This week I noticed that the Knock Out roses had a few aphids on them. If you grow plants, the odds are you’ve had many encounters with aphids. I know how to control aphids but I didn’t know much about their biology, so I went on a strange and fascinating journey learning more about this ubiquitous pest.
There are over 4000 species of aphid, all within the family Aphididae. A few hundred of these are pests in agriculture, forestry and the home garden. Aphids are small, soft bodied insects that can be black, brown, pink, green or colorless. They feed by inserting a sucking mouthpiece into a plant and sucking out the sap of the plant. Because of this way of feeding, you usually find aphids on the soft, new growth of a plant. One anatomical feature that’s used to distinguish aphids from other similar insects is a pair of cornicles on the lower abdomen which are used to secrete defensive fluids.
When aphids fed on a plant, they can cause the leaves and stems to distort. However the biggest problem with aphids is that they are often vectors for viral diseases. The aphid can carry the virus within them and when they insert their mouthpiece into a plant, they transfer the virus to the plant. This can make them very damaging to farmers and gardeners.
The most shocking thing I learned about aphids has to do with their reproductive biology – it’s just plain weird and kind of wonderful!
In the spring, eggs that have overwintered hatch and all the nymphs are female, each of which is a viviparous, parthinogenetic fundatix. Now that’s a mouthful! Viviparous means that the female gives birth to live offspring – human beings are viviparous. Parthenogenetic means that the female produces offspring by a process called parthinogenesis – a word that comes from the Greek and literally means “virgin birth.” The female gives birth asexually – her eggs don’t undergo meiosis and there’s no fertilization of the egg. The female aphid just gives birth to live clones of herself. Calling this newly hatched aphid a fundatrix means that she’s the founder of the aphid colony and all of the aphids that will cluster on that plant stem will be born live and produced by “virgin birth!” (I said their reproductive biology is amazing – but it gets even better!)
An aphid can produce 40-80 offspring during its 20-40 day lifespan. In less than two weeks a newborn aphid is ready to start giving birth to its own offspring. You don’t have to be a math wizard to understand that one aphid can turn into an infestation in a very short time. Every one of those aphids is a clone and they’re all female.
Most aphids are born wingless but sometimes, especially if they become crowded, a winged aphid will be born. Aphids aren’t strong fliers but with the aid of the wind, this winged aphid can leave her colony and start a new colony on a different plant or a different part of the same plant.
This asexual reproductive cycle goes on all summer. (In greenhouses this cycle can go on undisturbed for years!) When fall arrives, something changes. It might be caused by shorter days, cooler temperatures or a change in food supply. But whatever the cause, aphids that have been producing live parthenogenetic offspring will now give birth to sexual male and female winged aphids – don’t ask me how! The winged sexual female will give live birth to a wingless sexual female who is now oviparous (egg laying) – again, don’t ask me how! The wingless sexual female and winged male will mate, eggs will be laid and the cycle will start again in the spring.
The aphid life cycle has blown my mind. Think of all the twists and turns of evolution needed to create this life cycle. I can think of about 100 different PhD dissertations that could be written about aphid’s reproductive biology! After learning all of this, I’ll never look at an aphid again without being amazed. In the aphid world, women rule and men are incidental, only necessary to fertilize eggs that can survive the winter. Virgin birth is an everyday occurrence. And everyone on a given branch or stem is a female clone, genetically identical to all the others.
Knowing all of this almost makes me want to leave the aphids alone and let them carry on their weirdly wonderful reproductive dance… almost!