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total nitrogen (2.33%) and higher soluble protein content (4.96 mg/g leaf tissue) when compared to leaves from controlfogged
plants (1.94% total nitrogen and 3.94 mg/g leaf tissue soluble protein) (Paine et al. 1993). There were no differences
between treatments in water content of the leaf tissues (74.8% for acidic-fogged plants and 75% for control-fogged plants).
Both larvae and adult beetles preferred to feed on leaf tissue from acid-fogged plants (Paine et al. 1993). The studies
demonstrated that the changes in Encelia foliage that alter insect consumption and growth indices can occur within 7 days of
treatment with acid fogs.
Acidic deposition has an indirect effect through E. farinosa upon the growth rate and biomass of T. geminata. Acidic
fogging (pH 2.75 vs. a control fog of pH 5.8) orE. farinosa resulted in a 34% increase in average larval biomass gain
(Control = 2.23 mg vs. Acid= 2.98 mg) and a 3 I% increase in larval growth rate (Control= 0.16 mg'mgl'd _vs.
Acid=0.21 mg, mg_-d_). The effect of fogging on growth rate and biomass gain was strongest for insects initiating feeding 14
days after treatment applications. The effect of acidic fogging and the duration of the effect (days post treatment) were
independent of each other (no interaction); the effect of acidic fogging upon growth rate and biomass gain was statistically
constant through time. For adult beetles, tissue consumption of acidic-fogged foliage was 65% greater than tissue consumption
of control foliage (Control = 0.0964 cm2 vs. Acid = 0.16 cm2). For larval beetles, tissue consumption of acidic-fogged
foliage was over 400% greater than control foliage (Control = 0.057 cm2 vs. Acid = 0.259 cm 2) (Redak et al. In Prep.).
Simultaneous Impact of Acidic Deposition and Drought as Multiple Stresses
Drought stress has been shown to mitigate the impact of gaseous air pollutants and, to a certain extent, acidic wet
deposition (acid rain and fog) on plant physiological and growth processes (Tingey and Hogsett 1985, Meier et al. 1990,
Bender et at. 1991, Showman 199l, Beyers et al. 1992, Temple et al. 1992). Our field data investigating the impact of acidic
fog upon the E. farinosa-Z geminata system are consistent with the theory that drought stress may ameliorate the impact of
acidic fog upon the susceptibility of plants to insect herbivory (Warrington and Whittaker 1990, Von Sury and Fluckiger
1991). Drought stress may make the plant less susceptible to the effects of the air pollution, or alternatively, the two stresses
may act independently but antagonistically on the plant so that the net effect on herbivore success is neutral. Using an
undeveloped area (remnant coastal sage-scrub habitat) on the UCR campus, we applied acidic (pH = 2.0) or control (pH =
5.8) fog treatments (3, 3-hour exposures, every other day for 5 days) to 40 mature E. farinosa plants (20 plants per treatment
application). Following tog exposure, we caged 50 second instar T.geminata larvae in nylon screen bags on each treatment
plant. Larvae were allowed to feed and grow on the plants for 16 days at which time they were removed and their fresh and
dry masses were determined. Additionally, 7 days after fog treatments, we conducted a 72-hour laboratory bioassay determining
the consumption and growth rates of third instar T. geminata fed foliage taken from the field treated plants. After
removal of experimental insects, foliage was collected from each plant and analyzed for total N and percent water.
Acidic fogging showed no effect on larval growth or consumption values or any plant foliage quality values (Table
1). For the field experiment described here, we feel the drought status of the experimental plants may account for the lack of
treatment effects (acidic fog effects) upon insect performance. At the time of the experiment, southern California was in the
sixth to seventh year of a sustained drought. For the year of this study, 1990, the UCR area had received less than 15%
(approx. 40-50 ram) of normal precipitation by the end of the growing season (CIMIS weather data). Indeed approximately 1
week after treatment applications, several experimental plants (of both treatments) began to show characteristic signs of
drought-induced leaf senescence (leaf curling, browning, dropping older more mature leaves), and by the second week
following removal of experimental insects, all of the plants were undergoing severe leaf-loss. Enceliafarinosa normally is a
drought deciduous plant (Ehleringer and Clark 1988); however, in this case, the period of leaf-drop began approximately 6
weeks to 2 months earlier than normal. Given the sustained 6 years of drought and the very early period of leaf abscission,
we are confident that all experimental plants were experiencing relatively severe drought stress.
The response of E. farinosa to drought stress includes (among other processes) an increase in leaf pubescence
(Ehleringer and Bjorkman 1978, Ehleringer 1982). We suspect that (1) the experimental plants were so severely drought
stressed that additional stress, in the form of acidic fog deposition, may have no further influence on the plants' suitability as
a host for 7: geminata and (2) the morphological changes that occur in E. farinosa with drought stress are such that the plant
may be physically protected to some extent from the impact of acidic fog (i.e., drought is functioning as a "filter" to the
impact of acidic fog). The dense covering of pubescence (as a result of drought stress) could have physically shielded and/or
buffered the plant from acid tbg deposition (Caporn and Hutchinson 1987, Musselman 1988).
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