Recently a team from the Food and Agriculture Organization of the United Nations (FAO), carried out a comprehensive literature review on major staple crops feeding the world and the major pests and diseases affecting them, and the impact of climate change on those crops, particularly.
Cassava is a major staple crop and crucial for food security in many countries of the world: around 800 million people worldwide eat cassava, and in Africa alone, 500 million depend on it for their staple. It is grown throughout all tropical regions and can tolerate drought and performs well at annual rainfall of 600-1500 mm and temperatures of 25 degrees Celsius. And cassava is a crop which will likely be highly resilient to future climate change stressors. A study from Jarvis et al. has shown that cassava production will vary from -3.5 percent to +17.5 percent within Africa under 2030 climate projections. Unfortunately, cassava will be vulnerable to pests and diseases. Over a third of attainable cassava yield is lost every year to pests and disease alone.
For example, an outbreak of the African cassava mosaic virus disease (CMD) in Africa in the 1920s led to a major famine. The virus is spread by the whitefly insect and by transplanting of diseased plants into new fields. Cassava brown streak virus disease (CBSD) has been identified as a major threat to cultivation worldwide. An overall loss of US$750 million a year caused by CBSD has been estimated across Kenya, Tanzania, Uganda and Malawi.
The whitefly is a pest of cassava and transmitter of pathogens, and its abundance is growing in Africa. High temperatures, before arriving at an upper extreme, can increase populations of this pest. In Côte d’Ivoire, Fargette et al. noted that high temperatures are associated with high fecundity, rapid development, and greater longevity of this insect. The impact of precipitation is mixed on the whitefly. In Kenya, precipitation is very beneficial to whiteflies according to Robertson but in Tanzania, Jeremiah et al. observed that abundance was higher with lower mean annual rainfall. In Africa as a whole, whitefly populations are favored by high temperatures and moderate rainfall according to Sseruwagi et al.
All three threats are expected to increase under climate change across Africa, Southeast Asia, the northern part of South America and the Mato Grosso, an area in west-central Brazil, reported Herrera Campo et al, 2011. The suitable area of whitefly will decrease, but it will expand into areas previously unoccupied by it. Similar trends are projected for CMD and CBSD. The countries most at risk of whitefly include the Central African Republic, Ethiopia, and Cameroon. For CBSD the countries at risk are the Democratic Republic of Congo, Uganda, Cameroon, Central African Republic, and Liberia. For CMD, the Central African Republic, Liberia, and Equatorial Guinea are at risk according to Jarvis et al., who also observed that there is a worry that individual CMD strains will combine into super strains under climate change.
In the short run, intensified monitoring, evaluation and research are highly recommended for all pests and diseases. In the long run, host plant resistance remains one of the best measures. Cassava resistant varieties to CMD have yielded positive gains in crop production, reported Ortiz. However, breeding cassava for CBSD resistance is very difficult because of certain inherent plant characteristics, and because different cultivars will have different resistances depending on the agro-ecological zone.
Currently, there are no fully resistant cultivars to CBSD, only partially resistant according to Tomlinson et al.. Cassava whiteflies usually don’t require control in the absence of recurrent population outbreak, however, their suppression can be achieved using locally occurring or introduced parasitic Hymenoptera.
In the past the impact of mealybugs was huge, particularly in Africa. Since then, the pest has spread to Asia and has continued to cause damage. Its natural enemy, Anagyrus lopezi, is an effective control measure both in Asia and Africa reported Tomlinson et al.. Nevertheless, the problem could resurface under climate change if A. lopezi is disrupted according to Yonow et al. This is of particular concern, noted Ortiz, because scientists were only able to identify A. lopezi as feasible, all other potential predatory control agents found not suitable for Africa.
Broadly speaking, excessive heat stress and cold conditions restrict mealybug propagation. In Africa, projections of the area at risk of mealybug infestation include Sub Saharan Africa with the exception of the most southern parts of this region, reported Jarvis et al. and Yonow et al. Northern Africa, where cassava is grown, will be spared of mealybug infestations due to excessive heat stress. Furthermore, cold stress will restrict mealybug expansion in high mountainous areas where cassava is grown. While China borders countries infested with the pest, cold stress may limit the pest from spreading into China. The pest will likely spread to warmer and wetter regions, if not kept in check by its natural enemy. Nevertheless, heat stress and rainfall are limiting factors to pest spread observed Parsa et al. In South America the population is and will be low due to high parasitism rates. Nevertheless, the area at risk will grow under projected climate change, particularly in the northwest part of the continent.
Climate change is not the only factor at play in mealybug spread, but human mediated trade as well noted Hennessey et al., and Parsa et al., making the spread to areas absent of A. lopez possible.
Overall, the pest has been well managed by its natural enemy A. lopezi. In addition to biological control, intercropping can also reduce the impact of mealybugs up to 60 percent. Sex pheromones are also being increasingly employed for their management. They are, nevertheless, able to adapt to pesticides due to their high reproductive capacities and high voltinism.
Unfortunately, information is still sparse on the impact of climate change on pests and diseases of major crops. And experts are unsure how it will affect crop production and yields, through its impact on land, soil, water, ecosystem services, precipitation, atmospheric temperatures, and pollinators.
FAO, together with the Chinese Academy for Agricultural Sciences, Institute of Plant Protection (IPP-CAAS) is developing a platform for knowledge sharing of smart integrated pest management-solutions for the most virulent pests and diseases affecting farmers’ livelihoods. The platform should be available by the end of 2020.
Photo courtesy of FAO/Riccardo Gangale
