The Dual Benefits of Smelling Like Frightened Aphids

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If you garden, you have probably dealt with aphids. These tiny sap-suckers not only drain the plant of valuable sap, they can also serve as vectors for disease. Plants must contend with the ever-present threat of aphid infestation throughout the growing season and have evolved some amazing defenses against these insects. Recently an incredible form of defense against aphids has been described in pyrethrum (Tanacetum cinerariifolium) and it involves smelling like a frightened aphid colony.

Aphids produce their own alarm pheromones when attacked. Because aphids form large, clonal colonies, these pheromones can help warn their kin of impending doom. Other aphids will also eavesdrop on these alarm signals and will avoid settling in on plants where aphids are being attacked. Aphids aren’t the only ones honing in on these scents either. Aphid predators and parasitoids will also use these compounds to locate aphid colonies. As such, these pheromones are helpful to the host plant because it can mean a reduction in aphid numbers.

An alate (winged) green peach aphid (Myzus persicae).

An alate (winged) green peach aphid (Myzus persicae).

The selection pressured imposed by aphids on plants is so strong that it appears that at least one species of pyrethrum has actually evolved a means of producing these pheromones themselves. Pyrethrum is a member of the aster family (Asteraceae) native to southern portions of Eurasia. Like all flowering plants, its flowers are the most precious organs. They are the key to getting their genes into the next generation and therefore protecting them from herbivore damage is of utmost importance.

It has been discovered that pyrethrums produce an aphid alarm pheromone called ( E )-β-farnesene or EβF for short. The pheromone is not produced in every tissue of the plant but rather it is concentrated near the inflorescence. What’s more, pheromone production is not constant throughout the duration of flowering. Researchers found that it production reaches its peak just before the inflorescence opens to reveal the flowers within.

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The production of EβF in pyrethrum appears to serve a dual function. For starters, it actually results in reduced aphid infestation during the early stages of flowering. When the initial aphid attack begins, these insects consume some of the EβF as they feed and release it as they excrete honeydew. Other aphids detect EβF within the honeydew and will actually avoid the plant, likely due to the perception that the aphids feeding there are already under attack.

That does not mean that predators are not to be found. In fact, the other benefit of producing EβF in the inflorescence is that it appears to lure in one of the most voracious aphid predators on the planet - ladybird beetles. The ladybird beetles are able to detect EβF in the air and will come from far and wide to investigate in hopes of finding a tasty aphid meal. The ladybird beetles were most frequently found on plants during the early stages of floral development, which suggests that EβF production in the floral tissues is the main attractant.

A 7-spot ladybird beetle (Coccinella septempunctata).

A 7-spot ladybird beetle (Coccinella septempunctata).

Interestingly, it has been found that constant production of EβF is less effective at deterring aphids than pulses of EβF. It is thought that just as humans can get used to certain background levels of scent, so too can aphids. If aphids are exposed to high levels of EβF for long periods of time, they simply recognize it as the safe background level and will continue to feed. This may explain why pyrethrum plants only produce EβF for a short period of time during the most crucial stages of floral development. Research like this not only improves our understanding of the myriad ways in which plants defend themselves, it also offers us new avenues for researching more natural ways of defending the plants we rely on from unwanted pests.

Photo Credits: [1] [2] [3] [4]

Further Reading: [1]


Grasses That Feign Infestation

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Given the option, most of us would rather avoid a salad riddled with insects or an apple chock full of worms. Much as we prefer to avoid insect-infested fruits and vegetables, so too do many herbivores. Some plants seem to be taking advantage of this. In response to strong herbivore pressure, some plant species have evolved insect mimicry. One such case involves grass and aphids. 

Paspalum paspaloides can be found growing in tropical regions around the globe. In many ways they are similar to other C4 grasses. When they flower, however, one may notice something interesting. All of the flowers appear to be covered in aphids. Close inspection would reveal that this is not the case. Those clusters of dark specks swaying the breeze are simply the numerous dark anthers of the inflorescence. This has led some to hypothesize that these plants may be mimicking an aphid infestation.

This observation begs the question: "what benefit is there in mimicking aphids?" There are two major hypotheses that have been proposed in order to explain this phenomenon. The first is defense against herbivory. As stated above, herbivores often avoid plant material that has been infested with insects. Aside from any potential palatability issues, large populations of insect pests can signal a decrease in the nutritional value of a potential food source. Why waste time eating something that is already being eaten? Evidence in support of this hypothesis has come from other systems. A wide array of herbivores, both mammalian and insect, have been shown to avoid aphid-infested plant material.

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The second hypothesis is one of avoiding future infestations. Aphids are clonal organisms with a short generation time. It does not take long for a few aphids to become many, and many to become an infestation. As such, aphids looking for a new plant to colonize habitually avoid plants that already have aphids on them. It could very well be that such aphid mimicry is a means by which the grass keeps actual aphids at bay.

If this is a form of true mimicry then the question is not a matter of which hypothesis but the relative influence of each. It seems that it very well could be driven by a mixture of both strategies. Still, all of this is speculative until actual experiments are carried out. Those who originally put forth these ideas have identified similar potential mimicry systems in other plants as well. The idea is ripe for the testing!

Photo Credits: [1]

Further Reading: [1] [2] [3]