This review examines the direct effects of climate change on insect herbivores. Temperature is identified as the dominant abiotic factor directly affecting herbivorous insects. There is little evidence of any direct effects of CO2 or UVB. Direct impacts of precipitation have been largely neglected in current research on climate change. Temperature directly affects development, survival, range and abundance. Species with a large geographical range will tend to be less affected. The main effect of temperature in temperate regions is to influence winter survival; at more northerly latitudes, higher temperatures extend the summer season, increasing the available thermal budget for growth and reproduction. Photoperiod is the dominant cue for the seasonal synchrony of temperate insects, but their thermal requirements may differ at different times of year. Interactions between photoperiod and temperature determine phenology; the two factors do not necessarily operate in tandem. Insect herbivores show a number of distinct life‐history strategies to exploit plants with different growth forms and strategies, which will be differentially affected by climate warming. There are still many challenges facing biologists in predicting and monitoring the impacts of climate change. Future research needs to consider insect herbivore phenotypic and genotypic flexibility, their responses to global change parameters operating in concert, and awareness that some patterns may only become apparent in the longer term.

Host plant quality is a key determinant of the fecundity of herbivorous insects. Components of host plant quality (such as carbon, nitrogen, and defensive metabolites) directly affect potential and achieved herbivore fecundity. The responses of insect herbivores to changes in host plant quality vary within and between feeding guilds. Host plant quality also affects insect reproductive strategies: Egg size and quality, the allocation of resources to eggs, and the choice of oviposition sites may all be influenced by plant quality, as may egg or embryo resorption on poor-quality hosts. Many insect herbivores change the quality of their host plants, affecting both inter- and intraspecific interactions. Higher-trophic level interactions, such as the performance of predators and parasitoids, may also be affected by host plant quality. We conclude that host plant quality affects the fecundity of herbivorous insects at both the individual and the population scale.

Summary1. The impacts of elevated atmospheric CO 2 and/or O 3 have been examined over 4 years using an open-air exposure system in an aggrading northern temperate forest containing two different functional groups (the indeterminate, pioneer, O 3 -sensitive species Trembling Aspen, Populus tremuloides and Paper Birch, Betula papyrifera , and the determinate, late successional, O 3 -tolerant species Sugar Maple, Acer saccharum ). 2. The responses to these interacting greenhouse gases have been remarkably consistent in pure Aspen stands and in mixed Aspen/Birch and Aspen/Maple stands, from leaf to ecosystem level, for O 3 -tolerant as well as O 3 -sensitive genotypes and across various trophic levels. These two gases act in opposing ways, and even at low concentrations (1·5 × ambient, with ambient averaging 34 -36 nL L − 1 during the summer daylight hours), O 3 offsets or moderates the responses induced by elevated CO 2 . 3. After 3 years of exposure to 560 µ mol mol − 1 CO 2 , the above-ground volume of Aspen stands was 40% above those grown at ambient CO 2 , and there was no indication of a diminishing growth trend. In contrast, O 3 at 1·5 × ambient completely offset the growth enhancement by CO 2 , both for O 3 -sensitive and O 3 -tolerant clones. Implications of this finding for carbon sequestration, plantations to reduce excess CO 2 , and global models of forest productivity and climate change are presented.

Human activity causes increasing background concentrations of the greenhouse gases CO2 and O3. Increased levels of CO2 can be found in all terrestrial ecosystems. Damaging O3 concentrations currently occur over 29% of the world's temperate and subpolar forests but are predicted to affect fully 60% by 2100 (ref. 3). Although individual effects of CO2 and O3 on vegetation have been widely investigated, very little is known about their interaction, and long-term studies on mature trees and higher trophic levels are extremely rare. Here we present evidence from the most widely distributed North American tree species, Populus tremuloides, showing that CO2 and O3, singly and in combination, affected productivity, physical and chemical leaf defences and, because of changes in plant quality, insect and disease populations. Our data show that feedbacks to plant growth from changes induced by CO2 and O3 in plant quality and pest performance are likely. Assessments of global change effects on forest ecosystems must therefore consider the interacting effects of CO2 and O3 on plant performance, as well as the implications of increased pest activity.

Abstract.  1. Although pollen is a vital nutritional resource for honey bees, Apis mellifera, the influence of pollen quality on their foraging behaviour is little understood. 2. In choice‐test experiments, bees showed no innate pollen‐foraging preferences, but preferred oilseed rape Brassica napus pollen over field bean Vicia faba pollen after previous foraging experience of oilseed rape. 3. The free amino acid content of oilseed rape and field bean pollen was compared using high‐performance liquid chromatography. Oilseed rape pollen contained a greater proportion of the most essential amino acids required by honey bees (valine, leucine, and isoleucine) than field bean, suggesting that oilseed rape pollen is of greater nutritional quality for honey bees than is field bean pollen. 4. Honey bee foraging preferences appeared to reflect pollen quality. The hypothesis that pollen amino acid composition affects the foraging behaviour of honey bees is discussed.

In future elevated CO2 environments, chewing insects are likely to perform less well than at present because of the effects of increased carbon fixation on their host plants. When the aphid, Aulacorthum solani was reared on bean (Vicia faba) and tansy (Tanacetum vulgare) plants under ambient and elevated CO2, performance was enhanced on both hosts at elevated CO2. The nature of the response was different on each plant species suggesting that feeding strategy may influence an insect’s response to elevated CO2. On bean, the daily rate of production of nymphs was increased by 16% but there was no difference in development time, whereas on tansy, development time was 10% shorter at elevated CO2 but the rate of production of nymphs was not affected. The same aphid clone therefore responded differently to elevated CO2 on different host plants. This increase in aphid performance could lead to larger populations of aphids in a future elevated CO2 environment.

Changes in atmospheric composition affect plant quality and herbivore performance. We used the Aspen Free Air CO 2 Enrichment (FACE) facility to investigate the impacts of elevated carbon dioxide (CO 2 ) and ozone (O 3 ) on the performance of the aphid Cepegillettea betulaefoliae Granovsky feeding on paper birch (Betula papyrifera Marsh.). In Year 1, we simultaneously measured individual performance and population growth rates, and in Year 2 we surveyed natural aphid, predator and parasitoid populations throughout the growing season. Aphid growth and development (relative growth rate (RGR), development time, adult weight, embryo number and the birth weight of newborn nymphs) were unaffected by CO 2 and O 3 . Aphid fecundity decreased on trees grown at elevated CO 2 , O 3 and CO 2 1 O 3 . Neither nymphal performance nor adult size were reliable indicators of future fecundity at elevated CO 2 and/or O 3 . Aphid populations protected from natural enemies were unaffected by elevated CO 2 , but increased significantly at elevated O 3 . Individual fecundity in elevated CO 2 and O 3 atmospheres did not predict population growth rates, probably because of changes in the strength of intraspecific competition or the ability of the aphids to induce nutrient sinks. Natural aphid, predator and parasitoids populations (Year 2) showed few significant responses to CO 2 and O 3 , although CO 2 and O 3 did affect the timing of aphid and natural enemy peak abundance. Elevated CO 2 and O 3 affected aphid and natural enemy populations independently: no CO 2 Â O 3 interactions were observed. We conclude that: (1) aphid individual performance did not predict population responses to CO 2 and O 3 and (2) elevated CO 2 and O 3 atmospheres are unlikely to affect C. betulaefoliae populations in the presence of natural enemy communities.

No abstract