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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.

How GMO’s can influence soil microbiology

On several occasions, we have observed GMO corn crops and GMO corn stalk mulch produce a soil environment that enhanced disease, sometimes dramatically. Why would it be the case that GMO crops produce a disease enhancing soil environment, where non-GMO corn produces a disease suppressive environment?

Other research has identified that GM plants have altered carbohydrate and amino acid profiles in the root exudates, which seems to be a probable mechanism for producing an altered rhizosphere microbiome.

Robert Kremer and I approached this conversation in our podcast interview:

John: Earlier you mentioned the impact of genetically modified plants themselves, apart from glyphosate and AMPA. How do GMOs impact the soil’s microbial community?

Robert: Well, there’s not a lot of information. We found with soybean, for example, that genetic modification can have what are called pleiotropic effects—indirect effects due to the genetic modification that are in addition to the intended effect. In other words, effects that are in addition to the effect of making the plant resistant to glyphosate. And so there are things that can happen in the root system—with some of the early genetically modified soybean varieties, anyway—that even without being treated with glyphosate, the roots seemed to release a lot more carbohydrates or soluble carbon and amino acids. This is problematic because it attracts a lot of microbes that readily use this material, and many of those can be potential pathogens. So you have a potential problem not only with some root pathology, but it’s also possible to build up these segments of the microbial population and carry them over from year to year.

Another situation where we find these effects is in corn. Not in all varieties, but in many varieties that had been genetically modified to be resistant to insects using Bt, there was a side effect where some of the corn stocks would have a lot more lignin than others. Lignin is very difficult to decompose. That’s one of the reasons we sometimes see a lot of that residue being carried over for two or three years in the field—there’s so much lignin that it can’t decompose very fast.

And I think there are other situations that can occur. I had a Brazilian student here who looked at some of the nutrient composition. Some of the omega fatty acid ratios were changed in soybeans due to the genetic modification; that kind of thing. Now, I can’t say for sure if that has changed with some of the more recent cultivars, because I haven’t been looking at that very closely over the last few years. But, as you know, in our commodity agriculture, these varieties change almost from year to year. Some of the varieties that we were using fifteen years ago are not available anymore. So that’s always another problem. You just don’t know whether the effects of these newer varieties are any better or any worse unless somebody has a research program that’s addressing it.

John: Your first point is very intriguing. In essence, what you’re describing is that these crops and these plants may have the capacity to actually develop a disease-enhancing soil profile—which is interesting when you consider the long-term implications.

Robert: Right. That was a completely unexpected result that we had. And we were comparing it to some of the old non-GMO varieties like Williams 82 and Maverick, and they had much lower soluble carbon and amino acid release. So it was quite interesting, to say the least.

2020-09-21T20:35:48-05:00September 22nd, 2020|Tags: , , , |

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