Houses for mites?

an intriguing relationship

One July day I was moseying along a streambank, looking at the alders. Both red and Sitka alders grew there, but I was focused on the Sitka alders; I examined many leaves and put a few into a plastic bag to take home. A passer-by asked if I intended to eat them. Well, no, to my knowledge, humans don’t eat alder leaves—I planned to inspect the under-surfaces of the leaves with a dissecting microscope in my study.

Whatever for?? I suspect that most of my hiker friends already think I’m a bit nuts, with my interest in the composition of bear scats, the details of floral structures, the strange habits of slime molds, an unusual bird song, a skanky salmon carcass squirming with maggots, or other arcane natural history matters that often make me stop along a trail. A concern with the underside of alder leaves would probably clinch their opinions. Oh well….

I am interested in Sitka alder leaves because some of them have little tufts of hairs in the vein axils, where the side veins come off the midrib. On many other woody species in North America, including some maples, oaks, cherries, and grapes, tiny structures in vein axils (usually tufts of hair in North America but often pits or pockets elsewhere) serve as domiciles for mites. They are called domatia (singular = domatium), which means ‘houses’. And they serve the functions of a house: mites retreat to them for shelter from environmental extremes and predators, to lay eggs, to defecate, and to molt (leaving their exoskeletons there). Years ago, by surveying many leaves of many species in the eastern forests of North America, we found that at least twenty-five percent of domatia (and often fifty percent) in most species showed signs of mite occupancy. Similar occupancy rates were also found in foliar domatia in many other parts of the world.

Sitka-alder-Domatia 2 Carol Griswold

It is important to recognize that domatia are made naturally by the plant itself; they are often visible in the developing leaf bud. They are not galls, which are induced by the activities of insects or mites. So it is reasonable to ask if they serve a purpose and what that purpose might be.

Almost all of the mites we (and others) found in the domatia were potentially beneficial to the leaves and the plants that made them. Most of the mites were predatory–eating other mites or small insects–or fungus-eating–consuming molds and mildews and fungal filaments. Many studies have shown that the abundance of such beneficial mites is higher on leaves with domatia than on leaves without domatia. So the observations clearly suggest that this is a mutualistic relationship, in which both participants benefit: mites get housing in the domatia and plants get protection of their leaves.

The idea that the mite/domatia relationship is mutualistic dates back to 1887, when a Swedish scientist first looked at this. A few biologists accepted the idea without further investigation. But as is often the case with any new idea, the mainline scientists of The Establishment viewed the idea with scoffing, scorn, and rejection. Someone even went so far as to suggest that domatia occurred on leaves for the benefit of plant taxonomists, because they turned out to be useful in identifying species.

Such foolishness aside, a few experiments more than a hundred years later eventually showed that leaf damage is less when mites occupy the houses and forage over the leaf surfaces. Further study revealed that associations of mites with foliar domatia go back many millions of years, showing up in fossils. By now, leaf domatia are known from woody plants in most major taxonomic groups all around the world.

So—back to my Sitka alder leaves: I wanted to know if there were mites in those domatia too. I looked at ten mature domatia-bearing leaves from each of five locations in Juneau, counted the domatia and inspected them for evidence of mites. In over four hundred domatia, I found only four mites and one possible mite egg.

In the course of searching many alder leaves for domatia, I also found that leaves on many branches and trees had no domatia at all. In short, development of foliar domatia is sporadic in Sitka alders and mite occupancy is apparently very low. It takes a while for mite populations to develop on new leaves each year, so possibly a census of domatia in August would show a higher occupancy rate than in July, although that begs the question of what good it might do to protect leaves so close to the time when they turn brown and fall off. Maybe the summer season here is simply too short to allow time for a good population build-up of beneficial mites? Furthermore, considerations of mite population levels do not address the question of why domatia occur so sporadically on these leaves.

Perhaps evolution is in process of eliminating useless domatia from Sitka alders…or, contrarily, increasing their potentially useful occurrence on that species. Would on-going climate change affect the outcome? More questions than answers, once again!

Flowery fun in Gustavus

an orchid show, and other floral delights

Lady’s slipper orchids are sometimes called moccasin flowers, referring to the shoe-like shape of the flower. One of the petals is modified to form an oval pouch with an opening on top. The edges of the pouch are rolled inward. A small shield-like structure hangs down into the back of the pouch and behind the shield are the sex organs. The flower offers no nectar to visitors, but at least some species have an attractive aroma.

Bees that visit these flowers enter the pouch, but the rolled-in edges keep them from crawling out. So, once in the pouch, the bees are obliged to crawl up behind the shield, in order to get out again. In doing so, they pass very close to the pollen-receiving stigma, leaving pollen from previously visited flowers, and the pollen-bearing stamens, picking up pollen on their bodies to carry to another flower. A very elaborate system for creating the next generation of lady’s slippers.

After pollination, thousands of dust-like seeds are produced. They are so small that they contain no nutrition for a developing embryo (this is true of orchids in general). Lacking a source of nutrition, the seeds have to rely on forming an association with certain fungi (mycorrhizae), in order to germinate and grow. Lady’s slippers are slow growing and take several years to reach the flowering stage.

There are dozens of species of lady’s slippers in North American and Eurasia. They belong to the genus Cypripedium. This name is derived from some ancient Greek words. Cypris is an old name for Aphrodite (a.k.a. Venus in Latin), the goddess of love and beauty. The ‘ped’ part of the name refers to foot or footwear, sometimes rendered as ‘sandal’. So Cypris/Aphrodite/Venus has a rather large collection of sandals in her wardrobe!

Lady’s slippers were familiar to me, from years spent in the Midwest, but I have never seen them in Juneau. So one of my hopes for a recent Gustavus trip was seeing these in bloom. We’d seen their leaves occasionally in the past, but the plants were not then in flower. On this June trip, with the help of a knowledgeable naturalist there, we located clusters of three species of Cypripedium. There was a large-flowered white one (C. montanum, or mountain lady’s slipper). A small-flowered, round white one with some brownish spots is called C. passerinum (sparrrow’s egg or northern lady’s slipper). A yellow-flowered species has often been classified as a subspecies of C. calceolus, but more recently botanists seem to consider it to be a separate species, C. parviflorum, the small yellow lady’s slipper.

June 22 Cypripedium passerinum Sparrow Egg orchid 2 resize
Cypripedium passerinum, sparrow’s egg lady’s slipper. Photo by Kerry Howard

Lady’s slippers and many other showy orchids are often collected from the wild by willful gardeners. But this practice has led to the near-extinction of some species. The slow-growing habit, low levels of pollination and seed set, and the need for mycorrhizal fungi make recovery of exploited populations slow and difficult. So these plants should never be harvested from their native habitats.

June 22 Cypripedium Yellow Orchid 2 resize
Cypripedium parviflorum, small yellow lady’s slipper. Photo by Kerry Howard

We found other orchids too. Tiny twayblades are much more common in Gustavus than in Juneau. They are pollinated by minute flies and wasps, as Darwin documented long ago. Coralroots and so-called rattlesnake plantain are common in Juneau as well as Gustavus.

Orchids were not the only flower show in town, however. Lupines created hills of blue on the beach dunes. Cow parsnips and buttercups brightened beachside meadows. Roses and irises added splashes of color. One meadow was thoroughly decorated with the small white inflorescences of Tofieldia, which is easier to say than the ponderous common name of sticky false asphodel. Sticky it is—the stem sometimes captures tiny insects. Apparently, some botanists thought the inflorescence resembled the European asphodel, which in Greek mythology grew in the meadows where the souls of the dead walked. Great stretches of forest understory were carpeted by the leaves of deerheart, which sent up its small white spires of flowers, and the nearly-luminous, wide, white flowers of bunchberry (one of my companions is alleged to have said that they lighted the way to the outhouse in the darker hours!).

Indian paintbrush provided the most stunning floral array. Here in Juneau we see some yellow-flowered ones and (especially at higher elevations, I think) a few red-flowered ones. But in Bartlett Cove we found a beach meadow simply covered with paintbrush flowers: yellow, red, orange, particolored, and every combination in between. Quite splendid.

Here and there in summer

alpine sights, body-checked by a grouse, some thoughts on bear viewing, and wildlife on the home front

–In early August I went up Gold Ridge in hopes of finding the big, blue, broad-petalled gentian in bloom. Being a rather impatient sort, I’d tried earlier, in July, with no luck. But on this warm, sunny day, there were a few in bloom and more with buds. Higher up on the trail, I didn’t spot any, and they probably bloom slightly later up there. However, the mission was successful on this day, and a search later in the month should find lots more.

broad-petaled-gentian-by-bob-1
Photo by Bob Armstrong

Even if there had been none of those beautiful gentians, the day was a good one. A mountain goat was silhouetted on the ridgeline; young marmots gamboled about, while a big adult lazed on a boulder. There were several bear scats along the trail and, of course, I could not resist inspecting at least the most recent one. It was full of salmonberry seeds, along with some vegetation fibers; because the salmonberries at this elevation were not yet ripe, I knew that this bear had been foraging down lower.

Bird life was not well-represented, however: a pair of curious ravens, a robin, and an invisible sparrow pip-pipping in the alder brush. It is always a little sad when the season of bird song is over for the year. Nary a grouse or ptarmigan to be seen, and I’d seen only one brood in July. Although apparently no official census has been conducted, they seem to be much scarcer on the ridge these days than they were a few years ago. Back in 2005, the area was opened to hunting, and it is very likely that hunting has reduced the grouse and ptarmigan populations. Many of those birds were habituated to people on the trails, and many of us thoroughly enjoyed seeing them and their chicks almost any time we ventured up the ridge. Shooting them would have been easy (and very unsporting). It seems that, for the sake of a few hunters, the pleasure of many observers was reduced.

–When the sockeye come in to Steep Creek, the bears can feast. This summer, the one we know as Nicky came down late, as usual, and she does not have cubs; she’s around eighteen years old and may be slowing down a little. The cubs of Bear 153 put on a good show one evening: swinging on the willows, tussling in the grass, getting startled by a big salmon thrashing upstream, tipping over the camera gear set (by permission) in the stream, cavorting in the shallows. I had dropped by, intending to stay just a few minutes, and ended up staying almost two hours.

The few times I have gone out there to bear-watch, the crowds have been quite well-behaved, not needing much guidance from the rangers about proper conduct in bear country. But with so many people visiting the area, someone (or someone’s dog) inevitably makes a wrong move that makes the mother bears nervous and concerned about their cubs’ safety. This is a time to be especially observant of bear body-language and to give the bears even more space than usual. These bears are quite used to people and normally behave extremely well; we can keep them that way, for all of us to enjoy, if we ourselves behave properly. A new guide to staying safe around bears, including some new information, is in the works; it will be available from ADFG.

–When we were in Bartlett Cove, Glacier Bay, one day in June, we stumbled upon a female grouse that clearly had chicks somewhere nearby. Standing on big rock, she clucked and fussed, even when we stood back to see what might emerge from the tall, dense beach grasses. I circled slowly back around her rock, hoping to see the chicks as they crossed a narrow path. Well! Mama did not like that one bit, and as I inched forward, she gave a body slam to my shoulder as she flew ahead, sounding the alarm. As far as that female was concerned, I had invaded her space and she was not going to stand for it! Then we saw the eight or so chicks—they had already crossed the path and were not close to the mother’s rock at all. Nice big chicks! They all took flight away from the presumed danger (us), followed by mama.

The next day, we managed to upset a pair of greater yellowlegs as we walked out into some extensive beach meadows. Both adults yelled and swooped at us, so we knew that there were chicks in the area. We never did see those chicks, well hidden in the tall grass, and the tumult subsided as soon as we moved out onto the open beach.

–My home pond was a happening place this summer. Four different broods of mallards made it a regular stop on their rounds through the neighborhood. First, there was a brood of ten ducklings (known as the Tens), then a brood of five (the Big Fives), a brood of eight (the Eights), and a later brood of five (the Little Fives). Seldom was there more than one brood on the pond at a time; if two broods happened to be there, one dominated the area under the hanging seed feeder. There was a nice rain of seeds falling from that feeder, as the juncos scratched among the loose seeds and the jays tipped the whole feeder off balance. This was manna from heaven! And not to be shared. The Eights would advance upon the Little Fives, pushing them into a corner of the pond, and go back to gobble up falling seeds. On another day, the roles would be reversed, the Little Fives winning the prize. The Big Fives sometimes charged at The Eights, relegating them to the far upper end of the pond, and went back to snarf up the seed rain.

Several broods of juncos (and their parents) grew fat on the seed offerings, and I watched the young ones gradually acquire their adult plumage. Bears wandered through but did not bother with the inaccessible feeder. I watched two predators with evil intentions about ‘my’ ducks, but they departed, still hungry. A roaming dog threatened one brood, and the mother duck led that dog a merry chase in her version of a broken-wing act: back and forth went dog and duck, the duck always just two or three feet ahead of the dog. She could have just flown away, but she was intent upon keeping that dog away from her young ones. The dog did not respond to orders from the shore, so eventually, my quick-thinking neighbor jumped in and grabbed the dog, and peace was restored.