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

==== 5.4.1.3 Observed impacts on pests, diseases and weeds ====

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AR5 and SRCCL (IPCC, 2019) indicated that more frequent outbreaks and area expansion of pests and diseases are serious concerns under climate change but are under-researched because of the difficulties in assessing multi-species interactions ( [[#Porter--2014|Porter et al., 2014]] ; [[#Mbow--2019|Mbow et al., 2019]] ). High-quality historical and current observational data to detect changes in pests and diseases attributable to recent trends in climate are still limited.

Bebber (2013) found significant poleward expansions of many important groups of crop pests and pathogens since 1960, with an average shift of 2.7 km yr −1 . Different pest species populations respond differently to ongoing climate change, with some shifting, contracting or expanding their current distribution range and others persisting or disappearing in their current range ( ''high confidence'' ). These asymmetric distribution changes can create novel species combinations or decouple existing ones ( [[#Pecl--2017|Pecl et al., 2017]] ; [[#Hobbs--2018|Hobbs et al., 2018]] ), but their consequences on future crop production and food security are hard to predict. Multi-species climate change experiments are rare ( [[#Bonebrake--2018|Bonebrake et al., 2018]] ), but one study shows that under future climates different pest assemblages of interacting species may alter levels of damage to crops compared with that by only one species ( [[#Crespo-Perez--2015|Crespo-Perez et al., 2015]] ). Some studies highlight the importance of location-specific species interactions for more realistic projections of pest distribution, performance and damage to crops, which in turn would allow more effective prevention and pest control strategies ( [[#Wilson--2015|Wilson et al., 2015]] ; [[#Carrasco--2018|Carrasco et al., 2018]] ).

Weeds are recognised as a primary constraint on crop production ( [[#Oerke--2006|Oerke, 2006]] ), rangelands ( [[#DiTomaso--2017|DiTomaso et al., 2017]] ) and forests ( [[#Webster--2006|Webster et al., 2006]] ). Climate change could favour the growth and development of weeds over crops with negative consequences for desired plants in managed systems ( ''medium evidence'' , ''high agreement'' ) ( [[#Peters--2014|Peters et al., 2014]] ; [[#Ziska--2016|Ziska and McConnell, 2016]] ). First, changes in temperature and precipitation alter the range, composition and competitiveness of native and invasive weeds ( [[#Bradley--2010|Bradley et al., 2010]] ). Second, rising concentrations of CO 2 enhance growth of C 3 species (~85% of plant species, including many weeds) ( [[#Ogren--1982|Ogren and Chollet, 1982]] ; [[#Ziska--2003|Ziska, 2003]] ), and increase plant water use efficiency with potentially strong effects on invasive plant species establishment ( [[#Smith--2000|Smith et al., 2000]] ; [[#Belote--2004|Belote et al., 2004]] ; [[#Blumenthal--2013|Blumenthal et al., 2013]] ).

Some invasive species within unmanaged areas will expand further, proliferate and be more competitive under climate change as they may benefit from increased resource ability (e.g., additional CO 2 , enhanced precipitation) ( [[#Bradley--2010|Bradley et al., 2010]] ; [[#Kathiresan--2016|Kathiresan and Gualbert, 2016]] ; [[#Merow--2017|Merow et al., 2017]] ; [[#Ramesh--2017|Ramesh et al., 2017]] ; [[#Waryszak--2018|Waryszak et al., 2018]] ), which will make chemical weed control more problematic ( ''medium evidence'' , ''high agreement'' ) ( [[#Waryszak--2018|Waryszak et al., 2018]] ; [[#Ziska--2020|Ziska, 2020]] ). The range of other invasive weeds may become static, or even decline ( [[#Bradley--2016|Bradley et al., 2016]] ; [[#Buckley--2017|Buckley and Csergo, 2017]] ). A recent meta-analysis also supports that invasive plants respond more favourably to elevated CO 2 concentrations and elevated temperatures than native plants ( [[#Korres--2016|Korres et al., 2016]] ; [[#Liu--2017|Liu et al., 2017]] ). Movement of invasive species into low-fertility areas, however, could provide resource opportunities, especially if agriculture in those areas is limited ( [[#Randriambanona--2019|Randriambanona et al., 2019]] ).

Rising CO 2 concentrations and climate change could reduce herbicide efficacy ( ''medium evidence'' , ''high agreement'' ). These reductions may be associated with physical environmental changes (precipitation, wind speed) that influence herbicide coverage ( [[#Ziska--2016|Ziska, 2016]] ) as well as direct effects of CO 2 on plant biochemistry and herbicide resistance ( [[#Refatti--2019|Refatti et al., 2019]] ). Increasing CO 2 levels and altered temperature and precipitation are therefore projected to affect all aspects of weed biology ( [[#Peters--2014|Peters et al., 2014]] ; [[#Ziska--2016|Ziska and McConnell, 2016]] ), including establishment ( [[#Bradley--2016|Bradley et al., 2016]] ), competition ( [[#Fernando--2019|Fernando et al., 2019]] ), distribution, ( [[#Castellanos-Frías--2016|Castellanos-Frías et al., 2016]] ) and management ( [[#Waryszak--2018|Waryszak et al., 2018]] ).

A warmer climate increases the need for pesticides ( [[#Shakhramanyan--2013|Shakhramanyan et al., 2013]] ; [[#Ziska--2014|Ziska, 2014]] ; [[#Delcour--2015|Delcour et al., 2015]] ; [[#Zhang--2018|Zhang et al., 2018]] ). Increases in temperature and CO 2 concentration may reduce pesticide efficiency by altering its metabolism, or accelerating detoxification ( [[#Matzrafi--2016|Matzrafi et al., 2016]] ; [[#Matzrafi--2019|Matzrafi, 2019]] ). Intense rainfall also reduces persistence ( [[#Delcour--2015|Delcour et al., 2015]] ). Invasive pests and pathogens impose an additional cost for the society ( [[#Bradshaw--2016|Bradshaw et al., 2016]] ). Rapid and large-scale dispersal of pests is already a major threat to food security, as exemplified by the recent outbreak of desert locusts (see Box 5.8), indicating the importance of international cooperation. Taken together, the need for control of pests, disease and weeds will increase under climate change ( ''medium evidence'' , ''high agreement'' ). The use of toxic agricultural chemicals also has human health and environmental risks ( [[#Whitmee--2015|Whitmee et al., 2015]] ; [[#IPBES--2019|IPBES, 2019]] ). Surveillance for monitoring pest distribution and damages, climate-relevant pest risk analysis, and climate-smart strategies for controlling pests with minimal impacts on human and environmental health are important tools in the face of climate change ( [[#IPPC%20Secretariat--2021|IPPC Secretariat, 2021]] ).

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