In this review, we test the hypothesis that abiotic stress increases the suitability of plants as food for herbivores. We conducted a meta-analysis that included 70 experimental studies in which insect performance was measured on woody plants subjected to water stress, pollution, and/or shading. Overall, plant stress had no significant effect on insect growth rate, fecundity, survival, or colonization density. We found great variation, however, in the magnitude and direction of insect responses among studies, most of which was related to insect feeding guild. In general, boring and sucking insects performed better on stressed plants, whereas plant stress adversely affected gall-makers and chewing insects. Reduction in performance of chewers was greater on stressed slow-growing plants than on stressed fast growers. Reproductive potential of sucking insects was increased by pollution but reduced by water stress. In some cases where sample sizes were small or the treatment periods short, apparent differences in insect responses to stress were probably artifacts due to inappropriate experimental design.

Insect performance on experimentally stressed woody plants: a meta-analysis.

We studied to what extent changes in the breeding performance of two insectivorous passerines, the Pied Flycatcher (Ficedula hypoleuca) and the Great Tit (Parus major) were associated with pollution-induced variation in the biomass of invertebrate prey (spiders and insects on the ground, lepidoptera and sawfly larvae in tree canopies) in an air pollution (copper smelter) gradient. At the nestling time of both species, larvae were scarce in Scotch pine (the dominant tree species) close to the factory complex, peaked in the moderately polluted zone 2–4 km from the factory, and tended to decrease farther away. Both bird species preferred pine, particularly in the moderately polluted zone, where the proportion of larvae in the diet of P. major was high. Ground-living arthropods were scarce near the factory, but among-site variation was high even in the cleanest area. Breeding success of both bird species correlated positively with prey biomass, but only in P. major was food abundance correlated with nestling growth. Also, the productivity of different-sized clutches was affected in P. major, but not in F. hypoleuca. Large F. hypoleuca clutches produced more fledglings than did smaller clutches in all parts of the pollution gradient. For P. major, this was true only in the moderately or slightly polluted parts of the gradient; in the most polluted areas, clutches of 6–11 eggs invariably produced 3–4 fledglings. The stronger impact of food on P. major than on F. hypoleuca probably resulted from a different diet. F. hypoleuca, which forages much from the ground layer, was susceptible to direct effects of pollutants at the egg stage, whereas P. major, a caterpillar specialist, suffered from the shortage of larvae in the late nestling period. Our results indicate that the reduced breeding performance of birds in the polluted area may be due to various reasons; one species may respond directly to toxicity, and the other species to reduced food supply.

Pollution‐related variation in food supply and breeding success in two hole‐nesting passerines

Plants produce a variety of volatile organic compounds (VOCs). Under abiotic and biotic stresses, the number and amount of produced compounds can increase. Due to their long life span and large size, trees can produce biogenic VOCs (BVOCs) in much higher amounts than many other plants. It has been suggested that at cellular and tree physiological levels, induced production of VOCs is aimed at improving plant resistance to damage by reactive oxygen species generated by multiple abiotic stresses. In the few reported cases when biosynthesis of plant volatiles is inhibited or enhanced, the observed response to stress can be attributed to plant volatiles. Reported increase, e.g., in photosynthesis has mostly ranged between 5 and 50%. A comprehensive model to explain similar induction of VOCs under multiple biotic stresses is not yet available. As a result of pathogen or herbivore attack on forest trees, the induced production of VOCs is localized to the damage site but systemic induction of emissions has also been detected. These volatiles can affect fungal pathogens and the arrival rate of herbivorous insects on damaged trees, but also act as signalling compounds to maintain the trophic cascades that may improve tree fitness by improved efficiency of herbivore natural enemies. On the forest scale, biotic induction of VOC synthesis and release leads to an amplified flow of BVOCs in atmospheric reactions, which in atmospheres rich in oxides of nitrogen (NOx) results in ozone formation, and in low NOx atmospheres results in oxidation of VOCs, removal in ozone from the troposphere and the resulting formation of biogenic secondary organic aerosol (SOA) particles. I will summarize recent advances in the understanding of stress-induced VOC emissions from trees, with special focus on Populus spp. Particular importance is given to the ecological and atmospheric feedback systems based on BVOCs and biogenic SOA formation.

Can forest trees compensate for stress-generated growth losses by induced production of volatile compounds?

The role of indirect interactions in structuring communities is becoming increasingly recognised. Plant fungi can bring about changes in plant chemistry which may affect insect herbivores that share the same plant, and hence the two may interact indirectly. This study investigated the indirect effects of a fungal pathogen (Marssonina betulae) of silver birch (Betula pendula) on an aphid (Euceraphis betulae), and the processes underpinning the interaction. There was a strong positive association between natural populations of the aphid and leaves bearing high fungal infection. In choice tests, significantly more aphids settled on leaves inoculated with the fungus than on asymptomatic leaves. Individual aphids reared on inoculated leaves were heavier, possessed longer hind tibiae and displayed enhanced embryo development compared with aphids reared on asymptomatic leaves; population growth rate was also positively correlated with fungal infection when groups of aphids were reared on inoculated branches. Changes in leaf chemistry were associated with fungal infection with inoculated leaves containing higher concentrations of free-amino acids. This may reflect a plant-initiated response to fungal attack in which free amino acids from the degradation of mesophyll cells are translocated out of infected leaves via the phloem. These changes in plant chemistry are similar to those occurring during leaf senescence, and are proposed as the mechanistic basis for the positive interaction between the fungus and aphid.

Microbial impacts on plant-herbivore interactions: the indirect effects of a birch pathogen on a birch aphid.

http://faculty.ucr.edu/~john/2008/ButlerTrumble2008.pdf

Effects of pollutants on bottom-up and top-down processes in insect–plant interactions

The effect of artificial acid rain on the reproduction and survival of the aphid Euceraphis betulae on silver birch was studied in Turku, southern Finland. Eight bioassays were done during 1984-1986. In four of the bioassays the aphids produced 40% to over 100% more progeny on birches watered with dilute sulphuric acid (pH 3.5) than on control trees. In four other cases the performance of aphids did not differ between the treatments. An index of aphid reproduction pooled over the whole study was significantly higher on acid-treated than on control birches. The reproduction of aphids on acid-treated birches was enhanced when precipitation was below long term average, suggesting an interaction between the stress caused by acid treatment and dry periods.

M. Lindgren

Papers

The effect of simulated acid rain on performance of the aphid Euceraphis betulae (Koch) on silver birch