How insects sense unhealthy plants
Plants constantly communicate with electromagnetic signals, and insects are tuned in to some of those signals. In the excerpt of our podcast interview, Tom Dykstra describes how insects are attracted to some plants and not to others. If you want to learn more about this fascinating topic, or have any questions, Tom is presenting a webinar on Leaf Brix and Insect Herbivory tomorrow at 1 PM EDT. Please join us.
John: This is a fascinating piece, Tom. I know that this is something that many growers are very interested in and want to understand much better. Would you be willing and able to dig a little bit deeper into describing how the olfactory senses work and why some insects are attracted to certain regions where others are not?
Tom: Generally, the insects are smelling with their antenna and with their palps, which are some of the mouth parts. So when they’re flying through the air, they pick up various odorants in the air. And these odorants are vibrating and giving off energy. They’re absorbing and emitting energy constantly. As long as they are above zero degrees Kelvin, which is absolute zero, they are vibrating.
And these electromagnetic vibrations can be easily detected by various equipment in the laboratory, as well as the equipment on an insect. They have various sensilla. I would call them tiny antenna on the actual antenna-proper of the insect. And it’s these antenna—whether it be the antenna-proper or whether it be the smaller sensilla on the insect—that are tuned into very particular frequencies. And these particular frequencies are all important for the particular insect.
Some insects would not be tuned into CO2. They would have no reason to be. Whereas other insects, like mosquitoes, would be tuned into CO2. There are certain floral compounds given off by plants that some insects are going to be attracted to—honeybees being prominent among them. And then you have certain scents that are not attractive at all because some insects don’t go after flowers. There are some plants that advertise themselves as unhealthy. And when they do so, they are picked up by other insects.
Insects are only tuned in to the unhealthy plant. No insect will ever attack a healthy plant. What they’re zooming in on is the unhealthy plant, because it’s digestible. Healthy plants are not digestible. Unhealthy plants are. Because they can’t digest a healthy plant, there’s no interest in even attacking it. It either has to be injured or it has to be unhealthy. And then, by doing so, it is now digestible, and this is what an insect is going to attack.
At the beginning of our conversation, I mentioned how once you get above twelve Brix, insects really aren’t causing any more issues with your plants. And once you get above fourteen, they’re really not even landing on your plant, unless they just want to rest on it, because they won’t be able to take a bite. Or if they do take a bite, they won’t be able to get through the cuticle or into the phloem tissue—it’s not going to be digestible to them. They’ll have to pull out and move on to a different source.
So all insects are flying above, looking for crop plants that are digestible to them. Plants advertise themselves as being unhealthy, essentially saying, “I’m unhealthy. Please come eat me.” And so the insect will come in and it will start eating the plant, because that’s their job.
Our job is to eat healthy plants. We have a much more elaborate enzyme system. Our digestive systems are designed to eat healthy food. We don’t do well eating Doritos all the time. We do well when we eat healthy plants, and this leads us to be healthy.
For the insect, it’s different. Insects do not do well on healthy plants. They can starve. You can take a Colorado potato beetle and put it on a healthy potato plant and it will not be able to eat it. But you can take an unhealthy potato plant and the Colorado potato beetle will go to town, because this is what it is meant to do. And so because insects are only attracted to unhealthy plants, the unhealthy plants will advertise themselves. They’ve got certain visible frequencies that insects can detect, especially from a distance. And they’ve got certain odors. And these odors can be picked up by the antenna and by the sensilla, and the insect will move in for the kill, so to speak.
John: When we look at plant health, we’re talking about healthy plants versus unhealthy plants. What are some of the compounds that serve as insect attractants that we could manage and monitor?
Tom: You can’t. It would really have to be a general thing. Generally speaking, ethanol is a universal odorant that advertises itself as being unhealthy. So a lot of the plants will release not just ethanol, but also various alcohol components. Not all alcohols, but many alcohols advertise a plant as being unhealthy; it’s a hallmark of fermentation. Fermentation produces the alcohol.
And so when a plant is degrading and it’s in trouble and it’s fermenting, even in a small way—even in an imperceptible way—it will advertise itself. If these odorants are being released, they will be picked up by insects. It will change how the plants are perceived. You can take satellite images of two crop plants and they look different on various images. It can be a visible image. It can be an infrared image. But they both may be corn. They both may be soybean. They both may be anything you could think of, but they will not have the same look under an infrared camera or under a visible camera.
This is something which is very profound in grasshoppers. You don’t find them so much in the United States, but on other continents, locust swarms are a problem. These locust swarms are not just millions of individuals, but billions—sometimes trillions—of insects. They descend upon a very particular crop and take it all the way down to the roots and then pick up and fly away. And they will leave a farmer’s field right next to that exempt. These are the remarkable things that you realize when you see stuff like this— the grasshoppers made a decision. They made a decision to eat one plant over another. Why? Why didn’t they just come down and eat everything? We’ve always been told that grasshoppers will eat anything, and yet there is direct proof in some of the images that I have seen and testimonies of others that, no—they actually are very selective.
Now, I should tell you that grasshoppers are less selective than other insects. Some insects will disappear by a Brix of eight. Other insects will continue to chew on your plants right up through ten, eleven, or twelve Brix. But once they get to about twelve, they will lose interest. And the grasshoppers are among them. You can find the grasshoppers among slightly healthier plants for that reason, but you’re going to find that with the aphids, the leafhoppers, some of the other hemiptera insects, once the plant gets to eight, they lose interest in the plant. You just won’t find hemiptera insects on a plant above eight Brix. And those are the ones that have the beak that they stick into the phloem tissue and take a sip from the sugar water that is flowing around the phloem tissue.
So every insect has its own cutoff. You really have a lot of insects fall off by the time you get to eight. And as I mentioned at the beginning of this, most of the crops are between four and eight. So a lot of plants are really susceptible to every single insect that is out there. But if you can get above eight, you pretty much can take care of your aphids and your leafhoppers and psyllids. The Asian citrus psyllid is down here in Florida, and other psyllids, because it’s very rare to find a citrus tree that’s above eight. We’ve tested a lot of them.