Fearless Aphids

Fearless Aphids Photo Credit: I, Luc Viatour [GFDL, CC-BY-SA-3.0 or CC-BY-SA-2.5-2.0-1.0], via Wikimedia Commons

Habituating crop-munching aphids to their own distress pheromone may make them more vulnerable to ladybugs.


Making pests ignore danger. I'm Bob Hirshon and this is Science Update.

As the boy who cried wolf learned, too many false alarms can make people ignore real danger. Now, researchers are hoping to apply this to crop pests called aphids.

Georg Jander, of the Boyce Thompson Institute at Cornell University, explains that when aphids get eaten by predators like ladybugs, the dying aphids release an alarm pheromone that warns other aphids away. So researchers tried breeding plants that produce that pheromone, hoping aphids would avoid them.

What we found, though, is that if the aphids are raised on the plants that produce the alarm pheromone, they become habituated, so they no longer run away from the alarm pheromone.

However, Jander's team found that the fearless aphids tend to get eaten more by ladybugs. So combining the engineered crops with natural predators may be the best bet. I'm Bob Hirshon, for AAAS, the science society.

Making Sense of the Research

When you hear a car alarm, you probably don't call the police, or look for the would-be thief. That's because car alarms are usually set off accidentally, so people often ignore them. The process of getting used to a signal until you no longer notice or respond to it is called habituation.

In this case, the aphid pests release an alarm pheromone, or chemical signal, when they're bitten by ladybugs. The same chemical, known as E-beta-farnesene (EBF), is also produced by some plants in large amounts. Jander's team wanted to know why. The simplest, most obvious answer is that the alarm pheromone protects the plant by scaring aphids away. British researchers recently tested this hypothesis by taking plants that didn't normally produce EBF, and genetically engineering them so they did. And in fact, in the short term, aphids did avoid the EBF-engineered plants.

But Jander's team found that the story wasn't so simple. They tried raising aphids on plants that produce EBF, and found that soon, the aphids became habituated, and stopped responding to the alarm pheromone altogether. Jander compares it to having too many fire drills at work or school: eventually, people start ignoring the fire alarms and staying at their desks. What's more, the habituated aphids actually grew a little faster than their non-habituated counterparts. So if anything, it seemed like making EBF would be bad for a plant. But if that's the case, why do some plants produce it naturally?

Janda's team decided to introduce ladybugs into the scenario, and that's when the advantage of EBF became clear. Yes, plants weren't scaring away the aphids by producing EBF—in fact, they were making the aphids unafraid of it. But the aphids' fearlessness turned out to be a disadvantage when real predators arrived. Since they couldn't distinguish between the plant's EBF and EBF from other aphids getting eaten, the fearless aphids got eaten more often than their EBF-responsive counterparts. To go back to the fire alarm scenario, people who hear too many false fire alarms might be in trouble when an actual fire occurs. 

Since ladybugs are already used as a natural pest control, Jander says that engineering crops to produce EBF may give the ladybugs a leg up. Right now, they're trying to insert the pheromone gene into potatoes, one of the aphids' favorite targets. Then they'll find out if these plants get munched on less in the field.

Now try and answer these questions:

  1. What is an alarm pheromone? What purpose does the alarm pheromone EBF serve in aphids?
  2. Why don't plants that produce EBF scare aphids away?
  3. Suppose all plants began producing EBF. Over time and natural selection, how might aphids adapt to this situation?

You may want to check out the August 20, 2010, Science Update Podcast to hear further information about this Science Update and the other programs for that week. This podcast's topics include: beating aphids at their own game, why some birds bob their tails, ancient terror birds of South America, and more.

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