Alien Invasions. Our Native Butterflies and the Intruders that Threaten Them

The invasive cabbage white Pieris rapae, a pest of cabbage and related plants.

When we see a white butterfly it is most likely an invasive European species, called a cabbage white. Its beauty often blinds us to the fact that it is an unwelcome intruder which behaves as a pest. Growers of cabbages, radishes and other plants in the mustard family hate the damage that its caterpillars do to plants. You may have seen that same damage if you grow ornamental cabbages in your garden. The cabbage white or Pieris rapae, is a sturdy fellow that does well in many habitats and keeps spreading throughout the continent.

West Virginia White (Pieris virginiensis), beleaguered by non-native plants and butterflies.

The North American native species of white butterflies are well behaved, feeding in a number of native plants, such as toothwort, watercress and other members of the mustard family. They are no threat to native nor cultivated plants as their numbers are not overwhelming and a good balance has been reached between butterflies and food plants.

There are four native species; they all look very similar but you can tell them apart by subtle differences. The lovely green veined butterfly or mustard white butterfly (Pieris oleracea) lives in woodlands across northern North America and in the Rockies and Appalachian mountains. The West Virginia white (Pieris virginiensis) is found in the Appalachian and Great Lakes regions. These butterflies face several threats to their survival; in addition to the more familiar one, loss of habitat, there is a more serious and insidious one due to the presence of an invasive plant, garlic mustard.

Garlic mustard (Alliaria petiolata)

Garlic mustard (Alliaria petiolata) is very familiar to most of us; an introduced invasive species originally from Europe and Asia; it was brought to America as food, although nowadays nobody uses it for this purpose. It promptly escaped cultivation and now is most commonly found in areas disturbed by humans. It can be found almost anywhere in the eastern United States, especially in the spring, growing vigorously in sun or shade, blooming and spreading its seeds. It belongs to the mustard family and this is the undoing of our native white butterflies. Females can't tell the difference between this plant and others of the same family and so they lay their eggs on this plant. Little do they know that garlic mustard is toxic to their babies; their larvae don't do well and die as a result.

One of the reasons for the successful spread of garlic mustard is that when brought to North America, it left behind most of its enemies, about 63 species of insects, including several species of butterflies. It is ironic that the one that tries to take advantage of it is being poisoned and killed by it.

You can learn more about the West Virginia White butterfly and conservation efforts in
Conservation Assessment for the West Virginia White (Pieris virginiensis Edwards) and garlic mustard and methods of control in Missouri Department of Conservation website.

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Bugs in winter

If you live in latitudes where winters are cold enough for snow and ice, you see most insects disappear without hardly any traces. And yet, they come again in full force next spring. Where do they go? Aside for just a small handful that migrates to warmer climates, the rest are left with no choice but to find a hidden place to spend the winter. Unfortunately for us, some of them have figured out that human surroundings are nice and cozy and find shelter inside our homes. Such is the case of a striped black and bright red bug which sometimes becomes a nuisance in the fall in its search for warm places. It is the boxelder bug, so called because it feeds in the juices of boxelders and some other maples. Fortunately it doesn't cause serious damage.


Most ladybugs are well behaved and don't give you any trouble; however there are a few species that seek the comfort of each other company and together invade human residences. One of them is the Asiatic ladybeetle also called the multicolored lady beetle for reasons that are obvious in the picture below. Not two are alike in color and pattern. This lady beetle was introduced intentionally in the United States from Asia to combat aphids so years ago there weren't such invasions of these beetles. It is interesting that the same thing is taking place in some South American countries where this lady bug was unknown just a few years ago.


Another insect that may show up inside your home in winter, although for entirely different reasons is a long horned beetle of a very respectable size, the banded ash borer. If you have a fireplace and bring logs into the house you may find several of them crawling around and heading for the light of the window. This one has not been looking for the warmth of your house, instead you brought it in unwittingly with the logs where it was spending the winter. It feeds on the wood of ash and oak trees and remains out of sight for most of its life.

Goldenrod galls, The Long Stemmed Ones

In addition to the more familiar goldenrod galls that we all notice, there a number of smaller galls of a variety of shapes. A very interesting one goes by the name of pedicellate gall. What this means is that it has a foot. I prefer to describe it like a long stemmed wine goblet. This curious small gall, less than an inch in length can grow on different places in the plant. The glass part is ribbed and it may have a purplish color.

You may have never noticed it before but you should have no trouble finding some of them now that goldenrods have reached their full size and are starting to bloom. So, look for them on leaves, stems or flowers. You may find them in isolation or in groups.

If you open them, you will find the small midge's larva or pupa inside. I have yet to take some home and keep them in a container with water and covered by mesh. I want to see the adult midge. So I will do this soon.



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Goldenrod Galls and their Fauna

There are so many different types of goldenrod galls! They are made by insects that induce abnormal growth of the tissues of the plant, providing protection and food to the gall maker and no benefit to the plant. Not only that but there are many parasitoids and inquilines, a whole series of ecosystems with very complex faunas.

Here are some of my photos, comments and links to information on goldenrod gall fauna.


This type of gall is caused by a small fly, a gall midge. Each small gall is like a blister in between two leaves of goldenrod. Inside the gall the midge larva grows surrounded by food and seemingly in safety.

The safety is only an illusion. Quite often the galls are parasitized by tiny wasps that end up killing the larvae of the gall maker.

This is one of the parasitic larvae.

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The Midge and the Chocolate Lover

What does a midge have to do with chocolate? More than you think. Chocolate comes from cocoa trees that grow in the tropics. These trees look quite different from most other plants. Instead of having flowers in the most conspicuous places, they have them on the trunk and lowest branches. These flowers are small and white and face down, the reason for all this is that they attract tiny flies, known as midges, rather than bees or other better known pollinators. The midges are ordinarily attracted to fungus and cocoa flowers smell somewhat mushroomy, too. In essence, for the cocoa tree to bear fruit, first it has to be pollinated by midges.
Think about it next time you eat a bite of that marvelous thing known as chocolate.

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Goldenrod Galls Fabulous Fauna

Goldenrods can be quite abundant in fields along roads and in forest clearings. They are rather non-descript, weedy looking plants through the spring and early summer, but before the onset of fall, they explode in vibrant, luminous yellows that give them their name. Each flower is tiny but they are abundant and massed together at the tip of branches creating an impressive display.

Another season in which this plant may arouse the curiosity of passers bye is winter; quite often their tall, dry canes sport some odd looking round thickenings, half way along the stems. These balls are about an inch in diameter and they are very tough. You may puzzle about them; evidently they are not fruits, their placement is quite strange but they are most definitely part of the plant and not something that has been glued to it.

They are tumors called galls and they are provoked by some creatures, the gall makers. In addition to the highly visible round galls there are other types. Galls come in a wide array of shapes and sizes and location in the plant. Many different plants present galls of one sort or another; goldenrods and oaks seem to be more prone to this sort of attacks than most other plants.

How are these tumors produced? Do they serve any purpose? What causes them? Only recently have scientist began to crack some of these mysteries. There are many different agents that can cause these tumors; most of them are insects, but also mites, nematode worms, fungi and bacteria can produce them. Fortunately, in most cases, they don’t seem to have serious effects on the plants they use as hosts; other than being somewhat unsightly in some cases. The ones that affect your rose bushes are not going to make you very happy.

Galls are very specific kinds of tumors; unlike cancer they don’t grow in an uncontrolled way, instead they have a very definite structure. This structure serves the purposes of the gall maker not those of the plant. So, galls will be different depending on the organism that produces them. In fact one species of plants may carry several different types of galls, each one caused by a different organism and each one with a very characteristic shape, size and location in the plant.

The remarkable thing about gall makers is that they are capable of hijacking, taking control of, the genetic machinery of the plant and forcing it to grow tissues and an entire organ unlike anything else on that plant. This highly organized structure or organ, called a gall, provides shelter and food for the gall maker or its progeny.

Goldenrod round gall, early summer

Getting back to goldenrod’s round gall; it is caused by a small fly known by some as the peacock fly, although this name is confusing because it also applies to an unrelated kind of fly. Therefore, the best name for it is goldenrod gall fly. Early in the spring, when goldenrods are growing, female flies pick their target, the stems of growing goldenrods.
The fly pricks the stem of the growing plant with the egg laying organ, also called an ovipositor, depositing an egg inside the tender stem. Soon a tiny larva hatches and starts feeding on the tender tissues of the inside of the stem. When doing so a chemical in its saliva starts acting on the plant’s tissues altering the way they grow and forcing them to develop an unusual structure.

Goldenrod fly freshly emerged from a gall in early spring

In this manner the growing larva, or maggot, builds a home for itself and a source of food. The thick, tough walls of the gall provide an excellent shelter and the juicy tissues provide nourishment. The center of the gall is a round chamber where the insect fits snuggly. It can remain there feeling safe and contented; all its needs are taken care of until the next spring. It will grow through the summer, sleep through the winter and then emerge fully grown and transformed into a winged adult. The fully grown maggot is no bigger than a grain of rice; the adult is just like a winged grain of rice. Before going through metamorphosis the larva has one final task; it chews a tunnel all the way to the surface of the gall and stops just before breaking the skin. It has to do that because the adult fly does not have teeth and would be trapped inside otherwise.

Fly larva inside its gall in the middle of winter

Mysterious as this process is, it is not the whole story of goldenrods galls and their flies. A gall can be a universe within itself. Despite the protection provided by the sturdy walls of the gall, the larva isn’t as safe as it would seem. Several enemies have found the way to penetrate the defenses and make a meal out of them. Some raging battles take place inside the seemingly peaceful round balls. There are at least three kinds of parasites that can invade the sanctuary and take possession of the helpless maggot. Two of them are called parasitic wasps, black in color and even smaller than the fly. Another parasite is a beetle belonging to a kind called tumbling flower beetles which also attacks the galls. There are probably others, but with so much to learn, scientists are still studying the universe of the goldenrod round galls.

Often more than half of the galls that you see in a field of goldenrods contains, not a fly, but one of its parasites. Is this all? Oh, no! In winter black-capped chickadees and downy woodpeckers, knowing that goldenrod galls contain a small morsel of food, go after them when sufficiently hungry. Next time that you walk through a field of goldenrods in winter look for galls and you will notice the ones that have been cracked open by these birds. With a good eye you could even figure out which have been visited by one or the other of these two birds. Woodpeckers use their strong and sharp chiseled bills with precision, making a clean hole while chickadees, with their smaller bills, need to keep picking at the gall, chipping away here and there, until they finally reach the core; thus their holes are very sloppy by comparison. Both birds show a preference for larger galls; they seem to know that the smaller ones are likely to contain one of the smaller parasites rather than the fat and larger fly maggot.

Gall opened by a chickadee next to whole galls, collected in winter

This is the story of just one type of goldenrod galls, the one you are most likely to notice when seeing a goldenrod field. But as I said there is a fantastic gall fauna in goldenrods. There are at least fifty different kinds of gall makers, each one with its whole intricate story of chemical manipulation, delicate timing, enemies and intrigue. Several types of flies and midges and some small moths are responsible for these galls. Each builds a unique type of gall and has its unique life style, rhythm of life, assortment of enemies and location of the gall. In summary if you count all the types of gall makers and all the parasites or predators you realize that it is a real zoo, with a great variety of species.

One wonders how the goldenrod thrives despite the attacks from such a wide range of gall makers. Fortunately for the plant these gall makers have many enemies which don’t allow the populations to build up to dangerous levels. It is also fortunate for the downy woodpeckers and black-capped chickadees that these gall flies exist and provide them with much needed food in the dead of winter. And we are lucky too because we get to enjoy the beauty of blooming goldenrods in the fall and those of us who are bird watchers get to see the chickadees and woodpeckers year after year.

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Syrphus fly, a pollinator and aphid eater

Syrphus fly on Queen Anne's lace. © Beatriz Moisset. 2009

Among the many flower flies there are some that are beneficial to the garden in more ways than one. Flies of the genus Syrphus and also some of their relatives are moderately good pollinators when compared to bees. They visit flowers frequently because they need nectar to fuel their flight; the females also need protein rich pollen for their eggs to grow and mature. The reason why they are not as good as bees is that their bodies have very little hair so pollen doesn’t adhere very well to them. However what they lack in efficiency they make up in sheer numbers. In some instances flies are more abundant than bees and may end up providing a better service to some flowers.

The genus Syrphus counts with several species in North America. They are very hard to tell apart; as a matter of fact they are also hard to differentiate from their close relatives: Eupeodes. Entomologists need to examine them under the microscope and look for minute details of wings and other body parts to identify each species.

Beginners have an even harder time and frequently confuse them with bees, although their single pair of wings should be a clue that they are flies not bees. They are considerably smaller than honey bees but there are many species of native bees about the same size as Syrphus. There are other ways of telling these flies apart from bees; their antennae are very short and with a funny shape with a little appendage called an arista; their eyes are enormous and their legs skinnier than those of bees. As I mentioned before they are nearly hairless. Although they may look like bees you don’t need to fear their sting because they have none.

Larva feeding on aphids on a milkweed's leaf. © Beatriz Moisset. 2009

What is interesting about these flies is that in addition to being pollinators they are very helpful to your garden as larvae because they eat aphids. The female lays her eggs near an aphid colony. The tiny maggots that emerge from the eggs are blind and legless but they don’t seem to need those organs to find their food.

They promptly go to work helping themselves from the colony. It is odd to see some completely unconcerned aphids near a maggot devouring one of its sisters; they may even come close to inspect it. Each maggot needs a large numbers of aphids and about a couple of weeks to complete its growth. It finds a place to pupate, under a leaf or on the ground and emerges in a few days ready to start raising another family of pollinators/aphid eaters. Gardeners familiar with these qualities of Syrphus flies strive to grow the plants that will provide pollen and nectar to them to ensure that they do their duty as aphid patrol.

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Guide to Pollinators and Other Flower Visitors

© Beatriz Moisset. 2009