Wednesday, June 11, 2014

Dynastes granti, the largest Arizona beetle

If all gas stations were this clean, I'd have more fun collecting Dynastes granti
The Hercules Beetles are flying! Even though some 'real' entomologists scoff at my appreciation of these big, shiny, horned giants, I am looking forward to their late-monsoon appearance every summer.

Apparently I am not the only one, as this photo of Bill Warner shows. He is one of our leading scarab specialists (photo by Ben Warner).

On my Beetle Safaris with clients, I usually look for Dynastes granti at  artificial light sources with a strong UV component, like those arrays of mercury vapor lights at some gas stations along the Mogollon Rim in central Arizona. The further away from civilization and other lights the better. Most of those locations are well known and draw beetle collectors from all over. Even though the competition is usually good-natured and one is at least rarely alone, it is not always pleasant to spend the night at a gas station in the middle of nowhere. But if the weather is right, a warm night after a storm is ideal, the beetle hunt in those locations is usually rather successful.

Young Ash Trees in a canyon close to Prescott
By far more interesting and esthetically pleasing are the natural gathering places of the big beetles:.   lush stands of ash trees along creeks in little canyons. The beetles love the sap of ash trees that is raising after the monsoon rains in August. To get this treat, male beetles chew through the outer bark into the cambium layer so the sweet sap begins to ooze. They pick young branches of less than 3 inch diameter that are still tender and green. The chew marks have a characteristic shape. Even years later when they are covered by scar tissue they are still easily recognizable. In fact, those left-over scars were all I could find at first. I also discovered that under some ash trees, someone had turned over the soil. Left over beetle wings told the story: Scarabs, not all of them Dynastes, had been attracted by the oozing sap at night and then burrowed into the soil in the morning. Some large mammals like javelinas or skunks had learned to search for snacks in this promising locations.   

Left and middle: fresh bark scrapings, right: old scar all on the same Ash tree
The scarab-tribe of Dynastini is mainly night active. Most species are big and muscular enough to shiver efficiently and warm up to operating temperature even when it's cool outside, and once warm, they are strong fliers. So at night they fly to their ash trees, prepare their sap licking sites, congregate and find mating partners.The males use their horns to grab rivals and toss them off the tree. I assume the winner gets the girl...but I have seen males push females off the food, too, so fermenting tree  juice may be even better than sex.

  I started my search in the early morning because I had been told that the beetles, once they had chewed a sap producing wound into the tree, would hang around during the day, mainly resting in place from their nightly activities. It was at first difficult to spot them because they were all rather high above my head, at about 12 feet. So I was staring up against the sky while at the same time trying to keep my footing on slippery rocks and not to stumble into the creek. Catching them was a whole other problem. The two first ones I spotted took off, buzzing like helicopters, as soon as  my net came close to their branch. In the act of mating, they were obviously alert and warm enough for immediate take-off.  They disappeared high into the blue sky. Night-active scarabs? 

 Something else moved. Disappeared behind the branch. So I scrambled across the creek to check the other side. The right size and color, but - a cicada.

Slowly a search image formed: the beetles are round and shiny like ripe chestnuts, just not brown but greenish like the ash leaves themselves. And unlike chestnuts, they were not going to eventually fall down. I had to make them. So I got a thin stick, long enough to reach the beetles while I was standing on a tall boulder in the creek. I found that single males could be encouraged to walk down from their perches by pushing the end of the stick between their two horns. By backing off, they may have reacted as they would when faced with a powerful wrestling partner of their own kind. So I coaxed several males within range. A single female fell into the creek and I fished her out.
Overall the technique was extremely exhausting but fun.

The result of an over 400 mile round trip
During the following night I collected a few more beetles at a gas station so I went home with an even number of males and females.
The beetles will augment my own breeding stock, and a few are going to other breeders. A Montessori teacher is building a school project around a pair, the insect photographer Alex Surcica ordered some as models, others will go to entomology classes, an insect festival and a museum exhibit. With some luck, they can outlive their wild brethren by any number of months.
It turns out that I did not get quite enough specimens this year.

Eggs and larva of Dynastes granti, pupa of Strategus sp. ( Strategus is another Dynastini, I have no Dynastes pupae yet)
But that's okay. Last year I kept eight females who produced eggs from September until the end of November. The eggs then rested until January when most of them turned into little c-shaped larvae. They began feasting on fermented hard-wood mulch (my own month-long preparation) and grew quickly into very substantial grubs. They are still eating and growing now, a year after the eggs were laid. For Dynastes granti, the cycle from egg, to larvae (3 instars), to pupae, and finally adult beetles can take up to 4 years. At the Arizona Sonoran Desert Museum a batch took only 1 year, but the resulting beetles were tiny and unsatisfying. According to anecdotal evidence, temperatures and the availability of protein in the larval food have an impact on the length of the development time and the size of the adult specimens. But how those factors are correlated is impossible to tell without rigorously controlled experiments.

A male Dynastes granti has 2 horns. One on his forehead, one on the pronotum. By moving his head up and down, he can use the horns like the jaws of a pair of pliers. He can grab another beetle around it's 'waist' and toss him over his shoulder

And I'm really just breeding beetles for fun ... but I can already see that I could be easily tempted to add a couple more species to my beetle breeding room - there is still space on the shelfs....

Cottonwood Stag Beetle, Lucanus mazana, with egg and young larva
By the way, I just discovered that a Lucanus mazana female (our only Arizona stag beetle) laid eggs in her container and the first larvae are hatching ...

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