The melting of glaciers, sea level rise and intensifying wildfires are some of the attention-getting manifestations of climate change in our world today. Less recognized are changes in ecosystems and populations that are correlated with, if not best explained by, warming of the Earth’s climate. Among these responses are those of organisms that threaten man’s welfare—in other words, pests.
There appears to be a general trend of range adjustments among pests throughout the world. Scientists at the Universities of Exeter & Oxford, have concluded that crop pests have been moving towards the North or South Poles at an average rate of 2 miles (3 km) per year since 1960. This is an average- some insects are moving northward at an average of 12 miles per year, some bacteria not at all. The database consisted of 612 species of pests and pathogens distributed world-wide. Most of the pests are insects, nematodes, bacteria or viruses. Many of these organisms represent serious, even life-threatening risks to human beings, either directly through diseases or indirectly such as by reducing food supplies.
Following is a brief description of a pest species long recognized as a problem that appears to have worsened in recent years. Additional pest categories such as mosquito-borne diseases and fungi, will be discussed in other posts.
The Mountain Pine Beetle (Dendroctonus ponderosae), native to western North America (Mexico to Canada) kills trees when the larval stage bores into living wood, including vascular tissues, thus cutting off the water supplied by the roots to the stem and leaves as well as nutrients needed by roots. An infected tree can be killed in a few weeks.
In the United States, Dendroctonus beetles attack pines especially ponderosa, lodgepole and limber pines, and to a lesser extent other species of pines. Periodic outbreaks, which are natural features of the beetle’s life cycle, can be very damaging and appear to be increasing in intensity and area covered in response to climate change. Outbreaks are often a response to disease, fires or drought-stress in trees with lower resistance to beetle attacks.
Frequently associated with the bark beetles is the blue-stain fungus (Grosmannia clavigera) whose spores reach the galleries formed in the trunk of the tree via special structures on the head of the adult beetle. Because the spores of the fungus are sticky, they are moved from tree to tree by wandering adults, assisting in dispersal of the fungus. In exchange, the fungus reduces the production by the infected tree of a toxic resin which inhibits or kills the beetle. This the fungus accomplishes by breaking down the toxins in the resin and even metabolizing them. As do all fungi, blue-stain fungi reproduce from spores that produce thread-like cells that develop into matted networks of tissues called mycelia. The fruiting body of this ascomycete is blue and the mycelia releases blue pigment in infected wood. In trees attacked by beetles, the mycelia can obstruct the conducting tissues, blocking nutrient supply from the tree crowns, killing the tree. In many trees, it’s impractical to conclude whether the beetle or the fungus has killed the tree.
Beetle larvae overwintering under bark synthesize glycerol in their tissues. The glycerol, which acts like antifreeze, protects the beetles from low temperatures. However severe freezes in autumn before larvae produce glycerol kills them- temperatures falling 300 below 00 Celsius for at least 5 consecutive days are needed for a large kill-off and are believed to be the historical factor limiting the pine beetles range. In early 2014, low-enough temperatures occurred but too late to affect many larvae as they had by then burrowed deep into the wood and produced plenty of glycerol.
U.S. Forest Service surveys shows that the “epic devastation” described for many western states in the later years of the previous decade due to a Dendroctonus infestation is continuing but at a diminishing pace. It appears, in many areas hard-hit by the outbreak, the beetle is running out of susceptible trees, especially lodgepole pine (climateprogress.org). Warmer winters had encouraged the beetle population build-up before the 2009 peak outbreak. Clearly warmer winter temperatures benefited the beetle but other factors that are an outgrowth of forest management in recent decades, including harvesting regimes and fire suppression, and intensified droughts, interact such that no single factor is identifiable as the primary cause of the massive tree loss.
Galleries (tunnels) made by bark beetle larvae in living wood of pine. Several larvae are visible. (ext.colostate.edu).
Nevertheless, at least in British Columbia and probably elsewhere, range expansion in Dendroctonus is limited not by host availability but by climate. Computer modeling using historic weather data, beetle distribution and climate effects on pine beetle brood development shows the species to be increasing in range northward, eastward and at higher elevations in British Columbia over the 1921 to 2000 period (Carroll et al 2003). In 2012, aerial overflights of outbreak areas revealed the magnitude of the problem- it was possible to fly over affected pine forests in British Columbia for an hour or more and see only a few living pine trees. In Colorado, meanwhile, during 2008-2010, the beetles were found to have emerged up to 2 months earlier than historically, enabling them to complete two instead of one generation before cold temperatures slowed their progress. The extra generation means a larger beetle population and a potential for a greater outbreak. In the Pacific Northwest states, climate change is reported to be already increasing fire risk and consequent insect and disease outbreaks and is projected to continue doing so at least until 2040.
Recent research has suggested how beetle outbreaks can affect forest carbon balance. Ordinarily a healthy, growing forest serves as a net “sink” for atmospheric carbon dioxide (CO2) because CO2 uptake via photosynthesis in leaves and subsequent fixation of the sugars and other carbon compounds in tree tissues (mainly wood) exceeds the release of CO2 from tree respiration and the decomposition of organic matter in soils. However, a study in British Columbia of a major outbreak in 2008 (an order-of-magnitude greater in area and severity than any known prior outbreaks) showed that net CO2 uptake by the forest was reduced by the widespread weakening and killing of trees which ordinarily absorb CO2 from the atmosphere while generation of CO2 from decomposition of the mass of dead material from the beetle-killed trees increased. In other words the forest was converted from a net sink to a net “source” of CO2 during and just immediately following the outbreak. At the regional scale, the magnitude of the net source was large-about 75% of the annual carbon emissions from forest fires occurring throughout all of Canada. By exacerbating the severity of the beetle outbreak, climate warming is thus seen to be reinforcing the positive feedback which accelerates the release of CO2 to the atmosphere (Kurz et al 2008). Similar shifts in carbon balance may be operating in other forested areas such as in the tropics where large tracts are deforested by humans, leading to accelerated carbon release from the burning of the dead biomass. The replacement land use, mostly agriculture, fixes relatively little carbon and is likely to operate as a net carbon source at least until the land is abandoned and supports trees again.
Carroll, A.L. et al. 2003. Effects of Climate Change on Range Expansion by the Mountain Pine Beetle in British Columbia. cfs.nrcan.gc.ca
Kurz, W.A., et al. 2008. Mountain pine beetle and forest carbon feedback to climate change. Nature vol 452: doi:10.1038/nature06777:
Reardon, S. 2011. When trees attack, fungus can parry. news.sciencemag.org, (Jan. 24, 2011.)
(Science/AAAS News 2012-03-16. news.sciencemag.org)