Effects of climate change on plant pests

Effects on pest species

Climate-change effects on pest species are complex and include direct and indirect effects and their possible interactions. At a given location, a shift in warming and other climate and atmospheric conditions may result in direct or indirect effects on insect pests, pathogens, and weeds. Possible direct and indirect effects on pests include: changes in their geographical distribution, such as range expansion or retreat, or increased risk of pest introduction; changes in seasonal phenology, such as the timing of spring activity or the synchronization of pest life-cycle events with their host plants and natural enemies; and changes in aspects of population dynamics, such as overwintering and survival, population growth rates, or the number of generations of polycyclic species (Juroszek and von Tiedemann, 2013a; Richerzhagen et al., 2011).

In general, all important life-cycle stages of insect pests, pathogens, and weeds (survival, reproduction and dispersal) are more or less directly influenced by temperature, relative humidity, light quality or quantity, wind or any combination of these factors. The physiological processes of most pest species are particularly sensitive to temperature (Juroszek et al., 2020). For example, plant viruses and their insect vectors may be particularly favoured by high temperatures until their upper temperature threshold is reached (Trebicki, 2020). In a three-year field experiment in maize under tropical climatic conditions, Reynaud et al. (2009) showed that the incidence of maize streak disease (caused by the Maize streak virus) and the abundance of its vector, the leafhopper Cicadulina mbila, were closely associated with temperature, both increasing quickly above 24 °C, but that temperatures of 30 °C and above might be detrimental for the leafhopper and related virus transmission (Juroszek and von Tiedemann, 2013c). It might be expected, therefore, that global warming will promote many insect vectors and the viruses they transmit, at least within a certain temperature range.

Indirect effects are mediated through the host plant or through climate-change driven adaptations to crop management (Juroszek et al., 2020). Warmer mean air temperatures, especially in early spring under temperate climatic conditions, may result in life-cycle stages in the host plant occurring earlier in the season (Racca et al., 2015). This can affect pathogens that infect the host during a particular life-cycle stage, for instance wheat pathogens such as Fusarium species that infect wheat during flowering (Madgwick et al., 2011; Miedaner and Juroszek, 2021a). Crop-management adaptations driven by climate change include the introduction of irrigation, cessation of deep soil tillage, shifting of sowing dates, and the cultivation of more than one crop per year. Irrigation of maize in south-east Africa, for example, has permitted year-round cultivation of maize, but has also led to an increase in insect-vector populations, culminating in increased Maize streak virus pressure in irrigated and subsequently also in rainfed crops (Shaw and Osborne, 2011).

Interactions between factors affecting pests may be complex. For example, experiments under real-world field conditions in FACE facilities have shown the complexity of interactions between weed growth and temperature, water and CO2 under changed environmental conditions (Williams et al., 2007), and other experiments have shown that water stress can alter the competitive relationships between weed and crop plants in terms of their response to elevated CO2 concentration (Valerio et al., 2011). Under well-watered conditions, the growth of the C3 tomato crop (Lycopersicon esculentum) benefits more from elevated CO2 relative to the C4 weed Amaranthus retroflexus, whereas under water stress A. retroflexus benefits more from elevated CO2 compared to tomato. Experiments such as these (Valerio et al., 2011; Williams et al., 2007), conducted under controlled and field conditions, therefore suggest that plant responses to elevated CO2 are not predictable on the sole basis of the type of photosynthetic pathway (C3 vs C4), because there are complicated interactions with factors such as water availability and temperature, among others. These conclusions are in agreement with a recently published meta-analysis (Vila et al., 2021), especially performed to understand the combined impacts of weeds and climate change on crops.