females of the species. Competing males will be driven off. Butterflies in the family Lycaenidae, such as the Great Purple Hairstreak, Atlides halesus, and the Gray Hairstreak, Strymon melinus, both pictured below, defend perches.

Another method employed by males is “lekking,” in which several males congregate in an area where they display to females. Females then choose what they perceive as the fittest male specimen and mate with him. A “lek” is the term for the “stage” on which the males “perform.”

Still another form of hilltopping is when a male “patrols” a route along the summit of a hill or ridge, hoping to intercept a passing female. Many butterflies in the family Pieridae exhibit this behavior, also known as “scramble competition polygeny.” Males don’t have territories, but will actively compete when two of them spy the same female. The battle may consist of spectacular vertical flights, the two males spiraling around each other until one ceases to ascend further, conceding defeat. On my own hike, I noticed several males of the Desert Orangetip, Anthocharis cethura patrolling along the ridge, together with what I believe were Sleepy Orange butterflies, Eurema nicippe. Also seen at the summit of the ridge was a male West Coast Lady, Vanessa annabella, that repeatedly returned to the same patch of ground.

Besides butterflies, major hilltopping insects include several species of Hymenoptera (bees, wasps, ants) and Diptera (true flies). Flesh flies in the family Sarcophagidae, such as the one pictured below, were present on my afternoon sojourn.

To see the most variety of species, I would recommend spending all day at the very top of a likely hill, ridge, or mountain. The fauna will change as the hours pass and conditions change. Here in the desert at least, some species hilltop for only a few hours in the morning, then disappear. Other species are present all day, or may be most active in the afternoon.

Especially at this time of year, early spring, hilltops may be the only places to find an abundance of insects. Not only will you see hilltopping species, but other species that come to prey on them. I’m sure that is the only reason that dragonfly was up there, perching on a cactus of all things.

NOTE: I highly recommend reading the works of Dr. John Alcock for more about hilltopping insects. His books include Sonoran Desert Spring and Sonoran Desert Summer, both being collections of outstanding literary essays.