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Food & Climate
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Weeds, diseases and pests
Climate affects not just agricultural crops but their associated pests as well. The spatial and temporal distribution and proliferation of insects, weeds and disease causing agents is determined to a large extent by climate, because temperature, light, and water are major drivers for their growth and development.
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Climate also affects the pesticides we use to control or prevent pest outbreaks. For example, the intensity and timing of rainfall events influences how long a pesticide remains active and temperature and light affect how quickly a pesticide is broken down. Most studies agree that, in a changing climate, pests are likely to become more active than they are currently and their geographical ranges are likely to expand. It is likely, therefore, that more and more agricultural chemicals will be used in the future. Increased use of these chemicals will be potentially damaging to our health and be ecologically and economically expensive.
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1. A multi-colored Asian lady beetle searches for prey on a blooming pear tree. Michigan State University. www.imp.msu.edu/asian ladybettle.htm
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Response to climate variables
Because different pests respond differently to changes in weather, it's hard to make any overall conclusions about the effect of climate change on pest growth. In general, however, most pest species prefer warm and humid conditions and a weak crop during a drought is more likely to become infected by a pest than when the plant is growing well.
Crop damage by pests is usually a consequence of complex ecological dynamics between two or more organisms. An example of this complexity is the multi-coloured Asian lady beetle. This beetle is native to Asia but has adapted to climates and habitats in many different countries. On one hand, Asian lady beetles are an important natural defence against aphids and other pests but large numbers cause damage to fruit crops late in the season. So to benefit from the natural defence against aphids provided by the Asian lady beetle, it is important to carefully target pesticide use or employ other management tactics. |
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Rainfall, whether optimal, excessive, or insufficient, is probably the most important variable that affects the way in which plants are affected by pests or diseases. Both direct and indirect effects of moisture on crops make the crops vulnerable to pest damage and this is particularly true in the early stages of plant development.
Pest infestations often coincide with changes in climate, such as early or late rains, droughts, or increases in humidity and all of these factors can, in themselves, reduce crop yields. Under these circumstances, attributing specific losses to pests can be difficult. Table 1. shows key weather conditions that critically influence major pest epidemics and gives examples of the crop damage which can result.
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Insects
Insects flourish in all climates. Their habitats and survival strategies are strongly dependent on local weather patterns and they are particularly sensitive to temperature because they are cold-blooded. Insects respond to higher temperatures with increased rates of reproduction. Warmer winters reduce winterkill, and, as a result, increase insect populations in subsequent growing seasons.
Drought changes the growth and development of the host species from which insects feed and can reduce populations of friendly insects and spiders and birds. This increases the impact of pest infestation. Abnormally cool, wet conditions can also bring about severe insect and plant pathogen infestations, although excessive soil moisture may drown soil-residing insects.
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2. Typical cereal aphid. The damage caused by the aphid to the cereal plant lowers the plant's defences against other diseases or even against adverse weather situations (20 x magnification).
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Weeds
Weeds compete with crops for soil nutrients, light, and space. Drought conditions increase competition for soil moisture between crops and weeds, while humid conditions increase the proliferation of weeds. Warmer temperature regimes have been shown to increase the amount of grass weeds. Increased temperatures and humidity increase the spread of diseases, as wet vegetation promotes the germination of spores, the proliferation of fungi and bacteria and influences the lifecycle of soil nematodes. Some pathogens (e.g., powdery mildews) thrive in hot, dry conditions as long as there is dew formation at night.
Most of the species classified as weeds are plants that can also be classified as pioneer species. These pioneer species have a great ability to adapt to a wide variety of environmental conditions, are the first plants to colonize new areas and can survive in tougher conditions than many commercial species.
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Table 1: Effect of weather on pest damage and examples |
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1. Floods and heavy rains |
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1.1. Increased moisture encourages epidemics and prevalence of pathogens.
- Rice leaf blight caused great famine in Bengal (1942), 2 million people died.
- Wheat stripe rust outbreak contriubuted to the 1960's famine in China.
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1.2. Water induced soil transport increases spread of pathogens to non-infected areas.
- Outbreak of soyabean sudden death syndrome in the north central U.S.A. (1993).
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1.3. Continuous presence of excessive water in soil causes long-term problems related to rot and increases damage by pathogens.
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2. Drought |
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Water stress reduces plant growth and alters the relationship of nutrients carbon and nitrogen lowering plant resistance to nematodes and insects. Attack of stems and roots by fungal pathogens favored by weakened plants. Dry and warm conditions promote growth of insect populations, increasing viral epidemics.
- Summer locust outbreak correlated to drought in Mexico (1999).
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3. Air currents |
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Air currents provide large-scale transportation for disease agents (e.g., spores of fungi) or insects from overwintering areas to attacking areas.
- Spread of the southern leaf blight of corn from Mississippi to the Midwest by a tropical storm in the Gulf of Mexico during 1970.
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4. Warm winters |
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Increase of over wintering populations of all pests and insect vectors.
- Increased populations and number of generations of Mexican bean beetles and bean leaf beetles in the U.S.A.
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3. Locust plague. Photo by Heraldo de Aragon.
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Recent expansion of weeds, diseases and pests
Increases in pest-induced losses have been seen throughout the world since the 1940's. Over this period there has also been a huge increase in both the amount and toxicity of pesticides used. Increased pest damage results from changes in production systems, enhanced resistance of some pests to pesticides and the increased growth of crops in warmer and more humid regions where crops are more susceptible to pest infestations.
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The ranges of several important insects, weeds and pathogens have extended and expanded northwards. Recent climate trends and extreme weather events may directly and indirectly contribute to increased pest damage. Whether the change in global climate has contributed to these observations remains unclear.
Even without climate change, pest management faces some serious challenges over the coming decades. The most striking of these are the increasingly high dependence on chemical treatments and rising costs due to environmental protection and public health policies. Improved climate forecasts can help farmers prepare for changing seasonal-to-interannual conditions, and optimize pesticide management while minimizing environmental damage.
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About this page:
Author: Marta Moneo and Dr. Ana Iglesias - Universidad Politécnica de Madrid, España
1. scientific reviewer: Alex de Sherbinin - CIESIN, Columbia University, USA
2. scientific reviewer: Lily Parshall - Goddard Institute for Space Studies, Columbia University, USA
educational reviewer: Emilio Sternfeld - Colegio Virgen de Mirasierra - España
last published: 2004-05-12
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