Today,
I took a walk in a forest near Tulsa. Spring came late this year for most of
the United States. The buds of most of the woody plants have begun to open, but
very few leaves have expanded. One kind of tree, the black cherry (Prunus
serotina), has opened its leaves. And as soon as the leaves opened, they
were eaten by tentworm caterpillars (Malacosoma americanum).
Mindlessly
and cruelly efficient, that’s what it was. Dozens of hungry tentworm
caterpillars hid inside of silk tents that they wove where branches diverged in
wild cherry trees. While it looked soft, the silk was actually very tough.
Though a bird would easily see the caterpillars through the translucent fabric,
it would take a lot of messy work for the bird to tear through the fabric and eat
them. At night, when the birds cannot see them, they slip out of their tents
and eat the young leaves. It seemed like a perfect arrangement for the benefit
of the caterpillars. Tents festooned cherry trees throughout the forest.


This
was not merely an interesting observation. It was observations like this that spawned
a whole branch of ecological research. Why is the world green? Given the
astonishing ability of insects to multiply their numbers, why have they not
eaten every leaf and sprig of grass on the planet? Outbreaks such as locust
plagues prove that they could do so, given the opportunity. What stops them?
The answer is, lots of things. The interaction between plants and the animals
that eat them (collectively called herbivores) is dynamic and constantly
shifting.
Despite
what seemed like an easy feast, there were lots of chances for things that
could go wrong for the caterpillars. Like most plants, the cherry tree produces
toxins in its leaves that inhibit the growth of herbivores. The cherry leaves,
like the leaves of all the other plants in the deciduous forest, are not a big
salad bowl. Toxin production, however, is metabolically expensive. To make the
toxins, the leaves must use energy and molecules that they would otherwise use
for growth and food production. That is, if the leaves defend themselves more,
they grow less.
Young
leaves are often tender and have relatively few toxins. This appears to be the
case with wild black cherry. If the tentworms are going to eat them, it is best
to do so early in the spring. If the eggs hatch too late in the spring, the
leaves may be tougher and more toxic. That is, the caterpillars must get their
timing right. I looked around me and saw that the leaves of most of the trees
were just emerging. Black cherry was one of the earliest trees to open its
leaves.
But,
aside from encountering leaves that may be harder to eat, what problems might
the caterpillars encounter if they emerge too late? Black cherry trees produce
nectar in their flowers (which open later in the spring), but also from
“extrafloral nectaries,” structures on their reddish bark that produce nectar.
Nectar inside a flower attracts pollinators, but what benefit might the cherry
tree get from producing nectar on its bark? In numerous other species,
extrafloral nectaries attract and feed ants. When the ants visit the cherry
tree, they do not just eat nectar. If they encounter big packages of protein,
such as tentworms, they will swarm over them and eat them. As the spring
progresses, ants become more common and they search a larger and larger area.
Late tentworms might find themselves under attack. They need to hide and pupate
soon if they are to have a chance.
But
the caterpillars must also not hatch too early. In a previous year in this same
forest, I found dozens of tents filled with caterpillars, and no leaves for
them to eat. The particular pattern of weather conditions that year had tricked
the caterpillars into hatching too early. That year, many or most of the
caterpillars probably starved. This event interrupted what might otherwise have
been a year-by-year population explosion of tentworms.
Herbivores
often specialize on certain species of plants whose toxins they have evolved to
tolerate. Some herbivores, such as gypsy moths, seem able to eat almost any
kind of tree leaf. But even they have their limits. They do not eat grasses,
for example. These tentworms, however, seemed to eat only black cherry leaves.
Perhaps this was because they were the leaves that were available at the right
time. I decided to look more closely to decide if this might be the case.
The
black cherry trees were almost, but not the only, early leaves. The invasive
Bradford pear (Pyrus calleryana) is the tree that wakes up earliest in
the springtime. Before the buds of any other woody plant open, the Bradford
pear is in full white bloom. By the time the tent caterpillars swarmed over the
wild cherry trees, the Bradford pear leaves were already out. Why were there no
caterpillars eating their leaves? Perhaps the pear leaves had toxins that the
tent caterpillars could not tolerate. This seemed unlikely, because the pears
and the cherries are closely related species in the rose family. The same is
true of the serviceberry leaves (Amelanchier canadensis). But I had
nothing to go on. All I knew was that the wild cherries had caterpillars and
the serviceberries and the invasive pears did not.
Or
did they? One of the habits of a successful scientist, whether professional or
amateur, is to keep looking closely, to not be satisfied with a quick glance.
After seeing dozens of caterpillar tents on black cherry trees, I finally found
one on a Bradford pear. The tent was small, and the caterpillars were short and
skinny compared to those on the cherry trees. They had not eaten very much, and
this meager diet would almost certainly cause them to starve before reaching
adequate size for pupation. I also found one tent, similarly small and with
scrawny caterpillars, on a red oak tree (Quercus rubra).

Tent
caterpillars have been widely reported to prefer cherry trees, and this is
certainly what I see every spring in this particular forest. But they have also
been found on other kinds of trees; my observation of tentworms on a red oak was
therefore unusual but not something to write home about. It is difficult,
without extensive research, to know why the tentworms prefer cherry trees.
Perhaps it is because the caterpillars often eat cherry leaves, and when the
adults emerge to mate, they look for cherry trees as places to lay their eggs.
This cycle of preference from one generation to another might maintain the
association between tentworms and cherries. This, however, is not a very
convincing explanation. As I saw on just a single day of exploration, the
tentworms occasionally hatch on and try to eat other kinds of trees. It would
not take long for the tentworms to spread to other tree species, if the leaves
were just as suitable a food for them as are cherry leaves.
Still,
if the tentworms begin their feast on the right kind of tree, not too early,
and not too late, they would seem to have it pretty good. But the natural world
is full of perils. Dozens of species of other insects attack or parasitize the
eggs, caterpillars, or pupae. Though I cannot find a published confirmation of
this, I suspect that some of the parasites may affect the nervous system of the
caterpillars in such a way as to alter their behavior. There are parasitic
worms that cause strange behavior in, for example, snails. In particular, the
worm makes the snail climb out on a twig tip where a bird can eat it. I have
seen a few tentworms, in the daytime, on the outside of their tents, where birds
could easily find and eat them. Was it because parasitic worms influenced their
behavior? Perhaps so. The caterpillars would occasionally twitch!
Finally,
the effects of the tentworms on the cherry trees may not be as great as it
would at first appear. I have seen hundreds of cherry trees infested and
completely denuded by these caterpillars, but I have not seen any of them die.
Since I did not mark the trees, I cannot be certain; but there are certainly
not very many tentworm victims. Since the tentworms must finish their work as
quickly as possible, well before the end of springtime, the cherry trees simply
grow a new set of leaves once the caterpillars have pupated.
No
matter what the cherry tree does, there is a cost. It could produce costly
toxins early in the spring, thus defending itself from tentworms; or it could
allow the leaves to be eaten, and grow them back. For reasons that at least I
do not know, evolution has selected the latter option for the black cherry.
All
this, from just looking closely at and thinking about something I saw while
walking through the forest.