*Insect Pests Dynamic Under Global Warming of Temperate Forests*

For the increase in global tree mortality under climate warming, biotic agents are important causes. Having mentioned the topic in the previous blog, now I will give more details in this post. Biotic agents that can cause tree death include insect pests and fungi pathogens. Insect pests usually can cause more damage than the fungi pathogens because insects are more mobile and more resistant to external environment. They both have high fecundity thus can evolve very fast but insects are especially good at the "trail and error" type of evolutionary adaptation (Hill, 2011). Foresters have already identified insect pests cause as mush as five times the economic impact than forest fire (Logan 2003), then to understand the dynamics of insect pests in temperate forests under global warming is essential for any study on forestry.


The most obvious (and also the most important) change of climate warming on the dynamic of insect pests is the disruption of phonological synchrony. Phenological synchrony is the match in phase of population development of insects, their predators and their nutrients. The nature has set these in a correct timing to ensure both the insects and their predator can enjoy sufficient amount of food to keep their population at carrying level, and without the danger of spoil their long-term prospects. If such pattern is disrupted by a warming climate, what might happened is the insects start to develop earlier that their predators and nutrients. Insects then would consume more nutrients earlier and avoid the risk of predation. This can make the pressure on plants very high (too much being eaten and too slow regenerations).


Another change equally important change is the extended range of activity of insect pests under a warmer climate. Evidences from North American temperate forests suggest that has already became a reality (Parmesan, 2006). Pests attack new land is not a new story. Over the past millenniums there are uncountable alternations of the biomes on the earth's terrains. However, if global warming can intensify the activities of native pests, as well as provoke invasions of new pests, it would make the whole pest dynamics more complex for the forest systems to cope. The contribution factor may also comes form evolutionary changes on the insects’ genetics. Warmer climate creates more favourable conditions for the breeding of insects, this in turn will lead to intensified genetic recombination of insect populations, and eventually could led to greater genetic fitness and make insects pest adapt into their environment more comfortably.


Having discussed the fecundity of insect pests, I will now review the changes on mortality under global warming. Over the past, chilly winter an important mechanism to reduce the pathogens in temperate forests. Severe temperature usually well below the freezing point of water in many forests can kill pests very effectively. Foresters identify this mechanism as “cold-limited mortality”. Global warming might change this, for example, the outbreaks of southern pine beetle have had their range extended northward for about 200km when the minimum temperature have increased by 3.3 degrees Celsius in the USA (Tran, 2007). The prediction for new pests population under warmer climate is the invasion of southern species towards north because of the reduction on cold-limited mortality (the reverse in southern hemisphere of course).


Finally, global warming might “normalize” insect pest outbreak. If the outbreak of insect pests intensified under a warmer climate and become a common trend, then we need to rethink our opinion on whether this should be classified as pest outbreak or not. However, there are self-destructive factors too in intensified insect activities, because an more active pest dynamic will rise the likelihood of forests fires as dead trees can serve as fuel wood. Forest fires of natural causes are good for the health of trees and they can kill pathogens (Parker,2006).

Reference

Dukes J. S. et al., (2009), "Responses of insect pests, pathogens, and invasive plant species to climate change in the forests of northeastern North America: What can we predict?", Canadian Journal of Forest Research, Vol. 39, pp. 231-48.

http://ddr.nal.usda.gov/bitstream/10113/26723/1/IND44171952.pdf

Hill J. K. et al., (2011), "Climate Change and Evolutionary Adaptations at Species' Range Margins", Annual Review of Entomology, Vol. 56, pp. 143-59.

http://www.annualreviews.org/doi/pdf/10.1146/annurev-ento-120709-144746

Logan J. A. et al., (2003), "Assessing the impacts of global warming on forest pest dynamics", Frontiers in Ecology and the Environment, Vol. 1:3, pp. 130-37.

http://www.esajournals.org/doi/abs/10.1890/1540-9295%282003%29001%5B0130%3AATIOGW%5D2.0.CO%3B2?journalCode=fron

Parmesan C., (2006), "Ecological and evolutionary responses to recent climate change.", Annu. Rev. Ecol. Syst., Vol. 37, pp. 637–69.

http://www.annualreviews.org/doi/abs/10.1146/annurev.ecolsys.37.091305.110100

Parker T. J. et al., (2006), "Interactions among fire, insects and pathogens in coniferous forests of the interior western United States and Canada", Agriculture and Forest Entomology, Vol. 8, Iss. 3, pp. 167-89.

http://www.sustainablenorthwest.org/bmfp/parker__clancy__mathiassen.pdf

Tran J. K. et al., (2007), "Impact of minimum winter temperature on the population dynamics of Dendroctonus frontalis", Ecol. Appl., Vol. 17, pp. 882-99.

http://www.esajournals.org/doi/abs/10.1890/06-0512