It is now widely accepted that climate change is taking place. Predictions made using different climate change scenarios, in the UK Climate Impacts Programme (UKCIP), suggest that it will become warmer in the UK, and that by the 2080s, the mean average temperature may increase by 3.5°C. In extreme cases, in the south and east of the UK, summer precipitation may decrease by 50 per cent or more by the 2080s, and winter precipitation may increase by up to 30 per cent.
The climate has profound effects on populations of invertebrate pests, such as insects, mites and slugs, affecting their development, reproduction and dispersal. Extreme weather events, including intense rainstorms, wind or high temperatures, also affect survival. The 1990s was the warmest decade on record, and records continue to be broken.
This has been expressed in changes in pest activity and abundance. For example, following 2006-07’s warm winter and spring, many aphid species started their spring migration much earlier than normal and the peach-potato aphid (Myzus persicae), in particular, was captured in unprecedented numbers in the traps operated by the Rothamsted Insect Survey.
The rate at which most pests develop is dependent on temperature, and every species has a particular threshold temperature above which development can occur, and below which development ceases. As temperatures rise, some pest species may be able to complete more generations in a year. This effect may be most noticeable in insects with short life-cycles, such as aphids and the diamond-back moth.
On the other hand, the temporary exposure of populations to extremely high temperatures may delay the development of surviving individuals and thus delay the subsequent generation. The increased frequency of heatwaves will undoubtedly be deleterious to the survival of some UK pests, such as carrot fly, which is particularly sensitive to a combination of high temperature and dry soil conditions during the egg and early larval stages. In contrast, an increase in winter temperatures will improve survival of some pests. Notable examples are the species of aphid that spend the winter as adults and nymphs (peach-potato aphid, cabbage aphid, potato aphid) and other pests, such as the silver Y moth and diamond-back moth, that are unable to currently survive UK winters in large numbers.
Too much water can be devastating for some pests. Raindrops can physically dislodge them from their host plant and behaviour patterns can be disrupted. Some pest infestations are suppressed by periods of rainfall, either because of physical effects or because high humidity leads to outbreaks of fungal disease, often observed amongst aphids on lettuce and brassica crops. Some pest insects appear to prosper during periods of drought - cutworms (caterpillars of the turnip moth) seem to be one such example. Indeed, the ADAS cutworm forecast is based on estimates of the effect of summer rainfall on the survival of young caterpillars, which are particularly susceptible to the effects of rainfall, as well as irrigation. It is anticipated that cutworm outbreaks may be more frequent in the future.
The English Channel is a relatively narrow stretch of water and many flying insects arrive from continental Europe. Some species are particularly well adapted to moving long distances in the air, and these include several species of moth, such as the diamond-back moth and silver Y moth. Since neither of these species survives particularly well during UK winters, new infestations are generally the result of migrations from continental Europe.
There has been increased migration of moths and butterflies to the UK during the last two decades, and recent research by the Centre for Ecology and Hydrology (CEH) has linked this to a pattern of rising temperatures in south-west Europe. The study suggests that for every 1°C rise in temperature in south-west Europe, 14 new species of moth or butterfly can be expected to arrive on the south coast of the UK, although most of these will never become pests. Most of these migrants originate from due south of the UK and will have flown over many kilometres of open sea. Wind direction, and high altitude wind in particular, is important to the flight of migratory pests. Although the current climate change models do not contain a robust wind component, wind is likely to have a strong influence on pests, both on the arrival of immigrants, and the timing of such events, and on their eventual spread within the UK.
Climate change may affect our ability to control pests. For example, high temperatures are reported to reduce the effectiveness of some pesticides. Humidity levels can also modify their efficacy, as can the timing and amount of rainfall following their application. On a simpler level, rain can affect growers’ ability to apply the pesticide at the time of most need, possibly an increasingly likely scenario. If pests are able to complete more generations in a season then this may lead to greater pesticide use, which in turn may lead to the more rapid development of pesticide resistance.
Strategies to avoid the development of resistance need to be planned in advance and rely on the availability of a range of pesticides and/or alternative control methods. The natural enemies of pests will have their own climate optima, although not necessarily the same as their hosts. It is pertinent to ask whether climate change will affect natural enemies to the same extent, or in the same direction, as the pests they control.
The advent of milder winters and warmer summers, more typical of other parts of Europe, has implications for the survival and reproduction of new pests in the UK. These may arrive as migrants, or on imported plant material. The Colorado beetle and the western corn root worm (Diabrotica virgifera) are good examples. The latter is a North American pest of maize, and since its introduction into Serbia in the 1990s, it has spread quite rapidly through Europe. A few individuals were found near Heathrow Airport in 2003. Non-indigenous pests may become established initially in protected crops under glass and then infest field crops subsequently, as the climate changes.
The UK Plant Health Service works together with the horticultural industry to ensure that alien pests are detected and dealt with appropriately.
Obviously, there is considerable uncertainty attached to climate change predictions, and this is also true of likely changes in pest pressure in the future. Some pests will inevitably become a more serious problem, whilst others may decline.
Whatever the long-term consequences of climate change, pest numbers will undoubtedly continue to fluctuate from season to season, depending on the particular combination of weather conditions that occurs each year.
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