Why is biodiversity so important? The case of the Tortrix, Part 1

As the on-call ecologist at The Center at Donaldson, I get pulled aside to see a lot of things. Dead birds, turtle eggs, strange insects, and albino squirrels, for example. Sometimes there is an unsolvable mystery. Other times, important clues that fill in the gaps to working hypotheses and ponderings.

“Adam, have you seen the tree in front of Lindenwood?”

On May 26th, I responded to another call and found that our beautiful American Plum tree had been taken over:

Think of your friend who is most grossed out by bugs, all things creeping and crawling. Imagine their worst nightmare. This would probably be it.

How about a close up?

I was naturally intrigued, so I stood underneath the tree and looked closely. Strings of worms several feet long were rappelling down off the bare branches on their own slime-ropes. There were so many that I could actually hear them chewing on the leaves. As several of the ropes started dropping around me, I figured it best to move out from under the tree. (I like insects a lot, but even I don’t want them crawling all over my scalp).

Ignore my comment about “bag worms,” which they were not… it was just the only vocabulary I had at the time.

Given that the tree was already 99% defoliated (and it was 4 pm on a Friday) there was really nothing to be done, other than try to ID the worm at hand.

I failed to grab and preserve a specimen (my first mistake!) but I posted the above photos and notes to a Facebook group dedicated to Insect Identification. These types of groups are a bit of a mixed bag. There are plenty of “know-it-alls” who are pretty misinformed, but I also personally know taxonomic experts who frequent these sites (often obsessively) and offer great scientific insight and identification. The internet has allowed “citizen scientists” to play an increasing role in making observations about the natural world, as well as allowing scientists to interact with a broader audience. We just have to be mindful about quality.

Someone in the group suggested that it was probably a Tortrid or Tortrix moth (from the insect family Tortricinae). They are commonly referred to as leafrollers. They further suggested it was of the Sparganothis genus.

Morphological differences between insect species can be so subtle that even experts cannot make a definitive identification of some without a microscope, sometimes not even without dissection. Unless it’s a scientific study or an intervention is planned, it is often enough to call something a “leafroller” and be done with it.

But… that won’t stop us from an educated guess!

I clicked through the Sparganothis species list on Wikipedia, looking for those who had populations in the Midwestern. From the Wikipedia citations I was directed to the Tortrix food plant database, which detailed many published observations on Tortrix moths. I wanted to see which species might be found of American plum.

There was an observation of Sparganothis directana feeding on a wild cherry tree (Prunus virginiana),  which is a close relative of American plum (P. americana). There were also are a few observations of other Sparganothis species feeding on Prunus species (S. reticulantana, S. directana, S. umbrana, and S. diluticostana). The first two species’ range includes Indiana. But that would be as close as I could get without professional help.

The moth “worms” that we saw were larva. They would then go on to the cocoon stage. I didn’t notice any cocoons in the tree, but then again, I didn’t look closely. Cocoons can also be made under the leaves, or even underground. The cocoon hatches and then moths emerge, fly around, mate, and die or get eaten. A few moth species overwinter as adults in the leaf litter. I don’t know about Tortrix, but some moth species (like butterflies) even migrate!

But back to our “outbreak.” We had one tree that was “taken over.” It was a “complete plague.” I was asked what I could do to fix this “problem.”

There is a lot of human culture hidden behind the language that we use.

As an agricultural species dependent on green growing plants, we humans have developed an inherently negative reaction to insects, which feed largely on green growing plants. It’s not surprising that we see any insect eating any plant (even if not a food crop necessary for our survival) as a “problem.”

Leaf roller moths, and many other insects, can be serious pests on agricultural crops. In order to provide food for ourselves, we need to think carefully about how and when to manage pest populations on our food crops. The tools used may be mechanical, biological, or chemical.

The new imperative of the Anthropocene is to look harder and longer at the web of connections around us, to consider the many ripple effects of our decisions.

Below is a chart detailing the phenology (seasonal timing) of a Tortrix moth that was observed in an outbreak in Michigan in the 1960’s (Wilson, 1972). I can’t be sure, but I wouldn’t be surprised if our Tortrix had a roughly similar life history. Wilson’s publication is pretty short and it’s an interesting read.This species covered hundreds of thousands of acres, primarily targeting red oaks. The initial boom started in 1965 and lasted until a population collapse in 1970. In some areas, “considerable dieback and tree mortality occurred.” A similar outbreak occurred in the region from 1956-1958.

I found these note interesting (emphasis mine):

Mature larvae exhibit typical torticid behavior when disturbed. They retreat rapidly in reverse and drop down on their silk threads. Those dropping over water attract trout and other fish. Some fishermen who noticed the activity of this “greeny-worm”, as they called them, tied trout flies to imitate them during the outbreak…

In urban areas such as in and  near the town of Mio, which relies heavily on tourism, the insect denuded and killed many of the predominant shade trees. Dead oaks along roadsides were aesthetically unpleasing to residents and tourists alike.”

Even in scientific journals, the issue of aesthetics cannot be avoided!

It is beyond the scope of the paper, but the many ripple effects of these outbreaks were not measured. Sun-loving, savanna plant species in these forests probably flourished after this outbreak, produced lots of pollen and nectar, and laid down seeds in the soil to wait for the next outbreak. Oaks temporarily on the decline left room for beech, maple, and pine trees to grow. Innumerable insect species flourished in the dead wood of the trees, as well as fungi to slowly decompose the trees and recycle their nutrients back into the biosphere. Woodcock, wood peckers, and other birds that relied on open habitat and dead trees probably flocked to the region and had several years of good reproduction, feasting on the insect buffet.

To the natural world, an outbreak at this scale wasn’t “bad,” it was just different. This difference in habitat availability at a regional scale lead to a more biodiverse web of life. Rather than a single blanket of mature oak trees, there was a patchwork buffet of many species and stages of growth, each variation attracting a different cast of characters.

Our own moth worm outbreak is a naturally occurring phenomenon, just like the huge population booms and busts of many insect species. So was it a “problem” to be “solved?”

Stay tuned for Part 2!