As you’ve strolled a riverbank in Yosemite, have you ever glanced up through the bright green, sun-drenched leaves of a White Alder? If you have, did you notice pale bumps or little galls on the leaves?

An exciting and beautiful microecosystem exists within the swollen gall tissue of the leaves of this alder (Alnus rhombifolia). A hand lens (10x) will reveal some of the minute inhabitants which are governed by the same biological principles that govern larger populations and communitites.

Spurred by inquiries of Park visitors, I began an investigation which eventually included the examination — with dissecting and compound microscopes — of 300 galls. The cycle of “who-ate-whom” emerged as four main food chains with alder leaves as primary producers, four minute plant feeders as primary consumers and one parasite and one predator as secondary consumers. The following diagram and food web delineate these interactions.

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Figure 2

The shade-tolerant White Alder is abundant along watercourses in Yosemite, especially between 4-5,000 feet (1,200-1,500 m.). Though a plant can fix only a small percentage of the energy of the sun, the producer, in this case, supplies more than enough energy for its arachnid and insect herbivores. Forty-three percent of the alders examined had been attacked by gall mites to some extent; however, even heavily infested leaves remained attached and partially photosynthetic, with only minor leaf curling.

Mite galls, cataplasmas composed of enlarged parenchyma cells, are 1-2 mm. across and irregularly globose, the heaviest concentration of galls occurring generally in the basal half of the leave. During primary stages of growth, the gall tissue is green. As feeding progresses and callous-like growth continues, the gall fades to yellowish-green, takes on a reddish cast from the plant pigment, anthocyanin, and finally turns black or brown, an indication of fungal activity.

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Cross-section

The Alder Gall Mite (Phytoptus laevis), an eriophyid mite (Koehler, 1976), causes the galls by its feeding. The mites are wormlike with two pairs of legs on the anterior portion of the body. They winter within terminal buds, where mating presumably occurs (Baker and Wharton, 1952). Eggs are laid on each leaf surface, and the population, by number of galls observed, seems to reach its peak in July. In older galls most eriophyid mites have been displaced by competing tarsonemid mites, possibly an undescribed species that will belong to the genus Eriotarsonemus (Koehler, 1976). These mites have a more classical appearance, with eight legs, a shorter, heavier body than the wormlike gall mites; however, the fourth pair of legs in the male is turned back and up and is used for clasping the female during copulation. As the tarsonemid mites prefer a humid, sheltered environment, the already-formed gall provides a favorable habitat for the invading mite. Outside the galls on the leaf surface, other primary consumers were spider mites of the family Tetranychidae, found free-roaming and also quiescent, beneath a web-like material, Wooly Alder Aphids (Prociphilus tessellatus), and unidentified green aphids, found mainly on the underside of the leaves.

Feeding on these primary consumers were some secondary consumers, including larvae of the parasitic chalcid wasp, family Eulophidae, enclosed within the galls, and predaceous mites, as yet unidentified, found within erupted galls and free-roaming. These organisms play vital roles in population regulation of herbivorous gall mites. Approximately twenty percent of the galls examined contained larval, pupal, or adult wasps, while thirty percent of those leaves examined held predaceous mites. Upon opening one gall, I watched an adult wasp emerge, dry its iridescent, translucent wings and walk from gall to gall in search of a feeding tube formed when eggs were oviposited earlier. It was fascinating to watch the tiny wasp clean itself, feed, whir its wings to warm up, then take off by catapulting itself into the air.

Gall-makers are very specific in selection of food plants. Galls are restricted almost exclusively to seed plants, except a few dipteran galls on fungi, mite galls on lichens, and nematode galls on algae and bryophtes (Brues, 1946). There is little similarity in plant families susceptible to gall-makers in Europe and North America. Since flora and insect fauna are quite similar on both continents, more uniformity would have suggested an earlier evolutionary origin of the gall-forming niche. In contrast, the “disparity leads one to believe that the various gall insect faunas must be of comparatively recent origin, and that they are undergoing speciation at an unusually rapid rate” (Brues, 1946). This statement seems true for my investigations. At least one mite may be a new species, and several aberrant individuals did not seem to fit into existing classifications. Though the forces of natural selection may have been blunted for contemporary, technological man, they are still acting on less shielded populations, shaping the evolution of minute creatures most fit to survive in gall tissue.

Though much of our attention in the last years has been directed outward toward the universe, there is much knowledge yet to be discovered in exploration of the worlds within the microcosm of an alder leaf.

Baker, Edward W. and G.W. Wharton, 1952, An Introduction to Acarology. The Macmillan Co.

Brues, Charles T., 1946, Insect Dietary, An Account of the Food Habits of Insects. Harvard University Press.

Koehler, C.S., Extension Entomologist-Plant Pathologist, August, 1976, Personal Communication.

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