Introduction

Global pests affecting crops, forestry and ecosystems

Since their domestication, around 10 000 years ago, crops have been threatened by a multitude of pests causing yield losses often leading to starvation and social unrest. On average, at a global scale, between 10 and 28 percent of crop production is lost to pests (Savary et al., 2019). Further post-harvest losses are observed, with the worst scenarios in developing countries. Moreover, besides losses, the presence of mycotoxins (toxins produced by fungi) in food and feed can severely threaten the health of humans and livestock (Magan, Medina and Aldred, 2011; Van Der Fels-Klerx, Liu and Battilani, 2016).

Historic and current examples clearly show the extensive damage that can be caused by pest outbreaks.

Among insect pests, two classical examples show the economic and social damage resulting from invasive expansion. One is the invasion and destruction of European vineyards by the insect phylloxera (Daktulosphaira vitifoliae) during the second half of the nineteenth century, and the second is the Colorado potato beetle (Leptinotarsa decemlineata) in the twentieth century, which rapidly colonized potato plots. Both of these pests originated in the United States of America. More recently, several native insect species from North America, with no prior records of severe infestation, have emerged as devastating pests of forest resources because of changes in their population dynamics. These include the aspen leaf miner (Phyllocnistis populiella), the leafblotch miner (Micrurapteryx salicifoliella) and Janet’s looper (Nepytia janetae), which have decimated millions of hectares of aspen, willows, and spruce-fir forests since the early 1990s (Bebber, Ramotowski and Gurr, 2013). Other native species that have become pests include the mountain and southern pine beetles (Dendroctonus ponderosae and Dendroctonus frontalis, respectively) and the spruce beetle (Dendroctonus rufipennis), which have recently expanded their distribution, infesting commercially important pine and spruce trees (Anderegg, Kane and Anderegg, 2013; Bebber, Ramotowski and Gurr, 2013).

Classical examples of the damage caused by crop and forest diseases include the Irish potato famine caused by Phytophthora infestans in the 1840s, the devastating impact of coffee rust in Ceylon caused by Hemileia vastatrix in the 1860s, and the Great Bengal Famine in 1943 caused by Helminthosporium oryzae (Schumann, 1991). Another major example not to be forgotten is the chestnut blight caused by Cryphonectria parasitica, which wiped out the American chestnut tree (Castanea dentata): by the 1950s, 80 percent of the chestnut trees had died (Schumann, 1991), severely affecting the landscape of an entire country. The threat persists. New and more virulent strains of the rust fungi Puccinia graminis (Saunders, Pretorius and Hovmøller, 2019) and Puccinia striiformis are currently spreading (Liu et al., 2017) and a new invasive lineage of Phytophthora infestans has rapidly displaced other late-blight genotypes (Cooke et al., 2012). The sudden wilt of olive trees caused by Xylella fastidiosa subsp. pauca, which has destroyed millions of olive trees in Apulia (Italy) and also threatens other European and Mediterranean countries, is an example of how a pathogen can affect a crop as well as the landscape of a region (Schneider et al., 2020; Sicard et al., 2018). In California and Oregon in the United States of America, as well as in other areas, Phytophthora ramorum, which causes sudden oak death, represents a threat to forest ecosystems (Rizzo, Garbelotto and Hansen, 2005), while other species of Phytophthora, such as P. kernoviae and P. agathidicida, are affecting the iconic and culturally important kauri in New Zealand (Scott and Williams, 2014) and P. pinifolia is damaging pine forests in Chile (Duran et al., 2008).

In addition to insects and plant pathogens, extensive damage to plants can also be caused by nematodes. According to Williamson and Gleason (2003), nematodes are among the most frequently occurring organisms on earth, affecting all ecosystems. Most of them are free-living and harmless to plants, for instance consuming microorganisms such as bacteria, but a small number of nematode species are obligate parasites of plants, and some of these plant-parasitic nematodes can pose a serious threat to managed and unmanaged ecosystems. In agriculture, the most economically important groups of nematodes are the sedentary endoparasites, including the genera Heterodera and Globodera (both cyst nematodes) and Meloidogyne (root-knot nematodes). In forestry, pine wilt disease, caused by the pine wilt nematode (Bursaphelenchus xylophilus), is one of the most devastating invasive diseases affecting pine trees (Pinus spp.), with significant impacts on natural ecosystems in Africa, North America, Asia, and Europe (CABI, 2021a). It is particularly devasting in eastern Asia, including China, Japan and the Republic of Korea (Ikegami and Jenkins, 2018).

Finally, some plant species are themselves pests. Weeds are “unwanted plants” in agriculture, horticulture, forestry and unmanaged ecosystems (Juroszek and von Tiedemann, 2013a; Korres et al., 2016; Wan and Wang, 2019). Thus, a weed is a plant prevalent in the wrong place or at the wrong time. Weeds have a range of properties that may be of benefit. Some weed species may provide useful ecosystem services, such as providing food for pollinators like bees, providing habitat for many beneficial organisms, and providing soil cover, thereby reducing soil erosion. They can also be primary colonizers following soil or ecosystem damage (e.g. fire, landslips), and in stabilizing riverbanks and sand dunes. In addition, some are traditional medicinal plants. However, weeds may cause contact dermatitis or incite allergies through their pollen, and they can be toxic to livestock (Ziska, Epstein and Schlesinger, 2009). They can also have a negative impact where they are not wanted. Many weeds have a wide environmental tolerance and a high level of phenotypic plasticity and evolutionary potential (Clements and DiTommaso, 2011), providing them with a very high competitive ability compared to crop plants, which have been selected to be homogenous. Weeds can therefore cause great losses in both the quality and quantity of crops and other plants and habitats, because they are competing for below-ground (e.g. water, nutrients) and above-ground (e.g. light) resources (Karkanis et al., 2018; Naidu, 2015; Peters, Breitsameter and Gerowitt, 2014; Ramesh et al., 2017). For example, the production of carrots (Daucus carota), even in a home garden, is impossible without weed control, due to the poor competitive ability of carrot seedlings.