As I write this post, it is currently a brisk -11°C (13°F) with a windchill of -17°C (2°F) outside, and scheduled to be even colder over the next few days. Most of us are likely going to stay inside, however, many other organisms do not have that option. Insects have a few evolutionary tactics that allow them to survive these harsh frigid conditions, including some invasive species.
Most insects do not survive winter, instead, they lay eggs and die when the cold hits, others use "freeze avoidance" or "freeze tolerant" strategies. Freeze avoiding insects can try and find a dry area to hibernate (or in the insect world, diapause, a dormant state). Others like the invasive Japanese Beetle grub will seek shelter and burrow in the soil (have been found up to 6 ft deep!), which has a higher heat capacity than air, thus warmer than then ambient air. Freeze tolerant insects have the unique ability to replace the water in their body with an anti-freeze like solution called cryoprotectants , which supercools the fluids within the insects body by increasing the solute concentrations within cells. Examples of cryoprotectants include sorbitol, mannitol and ethylene glycol, however, one of the more common ones used by insects is glycerol. Freeze tolerant insects can survive ice formation within their bodily tissues to some degree.
In North America, the invasive insect, Emerald Ash Borer has changed the landscape in states such as Michigan and Minnesota by killing thousands of Ash trees. They were thought to have arrived from Asia via wood packing materials and have been able to survive the cold temperatures found in Michigan. Researchers found that the Emerald Ash Borer was found to have a supercooling point (the temperature when insects do freeze). A few different studies found that that the supercooling point to be -30.6°C (-23°F) in Ontario (Sobek et. al., 2009) and between -26.4 to -23.0°C (-15.5 to -9.4°F) in China (Wu et al., 2007). Venette and Abrahamson (2010) found the average supercooling point for larvae in Minnesota was -25°C (-13°F).
The sap sucking Hemlock Woolley Adelgid (HWA) is another invasive species that originated from Japan and China, and has been detected in Michigan. They possibly came to Michigan from trees that were already infested and transported in from other states. HWA reproduce asexually and have two overlapping generations in one year (progrediens and sistens) with the sistens being the generation that spans early summer to mid spring and produce approximately 300 eggs. They are able to be transferred to other trees via wind and other animals. Sistens feed and develop during the winter, but are susceptible extreme cold weather as well, 90% mortality was found if temperatures dropped below -22°F (McCullough, D.G. 2015).
While cool weather can kill potential invasive insects, it is not something we can count on to completely eradicate invasive species from the area. In the case of the Japanese beetles, unless there is a quick freeze that prematurely hardens the soil and prevents the grubs from burrowing, they can survive cold temperatures. Some insects that utilize trees for protection may survive cold temperatures as not all sections of a tree cools equally. Sunshine, wind, direction the tree is facing may protect insects from mortality. There exists the potential for cold tolerance to be genetically linked. In this case, the few individuals that survive may pass on this genetic feature to future generations, and can quickly reproduce and rebuild their numbers, producing a population that is resistant to cold weather than previous populations.
The bottom line? Insects have evolved pretty interesting strategies to survive winter. Cold weather may help decrease insect numbers, but is not a permanent solution for large scale infestations. Prevention is important and this means checking your gear, equipment and shipments for invasive species. In the case of the two species mentioned above, transporting firewood from other regions can also bring in these insects and should be avoided!
For more information:
McCullough, D.G. 2015. Hemlock Woolly Adelgid: A little insect that means big trouble for hemlock trees in Michigan. MSU Extension Bulletin (E-3300). Found: https://www.michigan.gov/documents/invasives/HWA_Bulletin_518429_7.pdf
Sobek, S., J.C. Crosthwaite, and B.J. Sinclair. 2009. Is overwintering biology of invasive insects affected by climate change? Plasticity of cold tolerance in the emerald ash borer (Agrilus planipennis). 94th Annual Meeting of the Ecological Society of America. August 2-7, 2009 (Abstract).
Venette, R. C. and M. Abrahamson. 2010. Cold hardiness of emerald ash borer Agrilus planipennis: a new perspective. In: Black ash symposium: proceedings of the meeting; 2010 May 25-27;. Bemidji, MN. Cass Lake, MN: U.S. Department of Agriculture, Forest Service, Chippewa National Forest. 5 p.
Wu, H., M.-L. Li, Z.-Q. Yang, and X.-Y. Wang. 2007. Research on cold hardiness of emerald ash borer and it’s [sic] two parasitoids, Spathius agrili Yang (Hym., Braconidae) and Tetrastichus planipennisi Yang (Hym., Eulophidae). Chinese J. Biol. Contr. 23: 119-122 (Chinese).
Shikha Singh is the coordinator for the JLW CISMA. She has a BSc. in Biology from University of Western Ontario, and her master's and PhD at Michigan State University from the Dept. of Fisheries and Wildlife. Her areas of expertise include water quality, water policy, invasive species, education/outreach and public speaking.