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Jul. 25, 2012

Message in a Cornicle

by The Bug Chicks

Click to enlarge images
People hate aphids because they’re incredibly successful pests. Aphids love our vegetable gardens and our flower beds. They hang out under leaves, safe from the sun, and drain a plant’s energy by drinking the sap. They can also transmit all kinds of viruses. Honeydew, or an aphid’s sugary excrement, coats the leaves and provides the perfect substrate for the growth of sooty mold, which blocks sunlight from chlorophyll. Aphids can reproduce sexually AND asexually--and often--so they can create huge populations in a very short time. On top of all this, they’re great communicators and have figured out a way to defend themselves against predation. You might say they have a trick up their cornicles…
 
Aphid cornicles are paired tube-like structures located on the fifth or sixth abdomen segment that look like little exhaust pipes. They can be cylindrical, conical, or some even have setae (little bristle-like structures.) Cornicles are sclerotized (hardened). They vary in length depending on the aphid species, and some species can even elongate their cornicles by flexing muscles in the abdomen. Regardless of size, length, or color, cornicles are morphological characteristics that all aphids share.
 
These hollow tubes secrete droplets of fluid when the aphid contracts tiny muscles located in its abdomen, under the cornicle.  The contraction forces the fluid from specialized sacs that line the inside of the cornicle. For years, scientists thought this liquid was honeydew; however it was later discovered that honeydew is a waste product that exits the aphid's body through its digestive system. It’s now understood that this second liquid actually contains lipids, hemolymph, and a substance called an alarm pheromone. 
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Alarm pheromones are chemical cues that function as a type of “emergency broadcast system” that other members of a group can detect. It’s given off most often when there is a predator nearby. The aphid alarm pheromone is a compound called E-β-farnesene.  A bit of chemistry: farnesene has six different forms. Each form has a different function. The one found in aphids, E-β-farnesene, is one of the only forms found in nature.
 
An aphid only produces E-β-farnesene after it’s been attacked. Once the alarm pheromone is released, any aphid within detection distance will stop feeding and walk or fall off the leaf it’s on. In species with long cornicles, the aphids will flex their abdomens and smear the pheromone onto the predator in the moments before death. This action ensures that wherever the predator goes on the plant, the other aphids know before it even arrives!  It’s totally sneaky. This behavior allows the other aphids in the cluster more time to escape predation.
 
Even though all aphids can produce this pheromone, it is produced more frequently in immature aphids than in adults.  According to a study done by Mondor et al. at Simon Fraser University in 2000, when attacked, aphid nymphs produce E-β-farnesene 100% of the time. Adults, on the other hand, only produce the pheromone about 50% of the time. You’d think that all developmental stages would want to survive, so why wouldn’t they produce equal amounts of the pheromone? Think of it in the context of aphid behavior. Adult aphids are more likely and better able to disperse over a plant than the immature stages. The nymphs tend to cluster along plant leaf veins, creating almost a smorgasbord for predators. So the more readily an immature aphid alerts the rest of the clustered group, the more members of that group will survive.
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Production of an alarm pheromone is an adaptation of aphids that effectively counterbalances the fact they are slow and delicious to predators. It’s a tough life out there for animals so small, but these defensive mechanisms give them the edge they need to survive in a big world.
 
Want to know more?  More in-depth information can be found in the references we used for this article:
Mondor, E., S. Baird, K. Slessor, and B. Roitberg.  2000.  Ontogeny of alarm pheromone secretion in pea aphid, Acyrthosiphon pisum.  Jour of Chem. Ecol. 26 (12): 2875-2882
Mondor, E. and B. Roitberg.  2002.  Pea aphid, Acyrthosiphon pisum, cornicle ontogeny as an adaptation to preferential predation risk.  Can. J. Zool.  80: 2131-2136
About The Bug Chicks

Kristie Reddick and Jessica Honaker are The Bug Chicks. They each have Masters Degrees in Entomology and love to teach people about insects and spiders.

The views expressed are those of the author and are not necessarily those of Science Friday.

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