Mountain pine beetle (MPB) is considered the most significant mortality agent in pine ecosystems of western North America, where it attacks and kills trees. The current distribution of MPB is limited by climate, but factors governing migration southward into warmer areas are unclear. While it is known that insects can respond favorably to warm temperatures, too much warming can be detrimental to population growth.
To simulate a changing climate, USDA Forest Service researchers conducted a field-based reciprocal translocation experiment. MPB from northern Utah (central MPB range) were reared in northern Utah and central Arizona (southern edge of MPB range) and central Arizona MPB were reared in central Arizona and northern Utah. Field results confirm laboratory research showing local adaptation and genetic differences between the populations in developmental response to temperature. Even though temperatures at translocation sites differed, both populations developed with a 1 year generation time, which is the most successful developmental strategy for mountain pine beetle outbreaks. Both populations were also reared at a lower elevation, out-of-range location in Arizona. A 1 year generation time was also observed at that site, suggesting that factors other than temperature limit MPB at low elevations in Arizona. We also showed, for the first time, genetic differences in MPB capacity to survive cold temperatures, wherein northern Utah beetles can withstand colder temperatures than Arizona beetles.
Population persistence in a warming climate is possible for MPB, but temperatures must not be too hot, but just right. To sustain a generation time of 1 year, late fall and winter temperatures must be cooler than ~15°C and summer temperatures must not exceed ~25°C for extended time periods. Population-specific models will be required for predicting future MPB responses and range expansion in a changing climate.