Hawk moths

master fliers and specialized pollinators

hawk-moth-adult-by-bob-armstrong.jpg
Bedstraw hawk moth (Hyles gallii) Photo by Bob Armstrong

If you walk through a field of fireweed, you might see spittle bugs and aphids and –if you are lucky—a hawk moth. They hover at flowers on their rather narrow wings, extending their ‘tongues’ to extract nectar. They can fly very fast, which may have given them the common name of ‘hawk.’ They’re also called sphinx moths: when a caterpillar is at rest, it raises the front part of its body and tucks the head down; someone with a vivid imagination saw a resemblance to the famous Egyptian Sphinx.

There are well over fourteen hundred species of hawk moth in the world. Some of them specialize on extracting nectar from orchids and often incidentally (to the moth) pollinating them. Many hawk moths have long proboscides (‘tongues’), suitable for extracting nectar from the long nectar spurs of certain flowers. When there is a good fit between the length of the spur and the length of the proboscis, the moth can pick up pollen on its eyes or face and transfer it to another orchid flower. If the proboscis is too short, the moth can’t reach the nectar in a long spur and is not likely to visit that kind of orchid very often. If the proboscis is too long, the moth’ head or body will not contact the place in the flower where the pollen is produced, so although the moth can steal some nectar, pollination is unlikely (unless it happens that the pollen is contacted by the proboscis itself).

Certainly the most famous of these relationships (as I mentioned in an earlier essay) concerns the Madagascar star orchid, whose prodigiously long (eleven inches or so) nectar spur caused Darwin to predict the existence of a suitable moth with an equally long proboscis. Sure enough, someone else eventually found that predicted moth, in action. Now there are rumors that a second species of long-tongued hawk moth can also reach the nectar and do some pollination of this orchid.

A little closer to home, in the swamps of Florida and Cuba, the ghost orchid lives high up on tree branches. The nectar spur is said to be about five inches long (but variable) and it is now known that several species of hawk moth can pollinate this species. Unfortunately, the orchid is now quite endangered, in part because of over-collecting by too-avid horticulturalists.

Slightly closer to Alaska, in the tall-grass prairies, the fringed prairie orchids are pollinated by hawk moths. The western fringed prairie orchid has a nectar spur over two inches long, which is said to be longer than most other North American orchids. It is known to be pollinated by four species of native hawk moths (perhaps more) and by one non-native species (that was introduced to North America from Eurasia to help control an invasive weed). This species of orchid is designated as ‘threatened’, largely because of habitat loss as the prairies were plowed under for agriculture. But in addition, the moths are at risk from pesticides drifting over from the agricultural fields. Some published accounts say that no seed set is accomplished in the absence of moth pollination, but others say that a little self-pollination without the help of the moths is possible. In either case, reproduction is generally poor.

In Alaska, little seems to be known about the relationships of hawk moths to flower pollination. Of the seven species on record in the state museum, only some are represented by more than a few specimens (thanks to the helpful entomologist, Derek Sikes, for this info!). I’ll summarize a bit about three of them. Here in Juneau, we sometimes see the bedstraw hawk moth (Hyles gallii) as the adults visit fireweed and other flowers. Two local orchid-watchers have seen and photographed this moth visiting the white bog orchid, carrying pollen on its fairly long proboscis (about an inch long). Is this moth a regular pollinator of this orchid? Hawk moths elsewhere are known to pollinate related species of orchid, but it has been thought that this orchid is pollinated mostly by moths that sit on the flower while they sip nectar (instead of hovering, as a hawk moth does). In any case, hawk moths seem to be uncommon around here, so their role in pollination may be occasional at best.

Another hawk moth that we occasionally see here is called the hummingbird hawk moth (Hemaris thysbe). The wings are clear, without any colorful scales. This species is known to visit many kinds of flowers, including some orchids similar to the white bog orchid, but how many of these visits accomplish pollination is not known. The proboscis is of medium length (less than an inch), so deep nectar sources are not available to this moth.

One other fairly well-represented species in the museum collection is the one-eyed sphinx moth (Smerinthus cerisyi), which sports pretty blue eyespots on the hind wings. Although I don’t know if it has been recorded in Juneau, there are records from coastal British Columbia and from near the head of Lynn Canal, so it seems possible that we might see it here. It has an extremely short proboscis (only a few millimeters long), and one source states that it is not functional at all. In that case, the adults would not feed and there would no pollination.

Hawkmoth caterpillars are often called horn worms, for the horn-like projection that sticks up from the rear end. The three Alaska species that I’ve mentioned all have ‘horns’, although not all hawk moth caterpillars do. The caterpillars are herbivorous, commonly eating a variety of leaves. Bedstraw caterpillars eat fireweed, plantain, enchanters nightshade, and many other things, in addition to bedstraw. Hummingbird caterpillars eat snowberry, blueberry, cherry, thistle, clover, and more. One-eyed sphinx caterpillars forage chiefly on willow and poplar, but occasionally other species too.

Good finds in Gustavus

stealthy spiders, ambitious amphibians, strange ferns, and more

A summertime walk through woods and meadows is almost always good—birds are singing, flowers are blooming, and there’s always nice fresh air. But sometimes all the little pleasures form a base on which rest some observations of particular interest. Here are a few good ones from a recent trip to Gustavus.

–Dandelions had mostly gone to seed, so fields that had been golden with flowers were now white with plumes on mature seeds ready to disperse on the wind. But here and there we found a laggard flower, still yellow and conspicuous on the background of white. On one of these late bloomers there was a bumblebee, a strangely immobile bee. Looking more closely, we saw a yellow crab spider with the bee in its clutches. Crab spiders are venomous (to insects), immobilizing their prey and then sucking out the juices. Dinner was in progress and the bee would fly no more. Crab spiders are generally ambush-predators; some of them lurk on flowers in hopes that a tasty insect will alight. The color of the spider often matches the color of the flower on which it awaits a victim.

crab-spider-howard
photo by Kerry Howard

–The ponds at the gravel pits are a great place to see shorebirds, swallows, and kingfishers. There were sticklebacks swimming around and, in June, there were gravid females full of eggs. One pond held many thousands of toad tadpoles, swarming in the shallows where the water temperature was salubrious. They came in a variety of sizes—some at least six times bigger than the smallest. A female toad can lay thousands of eggs; the hordes of tadpoles that we saw undoubtedly had many mothers, which probably laid their eggs at somewhat different times, accounting for the size variation. None of them had begun to transform into toadlets; no little legs were visible. A dense pack of tadpoles clustered around a silvery object, each one trying to grab a mouthful. Looking closely, we discovered that the silvery object was a dead stickleback. Toad tadpoles commonly feed on algae and detritus, but they are also known to scavenge carrion and even the dead bodies of their comrades. Toad (and frog) populations have declined dramatically almost everywhere, and it was heartening to see this large aggregation of juveniles.

–Gustavus is noted for (among other things) its wide sandy beaches. On our way out to one of them, we heard some odd sounds, rather like the hooting of a small owl. As we listened carefully, however, it became apparent that several snipes were performing their aerial territorial display. It’s called ‘winnowing’, and it’s made by the rapid passage of air over the spread-out tail feathers, usually as the bird dives toward the ground from high in the air. Usually the male does this but sometimes females do too. I hadn’t heard this display for a long time and it was a gladsome sound.

–Out on the sandy beaches we found windrows of long, flexible tubes that were the former housing of certain marine worms. The worms were long gone, possibly starved at the end of winter when food is scarce. Then the tides presumably stripped the empty tubes from their attachment points and piled them up on the beach. This observation stimulated a lot of conjecture but no concrete answers.

–On the vegetated sand dunes there were lots of the strange little ferns called moonworts (a.k.a. grape ferns). They don’t look at all like ferns to the layman’s eyes, because the fronds are generally not very lacy or branched. We found many that appeared to be the common moonwort, but there were also a few much more robust individuals that were certainly a different species. On one of the postglacial-uplift meadows we found another kind, one that is now classified in a different genus; the fronds on this one (so-called rattlesnake fern) are somewhat more ‘fern-y’. All three of these are widespread species in North America and even beyond, but they are so odd that it is always fun to find them.

–The pilings of the public dock usually offer something even to a casual observer. Enormous white anemones, far larger than any we usually see in the rocky intertidal zone, wave their tentacles if the tide is in. Sea stars cling to the vertical surfaces too, but the largest ones have trouble hanging on when the tide goes out and leaves them above the waterline. Colorful sponges and tunicates add to the array. Sometimes there’s a giant whelk laying a coil of egg cases. Small fish sometimes gather under the docks and are visible between the pilings. And while one inspects the fauna on the dock, barn swallows are swooping overhead, gathering flies and mosquitoes for their chicks.

–We searched for lady’s slipper orchids (of which more, later). One clump of flowering stems still included a stalk with last year’s seed capsule, well dried. Someone opened the capsule to see if any seeds were left and found, instead, a tiny spider guarding her minute ball of orange eggs. We were sorry to have destroyed her safe-house!

–Sweetgrass grows in many Gustavian meadows and some of us stopped to braid some stems. Braided sweetgrass is used, especially by Native Americans, to construct baskets and decorative items, and we had to try just a simple braid. As we concentrated on our task, we heard thundering hoofbeats, getting rapidly closer. Turning around, we saw a fast-trotting female moose, followed by a young calf. They were so intent on getting away from whatever startled them that they ignored us and passed by, barely thirty feet away, and off they went, full tilt.