Written by: David Hollister
Recent studies have shown that average global temperatures have risen at an unprecedented rate over the last hundred years due to anthropogenic activities resulting in the release of carbon dioxide into the earth’s atmosphere. This increase will lead to an acceleration in the coming years, leading to global mean temperatures as much as 4.1°C higher than pre-industrial temperatures by 2100 (Cannon 1998). One consequence of climate change that is of particular interest to humans is the adaptation of agroecosystems to warmer temperatures and the possible reduction in crop yields due to the changes in the behavior, phenology, morphology, and life cycles of the insects that play an essential role in these communities.
Researchers can determine the loss of crops due to insect activity by the herbivory of insect pests, the efficiency of parasitoids, predators at controlling pests, and the susceptibility of the crop plant itself to herbivory. Changes in any of these three categories could negatively affect output. While it is difficult to predict how these factors will change in response to climate change, their important implications for the global food supply have led to extensive scientific research and discussion.
The response of Agricultural Pests to Climate Change
Herbivory by insect pests is the most direct way that insects’ adaption to climate change will influence crop yield. Predictive research and analysis of emerging trends have suggested that insect pests will most likely benefit from increased temperatures and become more challenging to manage (Michaud 2010). One reason for this is that the development of most insects is temperature-dependent. Increased temperatures may lead to earlier flight times in certain species, as has already been observed in aphids in the United Kingdom that are now developmentally able to fly a month earlier, causing an additional month worth of crop damage (Cannon 1998).
The number of pests may also increase as insects that are no longer restricted by low temperatures can continue mating year-round and produce multiple generations per year (Michaud 2010).
Another anticipated effect of climate change is the Northward shift in pest distributions (in the Northern hemisphere) as warming higher latitudes become suitable for colonization. Some insect species, such as the brown argus butterfly, have already shown an expansion of their ranges. Researchers predict their ranges to spread northward shortly (Thomson et al. 2010). While pest species’ introduction to new latitudes will inevitably lead to increased agricultural damage in some systems, researchers can mitigate the damage done by pests by natural processes.
Most of the damage is in situations where the parasite escapes its enemies and arrives in an area in which no indigenous species can parasitize or predate it. Still, its enemies may be able to follow it, native species will adapt to utilize the species, or the pest will move out of the range of the crop in the northern part of its distribution or by the contraction of its southerly range (Thomson et al. 2010). Ultimately, it is the reactions between the pest and its natural enemies that determine the extent of the damage done by insect pests as a result of climate change.
The Effect of Climate Change on Beneficial Insects
Many pest species are controlled by predators that depend on them for food or parasitoids that rely on them for reproduction. Still, evidence has shown that these insects may become less efficient as a result of climate change. For example, in addition to increased temperatures, various regions are expected to exhibit higher climatic variation, which, as shown by Stiremen et al., could lower the parasitism rates of beneficial parasitoids (2005).
Another source of decreased parasitism could be a loss of synchronization between the host and parasitoid phenology. The growth and development of individual species are affected differently by climate change; this could result in a parasitoid emerging at a time in which its preferred host instar is more abundant, leading to higher parasitism, but it is more likely that loss of synchronization will result in parasitoid species emerging too early or late to successfully parasitize its host (Stiremen et al. 2005); this will likely be less detrimental to generalist predators and parasitoids with multiple hosts. Additionally, parasitoids and predators may experience increased water loss as they forage longer (Michaud 2010); this is because of the increased pest habitat size created by plant foliage response to higher atmospheric carbon dioxide levels (Thomson et al. 2010).
The researchers suggest that other plant reactions to climate change will play an essential role in deciding the impact of changing pest and natural enemy populations.
Plant Susceptibility to Pests Under Climate Change
Many changes in plant growth associated with global warming can make crops more susceptible to damage by pests. Augmented productivity caused by inflations in temperature and carbon dioxide leads to a decrease in nitrogen concentrations in relation to the plant’s carbon content, requiring herbivorous insects to graze more to obtain the same amount of nutrition and, therefore, increasing crop damage (Cannon 1998).
In some cases, the pest insect may not be able to fully compensate for the loss of nitrogen even with increased intake and be less healthy; this could lead to the reduced fitness of parasitoids and more crop damage, as many need healthy hosts to reproduce successfully, or to increased predation by predators which may result in no net change in crop yield (Thomson et al. 2010).
Elevated carbon dioxide levels prove to hurt the expression of insect resistance genes in some plants. In raspberries, for example, the presence of added carbon dioxide led to the reduced expression of genes coding for cysteine proteinase inhibitors that would generally make them less appealing to insect herbivores (Martin and Johnson 2011). Crops are also likely to become less insect resistant because of drought stress brought on by water shortages associated with rising global temperatures.
Insects will often prefer to feed on plants afflicted with drought stress as they will contain less defensive compounds or have reduced waxy leaf cuticles that would otherwise limit herbivory (Cannon 1998).
Cannon, R.J.C., 1998. The implications of predicted climate change for insect pests in the U.K., with emphasis on non-indigenous species. Global Change Biology 4: 785-796.
Martin, P., and S.N. Johnson. 2011. Evidence that elevated CO2 reduces resistance to the European large raspberry aphid in some raspberry cultivars. Journal of Applied Entomology 135: 237-240.
Michaud, J.P., 2010. Implications of climate change for cereal aphids on the Great Plains of North America. Pp. 69-89. In: P. Kindlmann, A.F.G. Dixon, and J.P. Michaud (eds). Aphid Biodiversity under Environmental Change. Springer Science+Business Media B.V., Dordrecht, Netherlands.
Stireman III, J.O., L.A. Dyer, D.H. Janzen, M.S. Singer, J.T. Lill, R.J. Marquis, R.E. Ricklefs, G. L. Gentry, W. Hallwachs, P.D. Coley, J.A. Barone, H.F. Greeney, H. Connahs, P. Barbosa, H.C. Morais, and I.R. Diniz. 2005. Climatic unpredictability and parasitism of caterpillars: implications of global warming. Proceedings of the National Academy of Sciences of the United States of America 102: 17384-17387.
Thomson, L.J., S. Macfadyen, and A.A. Hoffmann. 2010. Predicting the effects of climate change on natural enemies of agricultural pests. Biological Control 52: 296-306.