In Part 1, we looked at the curious case of a webworm infestation of one unlucky American plum on our campus.
Here was the poor tree on May 26:
Certainly a blow to the tree. I had a couple people asked me what I planned to do, and I said, “Let’s wait and see.”
Here it was on July 12, less than 7 weeks later:
A few thin patches, but not too bad, considering that it was completely defoliated. Often the best prescription for ecological issues is patience.
What you can’t see in the above photo is the flowering dogwood tree in the back. On May 26, the plum was devastated, but this particular species of webworm wasn’t interested in the dogwood tree at all.
This highlights a very important theme in the plant/herbivore relationship. Plants want to grow and not be eaten. Herbivores want to eat plants. We have a recipe for an evolutionary arms race.
Plants wear thorns. Giraffes evolve long tongues. Other plants are tough and sandpapery. Cows evolves thick, tough tongues.
In addition to physical barriers, most every plant species has also evolved a chemical cocktail that herbivorous insects (or birds, mammals, etc) find unpalatable, indigestible, or outright poisonous. Here the herbivores are faced with a fork in the road: either eat just a little bit of every kind of plant, or specialize on just a few plants and evolve resistance to those particular chemical compounds. Renaissance man (insect) or highly specialized expert.
Since there are thousands of plants with different chemical signatures, it’s “easier” for any given insect species to pick one small group of plants and specialize, evolving defenses only for that chemical compound. It appears that the majority of insects have chosen to specialize rather than generalize. The monarch butterfly is perhaps the most famous of example, with it’s preference for milkweed. It can feed on several milkweed species in the Asclepias genus. It has evolved the ability to withstand the milkweeds’ sticky, toxic sap as it chews, as has the milkweed beetle, milkweed bug, and the milkweed tiger moth.
Back to our Tortrix moths (or webworm, or leafrollers). At the risk of demolishing my own argument, it appears that most moths in the subfamily Tortricidae are actually polyphagous (that is, they eat several different plants). But I was struck by the fact that only our single American plum, surrounded by several other plant species, was chosen for this moth buffet.
This has implications for our management of natural areas, and of more landscaped settings.
#1: The more biodiverse our flora (plants), the more biodiverse our fauna (animals). If each plant species can host several new insect species, the more players we have in the game. These of course form the base of the food chain. As populations ebb and flow, higher predators (like birds, mammals… and humans!) have options to fall back when circumstances inevitably change.
Following from #1…
#2: Biodiverse communities are resilient communities. The emerald ash borer, Dutch elm disease, Hemlock woolly adelgid, Chestnut blight… each of these insects or pathogens were introduced to the U.S. and devastated populations of native trees that had not co-evolved defenses with the alien newcomers. Beautiful neighborhoods with row after row of only mature ash trees were reduced to tree-less lawns. Squirrels had nowhere to nest, shade was non-existent, baby birds have now leaf-eating caterpillars to eat. Having a large cast of players ensures that if one is down for the count (due to a disease outbreak, drought, etc), another species can fill in for it. Ripple effects of such a trophic cascade are minimized.
This is precisely what I was thinking when I first saw our campus. I saw lots of mature silver maples and only a few other species, and only a few young trees. We made the difficult decision to selectively remove some large silver maples that were posing a danger to people or property and replaced them dozens of new trees comprised of over 20 species. We should be better positioned for the next unforeseen outbreak.
Following from #2…
#3: Each species has a role, and we are ignorant not only of what those roles are, but even how many or which members are around. Most of the species have been evolving for thousands or millions of years to get where they are today. We are dabbling with mysteries that we are only vaguely aware of.
On our campus, we have plenty of migratory and resident birds that fill the air with song and flashy displays of aerial stunts. Undoubtedly, some of them rely on moth caterpillars, cocoons, or flying adults. To varying degrees, they depend on the life cycles of various insects.
Following from #3…
#4: The above principles suggest that we should default to the precautionary principle. That is, before we do anything, let’s do no harm.
I’ll stop the train of thought there, but that’s a part of the process of beginning to “think ecologically,” not just with the surficial concerns that are immediately obvious.
John Muir put it another way: “When we try to pick out anything by itself, we find it hitched to everything else in the Universe.”