Sublethal chemical stimulation of arthropod parasitoids and parasites of agricultural and environmental importance.

An increasing number of studies have reported stimulation of various organisms in the presence of environmental contaminants. This has created a need to critically evaluate sublethal stimulation and hormetic responses of arthropod parasitoids and parasites following exposure to pesticides and other contaminants. Examining this phenomenon with a focus on arthropods of agricultural and environmental importance serves as the framework for this literature review. This review shows that several pesticides, with diverse chemical structures and different modes of action, applied individually or in combination at sublethal doses, commonly stimulate an array of arthropod parasitoids and parasites. Exposure at sublethal doses can enhance responses related to physiology (e.g., respiration, total lipid content, and total protein content), behavior (e.g., locomotor activity, antennal drumming frequency, host location, and parasitization), and fitness (longevity, growth, fecundity, population net and gross reproduction). Concordantly, the parasitic potential (e.g., infestation efficacy, parasitization rate, and parasitoid/parasite emergence) can be increased, and as a result host activities inhibited. There is some evidence illustrating hormetic dose-responses, but the relevant literature commonly included a limited number and range of doses, precluding a robust differentiation between sub- and superNOAEL (no-observed-adverse-effect level) stimulation. These results reveal a potentially significant threat to ecological health, through stimulation of harmful parasitic organisms by environmental contaminants, and highlight the need to include sublethal stimulation and hormetic responses in relevant ecological pesticide risk assessments. Curiously, considering a more utilitarian view, hormesis may also assist in optimizing mass rearing of biological control agents for field use, a possibility that also remains neglected.

Elevated ozone alters long-chain fatty acids in leaves of Japanese white birch saplings.

Long-chain fatty acids (LCFAs) in leaves have attracted attention as nutritious phytochemicals and olfactory signals that influence the behavior and growth of herbivorous insects. In recognition of the negative effects of increasing tropospheric ozone (O3) levels on plants, LCFAs can be altered through peroxidation by O3. However, how elevated O3 changes the amount and composition of LCFAs in field-grown plants is still unknown. We investigated palmitic, stearic, oleic, linoleic, linolenic LCFAs in the two leaf types (spring and summer) and two stages (early and late stage after expansion) of Japanese white birch (Betula platyphylla var. japonica) after a multi-year O3 exposure on the field. Summer leaves exhibited a distinct composition of LCFAs under elevated O3 at the early stage, whereas both stages of spring leaves did not exhibit significant changes in LCFAs composition by elevated O3. In the spring leaves, the amounts of saturated LCFAs significantly increased at the early stage, however, the amount of total, palmitic, and linoleic acids at the late stage were significantly decreased by elevated O3. Summer leaves had a lower amount of all LCFAs at both leaf stages. Regarding the early stage of summer leaves, the lower amount of LCFAs under elevated O3 was possibly due to O3-suppressed photosynthesis in the current spring leaves. Furthermore, the decrease ratio of spring leaves over time was significantly increased by elevated O3 in all LCFAs, whereas summer leaves did not exhibit such an effect. These findings suggest that further studies should be conducted to reveal the biological functions of LCFAs under elevated O3, considering the leaf type- and stage-dependent changes of LCFAs.

Plant-insect communication in urban forests: Similarities of plant volatile compositions among tree species (host vs. non-host trees) for alder leaf beetle Agelastica coerulea.

Behavior of insects, such as pollination and grazing, is usually determined by biogenic volatile organic compounds (BVOCs). However, particularly in O3-polluted urban forests, the BVOCs-based plant-insect communication can be disrupted by the reaction of O3 with leaf-emitted BVOCs, such as between Japanese white birch (Betula platyphylla var. japonica) and a leaf beetle (Agelastica coerulea). To understand plant-insect communication in O3-polluted environments, it is necessary to identify chemical species of BVOCs that contribute to attractiveness toward insects but are diminished by elevated O3. In this study, we conducted olfactory response tests and gas chromatography mass spectrometry (GC-MS) analyses to clarify whether there is a similarity of BVOC components among Betulaceae host trees that can explain the attraction of the stenophagous insect A. coerulea. The olfactory response tests indicated that Betulaceae host trees attract A. coerulea via leaf-emitted BVOCs, while there was no preference of the leaf beetles to non-host trees (Sorbus commixta and Morus bombycis). However, GC-MS analyses indicated that the composition of BVOC blends considerably differed among Betulaceae host trees, although alders (Alnus hirsuta and A. japonica) had a similar composition of BVOC blend in each season (June and September) during which the adult leaf beetle is active. A distinct characteristic of the emission from B. platyphylla was that 2-carene and limonene, which are O3-reactive species, were emitted with a high monoterpene ratio irrespective of the season. Thus, these volatiles and the blend could be expected to lead the disrupted communication found between B. platyphylla and A. coerulea under elevated O3 in previous field studies. In addition, our results indicated that A. coerulea is attracted to more than one blend within Betulaceae host trees, suggesting that grazing damages can be affected by different host preferences and O3 reactivity with specific BVOCs in the field. BVOCs-based plant-insect interactions should be further studied in multi-species communities to better understand plant-insect communication in O3-polluted environments.

Ozone disrupts the communication between plants and insects in urban and suburban areas: an updated insight on plant volatiles.

Plant-insect interactions are basic components of biodiversity conservation. To attain the international Sustainable Development Goals (SDGs), the interactions in urban and in suburban systems should be better understood to maintain the health of green infrastructure. The role of ground-level ozone (O3) as an environmental stress disrupting interaction webs is presented. Ozone mixing ratios in suburbs are usually higher than in the center of cities and may reduce photosynthetic productivity at a relatively higher degree. Consequently, carbon-based defense capacities of plants may be suppressed by elevated O3 more in the suburbs. However, contrary to this expectation, grazing damages by leaf beetles have been severe in some urban centers in comparison with the suburbs. To explain differences in grazing damages between urban areas and suburbs, the disruption of atmospheric communication signals by elevated O3 via changes in plant-regulated biogenic volatile organic compounds and long-chain fatty acids are considered. The ecological roles of plant volatiles and the effects of O3 from both a chemical and a biological perspective are presented. Ozone-disrupted plant volatiles should be considered to explain herbivory phenomena in urban and suburban systems. Supplementary information: The online version of this article contains supplementary material available at (10.1007/s11676-020-01287-4) to authorized users.

Leaf defense capacity of Japanese elm (Ulmus davidiana var. japonica) seedlings subjected to a nitrogen loading and insect herbivore dynamics in a free air ozone-enriched environment.

Japanese elm (Ulmus davidiana var. japonica) is a native species in cool-temperate forests in Japan. We investigated growth, physiological reactions, and leaf defense capacity of Japanese elm seedlings under nitrogen (N) loading (45.3 kg N ha-1 year-1) and seasonal insect dynamics in a free-air ozone (O3)-enriched environment (about 54.5 nmol O3 mol-1) over a growing season. Higher leaf N content and lower condensed tannin content in the presence of N loading and lower condensed tannin content in elevated O3 were observed, suggesting that both N loading and elevated O3 decreased the leaf defense capacity and that N loading further enhanced the leaf quality as food resource of insect herbivores. Two major herbivores were observed on the plants, elm leaf beetle (Pyrrhalta maculicollis) and elm sawfly (Arge captiva). The peak number of observed insects was decreased by N loading. Visible foliar injury caused by N loading might directly induce the reduction of number of the observed elm sawfly individuals. While elevated O3 slightly suppressed the chemical defense capacity, significantly lower number of elm leaf beetle was observed in elevated O3. We conclude that N loading and elevated O3 can alter not only the leaf defense capacity of Japanese elm seedlings but also the dynamics of elm leaf beetle and sawfly herbivores.