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ACIDIC DEPOSITION, DROUGHT, AND INSECT HERBIVORY IN AN ARID
ENVIRONMENT: ENCELIA FARINOSA AND TRIRHABDA GEMINATA IN
SOUTHERN CALIFORNIA
T.D. PAINE, R.A. REDAK, and J.T. TRUMBLE
Department of Entomology, University of California, Riverside, California 92521, USA
INTRODUCTION
Except at the higher elevations, much of the arid southwestern United States and northern Mexico is not forested.
The plant communities are dominated by widely spaced shrubs that are 1-3 m in height. Because these areas receive less than
20 cm of annual precipitation and mean maximum summer temperatures of 38-43 °C, plant growth is often limited by the
availability of moisture. Many of the species that are components of the Creosote Bush Scrub community are droughtdeciduous
and remain dormant during the dry periods (Munz and Keck 1968). Even in the slightly more moderate Coastal
Sage Scrub community that receives up to 40 cm of annual precipitation and less extreme mean maximum summer temperatures,
the plant species are adapted to extended periods without precipitation. Growth and flowering are often limited to
those periods with available moisture.
However, as the population of urban areas adjacent to these biotic communities grows, plants in those communities
are subject to additional stresses arising from human activities. One of the most important anthropogenic stresses on plants is
air pollution (Cowling 1982, Lee 1982, Linthurst et al. 1982, Treshow 1984, Shriner 1986). Although there are many
different atmospheric pollutants capable of affecting plants, including ozone, NO x, SO, and PAN (peroxyacetyl nitrate), the
impact on plants may be enhanced when the pollutants are combined with moisture. Wet acidic deposition in the form of
acidic fog is of particular concern in southern California. Acidic fog episodes in southern California last from 2 to 12 hours
and often have pH's of 2.0-3.0 (Waldman et al. 1982). Physical changes in leaf surface structure, lesions, leaching of foliar
nutrients, decreased photosynthesis, and reduced growth and yield may result from long-term exposure to acidic fogs
(Granett and Taylor 1981, Granett and Musselman 1984, Musselman and Sterrett 1988, Musselman and McCool 1989,
Paoletti et al. 1989, Takemoto et al. 1989, McCool and Musselman 1990, Mengel et al. 1990, Trumble and Walker 1991).
Pollution-stressed plants also may be more susceptible to insects and diseases (Endress and Post 1985, Trumble et al. 1987,
Trumble and Hare 1989, Jones and Coleman 1988, Paine et al. 1993). Consequently, if acidic deposition in the form of acidic
fog occurs when the plants are actively growing and when the herbivores also are active, the pollution stress may have a
significant impact on the fitness of the plant.
We have studied the influence of acidic deposition on the interactions between brittle brush, Enceliafarinosa Gray
(Asteraceae), a dominant shrub in the Creosote Bush Scrub and Coastal Sage Scrub communities of southern California, and
its primary herbivore, Trirhabda geminata Horn (Coleoptera: Chrysomelidae), a leaf beetle that feeds in both larval and adult
stages on E. farinosa. The system is important because it is both widespread and an indicator system for damage that may be
occuring in these fragile communities occupying harsh environments. The impact of pollution stresses on the interactions
between plant and herbivore has been much less studied in desert than in forest ecosystems (e.g., Johnson and Siccama 1983,
Jones and Coleman 1988, Mengel et al. 1990); however, there may be much less environmental buffering in these desert
communities for additional stress imposed by anthropogenic encroachment.
Acidic Deposition as a Single Stress
Our previous studies have demonstrated significant effects of acidic deposition on the nutritional quality and palatability
of E. farinosa to Z geminata in laboratory tests. Plants fogged three times for 3 hours each time at pH 2.75 were
compared to plants fogged with a control fog at pH 5.80. Acidic-fogged foliage had significantly higher concentrations of
Mattson, W.J., Niemel_i, E, and Rousi, M., eds. 1996. Dynamics of forest herbivory: quest for pattern and principle. USDA
For. Serv. Gen. Tech. Rep. NC-183, N.C. For. Exp. Sta., St. Paul, MN 55108.
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