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Hunting For Aphids in the Himalayas
Peter Fritsch, Ph.D., Assistant Curator of Botany
I visited Nepal in 1994 to collaborate with researchers on developing molecular markers for use in economically important Nepalese crop plants. While I was there I was also able to spend six days in the eastern part of the country in search of an unusual tree, Styrax serrulatus , and the fascinating insects that parasitize it.
The genus Styrax, commonly called the snowbell family, is a group of approximately 120 species of flowering trees and shrubs. The tree Styrax serrulatus is particularly interesting for its intriguing association with gall-forming aphids. Aphids are small insects that suck saps and fluids out of the plants on which they live - gardeners know aphids for their habit of infesting rose bushes. Aphids and some other insects can modify their host plant through physical and chemical process to produce a gall.. This abnormal swelling of plant tissue is used by the aphids as a nursery for their young and looks vaguely similar to tumors found in animals. On Styrax serrulatus , aphids produce trumpet-shaped, four-inch long, bud galls on various parts of the plant. Aphids attack all species of Styrax in Asia, but even though Styrax species are found throughout the New World and in Europe, no aphids are found there. The aphids are odd because there appears to be a one-to-one correspondence between aphid species and their Styrax hosts in which a species of aphid shows a loyalty to one species of Styrax and always returns to that species of tree to breed and propagate. They also are one of the only groups of aphids that form a social structure similar to ants, with a colony founder (called a fundatrix) and many sterile soldiers. To understand the nature of speciation in the aphids and their hosts, and to determine what effect the soldier caste has on the speciation pattern, I wanted to collect both the plants and the aphids occurring on them for a molecular analysis.
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Nepal is one of the poorest countries in the world. Most of the people of eastern Nepal practice subsistence farming, mainly cultivating upland rice in terraced fields and grazing livestock. Cutting trees for fuel and slash-and-burn agriculture has pretty much devastated the low and mid-elevation forests, especially the upland mountainsides. Throughout the one-hour plane flight from Katmandu to Kadvari, it was clear that there is little forest left. Large stretches of orange clay soil impart a desert-like appearance to the mountainsides. |
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Because much of the forest in Nepal is gone, I had to travel to the eastern extreme of Nepal, the Arun River basin, to search for the plant in which I was interested. I also needed to travel in the monsoon season, when the Himalaya Mountains are obscured by clouds nearly all the time. The clouds prevented spectacular mountain views, but it did allow me to see some very interesting plants in flower when my glasses weren't fogging-over! It also meant the height of the leech season. |
The trip was exhausting. There are no roads in Eastern Nepal, so we walked for six days. The minor trails are not maintained during the monsoon season because all of the native people are in the fields. Traversing these unkempt trails is a daunting experience. They are often hidden by vegetation, slippery from a constant rain and years of wear on the rocks, narrow and steep. It is especially dangerous for those with tender feet - which means most Westerners. The Nepali walk with flip-flops for traction, but that's impossible for anyone without leather soles (of one's feet, that is). The main trails often used bridges to cross the many rapids occurring throughout the countryside. They are sturdy, but in order to cross safely, it's best not to look down!
It turns out that Styrax serrulatus is very rare in Nepal, only occurring in the lowland sub-tropical forests. It became apparent that to find the plant would be difficult enough, but to find one with living aphids would be quite an occasion. We got lucky on the fourth day. We discovered six plants on an unplanned shortcut - they were the only plants we found during the entire six-day trek. Amazingly, one of the plants we observed possessed the aphid galls for which we searched, and one gall had living aphids! Further study indicates that the aphids collected are in all likelihood a new species. This is not that surprising, since the aphids have only been well-studied in Southeast Asia, and the galls are known to occur throughout east Asia.
Styrax serrulatus was subsequently analyzed for DNA sequence variation with respect to the other members of the genus that have been looked at thus far. Although the data is still preliminary, it appears that this species' closest relatives also occur in Asia and this whole group is most closely related to the two species of Styrax native to the southeastern United States, an example of intercontinental disjunction. The disjunctions in the genus are thought to have arisen through the range contraction of once widespread species beginning as early as the Tertiary period and extending to the Pleistocene. The information gathered on my trip to Nepal will help scientists to understand the composition, relationships, and biogeographic history of forests in the northern hemisphere.
California Academy of Sciences Botany Department
The Families of Flowering Plants: Styracaceae
Royal Bardia National Park, Nepal
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