Play the latest episode:


Brought to you by:


“You put some of this fermenting dough out in the field and instead of you going to find the slugs, the slugs come to you because you've put the dough out.”

 — Rory McDonnell, Associate Professor, Oregon State University 

In this episode of the Cover Crop Strategies podcast, brought to you by Montag Manufacturing, listen to a conversation with Rory McDonnell, associate professor at Oregon State University, as he discusses his innovative research on combating slugs and snails in cover cropped fields using bread dough. 

Related Content

Subscribe to Google Play
Subscriber to Stitcher
Subscribe to TuneIn

The Cover Crop Strategies podcast series is brought to you by Montag Manufacturing.

Montag precision metering equipment is helping producers achieve their yield goals while saving on seed and input costs. For establishing cover crops, Montag’s family of seed platform equipment adapts to a variety of major brand delivery systems that will conserve seed and nutrients along with soil and water. Explore new options for your production and conservation goals with your Montag dealer, visit or call Montag at (712) 517-2775.

More from this series


Full Transcript

Mackane Vogel:

Welcome to the Cover Crop Strategies Podcast, brought to you by Montag Manufacturing. I'm Mackane Vogel, Associate Editor at Cover Crop Strategies. In this episode, listen to a conversation with Rory Mc Donnell, Associate Professor at Oregon State University, as he discusses his innovative research on combating slugs and snails in cover crop fields using bread dough.

Tell our audience a little bit about yourself, and how did you first get into the world of agriculture, and what are some of your earliest agriculture memories?

Rory Mc Donnell:

Great. Great to be here. Thank you for inviting me. I'm an Associate Professor at Oregon State University on the West Coast. As you alluded to, my program is focused on slugs and snails and developing novel approaches for their management. I'm not from the United States, I was born in Ireland. That's where I grew up. I lived in a suburban area right on the edge of lots of agricultural production. My parents were not farmers or growers, so I didn't grow up in that environment, but it is an area that always fascinated me, and particularly how to grow crops most efficiently and how to manage pests in a more sustainable way. I have worked on slugs and snails here in North America, in Australia, in South America, and even in Africa and Europe as well, so I know them quite well.

Mackane Vogel:

All right. Why don't we get into slugs a little bit? Obviously, got a lot of no-tillers, a lot of cover croppers listening, and that demographic often faces greater slug pressure. Let's talk about why that might be. What is it about no-till and cover crops that might be more attractive to slug populations?

Rory Mc Donnell:

Sure. When I think about slugs in agricultural systems and in natural environments, the best locations for them are areas where they can take cover, so where they get away from adverse conditions on the surface, [inaudible 00:02:14], high temperatures, dryness, and very cold conditions. In systems where there's a lot of surface residue, you're creating ideal conditions for slugs to live happily, and you're also creating lots of food for them, and they're able to reproduce at a much faster rate. If you've got a lot of surface residue, and the humidity under that residue is a lot higher, slugs and snails like to feed on decomposing organic matter, so again, you're providing a food source for them. They like to lay their eggs in dark, damp places. Again, having that surface residue provides ideal places for the slugs to lay their eggs, but also and for those eggs to develop and their little slugs to hatch.

Mackane Vogel:

All right. Obviously, for those planting cover crops and no-tilling, doing so for soil health purposes a lot of the time and not wanting to sacrifice that, what are some ways, other than tilling obviously or planting less covers, what are some ways that they might be able to combat slug pressure, some of the stuff that you've found in your research?

Rory Mc Donnell:

Yeah. That's a great question and a question I'm often asked when I give interviews and when I give presentations. When we look at slug and snail management throughout the world, the number one approach that growers use is chemical control, so the use of pesticides here in North America for field crops, and there are three active ingredients. There's metaldehyde-based baits, iron phosphate-based baits, and also sodium ferric EDTA-based baits. They all work relatively well under certain conditions, but their efficacy is extremely variable. One week, they can work extremely well, and then a month later, you can do the exact same thing in the exact same field and you get complete crop loss. The effectiveness of these products is tied to environmental conditions. Things like air temperature and soil temperature, soil moisture content, and even wind speed all dictate how effective an application of bait will be.

Most growers will use baits. Metaldehyde-based baits tend to cause a quicker kill. They paralyze the slugs, and you see the slug mortality in the field. Iron-based baits are softer chemistries, so they basically just make the slug sick, but it takes a few more days for the slugs to die. That's one component of control. Another important approach is natural biological control. One of the advantages of having cover crops, and having no-till systems, and having this surface residue is that, in addition to slug populations building up, it also allows natural enemies of slugs to build up as well. Things like ground beetles, which are important predators of slugs in systems, also their populations increase as well. But their populations increase at a slower rate to this pest populations, so you have to be a little bit more patient.

Here in the Pacific Northwest, I've spoken with lots of growers who have been a no-till for 30, 40 years. They do not have to use any baits on their fields whatsoever, because the natural enemy populations have built up to such a level that keep the pest slugs in check. But that, again, of course, doesn't always work. Sometimes you have to apply broad spectrum insecticides to kill insect pests. If you do that, you're going to kill the natural enemies. One of the important things in my program is we're looking at these novel ways for helping growers to control slugs. One of the interesting things we've discovered over the past couple of years is that there are some food sources that slugs are very attracted to. We have found that fermenting bread dough, so simple bread dough mix, is extremely attractive. That could be used in mass trapping scenarios.

It could also be used in attract-and-kill approaches. You put some of this fermenting dough out in the field, and instead of you going to find the slugs, the slugs come to you because you've put the dough out. You target those areas within your crop where the bread dough is. We're working with a scientist in Canada at the minute, and we're trying to develop a synthetic equivalent of the bread dough so that we can then incorporate that into existing baits to make them more attractive. I do understand that farmers and growers are very busy people. Some of them may not have the time to make up a whole bunch of dough and just basically apply that to their field. It's not very practical, I certainly get that, so that's why we're interested in developing this synthetic alternative where we can incorporate it into a bait to make the baits more effective overall.

Mackane Vogel:

Yeah, that's super interesting. I understand you guys experimented with a number of different novel things. I think there was some fruit and some beer involved as well.

Rory Mc Donnell:

Yeah, exactly. Being a good Irish man, I enjoy my beer obviously. For me, pouring beer out into a cup and trying to attract slugs, I much prefer to drink it, if I'm honest. We were looking at alternative approaches, alternative attractants to beer. Beer, obviously, is a fermenting substance, and that's how we got onto the bread dough side of things. But as you said, we did look at other sources of attractants. In the initial stages of this work, we found that cucumber was extremely attractive to a wide range of pest slugs and snails. Again, you just had to chop the cucumber up and deploy it, and slugs and snails would come crawling in.

We looked at some tropical species as well. Papaya oil, papaya-flavored oil, was very attractive to some very damaging tropical species. There are probably other attractants out there as well that we are interested in. But at the minute, the fermenting bread dough was, by far and away, the best performing attractant that we had. We've just started a project with some collaborators in the Mid-Atlantic and Midwest, where we're looking at slugs in soybean. We're trying to determine whether we could use this bread dough attractant in soybean to help growers. Again, particularly no-till systems have managed these pests, because slugs are becoming much more of a pest in soybean, again, because of the no-till strategies that growers are using.

Mackane Vogel:

What is it about the bread dough that is so attractive from a scientific standpoint, I suppose?

Rory Mc Donnell:

Yeah. That's a great question. To be honest, we do not know the answer to that just yet. My suspicion is that it is probably some of the volatile compounds, so some of the older compounds that are being given off during the fermentation process. We mix a simple instant dry yeast with enriched whole wheat flour and water. We just mix it up, and you stir it. After a few minutes, you can get that distinctive odor of bread dough or that distinctive odor of fermentation. We're pretty confident that it's something within that smell that the slugs are attracted to. Now, whether it's all of the chemical compounds that make up that smell that they're attracted to or just select components of that odor that they're attracted to, we don't know.

But as part of the project that we're working on to trying to develop this synthetic bread dough odor, we have to be able to characterize what the original odor is first. We do that using gas chromatography and mass spectroscopy, where, as I said, we're collaborating with scientists at Simon Fraser University for that work. Once we know what the odor is, we basically start taking out odor compounds from the odor blend to try and figure out which of their compounds are most attractive. We don't know the answer, but we're pretty sure it's something to do with the fermentation process.

Mackane Vogel:

We'll come back to the discussion in a moment. But first, I'd like to thank our sponsor, Montag Manufacturing, for supporting today's podcast. Montag Precision Metering Equipment is helping producers achieve their yield goals while saving on seed and input costs. For establishing cover crops, Montag's family of seed platform equipment adapt to a variety of major brand delivery systems that will conserve seed and nutrients along with soil and water. Explore new options for your production and conservation goals with your Montag dealer. Visit or call Montag at (712) 517-2775. Now, let's get back to the discussion with Rory Mc Donnell.

Another important detail, that I think I didn't realize for a long time, is that, obviously, slugs reproduce at a pretty rapid rate, is that right?

Rory Mc Donnell:

That is right. That's correct. One of the unusual things about slugs and snails for that matter is that they're hermaphrodite, so they have both male and female reproductive organs. If you've got one slug that enters a farmer's field, technically, that slug can lay viable eggs and produce a viable population. That's one of the reasons why they're such a successful invasive species. Gray field slugs, for example, which are one of the most important pests throughout North America, they become reproductively active depending on where you are, usually within about nine months, let's say, and they will lay multiple clutches of eggs over a 3, 4, 5-month period. Those eggs hatch anywhere from 10 days to three weeks depending on temperature in the field. Each of those slugs, again, is hermaphrodite, so they can essentially self reproduce also.

Mackane Vogel:

In these experiments where you guys have placed the bread dough out in the field, how long does that stay effective, and how frequently do you have to change it out? I guess, what are some of the numbers you've seen of slugs being attracted to? Are you just putting them in different containers? I guess walk us through the logistics of some of that, if you will.

Rory Mc Donnell:

Again, that's a really good question. What we term that field life, so what is the field life of the attractant. One of the things we've noticed with bread dough is that when we put it in the field, we often just deploy it in open containers. Like a small, let's say, condiment container that you might get ketchup in at a university, something like that, but maybe a little larger. We put some of the fermenting bread dough into that. We place it inside a trap called a snailer trap so that it's protected from the environment. A snailer trap is just a cylindrical green trap that has got slatted doors. Slugs are attracted into the trap or snails are attracted into the trap. We also put a toxicant on the bottom, because we want to be sure any slugs that are attracted into the trap are killed. Otherwise, they'll come in, eat or feed of bread dough, and then crawl out and disappear. We have to put something in there that will kill them.

One of the things we've noticed is that, after 24 hours, the dough starts to develop a kind of a crust on top of the upper surface, largely because it's just drying out. When we first saw that, we were concerned. We thought, "Okay. If that crust is starting to develop, the chances are the attractiveness is going to drop way off." Initially, we decided we would test it for a week. We pre-aged the dough in the field for a week and then tested it after one day, two days, all the way up to seven days, with the expectation that the fresher the dough was, the more attractive it would be and the more slugs we would catch. But to our surprise, after seven days, the dough was every bit as attractive as it was after just 24 hours, even though it had this quite thick crust on the upper surface.

After that, we thought, "Okay, let's push it out to two weeks. Shortly after two weeks, and it's going to have lost its attractiveness." But again, to our surprise, after two weeks, it was every bit as attractive as it was after 24 hours, which was a real, real surprise for us. From a field life perspective, that's really important, because you alluded to it in your question, growers do not want to be going out every day and replacing this bread dough if it's going to become a viable tool. They want to put something out, forget about it, and then know that it's doing its job. It looks like bread dough is going to be able to do that. The field life is at least two weeks. We have experiments planned in this spring. We're going to test it for a month. We're pretty confident that, after a month, it's going to have lost its attractiveness.

But again, we just don't know. These organisms continue to surprise us. One of the things where we thought about as well is that when the attractiveness does drop off, if we were to add a shot of flour back into the mix, is that going to be enough to start the attraction again? We feel it probably will be, because the yeast are feeding on the flour, and then that generates the other compounds that they're attracted to. It may not be necessary to actually make a whole new batch of dough, rather bring out a shot of flour, mix it back into the dough, and then we're off to the races again essentially. That's how we do it. Just to give you an idea of how effective it can be, we worked on a snail species in Montana, and it's called the Eastern Heath Snail.

We tested our bread dough in Montana. After we deployed, I believe it was 60 traps with the bread dough, 60 traps with just water, which was our untreated control, and we collected snails over 48 hours. After 48 hours, with the bread dough, we'd collected 18,000 snails in 60 traps versus four or 500 with the water control traps. I think that really speaks to how attractive this material is for these organisms. Counting 18,000 snails, I can tell you, was absolutely no fun whatsoever. Most of my staff were furious with me after the experiment, but I was trying to explain to them, "This is such an important discovery. We can tell people that this is attractive, and people may believe us. But if we present this data to them, it's irrefutable how attractive it is."

It is highly attractive. We've tested it against species in Oregon. We've tested it against species, as I said, in Montana, even tropical species in Hawaii, and some colleagues have tested it on species in Florida, and whatever species we tested against, and it appears to be highly attractive. It's got tons and tons of potential. We're very excited about it.

Mackane Vogel:

Wow. Yeah. Those are some impressive numbers, for sure. I want to talk a little bit, too, about just the environment that's most ideal for slug trapping or when you're most likely to see them, I guess, and if there's certain weather patterns as well that come up frequently.

Rory Mc Donnell:

Yeah, good question. Yeah, there are. Slugs, despite them being associated with damp and moist areas, they do not like it when there's driving rain. They just do not like that at all. We see our best slug numbers when there's a light rain and light drizzle, even dense fog. You've got the high humidity without that driving rain, which they're not active in. Also, it has to be calm. If it's windy, so I would say anything greater than five miles per hour, slugs are not going to like it. One of the things slugs are constantly doing is they're constantly fighting water loss. Unlike snails, they're having the luxury of retracting into a shell to get away from dry conditions. If it's windy, they start to lose a lot of moisture. If it's very windy, they will not come out on that. It's very, very humid.

We tend to see biggest slug numbers and most activity in field crops and in other crops shortly after sunset, and also early in the morning, and throughout the night for that matter. But they will also be active during the day if conditions are right, if it's overcast, if it's damp, and so on. Temperature range, I would say ideally somewhere between 50 and 65 degrees Fahrenheit are probably where you're going to see they're most active. Once it starts to get into the 70s, again, if it's damp and moist, they're probably going to be okay. But if it's not damp and it starts to get warm and it starts to get sunny, they're just going to get out of there, because again, they're trying to fight this water loss. They can be active even under very cold conditions here in Oregon. We often see them active even when temperatures are below zero, and particularly if there's a canopy where they're getting some protection from the colder conditions. Yeah. Dark, damp, and calm conditions are the conditions that they like best.

Mackane Vogel:

Well, this has been really interesting. I just want to give you the opportunity to note or touch on anything I might've missed that you think is important for this conversation.

Rory Mc Donnell:

Yeah, thank you. It's been great. Enjoyed speaking with everybody. Of course, feel free to reach out to me. If you type Rory Mc Donnell, Oregon State University into the internet, you'll come up on my webpage. I always enjoy hearing from growers if they need some assistance with slugs identifications or assistance with management. I'm very happy to assist. We are also working on another fund project where we're interested in using nematodes as biological control agents. In Europe, growers use nematodes as a very effective bio control agent. It's a product that BASF have marketed in Europe, and they're actively trying to get it into the United States as well. That's another potential tool to keep your eye on for the future. Again, great to be here. Thanks for the opportunity to talk about slugs. I always enjoy it.

Mackane Vogel:

That's all for this episode of the Cover Crop Strategies Podcast. Thanks to Rory Mc Donnell for that great discussion. The full transcript of the episode, as well as our archive previous podcast episodes, are available at Many thanks to our sponsor, Montag Manufacturing, for helping to make this Cover Crop Podcast series possible. From all of us here at Cover Crop Strategies, I'm Mackane Vogel. Thanks for listening, and have a great day.