By: Lacy Sellent, Writing Fellow at the Cable Natural History Museum
A classic sign of the end of summer and the beginning of fall is apples ripening on the trees in our yards, along back roads, and in orchards. Throughout the state, thousands of apples are ready to pick, and some are already in a bowl on my table! Fallen and rotting apples can sure make a mess (that deer and wasps love!), but if you want non-wormy apples for eating, then you might want to keep the area around your favorite trees clean. Leaving fallen apples beneath a tree may cause an insect infestation. One of the biggest pests to apple trees is the apple maggot (Rhagoletis pomonella).
The annual life cycle of the apple maggot begins in July, usually after a good rain. This is when the adults emerge from their winter pupal form and make their way up out of the ground. Although named for its larval stage, adult apple maggots are actually flies. Each five millimeter long fly has black and white colored wings and a distinctive white dot on its back. With a little imagination, black markings on the wings resemble the letter “F.” The apple maggot adult’s coloration resembles the forelegs and pedipalps (grabbing and sensing mouthparts) of one type of jumping spider. This method of defense, where a harmless species mimics one that is better protected, is called Batesian mimicry. Viceroy and monarch butterflies are a classic example of this.
After being above ground for close to a week, female flies will begin the search for a place to lay their eggs. Although adults can fly up to a mile, they don’t usually travel far from the trees that they were burrowed under. Apple maggot flies originally deposited their eggs in thornapples (hawthorn), but have gained notoriety by also laying their eggs in domestic apples. It is not a very healthy experience for the apple. Wherever the apple maggot fly lays her eggs, that area of the apple will stop growing. This causes the apple to look all lumpy. Then, newly-hatched maggots munch and burrow their way out. Wherever the small white maggots tunnel, the apple begins to rot. Brownish trails appear throughout the apple’s flesh. It is this tenacious tunneling that earned apple maggots the nickname “railroad worm.”
After the maggot-infested apple falls to the ground, it is time for the next stage. Maggots exit the apple and enter the soil. Once safely burrowed several inches below the surface, the maggots form a brown, quarter-inch long, oblong case called a puparium. This pupal form allows them to go into a type of hibernation over the winter. They don’t eat until the next spring, when they emerge from the ground as apple maggot flies—bringing the cycle full circle.
The apple maggot is native to North America, but its original range was confined to the eastern United States. It fed mostly on the fruit of hawthorn trees, sometimes know as “thornapples,” which look like tiny apples. About 150 years ago, some of the hawthorn-eating flies began to eat domestic apples planted by settlers and our old friend, Johnny Appleseed. What fascinates scientists (and me!) is that the apple maggots eating domestic apples are adapting to their new host plant, and not reproducing with the apple maggots still feeding on hawthorns. They are suspected to be in the early stages of diverging into two separate species. This process, known as speciation, is usually thought to happen because of a physical barrier – like a mountain range or an ocean – dividing a population. There is no physical barrier in this case, so it is an example of “sympatric speciation.” This is a unique opportunity for scientists to study the genes of these critters as they are in the process of changing!
Unfortunately for us and our many orchards, apples provide more protection from predators for the apple maggots than hawthorns do. Two species of wasps parasitize the maggots by depositing their own eggs inside the maggot, giving the wasp larvae a protein-rich meal when they hatch. In hawthorn fruits, apple maggots feed close to the surface, partly because several species of caterpillars prefer to eat the core, and partly because the smaller fruit just doesn’t have as much area away from the skin. In domestic apples, the maggots can feed farther in, out of range of the wasp’s ovipositor. This means that there are fewer natural predators for the apple-eating population of Rhagoletis pomonella. Removing dimpled apples from your orchard before the maggots exit into the soil can help reduce their population.
As the old joke goes, the only thing worse than finding a worm in your apple is finding half a worm. Next time you see a small white apple maggot tunneling through your fruit, I hope you’ll take a moment to appreciate the interesting life history of this tiny pest!
For over 44 years, the Museum has served as a guide and mentor to generations of visitors and residents interested in learning to better appreciate and care for the extraordinary natural resources of the region. The Museum invites you to visit its facility in Cable at 13470 County Highway M. The new exhibit, The Joy of Birds: Feathers in Focus opened in May, 2011. Find us on the web at www.cablemuseum.org to learn more about our exhibits and programs. Also discover us on Facebook, or at our blogspot, http://cablemuseumnaturalconnections.blogspot.com/.
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