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Chemical Warfare and Other Ways Plants Defend Themselves

A nasty bug eyes a healthy plant. The leaves are luscious. A tasty meal, the bug thinks. It sinks its mouth parts into a leaf and begins to chew.

Is there any hope for the poor plant, rooted to its spot with no way of running?

You may be surprised at what happens. Plants are not as defenseless as we think! Plants actually protect themselves in multiple ways. Some defenses are obvious to us, such as spines or thorns. There may be a waxy cuticle on the leaves, making them tough to chew through, or they may contain sticky residues that gum up the body parts of predators. The leaves may have stinging hairs which burn.

Other defenses may be less noticeable to humans. When under attack, many plants produce chemicals that act as insect repellants or fungicides or else reduce plant digestibility to herbivores. Some plants are outright poisonous, containing strychnine or cyanide.

Yuck! The bug decides to try another plant instead.

But plants also begin to pump out a gas through their leaves that sends a message through the air to nearby plants, warning of the predator. The neighboring plants ramp up their defenses as well. Forewarned by the original plant, the next plant is already pumping defensive chemicals into its leaves when the bug takes a bite. Research shows that plants don't have to be related to communicate with each other. A corn plant may warn a bean plant of the invader!

Nasty stuff! The creature spits it out. The bug may have to wander more than fifty yards before finding a plant it can eat.

Plants may also launch a counteroffensive by calling on its insect allies. Say a bug chomps down. The plant begins to emit a  chemical or odor that attracts friendly bugs. The natural enemies of the predator soon arrive to take care of the problem. The amazing thing about this is that the plant can even identify what kind of bug the predator is and emit different odor molecules to attract the appropriate ally. 

Gardeners should know two things: First, about 95% of bugs are good guys, beneficial to the garden. Second, artificial pesticides or insect repellants may interfere with a plant's natural defenses, making them more susceptible to insect damage when the artificial chemicals wear off and creating a situation where the plants become "hooked' on the chemicals. There are some eco-friendly products that work by boosting a plants own defensive hormones and other organic products that minimize damage to the environment. The best defense really is a healthy plant, grown in good soil. Stressed-out, weak plants are less able to defend themselves. Have you ever noticed how grasshoppers thrive on draught stricken plants?

If you are interested in reading more about the astonishing world of plants and soil, you may like my previous posts:

How I Spent My Christmas Making Mortar

Down in the Dirt

A New Year's Resolution

By the way, I don't know what's been eating my hosta and tropicana canna leaves shown above, but it is definitely not this snail!

My new snail pot, just planted with corkscrew rush

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Reader Comments (19)

I never use those chemicals Deborah - but I never knew much until I read your post on how the plants launch a counteroffensive. Really interesting information to glean from :) Rosie

August 5, 2010 | Unregistered CommenterRosie

What a lovely way of putting it all Deb, I love the idea of al my plants communicating with each other to build up their defenses!

August 5, 2010 | Unregistered CommenterGillian

By the looks of your hosta, it may be a snail of that size! What hosta is that? Pretty color.

August 5, 2010 | Unregistered CommenterJeff

Rosie and Gillian, thank you for your comments! Jeff, I believe the hosta is a cross between 'Francis Williams' and Sieboldiana 'Elegans'. I have both of these growing nearby and several seedlings have sprouted, including this one which is my favorite. It seems to be some bug's favorite as well. Interesting, the other hostas in the area are untouched!

August 5, 2010 | Registered CommenterDeborah Elliott

Very cool post! I had no idea. That sort of explains why I have less trouble with the nasties than some of my neighbors...

August 5, 2010 | Unregistered CommenterTown Mouse

This was so interesting, I had never heard about plants having their own defenses other than things like thorns. I think my Hostas though, have been forgetting to warn each other :)

August 5, 2010 | Unregistered CommenterCatherine

Wow, that was some big bug biting your hosta, I'd be hiding!

Interesting! I have been reading about plant communication and chemical signals, and it's increasingly being verified that they really do defend themselves and nearby plants as you have described. Even against the big destructive tearing machine of a deer's jaws, there is a defense, as some plants have developed to be icky tasting to deer. Great post.

August 6, 2010 | Unregistered CommenterLaurrie

Dear Deborah, I do so agree with what you say here that a healthy plant is its own best defence against predators. However, I had no idea about some of the defence mechanisms that plants have which you describe here in such fascinating detail. This has been a most interesting posting to read and I shall keep it all in mind when I am next tempted to reach for the chemical spray when my prize plant is reduced to a hairnet!!

August 6, 2010 | Unregistered CommenterEdith Hope

beautiful post, interesting discussion in very simple terms. In school we call it Allellochemics, i might have the wrong spelling though (It's been a long time), allellopathy and chemicals.

August 6, 2010 | Unregistered CommenterAndrea in this Lifetime

I didn't know that plants produced chemicals like this, or that they communicate with neighboring plants. How interesting! I wonder if this defense response stresses the other plants? Thanks for the post!

August 6, 2010 | Unregistered CommenterJoseph

Hi! Your canna is being eaten by a Canna Leafroller. They are little caterpillars that turn into skipper-type moths. They are the reason I quit growing cannas. Here, it's warm enough for them to live through the winter The holes lined up like that are a dead giveway. They like to eat the developing leaves, rolled up tight. You can try bt if you want to.

I heard a rumor that it's too cold in Chicago for them (the leafrollers). Probably not true!

Thanks for your grafting comment. I hope you're right!

August 6, 2010 | Unregistered CommenterElizabeth Barrow

I know what's eating my hosta leaves and it is the snail's wicked cousin (or I should say cousins because there seem to be thousands) the slug. Haven't ever seen anything eating slugs in my yard before, which is too bad.

Christine in Alaska

August 6, 2010 | Unregistered CommenterChristine B.

Deb, what an interesting and fun post! I learned a lot of new things from it. While visiting great gardens I noticed that their hostas show signs of slug attack, but the owners do nothing to fight the slugs. Maybe, sometimes we need to let our plants to do the defence work! Thank you for such great post!

August 7, 2010 | Unregistered CommenterTatyana

I never used chemical pesticides and I never knew plants are so clever after all, until after I read your post.

August 7, 2010 | Unregistered CommenterAutumn belle

Deb - what a great post! This is such cool info - I had been fortunate to never use "chemical warfare" on bugs until this year.

We found these nasty (and very creepy) small 'cotton-like' balls on our mature Sugar Maple. The tree was infested on the branches - though I had never ever in 23 years seen them on the tree before. We called out an Arborist that identified it as "Cottony Maple Scale", who said if untreated would kill the tree in 2-4 seasons. Typically, there are natural predators -- wasps & fly parasites, but the infestation was so significant this was the only way to save the tree. :(

August 8, 2010 | Unregistered CommenterShyrlene

Hi Deb, Plants are highly evolved, as your post illustrates. (You may enjoy a book titled "The Secret Power of Plants." Though you most likely know it already.) It is rather like our own immune system, where if it is in good condition, we can fight off bacteria and other nasties that would invade us. It seems however most plants have no defense against rabbits - well, they do not like salvias and marigolds! The losses here this year are making me frown. Terrific post! ;>)

August 8, 2010 | Unregistered CommenterCarol

Ok Deb. Why do some plants are able to defend themselves and others can't? Shouldn't all plants evolve a defense?

July 9, 2017 | Unregistered CommenterJim

Hi Jim,
I think that all plants do have natural defenses, and in a perfectly ecologically balanced system, their defenses would work well. However, we know that is just not the way the plant world is, thanks in good measure to human intervention. As example, I wonder if one of the reasons non-native plants sometimes struggle more than natives is because their defenses are not as well adapted to the foreign environment.

Thanks for stopping by my blog and taking time to leave a comment!

July 9, 2017 | Registered CommenterDeborah Elliott

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