Editing IPCC:AR6/SRCCL/Chapter-5 (section)

==== 5.2.2.3 Impacts on pests and diseases ====

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Climate change is changing the dynamics of pests and diseases of both crops and livestock. The nature and magnitude of future changes is likely to depend on local agroecological and management context. This is because of the many biological and ecological mechanisms by which climate change can affect the distribution, population size, and impacts of pests and diseases on food production (Canto et al. 2009 <sup>[[#fn:r310|310]]</sup> ; Gale et al. 2009 <sup>[[#fn:r311|311]]</sup> ; Thomson et al. 2010 <sup>[[#fn:r312|312]]</sup> ; Pangga et al. 2011 <sup>[[#fn:r313|313]]</sup> ; Juroszek and von Tiedemann 2013 <sup>[[#fn:r314|314]]</sup> ; Bett et al. 2017 <sup>[[#fn:r315|315]]</sup> ).

These mechanisms include changes in host susceptibility due to CO <sub>2</sub> concentration effects on crop composition and climate stresses; changes in the biology of pests and diseases or their vectors (e.g., more generational cycles, changes in selection pressure driving evolution); mismatches in timing between pests or vectors and their ‘natural enemies’; changes in survival or persistence of pests or disease pathogens (e.g., changes in crop architecture driven by CO <sub>2</sub> fertilisation and increased temperature, providing a more favourable environment for persistence of pathogens like fungi), and changes in pest distributions as their ‘climate envelopes’ shift. Such processes may affect pathogens, and their vectors, as well as plant, invertebrate and vertebrate pests (Latham et al. 2015 <sup>[[#fn:r316|316]]</sup> ).

Furthermore, changes in diseases and their management, as well as changing habitat suitability for pests and diseases in the matrix surrounding agricultural fields, have the ability to reduce or exacerbate impacts (Bebber 2015 <sup>[[#fn:r317|317]]</sup> ). For example, changes in water storage and irrigation to adapt to rainfall variation have the potential to enhance disease vector populations and disease occurrence (Bett et al. 2017 <sup>[[#fn:r318|318]]</sup> ).

There is ''robust evidence'' that pests and diseases have already responded to climate change (Bebber et al. 2013 <sup>[[#fn:r319|319]]</sup> ), and many studies have now built predictive models based on current incidence of pests, diseases or vectors that indicate how they may respond in future (e.g., Caminade et al. 2015 <sup>[[#fn:r320|320]]</sup> ; Kim et al. 2015 <sup>[[#fn:r321|321]]</sup> ; Kim and Cho 2016 <sup>[[#fn:r322|322]]</sup> ; Samy and Peterson 2016 <sup>[[#fn:r323|323]]</sup> ; Yan et al. 2017 <sup>[[#fn:r324|324]]</sup> ). Warren et al. (2018) <sup>[[#fn:r325|325]]</sup> estimate that about 50% of insects, which are often pests or disease vectors, will change ranges by about 50% by 2100 under current GHG emissions trajectories. These changes will lead to crop losses due to changes in insect pests (Deutsch et al. 2018) and weed pressure (Ziska et al. 2018 <sup>[[#fn:r326|326]]</sup> ), and thus affect pest and disease management at the farm level (Waryszak et al. 2018 <sup>[[#fn:r327|327]]</sup> ). For example, Samy and Peterson (2016) <sup>[[#fn:r328|328]]</sup> modelled bluetongue virus (BTV), which is spread by biting ''Culicodes'' midges, finding that the distribution of BTV is likely to be extended, particularly in Central Africa, the USA, and Western Russia.

There is some evidence ( ''medium confidence'' ) that exposure will, on average, increase (Bebber and Gurr 2015 <sup>[[#fn:r329|329]]</sup> ; Yan et al. 2017 <sup>[[#fn:r330|330]]</sup> ), although there are a few examples where changing stresses may limit the range of a vector. There is also a general expectation that perturbations may increase the likelihood of pest and disease outbreaks by disturbing processes that may currently be at some quasi-equilibrium (Canto et al. 2009 <sup>[[#fn:r331|331]]</sup> ; Thomson et al. 2010 <sup>[[#fn:r332|332]]</sup> ; Pangga et al. 2011 <sup>[[#fn:r333|333]]</sup> ). However, in some places, and for some diseases, risks may decrease as well as increase (e.g., drying out may reduce the ability of fungi to survive) (Kim et al. 2015 <sup>[[#fn:r334|334]]</sup> ; Skelsey and Newton 2015 <sup>[[#fn:r335|335]]</sup> ), or tsetse fly’s range may decrease (Terblanche et al. 2008 <sup>[[#fn:r336|336]]</sup> ; Thornton et al. 2009 <sup>[[#fn:r337|337]]</sup> ).

Pests, diseases, and vectors for both crop and livestock diseases are likely to be altered by climate change ( ''high confidence'' ). Such changes are likely to depend on specifics of the local context, including management, but perturbed agroecosystems are more likely, on theoretical grounds, to be subject to pest and disease outbreaks ( ''low confidence'' ). Whilst specific changes in pest and disease pressure will vary with geography, farming system, pest/pathogen – increasing in some situations decreasing in others – there is ''robust evidence'' , with high agreement, that pest and disease pressures are likely to change; such uncertainty requires robust strategies for pest and disease mitigation.

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