The Case of the Missing Budworms

Spruce budworm is a caterpillar for most of its life, then briefly a moth which feeds on the buds and young expanding shoots of several boreal conifer species, especially white spruce. Spruce budworm is distributed across Canada and irregularly appeared in marginal numbers in eastern interior Alaska. Until recently, that is.

Budworm numbers erupted (in biology such a population outbreak is an “irruption”) in central Alaska in 1989-1990, 1993, 1995, and for sure in 2006-2007. Across Canada, spruce budworm (OK, let’s call it SBW like the pros) is responsible for 40 percent of the volume of commercial timber killed by all insects and diseases, or at least all that are monitored. So the appearance of SBW is a big deal, and for white spruce, a bad deal.

Why now in Alaska? Surprisingly, it’s not actually one of those mysteries of science. SBW kills so much timber that it’s been studied closely and well. The basic answer is the warmer the weather at critical stages during its development, the faster it develops. This means that during warm years a higher percentage of the class of hatched eggs graduates to the tree-eating and reproductive stages of life before they run out of time and are terminated by the change of seasons. And, of course, with more bugs you get more chewed-up, dead spruce.

Warm Augusts give the SBW first larva stage a head start on their second larva stage. Big deal, you say! Well, yeah, actually. Because when SBW are in the fully developed second larva stage their numbers are not, repeat not, significantly reduced by winter cold no matter how severe.

So, by contrast, cold Augusts won’t let the SBW reach full second larva stage development before the first frost thins the herd. So, after a warm August a warm May/June the next year speeds extra high numbers of the previous year’s SBW class on to reproductive adults. Now you’ve got an “irruption” on your hands.

So, here’s they mystery. August 2007 was fourth warmest in the 104-year Fairbanks record. And if you shook a spruce tree in May or June 2008 in much of central Alaska various SBW larva stages fell out. OK, on track for irruption. But by early July 2008 SBW were hard to find. And you will look long and hard to find a 2008 pupa case that a reproductive adult moth cracked open and flew from.

So what do you think happened?

Also, August 2008 weather in central Alaska - lack of warmth in the daily high temperatures but not particularly cold overall - is really poor budworm weather. So your spruces should get a break in 2009. But a new El Niño in the Pacific may give a boost to Alaska summer 2009 temperatures, so that you shouldn’t count the SBW out for 2010.

post by Glenn Juday
Professor of Forest Ecology

Further reading:

• Wikipedia entry on Spruce Budworm
• Alaska's Changing Boreal Forest, by Francis Stuart Chapin, et al. 2006. Oxford University Press.
• Alaska Division of Forestry Alaska Forest Health Protection Program
• "Spruce Beetles, Budworms, and Climate Warming," by Glenn P. Juday, global glimpses v. 6 n. 1, April 1998. Center for Global Change & Arctic System Research.