The Most Invasive Butterfly: Cabbage White Butterfly (Pieris rapae)

A flock of cabbage butterflies

(Pieris rapae)

© Beatriz Moisset

If you see a pretty white butterfly, more likely than not you are seeing an invasive species that was accidentally introduced to this country and to many other parts of the world more than a century ago.

The cabbage white butterfly is noticeable, pretty and soft, it visits gardens, meadows, you name it. Perhaps the only places beyond its reach are higher elevations. It starts flying early in the season and produces several generations throughout the summer and fall. No wonder it is seen so often.

Cabbage butterfly

(Pieris rapae)

© Beatriz Moisset

The name refers to the plant its caterpillar feeds on, cabbage. It also eats other members of the mustard family, such as cauliflower and kale and it is regarded as a serious pest. It also feeds on a number of wild, non cultivated mustards, including the invasive garlic mustard.

The cabbage butterfly is related to a group of native butterflies, the so called whites, yellows and sulphurs or family Pieridae. Their colors, as the names indicate are either whites or yellows. The caterpillars of most of them also feed on plants of the mustard family. A total of 58 species lived in North America before the arrival of this newcomer. Nowadays, the cabbage white is seen as often as all of them combined. One wonders if the growing numbers of this invader are having an impact on the populations of native Pieridae butterflies.

Orange sulphur butterfly

(Colias)

© Beatriz Moisset

In summary, it has the qualities of so called weedy species, with high rates of reproduction, very adaptable to different food sources and a variety of habitats.

Virginia white butterfly

Pieris virginiensis

© Beatriz Moisset

The cabbage butterfly reminds me of several insect species that were brought to this country intentionally or accidentally and proceeded to become incredibly widespread in a relatively short time. Now, some of them are more abundant than any of their native relatives. Here are a few examples: the Asian ladybug, the Chinese mantis, the giant resin bee, the European paper wasp, the drone fly, and the brown marmorated stink bug.

David Quammen, a distinguished journalist who has written extensively on conservation and ecology, has coined the term “Planet of Weeds,” meaning that the ecological changes caused by humans are having a severe impact on the flora and fauna of the entire planet. We had transported unprecedented numbers of species of plants, animals and bacteria beyond their natural distributions with the consequence that many of them have become established in their new surroundings. Not only do they become established but spread out from the original place of introduction, thus deserving the name of invasive species. They impact the local flora and fauna, bringing up the extinction of many species. In this way we are causing a mass extinction, comparable to some of the most serious mass extinctions of the past.

Specialist species are more vulnerable to extinction. The ones that have the best chance for survival are the weedy ones, the adaptable generalists. We are creating an impoverished world, gradually losing precious biological diversity, biodiversity for short. It may take millions of years for this biodiversity to raise back to the present levels. The human species may never see that.

Drone fly (Eristalis tenax)

One of many introduced, widespread species

© Beatriz Moisset

List of articles

Beginners Guide to Pollinators and Other Flower Visitors

© Beatriz Moisset. 2016

The Great Impersonators, flower flies

Eristalis tenax, the drone fly, a honey bee mimic

© Beatriz Moisset

Why would one insect want to look like another? There must be some advantage in doing so. When it comes to flower flies the answer becomes apparent after a little observation.

These flies are found visiting flowers rather than garbage or dead meat like some of the flies you are most familiar with. They are rather pretty, with patterns of brown and yellow or black and yellow which easily remind you of bees. I have seen many a photographer deceived to the point of posting a picture of a flower fly claiming that it is a bee.

The goal of this remarkable imitation is not to fool photographers, of course, but rather hungry predators. Flower flies are delicious morsels of food that any bird would readily accept, but the fear of being stung by this bee mimic may lead him to think twice and skip this prey.

Some flower flies are about the size of a honey bee. The mimicry is so convincing that one of them is called the drone fly. Others are smaller and they may be imitating some of the lesser known and very abundant solitary bees. A few are long and thin and look like wasps, rather than bees.

Mallota bautias, a bumble bee mimic

© 2005 Beatriz Moisset

They can be distinguished from bees or wasps by the number of wings. Bees and wasps have four; flies have only two. The back wings have been reduced to little knobs, called halteres, used for balance. The halteres and the number of wings are hard to see when the insect is flitting about. Even when it rests on a flower, you continue to have trouble because a bee's front and back wings hook up appearing like a single unit.

Flower fly (left), bee (right)

Compare the size of the eyes

and the size and placement of the antennae

© Beatriz Moisset

Other differences are more obvious if you train your eye to see them. Flower flies have enormous eyes and tiny antennae that emerge from the front of the head, rather than higher up as in bees. Also flies are almost hairless and their legs are skinny when compared with those of bees. I hope all this helps you and saves you from embarrassing mistakes. I have seen business cards, article illustrations and even a book cover with a fly passing for a bee. So, if you still don't get them right, at least remember that you are in good company.

Toxomerus, mimics of small native bees

© Beatriz Moisset

In England, flower flies are called hover flies, an excellent description of their behavior. Some people in the US are adopting this name, so you may find either term in the growing literature on these interesting and useful insects.

A colorful flower fly

Helophilus, the sun lover

© Beatriz Moisset

These flies feed on nectar; sometimes they also eat pollen, especially the females who need this protein-rich food to produce eggs. This is why they spend so much time visiting flowers. Because of this habit they often end up carrying pollen from one blossom to another. They may not be as good pollinators as bees, but their role is not insignificant and deserves recognition. For instance, the so called drone fly is used to pollinate greenhouse sweet peppers.

Spilomyia sayi, a wasp mimic

© Beatriz Moisset

Let us applaud the bee impostors, flower flies or hover flies, for their role as pollinators.

For more on pollinators and other flower visitors read the e-book:
Beginners Guide to Pollinators and Other Flower Visitors

List of articles

© Beatriz Moisset. 2014

A Busy Pollinator and a Relaxed One

An industrious little bee gathering food for her babies and pollinating flowers along the way. It methodically collects pollen without missing any and then promptly flies to the next flower.



The bee is very industrious and visits many flowers in a very short time. The flower fly, on the other hand, takes its precious time gobbling up pollen, so it visits fewer flowers overall.

FLOWER FLIES. Very Good Friends of the Gardener

There are some little flies that visit flowers with great frequency. Naturally, they are called flower flies; although the British prefer the name "hover flies", which describes their behavior very accurately. They are very numerous in the garden; but if you dislike house flies you don't need to worry about flower flies; they are rather pretty. You probably didn't notice their presence thinking that they were bees, rather than flies.They are excellent impersonators of stinging insects such as bees and wasps and they probably fool hungry birds into thinking that they may get stung but these little "bees". This strategy seems to work very well for them, so they can go feeding on flowers unmolested by these winged predators.

Sphaerophoria

In their visits to flowers they accomplish some pollination. They may not be as efficient as bees, but their sheer abundance makes them important pollinators of wild flowers. But there is something else that these flies do that makes them very welcome in the garden: The larvae of some of them feed on aphids.


This is such an important task that they deserve more attention than they get from plant lovers. We don't appreciate them enough.