Mow less: Studies show less is more when mowing noxious weed

Two recent studies have revealed that mowing silverleaf nightshade, a poisonous and noxious weed, enhances its fitness and defenses, making it more robust and resistant to insect predation. 
 
Rupesh Kariyat, an associate professor in the department of entomology and plant pathology, has provided valuable insights into this research, shedding light on the properties of this weed, which is also known as white horse nettle, tomato weed, or bull nettle.

First study: Local adaptation to continuous mowing makes the noxious weed Solanum elaeagnifolium a superweed candidate by improving fitness and defense traits -
https://www.nature.com/articles/s41598-021-85789-z

Second study: Continuous mowing differentially affects floral defenses in the noxious and invasive weed Solanum elaeagnifolium in its native range - https://www.nature.com/articles/s41598-024-58672-w

Transcript

 [00:00] Intro

Welcome to the Arkansas Food, Farms and Forests Podcast, the podcast bringing you the latest on food, fiber and forestry research from the Arkansas Agricultural Experiment Station. The research arm of the University of Arkansas System Division of Agriculture.

[00:17] Jenifer

Welcome to food, farms and forests. I'm your host, Jenifer Fouch. Today we are learning about silverleaf nightshade, a noxious and poisonous weed, and two specific studies that show continuous smoking can make silverleaf nightshade stronger in specific ways. Joining us to explain the research and the results is Rupesh Kariyat, associate professor in the department of entomology and plant pathology and the corresponding author of both of these studies. Thanks for being here with us.

[00:48] Rupesh

Oh, thank you for the invitation. It's always nice to talk about some of this interesting stuff in the lab.

[00:54] Jenifer

So let's start with the silverleaf nightshade. It's also known as white horse nettle trompillo, tomato weed and bull nettle. Can you give us a little background on this plant?

[01:07] Rupesh

Like the name says, it's a member of nightshade family, which is actually Solanaceae. That is the botanical family name. It's the same family where we have your tomato, tobacco, eggplant, peppers, petunia, all these plants are in this, in this family. It's a very important family. Probably more than 2000 species in this... or at least in this family. So, and it produces purple flowers. So 95% of all the flowers are purple or, you know, light purple. And also there is a mutation which also makes them white. So you can see these three colors.

[01:50 ] Jenifer

How did you start looking into it and how mowing affects it.

[01:55] Rupesh

My interest in this plan started actually in 2013. I was a postdoc at Penn State, where I just had graduated from my PhD program and my phD was on a different weed, Solanum Carolinians, which is actually not in Arkansas. In fact, I have one of it in my front yard and I wanted to continue working on this insect bland interactions in weeds. So I started to look into a species which is present in Europe that I could potentially work on. So then I came across this weed, and I was really surprised and sad that I didn't know about how bad this really is actually, at that time. One of the places that this is such a big concern is in Greece, Greece, Turkey, but also in other African countries like Tunisia, Morocco, and in the southwestern United States and in Mexico. So Texas is in the native range, and also, parts of Mexico and many other places too. I was able to find a collaborator, in Greece. So when I went to Greece, it's so funny that the city of Thessaloniki, when you you're on the flight and the flight is going to land, if you look through the window, this is all you see right there. You can you can get a sense of how bad it is. They grow a lot of olives, right? When you go to Olive garden's, you talk to farmers. Then you start to learn that this they are miserable because of this weed. This weed grows understory of olive plants. And it would suck all the nutrients from them and eventually would kill, these trees. 90% of biology is good observation. And so you start to observe, you have a group of people who are also really good at this. You start to see that the plants, which are grown in places where there is not much of human activity, like, mowing or anything, they are big, they are bushy, but they look different than the places or fields where there is constant mowing. Right, people disturbance. And the classic thing about this weed, and people in South Africa has done a lot of work on this, is that the plant really likes disturb... to be disturbed. It's like complete opposite of other people it likes and probably likes when it is disturbed. So that's where I did a lot of my field work on this plant. But my interest is mostly in understanding how plants respond to insect damage, when insects feed on them. How do insects do on the plants? But also how do plants respond. So, I came to Texas. I got this job interview at the University of Texas, Rio Grande Valley, the southernmost tip of Texas. It's like six miles away from border to Mexico. And then, I land, an airplane and then, this, professor picks me up, and then he takes me to the campus, and then I step out of the car. The first thing I see is this weed. This is the same weed that I have seen in Greece. Then the other thing, like, I'm more trained as an entomologist in addition to be a plant biologist. So, there is this caterpillar called the tobacco hornworm, which you guys see on your tomato plants all the time. So, and then I walk around, Texas, so this is in McAllen, Edinburg, McAllen, South Texas walk around and I flip few leaves and then the fourth or fifth leaf I flipped I see an egg and that egg by looking at it, I know that it is a manduca sexta tobacco hornedworm egg. So I have the plant I had been working and the insect I had been working, and the, it’s like match made in Texas. So, one of the main reasons that I took the job was that I could work on this system. And it may be that I don't need permits. I don't need any paperwork. I can just go to my garden, and then collect the plants and collect the insect and actually do experiments on it. So this would be a great time to introduce two key people who are involved in this. One is Jesus Chavana and he is the lead on the first paper in 2021. So this is his master's thesis. I also had this great opportunity of having another student who came through a USDA internship program. His name is Alejandro Vasquez, and his master's thesis was the second paper that you guys are interested in. And he is also a doctoral Dissertation Fellowship awardee from University of Arkansas. Going back to the story, we drive around and then we started looking in South Texas, different populations of this plant. And in South Texas, again, we were able to identify these batches or populations of this plant where they are under a lot of disturbance, especially by moving by the city of McAllen and Edinburg. So they would come and mow, and then there are places that they don't mow. So it was a natural, sort of natural, it's not, semi-natural, let's say, where you can see these patches where nobody touched them and then you can see constant mowing. So from 2018 we started to collect data on during an year, how many times do they more? When do they mow? And what happens to the plants after they mow? And how does that compare to the plants that are unmowed? Right?

[07:21] Jenifer

So essentially a very broad explanation is that your research found that mowing and continuous mowing can make them stronger in different ways. So the first research focused on fitness and defense traits. Can you explain the results from that 2021 research? The first study.

[07:40] Rupesh

So Jesus, was very interested... Jesus is from that area, he had been living there for a while. He knows that this is such a concern for producers. And we had been talking to a lot of farmers in the Valley. And then we had this very interesting observation that there are places which are mowed multiple times a day by the city or by, you know, people. And then we have patches in the same area because soil is an important concern. Right? You cannot look at these populations if they have completely different types of soil. So we need to find patches were very close but you have mowed and unmowed. So we started to look at that. And when we started to measure some of the very basically it's like height of the plants, you know, number of leaves and all these things. But then we started say something interesting, which is when there is continuous mowing, it actually makes the plants stronger. They may not produce the same number of fruits, but there is no difference in the seed number. So making big fruits okay, it's a big deal, but it's actually the seeds, right the number of seeds. But then the most interesting thing was when you germinate the seeds, we saw that the seeds from mowed population germinated more. But also they had this germination tendency, which is very common in weeds, that they are don't germinate at the same time. So it's called a staggered germination. So if you put 100 seeds, 25 of them will germinate in a week, another 15 weeks will germinate after two weeks. So the seed bank stays for much longer. So the staggered germination was much more pronounced in mowed population. Then we collected the leaves, and then we put caterpillars and beetles and aphids on them from, so leaves from mowed populations and leaves from unmowed populations, let's say they are control population, right. And then, so some of these insects do poorly on the mowed ones, which means that the mowed ones are better defenders, too. So every time they get mowed, they are actually buffing up... Right. So they are getting better to defend.
 
 

Most of the plants you don't know that they are they are toxic, right? But if you look at a plant which has a lot of spines, you know that that is not something that other other organisms would enjoy feeding on. And guess what the mowed plants have? More spines. And we also found that they have this tiny hair. So we have more of that. So when you take this all together, increased germination, not much difference in seeds, and the higher defenses, our main finding is we get that you are trying to mow these plants so that the plants are getting eliminated. But what you are actually doing here, you are making them much worse, much stronger. The seeds that are from these plants, which are mowed more, are even worse, right? They are much more resilient. And they can germinate in a much staggered way so that the seed bank stays in the soil much longer. 

But what we see is if you keep on mowing some of these insects, we are able to put them in check becomes less efficient in that. So they cannot do so well... when you continuously mow the plants. 

And then Alejandro and Jesus would come back and say that we should also look at these flowers. Right. So these flowers and that they can produce hundreds of flowers, if it is an unmowed plant. And the flowers are very interesting because obviously if you lose flowers, you lose all your fitness because you cannot produce fruits and seeds. But then that some of these insects actually feed on flowers too, so they actually feed on the sexual organs of plants. And if they kill that or eat it, then that is that is no fitness. And the plants are in that dilemma where you cannot make the flowers too toxic. If you make the flowers too toxic, then, if there is toxins in the pollen and floral organs, then you cannot get a lot of pollinators, right if it is toxic to them. And so Alejandro wanted to ask this question where he wanted to see whether mowed flowers, so flowers produced on mowed plants, are different from flowers produced on unmowed plants.

[12:27] Jenifer

So when you first studied the leaves, yeah, you found your results. And then there was curiosity on the flowers. Once you looked at the flowers and did the research, were the results the same that mowing made them stronger, better?

[12:43] Rupesh

So Alejandro's question was like, how do these flowers differ between mowed and unmowed plants? And does that have consequence for the insects that actually feed on them? Right. And the third question would be like, is it similar to what we found with the leaves? Because if you take plant as a whole, just leaves being highly toxic may not always be enough, because if the flowers are pretty good nutritious, that would attract a lot of insects that would feast on them and kill the plant. There would be zero fitness. So that's how, that was the motivation for that study. And he found that the mowed the flowers were actually smaller or like lighter, than the unmowed flowers.  Then he started looking at damage on these petals. So you have the anthers, then you have the petals like the corolla, right. The, the purple or the white part of this plant. So if you if you remember about the first study, we would expect that the unmowed plants, which the caterpillars were doing better. Right. So mowed plants were much more puffed up or have more toxins. So Alejandro's question was does that actually translate into sexual organs to, right, in the flowers and the ... voi la, yes. So the mowed the flowers had lower damage than the unmowed flowers, which, again, sort of confirmed our first story. And it also says that these defenses, cascade into other organs of the plant and not just, in the, in the leaves. Then, he also looked at the mass gain of the caterpillar. So we can measure their mass very early. Then you put them on the plants, or you make artificial food with adding these petals. So he would grind the petals and the anther from mowed and unmowed. And he would supplement the artificial food like formula, baby formula. he would add that so you can sort of disentangle other effects, right, other environmental variables and just ask whether this food is better or worse. Another. And he would weigh them after 48 hours. And we can say that over 48 hours, this caterpillar on this diet made more mass or put on more mass when compared to, another diet or controlled diet where there is no plant material. Why is this significant? It is significant because on average, a caterpillar within 20 days of their growth as caterpillar before they’re pupa puts on close to 800 times their initial body mass. And just to convert that into humans or other animals, right, the baby before it gets mature if it puts on 800 times its body mass - so insane. So they have a very short window of growth, and they have to, put on a lot of mass so that they can successfully pupate and become a good adult. Select again, made and disperse, so on. So he looked at that, and he found that the more you mow, even the flowers, are getting more and more difficult for the insects to feed on. The, you know, what I said that the reason why these plants are kept in check is because they get constantly fed upon by insects. Right. And the reason, why they, they may not be producing to their capacity the number of fruits and seeds is because their floral organs or sexual organs gets eaten by these caterpillars, which is very clear that they can do that. So but if you keep on mowing, they cannot eat as much as they can. And like I said, they have a limited window. So they would rather feed at a place where it is less toxic. So eventually the more more plants ridden again. But then he also did one thing which was interesting is, if you remember, I talked about the spines on the plant, right. In mowed plants they have more, but the flowers are on a pedestal, right, there’s a small piece of stem on which the flower stand, and that if the caterpillar wants to get to the flower, it has to go through the stem with spines. So he looked at the density of that spine, in that, just that pedestal. The logic is that if there are more spines on that, it makes it difficult for them to maneuver through that. So he found that in mowed flowers, flowers produced on more plants have more spines, too. So let's take a step back and understand that from a caterpillar perspective, I am interested in feeding on the flower, right? I need to get to the flower because the mother doesn't lay eggs on flower. It lays eggs on the leaf, so it starts walking. It's needs to feed a fast, to feed a lot to get to the flower, but the route is pretty bad. It's a lot of spines when compared to the next one, which does not happen. Okay, I am going to maneuver through that. I'm going to get poked out of my body. I'm going to bleed a little bit. But if I bleed my blood, which is hemolymph is actually a signal for ants and other things that you have a prey here, so come and pick me up. All right. So all right. But I'm going to do it anyway. Walks up to the flower and starts feeding, which is again more toxic than the other. So continuous moving in this species. I want to be very clear about that. Like I'm we are not making any claim about all the species, every weed or anything. And in this species, continuous mowing leads into producing plants that can eventually lead into a situation where they are so tough and toxic that the insects which have been co-evolved with them, so they know to process these toxins will still be difficult to feed, and the keep them in check. I think that's what we are trying to say. Again, this is only two lines of evidence. We need to collect... there is a whole different story happening under the soil with these roots, right? Because they can spread. And like I said, when you blow every piece becomes a new plant and all these things. But it it tells us how our management practice can have a significant effect on plant growth and development, especially in weeds, but have consequences beyond that for insects that feed on them.

[19:18] Jenifer

So, so far, and might be too early to tell. And it might be, as you mentioned, this is specifically for ... this species ... This plant, this species. What do we do with this information then? In Texas, for people in Texas, is this enough information to say, okay, maybe we should look at our our management? What are we doing? Should we stop mowing? And then does it also translate to maybe other species? There's something we can learn from it, maybe not translate directly, but is there something we can learn from this, from this management style, from this research that can be applied to other weeds in other places, for example, here in Arkansas where this plant is not native, but we may have other plants that we have issues with. Do we learn from this research and maybe is the mowing also an issue?

[20:13] Rupesh

Excellent question. I think we yeah, the first thing I would say is we need to look into our management practices. 
 
 What our study, or not just us, but a group of people who work on similar species realize that management practices needs to be better understood by using the ecology and biology of the species and the other species which interact with them. What I mean by that is, maybe instead of mowing 6 to 8 times, we may have to reduce the frequency of mowing. That's what Alejandro clearly shows, that frequency of mowing is also important. There are other cultural things that, makes us want to mow. You don't want your lawn or a place of public interest or a park where there is just wild plants and they are toxic, right? So if you eat the berry, you can get sick. so it's really hard to say how, but I think what we are telling is that this is something that not many people have thought about. and this should be something that we should consider when we make these management plans. Maybe mowing is less or mowing is more. So that it gives us more questions. Any scientist you ask, any paper that they publish, they would say that this gave us more questions than answers. Like that's a given. So we are exactly the same. 

[21:34] Jenifer

So this is one piece of the puzzle. And that the start of a bigger conversation of now we know that mowing and mowing continuously makes the plant stronger, tougher. Yeah. So essentially the solution is leave them alone as much as possible. Or do we try to pull them.

[21:21] Rupesh

Pulling them is is not a bad strategy but it's a horrible time sink and energy sink, right? So, in our garden we can do that and we don't necessarily like it. I don't know, I don't know. I don't really have an answer for that. That's for the weed scientists to figure out. I, I, I rest my case here. Or we... rest my case. I just want to make sure that the real shout out for this whole thing is to Jesus and Alejandro and a few others who worked on it. I just want, everyone to know that that includes Sukhman Singh, who is actually a phD student at Penn State now, who graduated from my lab. Doctor Bradley Christofferson, who is an associate professor and a dear friend, he is still at University of Texas. He's an eco physiologist, a very strong eco physiologist. Alexis Racelis. Dr. Alexis Racelis,, who is a full professor, agro ecology, and a mentor, to a lot of young faculty. I still consider myself young. and then, Alexa Alaniz was an high school student who wanted to work with us. And shout out Alejandro and did a lot of experiments with him. So this is weighing and making diet and then cleaning them, we have to change their diapers every day. Like it's also a whole process. Doctor Robert Dearth, who was that department, who was the department's chair of biology, who is the director for that school no. He was also a huge influence on this study, but also helped us with the navigating through this publication process. And, conversations with him is always intellectually stimulating. So these are all the other people who worked on it.

[23:38] Jenifer

Very good. I appreciate that. I'm sure they appreciate the shout out. Thank you so much. Thanks for sharing your knowledge with us and, for discussing this topic and explaining the research.

[23:48] Rupesh

Thank you for inviting me.

[23:52] Jenifer

That was Rupesh Kariyat, professor in the Department of Entomology and Plant Pathology, talking to us about his research into silver leaf shade and the effects of continuous mowing. Thanks for joining us for this conversation. I'm Jenifer Fouch. Don't forget to subscribe!

[24:08] Outro

The Arkansas Food, Farms and Forests podcast is produced by the Arkansas Agricultural Experiment Station, the research arm of the University of Arkansas System Division of Agriculture. Visit aaes.uada.edu for more information.