Pandora Moth defoliation returns to the Kaibab

The Pandora Moth (Coloradia pandora) is a native defoliator of ponderosa, Jeffrey and lodgepole pine trees of the Western US. During most years, populations and defoliation is minor, but periodic outbreaks cause widespread defoliation (Speer and others 2001). The outbreak shown here in Arizona's Kaibab National Forest first caused significant defoliation in June-July of 2013 and defoliating caterpillars have now returned in 2015.

One of the more peculiar aspects of outbreaks is that while large moth populations persist for half a decade or more, their two-year life cycle causes severe defoliation by larvae to occur every other year. Pandora moth populations are eventually checked by a virus (the nuclear polyhedrosis virus, NPV) as shown by these photos taken in 2013. In large part because of this, defoliation follows a shifting landscape mosaic among years during the outbreak. The area that is defoliated first appears to be at lower risk of severe defoliation two years later. As shown in these maps, the outward-moving pattern of the 2013 and 2015 outbreak has parallels with what occurred in this same landscape during the early 1980s (Schmid and Bennett 1988).

The pattern of alternate year defoliation means that while annual tree growth is reduced--as is the "greenness" measured by NDVI---tree mortality is minimal. In Oregon, old growth ponderosa pine trees show centuries of recurrent outbreaks in their tree rings and tree mortality was minimal after an outbreak in 1992 and 1994 (Speer and others 2001). In the prior outbreak on the Kaibab, tree mortality was largely limited to those that were already stressed by mistletoe (Schmid and Bennett 1988). Recent drought may contribute to tree mortality during this current event. As these outbreaks last for about six years, we can expect a new defoliation, perhaps in the areas beyond where it currently occurs, come 2017.