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]


Giant Hogweed And Other Toxic Plants

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Everybody run, giant hogweed is coming! I am sure by now, many of you reading this will have picked up a story or two about a nasty invasive plant that will render you blind and nursing third degree burns. Indeed, giant hogweed (Heracleum mantegazzianum) is a plant worth learning how to identify. However, the tone of these articles is often one of hysterics, leaving the reader feeling like this plant is more like a Triffid, actively uprooting itself to hunt down unwary humans. Is giant hogweed worth all of this anxiety?

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Let's start with the plant itself. Giant hogweed is a member of the carrot family (Apiaceae). Its native range encompasses much of the Caucasus region and into parts of central Asia. It was (and probably still is in some areas) considered a wonderfully large and unique addition to a temperate garden. And large it is. Individual plants regularly reach heights of 6 feet (2 m) or more and some records indicate that individuals over 10 feet (3 m) in height are not unheard of.

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Because it was once a popular garden plant, this species has been introduced far outside of its native range. For many decades, giant hogweed probably lurked in the background unnoticed, its seeds finding favorable spots for germination among other weedy plants along roadsides, fallow fields, and abandoned lots. In the last few years it has grown harder to ignore. More and more plants are showing up where they shouldn't. Indeed, it seems that giant hogweed is yet another invasive species we need to get on top of. But what about all of that panic? Certainly its invasive status alone isn't what all the hype is about.

Well, like all members of the carrot family, giant hogweed produces an impressive array of chemical compounds. Many of these compounds serve to protect the plants from hungry herbivores and a plethora of microbial infections. Some of the compounds in the giant hogweed arsenal are a group known as the furocoumarins. These compounds defend the plant in a rather alarming way. These furocoumarins are phototoxic, which means when the sap gets on the body of an animal and is exposed to sunlight, they cause severe chemical burns.

Giant hogweed when not in bloom.

Giant hogweed when not in bloom.

Stories of people being hospitalized due to an unfortunate run in with this plant make headlines wherever it pops up. That being said, simply touching the plant isn't going to hurt you. The chemicals are sloshing around in the sap of giant hogweed and the plant needs to be injured in some way before they will leak out onto whatever is hurting it. For humans, this usually occurs while mowing or weed whacking, or if a child mistakenly uses the hollow stem as a pea shooter.

With stories like this floating around, it is no wonder then why people get so upset when this plant shows up. However, I can't help but feel that this is being fed on a bit by media fear-mongering. It is worth putting giant hogweed into some practical context. It may actually alarm you to know just how many plants on the landscape have the ability to cause you harm if handled the wrong way.

Wild parsnip ( Pastinaca sativa )

Wild parsnip (Pastinaca sativa)

Even hogweeds less robust relatives are capable of causing phototoxic reactions. I once weed whacked a large patch of Queen Anne's lace (Daucus carota) and wild parsnip (Pastinaca sativa) and ended up covered in nasty blisters the next day. I recovered but I sure did learn to give those two species more respect whenever I encountered them. Plants like poison ivy, oak, and sumac certainly cause their fair share of misery but even these do not get the sort of media attention that giant hogweed does.

Even more interesting are some of the species we actively plant in our gardens. For instance, castor bean (Ricinus communis) is quite popular among gardeners and it is responsible for producing ricin, a protein with enough killing power to bring down an adult human many times over. Take a bite out of the castor bean in your garden and it will be the last thing you ever eat. Even more potent than ricin is aconitine, an alkaloid produced by beloved garden plants like the monkshoods (Aconitum) and the larkspurs (Delphinium). This powerful alkaloid causes your nervous system to endlessly fire, leading to convulsions and death.

Castor bean ( Ricinus communis )

Castor bean (Ricinus communis)

Similarly, a few different species of Datura are commonly grown around the world. Datura posioning is nothing to mess with and symptoms include "a complete inability to differentiate reality from fantasy; hyperthermia; tachycardia; bizarre, and possibly violent behavior; and severe mydriasis (dilated pupils) with resultant painful photophobia that can last several days." Even plants we grow for food can hurt us in bad ways. Most members of the tomato family produce a multitude of toxic alkaloids like solanine. That is why only ripe tomatoes and eggplants should ever be consumed.

Jimsonweed ( Datura stramonium )

Jimsonweed (Datura stramonium)

In reality, I could devote an entire blog and podcast series to the chemical warfare plants have taken up during their long and complicated evolutionary history. Long story short, plants are sessile organisms that must defend themselves in order to survive and toxic chemicals are really great means to do just that. The reality is that we welcome many toxic and potentially harmful plants (both knowingly and unknowingly) into our lives and it seems slightly odd that species like giant hogweed warrant such fervor from media outlets. That being said, it is important to treat these plants with the respect they deserve. Don't bother them and they won't bother you.

So, is giant hogweed coming to attack you and your family? No. Is giant hogweed a plant worth learning to identify? Yes. Is giant hogweed dangerous to humans? Yes, but only under certain conditions.

Plants like giant hogweed are the perfect reminder as to why we must give plants more respect in our society. Teaching friends and family which plants can feed them and which plants can hurt them is something everyone should invest some time in doing. If you find giant hogweed in your area and you do not live in the Caucasus or central Asia, don't be a hero. Call a professional to come and deal with it. Otherwise, stay calm and keep on botanizing. Giant hogweed is not out to get you.

Photo Credits: [1] [2] [3] [4] [5] [6] [7]

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

Got Herbivores? Turn Them Into Cannibals!

Plants have to deal with quite a lot in their day to day lives. They can't get up and move like animals can. Due to their sessile nature, plants rely on a suite of physical and chemical traits for defense. The world of plant chemistry is quite amazing and thanks to new research published in Nature Ecology & Evolution, it has gotten even more interesting. Under attack by herbivorous insects, some plant species are able to turn their vegetarian predators into cannibals. 

Cannibalism in insects is not unheard of, even among the herbivorous species. When the going gets tough, why not eat your sibling or your neighbor? Well, research using tomatoes and the army beetworm (Spodoptera exigua) suggests that plants might be able to induce this behavior in caterpillars long before it would happen naturally. It makes sense too. Plants that are able to induce cannibalistic behavior via chemical means not only reduce grazing pressures on their own tissues, they also reduce the number of herbivores in the system.

The chemical in question here is called methyl jasmonate. It is a volatile organic compound produced by a plethora of plant species and is thought to play in role in a diverse array of biological functions such as germination, root growth, fruit ripening, and defense. It is often released when a plant becomes damaged. Neighboring plants are able to pick up on this compound and will begin to beef up their own defenses in response. After all, if your neighbor is being attacked, there is a decent chance you will be too. 

Researchers investigating the effects of this chemical on the beetworm (a common aggricultural pest) found that plants that were treated with methyl jasmonate induced beetworms to turn on one another through cannibalism. Caterpillars hanging out on plants that were not treated with methyl jasmonate only turned to cannibalism after they had consumed all of the leaves available, if at all. 

The researchers are now gearing up to figure out whether inducing cannibalism also helps to spread disease among caterpillars. This exciting new form of plant defenses opens up doors to many new questions and potentially safer forms of pest control. Considering the near ubiquity of methyl jasmonate in the botanical world, it begs the question as to how common this form of defense really is. 

Photo Credits: [1] [2] 

Further Reading: [1] [2]

Cannabis Ecology

Weed, pot, hemp, marijuana, Mary Jane, dope... Regardless of name, we are all quite familiar with the plants in the genus Cannabis, at least in the context of their relationship to humans. With Colorado and now Washington being the first states in America to legalize its recreational use and plenty more states slated to do the same, the genus Cannabis is as hot of a topic today as it has ever been.

Admittedly a lot of what is said about these plants is based in misinformation and senseless hysteria, however, there is no denying just how significant this genus has been to humans over recorded history. Despite the celebrity status, it is staggering to realize just how little people know about the ecology of these plants. When you strip away all of the anthropogenic uses and their maligned reputation, you will find a very interesting natural history indeed.

Originally native to the Caucasus region of eastern Europe, India, and Iran, as well as parts of Africa, there are purported to be 3 species in the genus Cannabis, though there is some debate over this. In the wild they are said to grow in open, disturbed habitats. This lack of specificity when it comes to habitat has allowed Cannabis to attain a near global distribution. It can now be found growing in mild climates throughout the world and can readily escape cultivation when conditions are right.

Cannabis was once placed in the nettle family and then the mulberry family, but DNA analysis has since moved it into its own family, Cannabaceae, of which hops (Humulus) and hackberries (Celtis) also belong. All species of Cannabis are wind pollinated annuals. Most of the time, plants are dioecious, meaning individuals are either male or female, however, it isn't uncommon for some individuals to be monoecious (male and female parts on the same plant). All 3 species will readily hybridize with each other, a fact that has been widely utilized by breeders. It is the female flowers that are most coveted for consumption as they contain the highest amounts of the chemical THC.

There are many hypotheses as to what function THC has for the plant. It is a secondary metabolite, meaning it does not serve a direct role in the growth, development, or reproduction. It is produced from glandular trichomes that can be found in the greatest abundance on the female flowers. Research indicates that THC plays a multifaceted role in protecting the plant from fungi and microbial infection as well as deterring herbivory. Recently it has been found that THC has high UV-B absorption properties, leading some to believe it may also protect the plant from sun exposure.

Whether you venerate this genus or despise it, you cannot deny the fact that this is one group of plants that has been quite serendipitous in its evolutionary trajectory. The wide array of uses attributed to the genus Cannabis has lead to near global dominance. Plants like this really make you question which species has the upper hand. Above all else, I would only ask that people on both sides of the argument approach this subject with an open mind. At least here in America, I think we are entering a new era with these plants.

Further Reading:
http://www.druglibrary.org/olsen/hemp/iha/iha01201.html