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In July 1646, John Winthrop, the governor  of the Massachusetts Bay Colony wrote: “Great harm was done in corn, especially wheat and barley, in this month by a caterpillar, like a  black worm about an inch and a half long.” “It was believed… that they  fell in a great thunder shower,   for… yards and other bare places, where not  one of them was to be seen an hour before,   were presently after the shower  almost covered with them….” “...the churches kept a day of humiliation, and  presently after, the caterpillars vanished away.” This apparent bug storm was just  the first recorded instance of   what would become known as “caterpillar years.” And about once a decade ever since,   massive population booms have continued to  slam different regions around North America. So what’s the deal with these  recurring caterpillar plagues? [♪♪ INTRO ♪♪] The Bizarre Beasts pin club is open  for subscriptions for the whole month! Sign up by September 20th and your  first pin will be a tent caterpillar.

Tent caterpillars are any of the couple  dozen species in the genus Malacosoma,   which live in temperate regions around the world. They’re moths, but they’re mostly  known for their larval stage when   they spin silk sheets across tree branches, so they end up being called some variation   of “tent caterpillar,” instead  of something more moth-centric. They eat leaves as larvae, just so many leaves, but they don’t eat at all as adults.

And as larvae, this genus is well  known for its social behavior. Butterflies and moths usually  tend to be more or less solitary,   but tent caterpillars represent part of the  small fraction that are pretty gregarious. They cooperate to build their silk shelters,   and they leave pheromone trails while  foraging to help other individuals find   their way to new food sources, and maybe  also to help the colony stay together.

Their aggregation habits are why these  species get so much attention to begin with, like, if you live somewhere that gets tent  caterpillars, you’ve almost definitely noticed. And a few Malacosoma species,  especially in North America,   have social tendencies that  are impossible to ignore. The eastern tent caterpillar is  maybe the most well known of these.

It lives in North America from the east  coast to almost the Rockies, and is the   likely culprit for that wormageddon reported  by Governor Winthrop back in the 17th century. And the western tent caterpillar  is found, as its name suggests,   throughout the western half of North America. These species, among others, are  known for living socially in silk   nests that engulf tree branches during  their earlier developmental stages.

And we humans often freak out when these appear,   but they’re not the signs of  an impending, invasive doom: just a natural part of the life  cycle of these native moths. The tents provide protection while the  caterpillars eat the leaves off of the   branches underneath, after which they start  periodically venturing out to find food,   and then returning to the tent to safely digest. Some species seem to prefer stonefruit trees,   but certainly won’t turn their caterpillar noses  up at any other tree they might come across.

As they get bigger and require more food,   they wander off and become mostly  solitary, and stop making tents. Meanwhile, forest tent caterpillars roam  around for their whole larval stage. They’re found all over North America, and weirdly  enough, they never actually make true tents: instead, they construct more  of a series of silk blankets   that they hide under together  when they stop to rest or molt.

Not having a central tent to return to each  day means that this species mass migrates   across trees as they feed, and they’re  the only tent caterpillars known to do so. The common factor for all  of these species, though,   is that the caterpillars don’t just  live together in groups, they swarm. These tent caterpillars have  loosely periodical population   explosions that cause them to absolutely  dominate the forests they live in.

During these explosions, they cover massive areas,   congregating so densely that up to 20,000  caterpillars have been reported in a single tree. And the scale of destruction  during outbreaks is even worse. We’re talking recurring, “completely cover entire   sections of forest and then eat every  leaf” levels of caterpillar-pocalyse.

And although forest tent caterpillars don’t do  the same tenting thing as the others, they can   massively wipe out whole contiguous sections of  forest on an even bigger scale than their cousins. Their outbreaks can cover a total of tens  of thousands of square kilometers at a time. Weirdly though, being fully defoliated  is something that most trees are able   to bounce back from in just a few  weeks without long-term damage.

And, luckily for the trees, this  isn’t an all-the-time thing. But it has been repeatedly happening for about  as long as anyone has been keeping track. Early records are pretty inconsistent,   making it hard to say for sure which species  had population spikes when or for how long.

But it’s generally clear that tent caterpillars,   especially in North America, have long  followed these plant-destroying boom cycles. They’re often cited as occurring every 10 years,   but it varies not only by species and by  region, but also within both categories. Depending on who you ask and  where you’re talking about,   species like the western tent caterpillar  reach outbreak numbers every 6 to 11 years,   while for forest caterpillars, it  can be more like every 9 to 13 years.

This timing is made even messier by the fact that   each caterpillar heyday lasts for  an indeterminate number of years, usually a few in a row, though. But in any case, when it does happen,  it seems like it hits all at once. After hatching, young caterpillars  quietly eat just a little bit at a time… until they’re big enough to go totally HAM.

After their final molt, they suddenly  and exponentially crank up their rate of   leaf consumption, eating over 80% of all the  food they’ll ever eat in their entire lives. This means that the full scale of outbreaks isn’t  really obvious until the last caterpillar growth   stage, when defoliation appears  to just happen out of nowhere. But why do these outbreaks  happen in the first place?

Short answer: we don’t know for sure. Sometimes, animals occur in overwhelming  numbers so that predators can’t keep up. One of the best examples of  this is periodical cicadas.

Adults in a local population all  come out pretty much simultaneously,   and do so exactly every 13 or 17  years, depending on the species. The suddenness and periodicity mean that there’s   just way more prey than the  predator population can eat. But tent caterpillars are  kinda around all the time,   and only happen to reach nightmare  proportions on an approximate timeline.

And more importantly, predator population  numbers seem to keep up with the caterpillars. So the evolutionary origins of tent caterpillars’  whole deal might actually be a lot simpler. Tent caterpillar moths lay eggs  in clusters on tree branches,   so the larvae that hatch in the spring  are already physically close together.

But those caterpillars staying together is likely   due to silk trails making it easier  for individuals to find one another. Even solitary caterpillars  often mark their trails,   sometimes to give themselves a  silk surface during their travels,   sometimes to make it easier to find their way back  to some choice leaves they found the day before. Other caterpillars finding and following these  trails can result in different colonies of tent   caterpillars joining together, different  species of tent caterpillars co-foraging,   or even tent caterpillars hanging out in  the tents of entirely different moth genera.

Grouping together helps caterpillars  to forage cooperatively, to build more   silk traction on slippery surfaces,  and to make those protective tents. So for social species like tent  caterpillars, what we’re seeing   may be something as simple as a successful  survival strategy paying off in a big way. It causes a gradual ramp up  toward a population boom – which is followed inevitably  by a naturally-occurring bust.

Because infinite population growth is impossible,   and outbreak-level numbers just  can’t hold up indefinitely. I mean if nothing else, they’re  bound to run out of trees, right? But the actual consequences of getting  so numerous are many and varied, and   there seem to be diminishing returns in adult egg  production after populations reach peak numbers.

This decline is connected to several concurrent  things that happen when it’s caterpillar o’clock. For starters, the caterpillars get eaten. A lot.

Some birds, mammals, and amphibians try  to partake of this tree-webbing bounty,   but the hairs that cover the larger  caterpillars don’t digest that well. A couple species of cuckoos seem  to have no trouble with this,   though, and tend to lay more eggs as  caterpillar populations increase – they’re predators who can  keep up with so many prey. And lots of different insects also eat tent  caterpillars during every stage of their   development, some of which even sync their  own life cycles up with the caterpillars’.

Parasitoids in particular tend to thrive  during boom years, and this makes sense: more caterpillars just mean more potential hosts,   which mean more parasitoids the following year,  who are able to lay eggs in even more hosts… etc. But this self-fulfilling feedback loop of  caterpillar population control is most apparent   with a nuclear polyhedrosis virus, which may be  the number one factor keeping outbreaks in check. It comes about pretty organically  as a biological control: the deadly virus is always present at  some level in any given population,   but it spreads much more easily between  caterpillars when their densities are highest.

So tent caterpillar outbreaks are kind of  automatically doomed to end in a viral crash. And interestingly, initial caterpillar survival  goes up as genetic diversity increases,   but overall resistance to the virus doesn’t. Diversifying the gene pool can often  make populations healthier, after all.

So an occasional diversity boost, maybe  from some new moths showing up on the scene,   might contribute to a rapid population  increase or even to the onset of an outbreak. But researchers have found that  this newly increased population   would still be just as susceptible  to viral infection in the end. And lastly, in the same way that  rising caterpillar populations get   increasingly sensitive to biological  factors like the ones we just covered,   they also get more susceptible  to circumstantial effects.

For example, when all the good  egg-laying spots get taken,   the remaining options tend to have  much worse winter egg survival. Being too successful for too long means  basically becoming a giant target for all   the synergistic factors that can  knock them back down a few pegs. Long story short, tent caterpillars are  really good at making more tent caterpillars,   but forests are just as good at not  being replaced by tent caterpillars.

These little guys are just doing what they do  best, until their ecosystem takes back control. From mesmerizing murmurations of  starlings, to a giant pile of crabs,   huge animal groups can be awfully impressive. But when they show up suddenly and start eating  everything, that awe can easily turn to horror.

So you might be wondering: is your area due for a caterpillar takeover?? Well, that depends on where you live! In British Columbia, you’re probably  in the clear until the early 2030s.

Go look for some wandering salamanders instead. In Wisconsin, on the other hand? You’re kinda overdue… But in general, if you live in  the tent caterpillar’s range,   your best bet is to just look up recent  local outbreaks and follow the pattern.

And when you finally do see those silk-covered  trees getting devoured by millions of tiny,   wriggling beasts, maybe now that  sight won’t seem quite so bizarre. Sign up for the pin club at BizarreBeastsShow.com  and help us keep this channel going. If you want a caterpillar to be your  first pin, sign up by September 20th.

Thank you for watching Bizarre Beasts! You help our non-profit bring more educational  videos to more people every time you join the pin   club and every time you watch, comment, share,  and tell people about a Bizarre Beasts video. And if you like wearing cool things and want  to let everybody know it, we made a new shirt!

This brand new shirt highlights twelve of our  favorite arthropods from Bizarre Beasts episodes   like Christmas Island crabs, book scorpions  and the tent caterpillars from this video! Show your support for Bizarre Beasts everywhere  and get yours right now at BizarreBeastsShow.com! And now, for some bonus facts… [♪♪ BONUS FACTS ♪♪] Tent caterpillars may seem  threatening if you’re a leaf,   but to many of their predators and  parasitoids, they’re an easy meal.

Birds have no trouble pecking through their  tents, and braconid and ichneumonid wasps   are eager to get their virus-y eggs into  their larvae and cocoons, respectively. But one of their biggest threats is a parasitoid  fly that specializes in tent caterpillars. In order to successfully parasitize a host,   the fly needs to lay its egg  squarely on the caterpillar’s head – anywhere else, and its would-be  victim can just bite it off.

As a result, tent caterpillars flick  their heads as a defensive move. Flailing around like this is actually  pretty common in caterpillars generally,   and can also serve as an effective way to just  bash potential nest-invaders out of the way. And when a whole colony gets a-thrashin’,  you’ve got yourself a much tougher target.

But cooler still, tent caterpillars have evolved  to react this way in response to specific sound   frequencies, based on the sounds most  likely to be associated with threats. However, this does have the unintended   consequence of making them pretty  responsive to other noises, too. For example, if you happen to cough near some  tent caterpillars, they may all briefly flip out.

Consider it one more thing to  keep in mind in order to make   your next Malacosoma encounter feel less bizarre. If you like the idea of having your very own silky  accessories like a tent caterpillar, but you don’t   have the time or the glands to make your own, you  should hear about today’s sponsor Manta Sleep. They make the world’s best sleep masks and  functional sleep accessories, like this one - the Manta Silk Sleep Mask, that  they were kind enough to send us.

It’s the most luxurious sleep mask on  the planet, crafted from super-soft,   ultra-premium mulberry silk. Which, if you’re craving  yet more caterpillar facts,   comes from the cocoons of silkworms that  are exclusively fed mulberry leaves.   Which is exactly the level of decadence you’d expect  to go into a mask that feels this luxurious. It’s fully adjustable and gentle on your skin,   and protects your eyes and  lashes like no other sleep mask.

Naps give you energy, focus,   strength and clarity that you don’t get  when you grind through the afternoon. So check out Manta Sleep and make sure to use  code bizarrebeasts for 10% off your order! And then, take a nap! [♪♪ OUTRO ♪♪]