Sunday, March 05, 2017

Introduced Species Develop Unexpected Relationships

Ailanthus webworm moth (Atteva aurea)
© Beatriz Moisset
Somebody I know lamented that the pretty ailanthus webworm moth (Atteva aurea), despite being native, got the name of the tree of heaven (Ailanthus altissima), a Chinese tree that, after being introduced as an ornamental, has become dreadfully invasive. The explanation of this state of affairs makes perfect sense.

The ailanthus webworm moth's original geographic range extended from Central America to southern Florida where its larvae used to feed on several trees, related to the tree of heaven. Probably it was known only by Lepidopterists, those who study moths and butterflies, and not by the general public. When the tree of heaven was introduced as an ornamental, the moth took a liking to it and found it quite digestible. Now it was free to move to other places, following this new host plant. With time it became far more abundant and widespread than originally.

Nowadays it is found throughout eastern and central United States and Canada, wherever the tree of heaven is grown. In fact, it cannot be regarded as a native species in such areas, but an introduced one. Political boundaries are irrelevant. What matters is the ecological distribution.

Even more complex is the story of theColorado potato beetle (Leptinotarsa decemlineata) which is neither from Colorado, nor a potato beetle. Originally this beetle lived on Mexico and southern United States and fed on local plants related to the potato.

Colorado potato beetle (Leptinotarsa decemlineata)
larva and adult
© Beatriz Moisset
The adult beetle is roundish with ten stripes along its body. This is what decemlineata refers to. The larva is also almost round, orange with black spots. The larvae feed on buffalo bur, so called because its fruits are burs that cling to the fur of animals. The plant uses this means of transportation to spread to other places. It turns out that the beetle larvae can catch a free ride and also expand its territory by this means. Buffalo bur is related to potatoes. They are both members of the same genus, Solanum.

Buffalo bur (Solanum rostratum)
The potato beetle started getting longer rides when cattle drives became common. This is how it eventually arrived in Colorado and Nebraska. Buffalo bur and a few other members of the Solanum genus were present there, so the beetle found enough food to make a living. And then, one day, oh joy! Potato agriculture started in the United States. Potatoes turned out to be a great food for these beetles, and they were available in abundance. What a bonanza!

Flowers of potato plant (Solanum tuberosum)
© Beatriz Moisset
This is how a beetle that had been hardly noticed until then became the exasperation of potato growers and this is how it earned its common name. It never had one before. Nowadays it has spread to most of North America and, not content with that, it has managed to go to other continents wherever potatoes are grown.

"The presence of aurea in the eastern United States and Canada and its association with Ailanthus altissima (Mill.) (Simaroubaceae) is an interesting subject to be investigated. This plant is an ornamental introduced from Asia and now considered one of the most serious weeds in the United States. It was first planted near the University of Pennsylvania, Philadelphia, in 1784 (W. Thomas, pers. comm.) and from there it spread over the entire country. Once it reached southern Texas, where presumably aurea was already present, the moth started to move north. By 1856 it had reached Georgia, as indicated by the material described by Fitch (1856: 486). Riley (1869: 151) found it common in Missouri, feeding on ailanthus. These records indicate that this showy and common moth was absent in the region before the introduction of ailanthus, and the approximately 70-year gap between the introduction of the host, to the first record of the moth by Fitch, is the time it took the plant to move south and the moth to move north." 

Monday, February 06, 2017

Funny Larvae

Firefly larva © Corey Kallstrom
Who would think that insect larvae can be interesting? They don't have much to offer when compared to their adult counterparts. We can't help but notice adult insects. They fly, they buzz; in fact they can make quite a racket during hot summer nights. Some blink their little green lights over the meadows. Others wear flamboyant colors that seem like works of art.

Glowworm (Phengodes sp.) © Ashley Bradford
Larvae, on the other hand, at least most of them, lead obscure, secluded lives, hiding in secret places. They are rather colorless and shapeless worm-like things, with stubby little legs or no legs at all, with tiny eyes or no eyes at all. None of this is surprising. Larvae have only one function in life, eating and growing bigger. They have nothing to do with the more exciting things like sex. That is left for the adults. Well, larvae actually have two functions. They possess strategies for fighting or evading enemies. Despite such dull lives some insect larvae manage to be quite interesting.

Lightning bug larvae have green lights along their bodies. They look like little trains, and that is what we call them in Argentina, trencitos. One wonders about the purpose of these lights. Adults use theirs to attract members of the opposite sex, but larvae have nothing to do with that. These lights must serve another purpose. It turns out that lightning bugs are highly toxic and bad tasting, so the lights are telling predators to stay away and avoid the unpleasantness.

Monarch caterpillar
© Beatriz Moisset
Some caterpillars use the same strategy. The monarch butterfly caterpillar wears bright black, yellow and white colors arranged in a bold pattern to advertise its toxicity obtained from feeding on milkweeds. Other caterpillars have a variant of this method. The eastern swallowtail butterfly is large and colorful with a pattern of black and yellow. The larvae change their appearance dramatically as they age. The nearly fully grown caterpillar is bright greenish yellow with two large eyes on its head, except that these are not real eyes but just look like them. It is thought that this feature serves to deter predators that erroneously assume that they are dealing with a larger and more powerful creature at the sight of these large eyes. They have another feature for defense against enemies. When threatened, they pop up two bright antenna-like threads. These appendages emit a nasty smell that seems to be effective in keeping other creatures away.

Eastern swallowtail butterfly caterpillar.
Showing its osmateria when threatened
© Beatriz Moisset
Some larvae feed on extremely poor food, such as leaves or wood. It is rather remarkable that they manage to extract nutrients out of them, especially the ones that feed on wood. Most of what they eat goes through their digestive system and comes out of the other end with little modification. A partially rotten log may contain long tunnels filled with what looks like sawdust. They are the holes made by beetle larvae as they grow and advance in search of more food. The sawdust is their poop, which takes the name of frass. Some caterpillars live inside a bunch of leaves which they tie together and curl up so they can hide from predators while they munch away. If you open one of these bundles you find a large amount of frass surrounding the caterpillar.

Caterpillar and some of its refuse or frass
© Beatriz Moisset

Larval tunnels on wood (possibly a beetle)
filled with frass
© Beatriz Moisset
At the other end of the spectrum, there are larvae whose mothers have provided with a rich, highly nutritious food. Many wasps and bees do that. In the case of wasps, the food is insects or spiders; bees, on the other hand, collect pollen and nectar for their brood. These larvae live in splendid isolation inside small compartments, called cells, built by their mothers. The amount of food supplied is slightly larger than the future adult bee or wasp. Almost all of it is converted into flesh and nothing is wasted. At the end of the larval stage, only a small pellet of fecal matter is produced. It gets the name of meconium, comparable to the first bowel movement of a newborn baby. The baby's meconium is the accumulated waste of a few months; so the similarity is strong.

Cell of Crabronid wasp larva (Trypoxylon collinum)
Opened cell reveals the fully grown larva and the pellet of meconium (left)
No remains of the spiders that fed this larva
© Beatriz Moisset
A lot more can be said about larval poop. A number of larvae use their own fecal matter to keep enemies away. Some carry their own feces as an umbrella, others build a little case. They hide inside it the way a snail hides inside its shell. Some insect larvae don't use real poop, but it looks like it. A whole group of caterpillars is called bird-dropping caterpillars. Still others fling their waste as far away as possible so as not to give away their position to possible enemies. I don't need to repeat these stories as I have written about them in two articles that you may find interesting: "The Poop Bug and the Golden Beetle," 2009 and "Poop Flinging and Other Poop Strategies," 2012.

Golden tortoise beetle larva carrying its feces
© Beatriz Moisset

Neochlamisus beetle larva and its fecal case
© Beatriz Moisset

List of articles
Beginners Guide to Pollinators and Other Flower Visitors

© Beatriz Moisset. 2017