Abstract
Interactions between above- and belowground herbivores have been prominent in the field of aboveground-belowground ecology from the outset, although little is known about how climate change affects these organisms when they share the same plant. Additionally, the interactive effects of multiple factors associated with climate change such as elevated temperature (eT) and elevated atmospheric carbon dioxide (eCO2) are untested. We investigated how eT and eCO2 affected larval development of the lucerne weevil (Sitona discoideus) and colonization by the pea aphid (Acyrthosiphon pisum), on three cultivars of a common host plant, lucerne (Medicago sativa). Sitona discoideus larvae feed on root nodules housing N2-fixing rhizobial bacteria, allowing us to test the effects of eT and eCO2 across trophic levels. Moreover, we assessed the influence of these factors on plant growth. eT increased plant growth rate initially (6, 8 and 10 weeks after sowing), with cultivar “Sequel” achieving the greatest height. Inoculation with aphids, however, reduced plant growth at week 14. eT severely reduced root nodulation by 43%, whereas eCO2 promoted nodulation by 56%, but only at ambient temperatures. Weevil presence increased net root biomass and nodulation, by 31 and 45%, respectively, showing an overcompensatory plant growth response. Effects of eT and eCO2 on root nodulation were mirrored by weevil larval development; eT and eCO2 reduced and increased larval development, respectively. Contrary to expectations, aphid colonization was unaffected by eT or eCO2, but there was a near-significant 10% reduction in colonization rates on plants with weevils present belowground. The contrasting effects of eT and eCO2 on weevils potentially occurred through changes in root nodulation patterns.
Highlights
Many studies report on plant-mediated interactions between spatially separated insect herbivores that live above- and belowground, yet few studies have considered these interactions in the context of global climate change
we observed that elevated temperatures (eT) negated the positive effects of elevated CO2 concentrations (eCO2)
Such negation of eCO2 effects by eT has also been found in aboveground insect
Summary
Many studies report on plant-mediated interactions between spatially separated insect herbivores that live above- and belowground, yet few studies have considered these interactions in the context of global climate change. While the effects of predicted increases in global average surface temperatures (by 1–4◦C within this century) and atmospheric CO2 concentrations (from current levels of 400 to over 550 μmol mol−1 by 2050) on insect-plant interactions have been characterized separately, only a handful of studies have considered more than one climate change variable simultaneously (Robinson et al, 2012; Stevnbak et al, 2012; Murray et al, 2013). All legumes form symbioses with rhizobial bacteria that fix atmospheric nitrogen (N2) and are carried in root nodules (Haag et al, 2013). It is widely reported that root nodulation (and biological N-fixation) increases in response to elevated CO2 concentrations (eCO2; Ryle and Powell, 1992; Lüscher et al, 2000) but decreases with elevated temperatures (eT; Munns et al, 1979; Zahran, 1999)
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