How Do You Like Them Apples?

It's hard to escape your enemies

Crab apple in flower, Cambridge, Massachusetts. Drawing by WRT.

The domestic apple that is so consistently recommended every day by doctors and that was shot off his son's head by the archer William Tell is an Old World domestication and was introduced to North America by immigrants from Europe about 400 years ago.  This was a lucky break for the apple because it pretty much left many of its pests behind for a couple of centuries. But nowadays, if you have an apple tree, you have pests, including one called the apple maggot fly, that costs a lot of money and effort to control. So how did this happen? We know it wasn't some dumbbell bringing infested apples from Europe because the apple maggot fly is native to North America.  How it came to switch to domestic apples is an interesting story, a story that came to mind one April on the streets of Cambridge, Massachusetts where crab apple trees were blooming profusely.  Although these lovely flowering street trees are not the main actors in this story, they are closely related and are also host to the apple maggot fly.

The apple maggot fly, Rhagoletis pomonella, one of the picture-wing flies of the family Tephrididae.

Before Europeans arrived, the North American apple maggot fly used the native haw (or hawthorn, as in Nathanial) as its host, piercing the skin of the fruit to lay its eggs.  Haw and domestic apple are closely related, both being in the genus Malus, so it would seem like a host switch would not be a very big deal.  The fly in the ointment (or the apple, as it were) was that the boy and girl flies meet, court and mate only on the fruit, be it haw or apple, at the right stage of ripeness, and to make this work, the flies (which don't live very long) have to emerge from their puparia in the soil at just the right time.  Too early, and there is no suitable fruit to lay eggs in, too late and it's, well... too late, and the fruit is rotting on the ground.  But domestic apples ripen later than native haw, so the haw flies emerge too early for apples, and for a couple of hundred years, apples were safe from the fly. But in the 1860s, apple growers discovered a new apple pest --- you guessed it, haw (now apple) maggot flies.  So, a few haw maggot flies must have emerged late enough to do apples, and because the date of emergence is heritable, their descendants, in ever greater numbers, joyfully colonized domestic apples, to the chagrin and cost of growers.

Since that time, the flies living on apples have diverged genetically from those breeding in haw, and nowadays, if you offer each the other host, they not only are not as attracted to the host's scent, but eggs laid in the other host develop poorly or not at all.  And of course, because mating takes place on the host fruit, there is almost no crossbreeding, because each is attracted almost exclusively to its "own" fruit, and the sexes simply don't meet.  In effect, they have migrated to different fruit-continents.  The two lines of descent have become, in essence, separate species, with greatly reduced gene flow between them. But lest you think that the switch to a new host rid the fly of its little wasp parasites, the parasites evolved right along with their host, so that now the wasp race that parasitizes the apple version of the fly does not parasitize or develop in the version whose host is the haw, and vice versa.

The moral of the story is, I suppose, it is truly hard to leave your troubles behind, be you apple or apple maggot fly.

An extra tidbit: Many tephritid fruitflies mate on the fruits into which the female deposits eggs, some doing little dances to create the right mood. I even see them on the tiny berries of pokeweed, and it’s fun watching them.