University of California

Ecology of gall-forming Lepidoptera on Tetradymia: III. Within-plant horizontal and vertical distribution


Hollister Hartman

Author Affiliations

Hollister Hartman was formerly research assistant with the Population Biology Program, Department of Entomology, University of California, Riverside, is with TRW, Ballistic Missiles Division, P.O. Box 1310, San Bernardino, CA 92402.

Publication Information

Hilgardia 52(3):28-39. DOI:10.3733/hilg.v52n03p012. February 1984.

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Gall insects differ from most other herbivorous insects in that they must remain in one place, chosen soon after eclosion, until adulthood. In a sense, the outcome of the ecological “game” is completely determined by the player’s first “move.” The organism’s phyletic heritage, like rules written eons in the past, sets the constraints for a life style at once specialist, cryptic, and sessile. Zweigelt (1930 in(Mani, 1964) argued that gall formation is advantageous for a plant in that it localizes the parasite in time and space, and forces it to extreme specialization.

Explanations for observed distributions of Gnorimoschema tetradymiella Busck 1903 and Scrobipalpa n. sp. gelechiid moths among Tetradymia stenolepis Greene 1885 (Asteraceae) host plants were sought in (Hartman (1983b)). The horizontal distribution of Scrobipalpa n. sp. leaf galls and G. tetradymiella stem galls within individual T. stenolepis was similarly studied to elicit patterns among plants, elevations, and years. A subsidiary objective was to identify the environmental processes governing these patterns in the New York Mountains, California (for description of study area see (Hartman, 1983b).

Vertical distribution was studied to determine the effect of leaf and stem gall densities on gall height. Correlative objectives included finding how T. stenolepis foliage distribution and quality were related to each other, and to gall height.

Horizontal distribution: Gall location among quadrants

Agricultural entomologists interested in obtaining accurate infestation estimates often note that insect density varies within a tree according to the cardinal points. Directional insolation, wind, and water flow create differential growth rates and vegetation quality among plant sectors (Friedrick, 1951 in(Dean, 1959); (Ebeling, 1950). Pattern-skewing (also called sector or quad bias) of galls on Tetradymia in the New York Mountains was explained as either: 1) Gall insect behavior biasing one or more gall crops toward a particular quad, or 2) gall attrition from animal browse, weather, and shedding of plant parts being least in this quad.

Wind patterns were analyzed to test the former alternative. Moth orientation to host-plant olfactory cues or mating pheromones requires stable, low-velocity breezes to maintain integrity of the aerial trail and/or continuous emission by a pheromone source (Farkas and Shorey, 1972); (Kaae and Shorey, 1972); (Sower, Kaae, and Shorey, 1973). Gall formation was investigated in terms of new galls being either distributed nearly evenly over the compass points, or concentrated in one or two sectors. Under concentration, the same sector may be chosen every year due to extrinsic environmental influences (termed “bias-clumping,” in the sense of a consistent skewing) or the insect may be innately aggregative, but with sectors varying between plants and between years (termed “random-clumping”).

Old galls, subject the longest to animal browse, weather, and shedding of plant parts were examined for attrition effects. Three patterns were identified: 1) Loss from a random sector, independent of gall density; 2) loss which was greatest where gall density was highest; and 3) loss which was dispersed evenly over all plant sectors. Animal browse produces

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Hartman H. 1984. Ecology of gall-forming Lepidoptera on Tetradymia: III. Within-plant horizontal and vertical distribution. Hilgardia 52(3):28-39. DOI:10.3733/hilg.v52n03p012
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