Over-informed on IPM - Episode 012: Intro to IPM - Establishing Thresholds

Anna dwells on the concept of acceptable levels of pests and talks to PhD student, Kadie Britt of Virginia Tech, about establishing pest thresholds in a new crop, industrial hemp.
aphids on a plant

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Show Notes


I want to take this episode to discuss an important concept in IPM theory, and that is the concept of acceptable levels of damage. Other than properly identifying the pest that is causing damage, I feel that accepting a certain number of pest insects in your crop is the most important aspect to accepting IPM into your heart.  Acceptable levels of damage can be situational, they can be based on economics, they can be based on emotional factors.  For example, your acceptable level of mice in your house is likely zero mice.  You would be willing to take out quite a few tools from the old IPM toolbox to keep mice levels below this threshold of zero mice. However, your acceptable level of mice in your barn might be a little higher, perhaps you would be open to a biological control, like a barn cat, to maintain that level, where ever it is for you. In commercial agriculture, acceptable levels are normally right at that point where the expense of going out and acting on that pest population is about even with crop losses due to pest damage. We call that economic injury level. You’re bottom line is better off not acting until the number of bugs chewing or sucking away in the field, grows beyond the level where you’re losing money if you don’t act.

Once acceptable levels are established in any given crop, we can develop economic thresholds or action thresholds. Thresholds are what you actually see or count in a crop, as far as pest number or pest damage, things that would trigger a pest management action. Like number of aphids per so many plants, number of moths in a trap per week, number of degree days after January one. Now developing thresholds is a pretty complicated process but, luckily for us insect population growth is normally pretty predictable, so lots of us nerds have developed all sorts of models for us to rely on to get in there and act when the time is right. For many systems these action thresholds can get pretty specific.

For example, I will read an excerpt from the New England Vegetable Management Guide’s recommendation for Colorado potato beetle scouting in potato:
“..Scouting should be done weekly through the mid-season. Walk the field in a V-shaped pattern and select 50 potato stalks at intervals, e.g., every 10 to 20 paces, depending on field size. Count adults, large larvae (greater than half-grown) and small larvae (less than half-grown) separately. If the number of beetle adults is greater than 25, small larvae greater than 200, or large larvae greater than 75, an insecticide should be applied. If the number of beetle adults is below 15, small larvae below 75, or large larvae below 30, no insecticide is required for that week. If the number of beetles in your sample is between high and low, no insecticide should be applied, but the field should be checked in 3 to 5 days.”

SO, pretty straightforward, right? If you’re going to use pesticide, time it for when it’s the most useful. It also makes sense to avoid unnecessary applications of pesticide that disrupt development of biological control on your farm. Why kill all those good bugs working on your farm if it’s not making a difference in the impact of the bad bugs, right? 

But what about when us nerds haven’t gotten around to developing those action thresholds? What about when a crop is so understudied, we don’t even know what bugs are feeding on it or what an acceptable level of these mystery bugs is in that crop?! What kind of crop would be so understudied? What about one that’s been illegal to grow for most of American history – I’m talking about industrial hemp.

As of December 2018, the Farm Bill permanently removed hemp from the Controlled Substances Act. However, the Farm Bill did not legalize growing hemp nationwide. It only allows states to regulate hemp if they choose to. While many states have created regulatory programs for producing hemp, New Hampshire is not one of them! Let me make this clear, hemp production is not legal in the state of New Hampshire. 

Whatever your feeling are on this issue, a new crop is very interesting to IPM specialists, especially a crop that is famous for it’s secondary compounds and potentially so well defended against pathogens and insects. To get over-informed on this I talked to an entomologist down in Virginia, where researchers had gotten special permission to study industrial hemp prior to the 2018 Farm Bill:

Kadie Britt, PhD student at Virginia Tech
Anna: I asked Kadie what kind of challenges she’s faced with working on such a controversial crop for her PhD and she says she has to take a lot of guff just doing normal stuff
Kadie: I’ve put my stuff in driers here on campus, to dry out plant material. People will say “I’m getting high! It smells like marijuana in here!”…and I’m like “heh, never heard those jokes before”… but I think the public perception is pretty good. I haven’t had too many problems with people accepting it. 

The challenges with the legality of it – the past few years, importing the seed has been a hassle. Like, last year we didn’t get to plant until June because the seed was held up at the Richmond airport. It was as simple as that – it was in customs.
It’s also weird – prior to the Farm Bill passing I had technically worked with a controlled substance. Luckily nothing ever happened, I never got pulled over in the process of doing my sampling work but, had that happened, it would have been something we’d have to deal with. It wouldn’t have been fun.

Anna: Before we get into Kadie’s PhD work we need to clarify the different types of industrial hemp crops because there are some major differences in what these plants are used for, how they are grown, and what parts of the plant need to be protected from pests. I had no idea until Kadie explained:

Kadie: Industrial hemp is a really multifaceted crop, it’s really three crops in one. The first use being for fiber, so t-shirts, ropes. Historically it was used for making ship sails – for ship sails and ropes. Hemp is known for being antimicrobial, so if it got wet it would be less likely to mold than traditional rope. 

You can also grow it for grain. You could call hemp a super-food. It has a lot of good fats in it – like avocado-type good fats. You can use the seeds and sprinkle them on oatmeal or yoghurt or smoothies. They have a lot of good health benefits. So that’s why you might grow hemp for seed – maybe also for bird seed or animal feed but then you’d get into all sorts of other situation with the FDA because its not regulated or tested, so there’s that. 

The grain can be pressed to make oil – and by that oil I mean cooking oil or for salad dressing, like olive oil. So you’re getting some good fats but you’re not eating the seed. A lot of people will buy the seed oil, thinking that it’s CBD oil. Again its got a lot of health benefits but its not the stuff with all the cannabinoids or other phytocompounds found in hemp. 

Some varieties are just for fiber – they can grow as tall as 15 feet, or more. Then there’s duel purpose, which you would grow for seed and fiber. These are the varieties I’ve worked with in the past few years.  
Anna: Did you hear that? Different plants for different products. Big tall plants for the stems. Other plants for the seeds and none of the plants that Kadie works with are grown for the flower, which is what is used to make CBD oil. This is a whole other ball of wax for another conversation. Let’s get to the interesting stuff…the bugs! I asked Kadie to describe her project so far:

Kadie: So I’ve got two summers in so far but the first summer was more of a survey. I visited the few hemp fields in Virginia that actually existed in 2017, just to see what insects were in fields. For me to get a feel for what hemp growing on a large scale looked like. From the surveys – kind of figuring out what the key pest species were. 

Anna: What are the insects you’re collecting? The main species?

Kadie: The key pest species we’ve been seeing – at least what I’ve seen for now – corn earworm for sure. Corn earworm targets pretty much everything, including hemp. I think its more damaging to the seed varieties because in can get in the top and devour seed or chew into the seed head and cause a lot of problems. I’ve also seen a lot of yellow-striped armyworm. 
Tarnished plant bug is there from season beginning to end. I’ve also seen a lot of brown marmorated stink bug, particularly here. This is a particular challenge because I don’t really know what they’re doing. I guess I assume they’re pests because they’re pests in every other system. But in hemp, the marketable portion in fiber is just the stem so if a plant bug fed on it, even if it left a spot, you’d still be able to use it to make what you want. In the seed varieties – they’re so small you can’t really tell if they’ve been fed from or if they’ve damaged the appearance of it. 

Kadie: Then this past summer in 2018, I started to actual experiments. Looking at the effects of defoliation on hemp growth and yield, quality type things. Then looking at the impact of different insect species on yield and quality. To do that we used plant clippers to cut off portions of the foliage – 0, 25, 50, and 75% defoliation. In my head that was to simulate “no” “low” “medium” and “high” levels of defoliation but it turns out – when I measured how much defoliation insects were actually doing – it never got higher than 10%. So all the defoliation I did was way over the natural mark, which was cool because, if this doesn’t have an effect on yield, they should really be safe from insects. But in Blacksburg maybe we didn’t really have a lot of pest pressure, so we’ll do that one again in Tidewater and see what happens.
Anna: With all the antimicrobial properties we’re talking about, do you think this plant is just better defended than most other crops?

Kadie: You know, I think that might be the case. So there are some rumors that go around – and people really like to latch onto this – that hemp is resistant to pest of all kinds, insects, pathogens, and weeds. But you take a walk through a field on a hot summer day and that’s not really true. There are weeds everywhere. There might be mold on it late in the season, especially in the east compared to Colorado. And there are insects.

I know that when you simulate insect defoliation it’s not the same as true defoliation. When insects feed, the plant responds differently than if the plant was just torn or ripped. – but still – you’re inflicting all this damage to the plant and its not affecting seed production or seed quality, what does this tell us? I really do think that it has more defense than say corn or wheat.

You know, as an entomologist I often wonder – this plant – we’ve been growing it for a couple of years, not that long. Maybe its still kind of in a wild state and as we domesticate it, we’ll take away some of its natural defenses? Being on the front lines of this research, I want to say “No! insects do affect this crop. Quit saying that!” but maybe the plant is a lot more robust and able to handle more than we realize. 

I am really curious about what happens over the next few years. Not to anthropomorphize insects but, when they get used to this being in the system – just like corn or soybeans – I wonder what the pest status will be then. So say a field is grown in hemp for ten years in a row, would those pests be more damaging in the years to come. 

But one of the key pests we’ve seen in hemp is called the cannabis aphid, which is a new record for North America. This says a lot because if this hasn’t been grown here for that many years and we’re already seeing this super specific pest on this crop, that’s pretty unique. 

So I think Kadie’s work tells a pretty interesting story about how we start with establishing economic thresholds. Who’s eating the plant? What kind of damage are they causing? How much damage can the plant handle before the crop is injured – before yield or quality takes a hit? For now, it seems like industrial hemp is pretty darn resilient but you never know!
That’s it for now. Thanks to Kadie from the Kuhar Lab in Virginia Tech’s entomology department. And thanks to Brentwood’s favorite son, Jason Lightbown, who wrote and performed our theme music.