How Climate Change May Affect Arthropod Pests
Factors attributed to climate change require careful observation of plant pests and their life cycles-and possible fine-tuning of pest control protocols.
The issue of "climate change," "global warming," or "environmental change" continues to generate controversy among scientists and non-scientists. However, this article will strictly discuss the potential impacts of climate change or the factors associated with climate change on arthropod (insect and mite) pests and how this may develop into effects on pest management.
Figure 1. Direct effects of climate change on arthropod pests are associated with the distribution and abundance of pest populations, with climate being the primary factor that limits geographical distribution of arthropods. In addition, temperature and moisture may influence survival, development and reproduction of arthropods.
Figures courtesy of Raymond A. Cloyd
Before we focus on pests, it is important to have some background information regarding the factors associated with climate change. This information, in most cases, cannot be predicted with any degree of certainty. However, some questions affiliated with climate change include the following:
Have human (anthropogenic) activities contributed to climate change, or is climate change a natural phenomenon? Well, the overall average temperature of the earth has changed less than a degree (for example, 0.6 to 0.7°C) over the past 100 years. In addition, the earth's geological record indicates numerous shifts in the climate throughout the ages.
What may be the possible factors contributing to climate change? It has been consistently proposed that human-made emissions of carbon dioxide (CO2) due to burning fossil fuel, and other gases that may trap infrared radiation emitted from the surface of the earth, are responsible for the so-called "greenhouse effect." However, it has been suggested that natural forces such as solar cycles and/or volcanic eruptions are also associated with contributing to the greenhouse effect.
What is the impact of climate change on biodiversity? There are predictions that any changes due to climatic or environmental factors may increase the risk of extinction of certain species.
Impact of climate change
What is the potential impact of climate change on plant-feeding arthropod (insect or mite) pests or herbivores? This is closely associated with "phenological synchrony." As such, climate change may increase asynchrony between host plants and herbivores resulting in adverse consequences.
Figure 2. The indirect effects of climate change are affiliated with host plants, competitors and natural enemies. For example, drought stress may cause changes in plant chemistry (such as concentration of amino acids) and plant structure that could either increase or decrease suitability as a host.
Also, it is important to understand that the effects of climate change may be direct or indirect. Direct effects (see Figure 1) are associated with the distribution and abundance of arthropod pest populations, with climate being the primary factor that limits geographical distribution of arthropods. In addition, temperature and moisture may influence survival, development and reproduction of arthropods.
The indirect effects of climate change are affiliated with host plants, competitors and natural enemies (see Figure 2). For example, drought stress may cause changes in plant chemistry (such as concentration of amino acids) and plant structure that could either increase or decrease suitability as a host. Furthermore, the distribution and abundance of natural enemies including parasitoids, predators and pathogens may be affected, which could influence their ability to regulate arthropod pest populations, thus resulting in potential outbreaks.
Figure 3. Insects may consume more when plants are exposed to elevated levels of carbon dioxide because less nitrogen is present in leaf tissues, and certain insect pests such as caterpillars may consume more leaf tissue (compensatory feeding) in order to acquire an equivalent amount of nutrients.
There are a number of factors associated with climate change that may impact the relationship among climate, arthropod pests, natural enemies and host plants. These include:
- distribution, abundance, and quality of host plant;
- pest physiological and behavioral processes;
- natural enemy effectiveness;
- plant growth rates and plant health;
- distribution and abundance of pest population; and
- the presence of competing species.
However, what may have the greatest effects on arthropod pests as it relates to climate change are carbon dioxide (CO2) and temperature.
Impact of carbon dioxide and temperature
There are a number of potential relationships or factors associated with climate change and arthropod pests. For example, increased carbon dioxide levels may:
- result in higher consumption rates by insects;
- allow invasive insect species to outcompete native insect species (for example, Argentine ant in California);
- encourage the increase in migration of invasive insect species and noninvasive species into new regions/areas;
- increase the duration and intensity of arthropod pest outbreaks due to extended frost-free periods; and
- create a higher potential for the occurrence of insect-borne human diseases such as malaria and dengue fever.
In regard to the last point, climate change may affect the incidence of insect-borne diseases such as malaria and dengue fever by increasing the range of insect vectors, extending seasons of transmission and enhancing reproduction and biting rates. Furthermore, development of the dengue virus inside the mosquito vector (Aedes aegypti) may be shortened under higher temperatures, thus increasing the proportion of mosquitoes that may become infectious at any given time.
It is important to understand the relationship between insect pests and increased carbon dioxide levels and temperatures. For instance, insects may consume more when plants are exposed to elevated levels of carbon dioxide because less nitrogen is present in leaf tissues, and certain insect pests such as caterpillars (see Figure 3) may consume more leaf tissue (compensatory feeding) in order to acquire an equivalent amount of nutrients. In addition, an increase in the carbon-to-nitrogen balance (ratio) in plants may influence a number of factors, including insect feeding, concentrations of defensive chemicals in plants, compensation responses by plants to insect herbivory, and competition between pest species. Temperature increases may also impact species diversity and distribution of certain insect pests such as the mountain pine beetle (Dendroctonus ponderosae). Nevertheless, current insect pests may extend their ranges into new areas by means of geographical movement northward.
So, what is the influence of elevated carbon dioxide levels on plant defenses? First of all, plants have two groups of chemical defenses: carbon-based and nitrogen-based. Carbon-based defenses include tannins, lignins and phenolics that are designed to slow insect growth and development. These types of defenses are present at higher concentrations under elevated levels of carbon dioxide. Nitrogen-based defenses include alkaloids and cyanogenic glycosides that are directly toxic or repellent to insect pests. These types of defenses are present at lower concentrations under higher levels of carbon dioxide. As such, chewing insect pests such as caterpillars and beetles that consume more plant tissues when plants are exposed to increased levels of carbon dioxide may actually ingest more toxins and thus may be killed faster and in greater numbers.
Figure 4. Additional factors to consider in regard to pest management include long-term harmful effects to plants (phytotoxicity) due to the amount of pesticide applications, and associated infestations of arthropod pests for extended periods of time. This may lead to natural infestations of multiple arthropod pest guilds such as chewers (caterpillars and beetles), suckers (aphids, scales and leafhoppers), and/or wood-boring insects occurring simultaneously on the same plant.
It is possible that climate change may increase the range of expansion of certain insects. Migration of new insect pests, both in terms of latitude and altitude, may result in a change in the ecosystem thus allowing populations of new species to increase. This may force other species to extinction because these new species may be better competitors at higher temperatures, although this may not always be the case. Population dynamics are not predictable as insect pests could be constrained by natural enemies, host-plant availability, and competition with other insect species. Furthermore, the rate at which insect species can establish populations permanently into new areas that are suitable will be limited by the rate of spread of host plants into new areas, which would impact specialist and generalist herbivores differently.
What about the effect of higher temperatures? This will likely benefit some insect species more than others, which may be due to the impact on life history parameters (for example, reduced offspring production) of natural enemies such as parasitoids and predators. A reduction in natural enemy populations (due to climate incompatibility) may lead to more arthropod pests present and thus plant damage, and more insect outbreaks as temperatures increase. It should be noted that population dynamics across new geographical ranges will be unpredictable with more insect pest outbreaks occurring due to ranges expanding quicker than natural enemies. As such, this may transcend into more pesticide applications (mentioned later).
Insect development, survival, distribution and abundance are directly affected by temperature because insects are cold-blooded, and as temperatures increase, these parameters also tend to increase. Thus, arthropod pest populations may develop faster and plant damage may occur more rapidly and possibly last longer than previously. Furthermore, higher temperatures may influence the effectiveness of insect pathogens (such as fungi, bacteria, and viruses) and natural enemies (for example, parasitoids and predators), which may negatively affect sources of natural mortality. In addition, an increase in temperature could lead to temporal and/or geographical separation leading to arthropod pest outbreaks. It is likely that higher temperatures will favor those arthropod pests with multiple generations more so than those with single generations. This may result in insect pests breeding throughout the year. Also, in regard to insect-vectored diseases, warmer temperatures may translate into additional insect generations, which may increase transmission rates of plant pathogens including viruses transmitted by aphids.
What about the impact on overwintering? Well, increased temperatures, which may lead to expanded warm seasons or shortened winters, could lead to earlier emergence and later overwintering of arthropod pests. This may also result in greater survival of arthropod pests during the winter.
What are the potential issues in the U.S. associated with the impact of climate change on arthropod pests? There are a number of factors that need to be considered, including
- expanded ranges of certain arthropod pests already present;
- increased arrival or migration of more arthropod pests;
- changes in ecosystems that may allow certain arthropod species or populations to reach outbreak proportions, which may result in extinction of other species; and
- expanded time period (earlier and later) in which arthropod pests would be present during the growing season. In the end, how will these factors and those described above influence pest management?
Impact on pest management
The questions to address at this point are: Will we have more problems with certain arthropod pests, and what will be the potential impact on pest management? In general, this could lead to increased pesticide use (in this case, insecticides and miticides) throughout the growing season with more frequent applications, resulting in higher incidences of resistance occurring in arthropod pest populations because of the increased selection pressure placed on these populations. In addition, this may directly and indirectly impact natural enemies thus influencing any natural mortality.
Additional factors to consider in regard to pest management include long-term harmful effects to plants (phytotoxicity) due to the amount of pesticide applications, and associated infestations of arthropod pests for extended periods of time. This may lead to natural infestations of multiple arthropod pest guilds such as chewers (caterpillars and beetles), suckers (aphids, scales and leafhoppers) and/or wood-boring insects occurring simultaneously on the same plant (see Figure 4).
Finally, what about the potential effects of climate change on pesticides? This may include decreased residues or persistence, reduced toxic action on arthropod pests, increased treatment rates or dosages required, increased number of pesticide applications, and fewer times suitable for pesticide application (influenced by temperature and wind, which may prevent or delay application).
In summary, although what has been presented in this article is mostly speculation, the fact that we know how arthropod pests will respond to changes in environmental conditions, especially temperature, allows us to make somewhat accurate predictions of what could occur in regard to changes in insect abundance, distribution and how this may impact pest management strategies. However, it should be noted that Mother Nature is going to do her own thing, because she knows what is best!
Raymond A. Cloyd, Ph.D., is Professor and Extension Specialist in Horticultural Entomology/Integrated Pest Management in the Department of Entomology at Kansas State University, Manhattan. He can be reached at firstname.lastname@example.org.
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