Integrated Pest Management, Weed Science

Episode 11: Weed Sampling

Join Emily and Nevin as they discuss the details about the data collected for Nevin’s palmer amaranth and dry edible bean studies.

This transcript has been edited for clarity.

0:19 Emily: Hi everyone, and welcome to Farm Sci-Ed, the show where we go into the science and education behind farming. I’m Emily Stine and today we’re talking to Dr. Nevin Lawrence, weed scientist, about the data he collects as part of his palmer projects. So sit back, relax and let’s go find out what data nevin collects.

So Nevin, what kind of data do you collect for your palmer studies and how do you go about collecting it?

Nevin: I guess we could classify the data in two different ways. We take weed control data and we take crop performance or crop yield data and we kind of think about that in two separate separate ways. So we have the, “what is the effect of the treatment on the weed population?” and then “hat is the corresponding effect of that particular weed population as controlled or not controlled by that treatment on on the crop?” And so we divide that into two – two different groups for uh weed control data throughout the year.

Maybe every two weeks we take a visual assessment of uh how good the weed control is and the way that we we do that is we always have a non-treated check plot, um and that’s a plot where we don’t do anything. And so our normal plot size for a dry edible bean study is 30 feet long and then uh usually four to six rows and we’re on 22 inch rows so that’s either seven and a third foot or that’s 11 feet and how wide the rows are but within that those treatments, we have one that we didn’t do anything and that one will usually have no dry bean yield because the weeds completely uh took over and the crops dead and non-existent. uh And so we have a yield of zero in those plots, but we also have weeds which are seven feet tall and that’s sort of our baseline of what happens if we do nothing and then when we take this visual assessment throughout the season, we kind of compare the other other plots to that one. So we we do this on a scale of zero to 100 and then we know what zero is and then if we have a really good treatment that we’re pretty sure is going to control all the weeds in in the study design we we keep it there and that’s what we call the the weed free check and that’s going to be 100 control of the weed.

But sometimes we don’t have that option especially in these minor and specialty crops we just don’t have good herbicide options a lot of times. And so we’ll have another treatment where we will go through and have to hand weed weekly which is which is gonna be a lot of work, but we’ll physically remove the weeds all use – all season long and so that way we have a known zero percent control and a known 100 control that we can use to compare the weed – the weed control data.

Additionally, besides in that visual control which which is helpful but it’s it’s not hard data, there’s a little bit of bias in that, is we will we’ll also go through and physically count how many weeds are within each plot and we do that depending on the study. It depends on how much area we’ll count, but we usually use what we call a quadrat which is a square, rectangle piece of pvc pipe or metal and those are various sizes and we’ll we’ll stick that in the plot and we’ll generally this is a rule of thumb – that it’s sort of just arbitrary – we try to count at least a square meter per plot of plants to get that population. So we’ll count how many weeds there are, and we’ll also count what the breakdown is of species.

That’s important because if we have a a weed control plot that fails early in the year, uh or it fails to control the weeds earlier in the year, we’re going to have a lot of early to emerge weeds in that plot so kochia, lambsquarter, those are the earlier weeds to emerge and that’s going to prevent later to emerge weeds from ever emerging because there’s there’s are going to be quite a bit of competition and presence of weeds in that plot. So what we can oftentimes see is if we’re looking let’s say, at palmer amaranth control, some of our worst performing plots actually won’t have any palmer in them but not because they controlled the palmer, it’s just because there was so much early to emerge weeds in them and so these counts are important to see not only what the density of weeds are but what species are coming up and when.

So we take these several times throughout the year um you know, probably three to five times we’ll we’ll do a weed count and just to kind of give some perspective on that you know a weed-free plot as i said earlier that’s going to have zero weeds in it, but the non-treated one can have hundreds, um sometimes even thousands we’ve seen weeds within a square meter, so it’s it’s pretty difficult to actually do that. You’re – you’re gonna be spending um 30 minutes on on one plot just on your hands and knees counting those weeds. uh Towards the end of the year we’ll also take biomass in those plots as well and so what we do with that is from that same quadrat we’ll go in and we’ll – we’ll cut using a knife – a rice knife actually – it’s a knife for hand harvesting rice but we’ll physically cut all the above ground biomass from weeds within that plot and then get then weigh that per species.

Occasionally the other thing we’ll do is we’ll also go in those plots and remove seed heads when they’re mature but before they shatter.
That way we can estimate seed production too. So from a given treatment, which is you know fairly small – you know we’re seven and a half feet by 30 feet at the smallest – we could say this treatment is going to produce or it has the ability to control, you know, this percentage of the weed population but you’re looking at under worst case scenarios, you know, potentially 30 palmer plants per square meter which is quite a bit and those 30 palmer plants each can produce let’s say 50 to 100,000 seeds. and so that can give a – when you extrapolate out to a whole acre, that gives the farmer an idea of what that treatment may or may not do in their field.

Right, as far as crop information that we collect, we we do stand counts throughout the year, so we’ll do stand count at planting and then we’ll – if we have zero yield in some treatments because it completely failed to control the weeds uh by the end of the year, there might not be any dry beans in there. So we can see when that stand was lost. We also take measurements of plant physiology – so how um tall is the lowest leaf, what’s the height of the – the pod height throughout the season and that’s going to be impacted by uh weed presence. So as we have more weeds, the plants going to potentially grow taller and so we’re going to have these these physiological changes and those could have yield impacts as well. So we look at what we call yield components and so, with dry beans that would be the number of pods per plant the number of seeds per pod so in a good dry bean uh plant you’re gonna have five to seven seeds per pod and uh when we have a high weed pressure, that might get reduced to one to three and so that’s a pretty significant yield loss right there.

We also look at test weight or the um the the weight of 100 seeds and so we’ll – we’ll physically take 100 seeds out, count them out 100 and then weigh those for each each treatment. Or we’ll sometimes do a thousand seed weight counts, and what that does too is it tells us we have a larger bean or a smaller bean generally you want a larger bean um but with a lot of wheat competition that the bean actually does become smaller and then finally just yield: how many pounds or how many tons per acre or tons per hectare of dry edible bean yield did we receive in those in those treatments?

8:17 E: Why do you collect so much data?

N: Yeah uh it is a lot of data. And not only is it a lot of data, but all the data we collect uh the majority of it’s probably in the last week of the crop so it’s it’s um it gets pretty busy that time of year. But what we want to do is get a thorough understanding of how the weeds are competing with the crop and what the impacts are.

So for example, um we might measure a yield reduction, but it’s important to know what the mechanism that yield reduction is. Is it because we’re having a smaller test weight? Are we having fewer pods being formed? Are we losing stand? Are we having fewer seeds per pod? All that sort of plays into what why the yield can be reduced and dry bean in particular. And so we’re trying to understand the nature of how that competition is occurring.

The other thing is we know what the end of the season um weed biomass is and density, and so sometimes uh plots that are looking good early in the year, they’re not looking that great at the end they’re not looking much better than some of the treatments been poor all year, but the yield impacts really aren’t that bad. And so what happens, is those plots because we’ve been tracking weed density throughout the year, we know that maybe those weeds didn’t come up until July or maybe even August and we’re harvesting the crop in September. And so although there’s there’s quite a bit of um emergence it was late enough that most of the yield inputs into the crop or the physiological processes that need to occur before yield, have taken place – that have taken place before those those weeds emerge.

And so there’s a lot of things we can do to link that up and that helps us know – um you know if I get a phone call from a dry bean grower that you know “we had a huge escape of of a particular weed, uh you know it’s getting harvest season what do I do?” depending on when those weeds emerged uh we’ll we can – we can inform him that you know if you harvest now there’s not gonna be that tremendous amount of uh yield loss, or we can tell them uh yeah actually um they’ve been growing all season long you’re not going to get much out of there. We also could tell them that hey your your test weight’s going to be a bit lower so you’re going to get dinged uh when you try to sell those beans at the elevator because they’re not as large uh the quality is not as good and so that’s some other components. It’s necessary to know this that we can inform growers of what to do and what to expect from different populations of weeds and what the value of using other treat – certain treatments are compared to other options they may be doing.

10:50 E: Well folks, there you have it. Today we talked to Dr. Nevin Lawrence about the data he collects as part of his palmer amaranth studies, why he collects it, and what he can then infer from that data. Join us next time as we continue examining what goes into the research in plant pathology, entomology, and weed science. Find us on twitter @TheFarmSciEd

or on our website at for transcripts and more information. Have a good one!

Entomology, Integrated Pest Management

Episode 10: Insect Sampling and Collection

Jeff talks with Emily about insect sampling and collection for his entomology studies.

This transcript has been edited slightly for clarity.

0:23 Emily: Hi everyone, and welcome to Farm Sci-Ed, the show where we go into the science and education behind farming. I’m Emily Stine and today we’ll be talking to Dr. Jeff Bradshaw about entomology sample collecting and the different processes he uses to get samples. So sit back, relax, and let’s go see what Jeff uses in the field.

0:42 So Jeff, can you explain the importance of sampling to entomological research?

Jeff: Insect sampling in general has a couple purposes, when it comes to pest management at least and some other disciplines as well, but there’s sampling for research essentially, and then there’s sampling to make a decision as in applied field crop pest management, and then sometimes you’re doing sampling for research for the purpose of making the decision. So the reason that’s important is there’s different levels of intensity of sampling that you might use, depending on if you’re a crop consultant and you’re trying to you know make efficient use of your time and not spend a bunch of time in the field sampling, you just need to make a decision at some level that you’re comfortable at versus when you’re doing sampling as a researcher – as a scientist – and you’re trying to use that data for other purposes, to learn more about the insect, when it occurs, how to control it at a high level of accuracy and precision.

So um sampling then for our study and for anything is one. There’s different tools that we use for sampling and then the purpose of the sampling. In our case, we’re collecting a number of different species and then using that sample to identify which species are present and then how many of those species are present within the sample.

2:06 E: What are the differences between sampling types?

J: Yeah so one other difference uh between sampling methods as I mentioned earlier: sampling for research or sampling for a decision. A lot of times we’re doing sampling for research, we’re using more than one tool. Basically attack a problem – if you will – from more than one angle, which is very common in science. Use more than one kind of method that you’re asking for, which you’re addressing, the same question because every sampling technique and tool has a slightly different bias, maybe towards different species or towards different catch efficiencies. Yeah, really what you want to do for research is you want to capture – kind of encapsulate all the different methods that you can, to try to see well, at the end of the day, if you have more than one sampling methodology or tool that you’re using, and they all kind of point in the same direction ultimately with their data, in terms of the conclusions that you can make, then you have a much more powerful conclusion because you have more than one method that you use to to arrive at it.

3:05 E: What do some of these methods look like?

J: Some of the methods that we’re using – specifically to our study – are vacuum sampling. So we have a leaf blower, which we put on the intake side of the leaf blower to use as a sucking tool and uh we cover the end with a mesh, and then move for a certain – well there’s different ways you can do it – you can move through a canopy for a certain period of time or certain distance or a combination thereof, so you have a standardized way of collecting with that tool. And so it sucks everything into the mesh and then you close the mesh around the around the sample and then put it into a plastic bag. So we’ve got vacuum sampler, got a sweep net that we use kind of similar type of sampling, you’re sampling the canopy of the plant just like you are with a with a vacuum sampler.

We have sticky cards that we leave out for a period of time and we’re targeting aphids or minute pirate bugs, aerial insects that are moving through the air and some of which are attracted to that yellow color color of this yellow sticky card. Then we use pitfall traps to ensnare organisms that are running across the soil. So essentially it’s a little more fancy than that, but it’s essentially a hole in the ground um and then the insects fall into the jar that’s within the hole and then we can pull the sample out of that.

And then we have pheromone traps that we use and those are much more specific. In this case, we’re using a pheromone to target the the pest of preference, in this case western cutworm and to trap them and to use that as just a general sampling tool to try to understand what their numbers are. And then a black light trap that we use again for for sampling purposes, and our purpose black light trap is really we’re trying to collect egg masses ultimately um from the western cutworms that are being attracted to that black light trap. And then other – you know – all those sampling tools are looking at ways to capture and then ultimately identify and count the specimens that are the species or organisms and numbers of organisms that are within the plot.

But we also have tools that we use to sample function, different tools that we use to sample function would be like sentinel prey. So we might use sentinel egg masses – so for example with a black light trap, we sample adults and then we go into the black light trap and we gather the females in the trap – well actually, gather all the western cutworms that are present in the black light, drop and then release them into a cage with corn or dry beans um and then give them 24 hours to lay eggs.

And we use those eggs that they lay on the dry bean leaves or on whole plants and we put those out into the field and then measure over a 24 hour period how many eggs are gone. You can put the eggs underneath the microscope and determine whether or not they’ve been sucked dry by a piercing insect or whether they’ve been chewed off by a chewing insect. And so that gives you just greater insight, not only the predation but maybe the type of predators that would be present and so that would sample the insects that are moving through the canopy of the plant.

Another layer of sampling would be to functionally sample the insects that are on soil – that we would use wax worms sometimes affectionately referred to as “truffles.” The way we lay out – basically clay um modeling clay with a pin that we attach the uh the wax worm to and then we leave it for a period of time, sometimes overnight, and then just we’ll have them in open cages and closed cages to expose them to the predation within individual plots. And then again, the the the data it will glean from that would be – you know – is the waxworm after 24 hours alive or dead? Is it still present? Has something ripped it off of the the pin and carried it off? um Has a piercing insect attacked it again? You can gain all those insights by – you know – kind of studying the specimen that you left, the sentinel prey that you left out with a microscope.

7:37 E: And what do you do with the samples after you’ve collected them?

J: All these different sampling methods whether we’re sampling for counts or sampling for function um take a lot of time. When we’re we’re sampling for individuals, like with the sticky cards or vacuum sample, you end up with a collection of individuals and so you have to go through the sample and identify the specimens that are there. Typically you’re not necessarily looking for everything that’s in the sample – sometimes you’re targeting specific groups or maybe specific orders.

And then at some point, you’re making educated estimate on which of those species might have high enough counts to give you some measurable differences when you start comparing treatments that is. If you have an extremely rare thing that only occurs in one sample, and it’s like one species of something that you’re you’ve rarely ever seen the sample, well, when you look at the data you’re going to have one and then a bunch of zeros and you can’t do anything with that comparatively. Whereas you may have a group – like when we’re sampling um for pitfall samples for example – again you have a bunch of insects in that sample, we know from prior research and prior soil sampling type work that ground beetles are typically have a lot of species or you can capture a lot of species with that method. And so that that family of beetles we might look specifically for and identify them to species, because we we think that that could tell us more, because we have more data associated with – this has more species or the species change between plots, where the numbers of individual species change between plot can then tell you something about either the habitat or the growth form of the canopy and different things different insights you can glean from that.

So when we collect them, they’re generally dead or we freeze them and make them dead. But generally we’re looking we’re looking at dead insects that we’re then counting. Sometimes on the sticky cards they might not be. The only sampling methods that we’re using where we’re actually looking at live insects are with the black-like trap sampling and again we’re we’re looking to keep them alive, so that the female moths can lay eggs and the eggs are alive. And so the sentinel eggs that we put out are living eggs, they’re not frozen so they they would be viable enough that they could even hatch within the plots and that again can give us some insight about that treatment and whether or not it had enough predators to kill enough the eggs that the larvae don’t hatch versus plots where you didn’t see that predation, now you have an infestation of western cutworm larvae that can then injure injure the plots.

10:45 E: What is the intent of all this collecting?

J: Yeah, so you know first we sort through it and there may be specific groups that we’re targeting. Within those specific groups, we might um identify them further to species uh or maybe the genus only and then from there you end up with a large database for each plot. Typically we might have three, four, maybe sometimes five sampling periods, so you could take that sample and repeat it throughout the season so you can multiply the numbers even more. And so so our data sheet would look like certain numbers, certain numbers of individuals, a certain species by date.

And so what we then hope to learn from that or things that you can learn are how treatments might impact the occurrence across the season of certain species. So in our wheat relay, or wheat only, we might anticipate the presence of cereal aphids and we might anticipate that they might start out pretty low in the season and increase as the wheat grows, we have more and more of those aphids and then once we harvest the wheat, probably won’t see that many of those aphids anymore since they’re pretty specific to grasses, in some cases fairly specific to wheat and and closer associated grasses.

So um so then that relationship we see with the with the ebb and flow across the sampling dates of the aphids, we can then look at the data set that has our predators and see if they track along that same line and whether or not there’s any relationship between that seasonal change in aphid numbers and the predators that are present in the field to see if kind of one begets the other, in a sense. We look across the season, we see an increase in pests and then the predators track in association with that increase and then we harvest the wheat. And those um across the season we can see that those aphids now are no longer present, but maybe the um predators stick around and then we can start to see if then those predator numbers that have maybe stayed in those plots after the wheat’s gone shifted over to western bean cutworm, or if that changed the nature of the predation rate of the sentinel egg masses, or if it changed the predation rate as a result of our wax worm truffles.

So once you have the data, there’s different ways of looking at it, you know. We could look at the species diversity of the predators, and see if that changed. We could look at species numbers to see if that changed by treatment. Obviously you could see if the treatments had no impact on either those whatsoever. And then we could – we’ll also look at correlative items like relationships between prey and predator and if any of those relationships changed as a result of the treatment or not.

13:58 E: What are you hoping to learn from it?

J: Yeah, so once we have that data set and we look at those relationships, we can look at higher order relationships between those ecological functions that we were trying to measure – the individuals and and species diversity we’re trying to measure – and see if there’s if they track at all with the yield that is reported out from the plots or the injury that we see on those beans. And usually there’s um obviously there’s a lot of variables in the field, a lot of other variables contribute to yield and quality other than just the things that we’re measuring. So typically what we would do is constrain the conclusion that we’re trying to make, so we don’t overblow the conclusion. We can say, well, this proportion of the population can account for 30 percent of the yield increase or accounts for 10 percent of the damage that we saw. We can say it’s related to this relationship.

14:50 E: Well folks, there you have it. Today we talked to Dr. Jeff Bradshaw about entomology sampling and what he does with the samples he collects from the field.

Join us next time as we go into more depth on our other topics in plant pathology entomology and weed science. Follow us on Twitter @TheFarmSciEd, be sure to check out our website at for transcripts and more information about this program. Have a good one!

Plant Pathology, Weed Science

Episode 9: Cercospora Disease Identification

Join Emily as she talks to Bob about how he identifies Cercospora leaf spot in the field and in the lab.

This transcript has been edited for clarity. 

0:22 Emily: Hi everyone, and welcome to Farm Sci-Ed, the show where we go into the science and education behind farming. I’m Emily Stine, and today we’re talking to Dr. Bob Harveson about cercospora diagnosis and what he looks for when trying to make a positive identification. So sit back, relax, and let’s go find out what he looks for.

0:37 Bob, can you tell me what cercospora looks like?

B: Well, it’s it’s a sort of an ash gray – it’s a lighter gray and they’re they’re oval to circular and they then they can coalesce and and really kill a large part of a leaf. They nearly always have some sort of a of a dark halo – dark brown to purple – that sort of thing. What you’re looking for, like with the hand lens, is is uh clear well they’re clear and they look kind of like cobwebs – you’ll see on the the lesion itself – you’ll see what looks like pepper pieces of little little pepper black and then you’ll see like the cobwebs and all that kind of stuff if it’s sporulating. If it’s not, then it’s kind of difficult to tell the difference.

2:27 E: What does cercospora look similar to in sugar beets?

B: Well I guess one of the things it can be is alternaria. Because that alternaria tends to be more circular and less oval than cercospora and it also doesn’t always have a border around it. There’s other several other diseases that foliar diseases like phoma that also can be confused if you don’t know what you’re looking for, and then bacterial leaf spot as well. When you have enough lesions that the bacterial leaf spot is starting to kill the the leaves, then that also can look like uh cercospora and been coalescing. And the reason that’s important to know is because the cercospora is the only one of this group that we really need to be concerned about. So if you if you mistakenly thought it was one of the other ones and made an application then you’re going to be wasting your money for that. Or if you thought it was one of the other ones and you didn’t make the application, then you also could take a hit from the pathogen that’s that’s damaging – or is cercospora.

2:27 E: How do you go about positively diagnosing cercospora infections?

B: You’re looking at the leaves you’re looking at it with with a hand lens, but honestly the only way to know for sure

in in most cases is to take it to the lab. And there’s just there’s a number of other things: you can plate it out, you can watch – watch, you know look at it under a microscope to see if you can see those spores, but it’s essentially you’re trying to make it sporulate so that you can identify it for certain with that. And sometimes it’s already doing that in the field sometimes it’s not.

2:57 E: And can you describe how you force cercospora to sporulate in the lab?

B: Something that’s called a humidity chamber is it’s just a paper towel that’s been dampened and put into a petri plate. And you just get a piece of the tissue that’s got the lesions on it, put it into that, close it up, and then it creates a really humid environment, which then in roughly 24 hours you can look at it again. It should be sporulating, then you can look at it in terms of that and and or you can put it put it onto media and let it grow out. And then in that way it would produce its uh the spores in that manner. Or you can also look at it directly uh with the with the microscope and see the spores themselves.

3:37 E: What do cercospora spores look like?

They they’re long and cylindrical, almost like a sword – a blade of a sword or something like that. It’s it’s very very long and thin.

3:50 E: How do the spores of Cercospora compare to the spores of other diseases?

B: Well it’s it’s hard to describe, but it’s – alternaria would to me look more like a club, you know it has a – it has a thin handle on it but then or more like maybe a tennis racket it’s got a bulbous end on one and then it’s got sort of like a handle on the other and with the cercospora, it would be cylindrical the whole way, like like a mop stick or something or broomstick.

4:20 E: What’s the next step after you have a positive diagnosis?

B: There’s not any kind of magic number, but I think in general, people look at this and say, “if you see three or more lesions on one single leaf then it’s time to it’s time to spray” or at least think about that. It also is important to know which part of the plant that – you’re not as concerned with it if it’s down in the lower part of the canopy as it would be up on the newer leaves, because that’s where the damage occurs. And and so then yeah because every time you look at that – you see three or four lesions, there’s probably more on the way that just haven’t formed yet.

4:55 E: Makes good sense to me! Well folks, today we talked to Dr. Bob Harveson about how he positively diagnosed cercospora in the field. We talked about what it looks like, what it might be confused with, and what he does in the lab to figure out if it’s cercospora or not. Join us next time as we go into detail on the other topics. Follow our twitter @TheFarmSciEd for updates on when new episodes are released and visit our blog at for transcripts and other information. Have a good one!