VINCE PATTON: Snows were melting late last year in Malheur National Forest. We'd been told to look very closely at the undersides of the branches. Sure enough, we found tiny daisy chains of eggs clinging to the pine needles.
LIA SPIEGEL, U.S. Forest Service: They lay their eggs on the needles and the eggs are there throughout the winter. She lays them right where the young are going to feed.
VINCE PATTON: The sturdy green needles bounce in the wind, but never bend. They protect the tiny, hearty orbs all winter long, but the needles are doomed.
Months later, the eggs hatch into a hungry horde. They devour the very needles that sheltered them.
ROB PROGAR, Pacific Northwest Research Station: they've eaten probably two thirds of the needle. It's going to be interesting to see what's left at the end of the season. I'm thinking this tree right back here.
VINCE PATTON: Rob Progar is an entomologist with the Pacific Northwest Research Station.
ROB PROGAR: Got it. Let's see what kind of story this branch will tell us. There's a number of caterpillars, one on this needle, one at the base of this needle. They're very cryptic. Like most creatures, you don't really notice them until they start moving.
VINCE PATTON: In a normal year, it would be hard to find these particular caterpillars, but this is year three of a massive outbreak.
VINCE PATTON: These outbreaks occur every 20 or 30 years. It's quite a natural phenomenon. Very few people actually get to witness them, because they're so infrequent. It's just one of the natural phenomena of the forest ecosystem. They're caterpillars for four to six weeks. It all depends on the weather.
VINCE PATTON: Their thin green bodies blend in so well with pine needles, it might be easy to overlook them. But the caterpillars have one final metamorphosis in store -- they drop their camouflage. And the outbreak fills the forest with more adult butterflies at once than most people will ever see.
ROB PROGAR: There'll be so many butterflies in the air that it will look like a snowstorm in August. It's simply something that everybody should come out and see.
LIA SPIEGEL: There's a lot of butterflies. It's amazing. There's a male and a female and now there's another male in there.
VINCE PATTON: Lia Spiegel studies insects for the U.S. Forest Service. She says pine butterflies die shortly after laying eggs.
LIA SPIEGEL: And last year, all the pines that I saw on this forest were covered with pine butterflies. All that white color, it's all butterflies. I've not seen anything like this, at least historically. They've never lasted more than three to four years.
VINCE PATTON: Pine butterflies live and die here naturally, in the Blue Mountains. This is a butterfly only found in the Pacific Northwest. In a typical year, just a few flit around the tops of the Ponderosa pines.
But for just a few extraordinary years, their numbers explode.
LIA SPIEGEL: It's the largest recorded outbreak of pine butterflies in the Blue Mountains ever. It's a continuous swarm, you know, not just around trees, but all between the trees, on the highways. You know, they're everywhere. If you look around here, you can see there are millions right now right around us.
So it's some astronomical number. I think it's pretty amazing that they can just pop up seemingly out of nowhere, you know, over a year or two.
VINCE PATTON: Another outbreak may not happen for another 20 or 30 years.
What happens to cause the butterfly numbers to crash for so long is just as big a mystery.
LIA SPIEGEL: We don't know what drives the populations to increase and we don't always know what causes them to decline. But they usually don't last very long.
VINCE PATTON: They really have no way to forecast how far the pine butterfly eruption will spread. In aerial services, the eaten trees have so few needles, they show up gray instead of green. The Oregon Department of Forestry estimates these soon to be butterflies ate the needles off trees across nearly 400 square miles.
LIA SPIEGEL: These butterflies can be seen for thousands of feet. Last year, we saw them up at 9,000 feet. So they're flying way up in the air.
VINCE PATTON: Warnings went up to pilots near John Day and Burns after aerial firefighting teams landed with their windshields half blotted out by smashed butterflies.
LIA SPIEGEL: From what I've seen on the ground, it's a much bigger area that's defoliated this year than last year. People that live here are kind of upset about it, because it looks like the trees are dead. I mean some of the trees, you look at them, hardly any needles. I bet a lot of people think they're dead.
But just like with fire, if the buds are still alive, they're not dead. They don't look very healthy, but most of them will probably survive.
VINCE PATTON: The butterflies, when they are voracious caterpillars, may not kill many trees outright, but they can stunt their growth for years and they do leave the pines extremely vulnerable.
ROB PROGAR: A tree of weakened vigor is more susceptible to attack by bark beetles. So the bark beetles will come through on these pine butterfly weakened trees.
Insects work together, so to speak.
VINCE PATTON: Rob Progar is noticing something odd in this third year of the outbreak.
ROB PROGAR: Dead caterpillars with the black heads.
VINCE PATTON: More pine butterfly caterpillars look like they're dying before reaching adulthood.
ROB PROGAR: So that's not a healthy caterpillar at all?
ROB PROGAR: But we really won't be able to determine exactly what killed them until we look at them under the dissecting scope. And then we have a little hemipteran here, which is a predacious sucking beetle or a bug. Actually, they're known to be predators to the caterpillar.
VINCE PATTON: Perhaps parasites and predators might contribute to the end of this explosion of pine butterflies. Maybe they'll starve because they ate themselves out of house and home.
In the meantime, healthy adult butterflies only have a month to live. They will do their best to mate and lay eggs on next year's food.
But in nature's mysterious cycle, the boom may go bust as abruptly as it began.
ROB PROGAR: It's spectacular. And this event only happens every 30 years, 20 years. It's something that I feel fortunate to be part of. Next time it happens, I'll be retired and hopefully still alive.