Pest Control Part 2: How Pesticides are Used in Integrated Pest Management

posted in: Science | 14
Doesn't this corn earworm larva look delicious? Image courtesy of Cyanocorax from Wikipedia Commons.

In Part 1 of Pest Control, I discussed what a pest was and how they were divided into categories as well as how those categories overlap. Identifying pests and how they cause damage is only one part of the puzzle. There’s another part of the puzzle that comes along when you start treating the crops and when talking about pesticides, it’s one that’s the most frequently overlooked. Economics need to be taken into account when treating crops because, believe it or not, going easy on the pesticides can actually be beneficial to farmers.

The latest paradigm for pest control in agricultural situations is called ‘integrated pest management’, which I’ll refer to as IPM from here on out. It takes an economical approach to pest management by sampling pests, looking at how they damage crops and what numbers of a pest are sufficient to damage a set of crops. This is much better than randomly spraying pesticides at anything which looks like it might be eating your crops because it takes into account how much money you’ll spend and save on treatments. It also encourages a conservative use of pesticides which not only lessens a pest’s exposure to pesticides and selection pressure for pesticide resistance but also lowers the amount of pesticides sprayed in the field. Although not all farmers use IPM (although most figures I see are well over 50%), it’s the best way to deal with pests because you know roughly how much money you’re saving by treating versus spraying randomly and you limit the amount of pesticides you spray on your fields.

When studying entomology, one of the things you begin to realize is that you’re probably never going to be able to completely eradicate any pest. There are, of course, some regional exceptions but completely eradicating a pest under most circumstances is impossible with chemical control, and difficult with other means. Some pests such as the infamous Colorado Potato Beetle, Leptinotarsa decimlineata, simply evolve too fast for us to be able to eliminate them with conventional pesticides alone. Others, such as Melanoplus differentialis, the Differential Grasshopper have a large host range and can live in many habitats other than farmland. Eliminating pests completely can be very difficult.

We also probably shouldn’t eliminate all pests, either. If you remember part one of this series, you’ll remember that a major theme in that article was that various insects could be good in one situation and bad in another. The Differential Grasshopper is a great example of this. While they are undoubtedly pests because they’re able to reduce fields of soybeans and corn to stubble within days, they play a vital role in nutrient cycling. Next time you walk through a vacant lot (if you live in their range, that is) pay attention to the sheer number of grasshoppers. Those grasshoppers end up as food for spiders, birds and other animals who in turn end up as food for other critters. Eliminate that link in the food chain and you’re in for some serious problems. Pest management is the key, instead of pest elimination.

The Colorado Potato Beetle is an example of an insect which causes indirect damage. They can wipe out an entire field of potatoes by defoliating the plants but they don't touch the tubers themselves. Image Courtesy of John F. Carr from Bugguide.net

However, there’s also something else to consider other than simply how easy killing a pest is. Let’s see you’re a farmer walking through a field and see an aphid. The question quickly becomes one of whether or not to spray for aphids. On the surface, it would seem like if you see pests you should spray but this isn’t necessarily the case. Just because you see insects feeding on your crops doesn’t necessarily mean they’re causing enough damage for you to take a loss.

Insects cause damage in a number of ways. Some, such as the codling moth, eat the product directly. Others, such as the European Corn Borer, eat parts of the plant which aren’t necessarily related to the product you’re selling. Others such as Aphis glycines, the soybean aphid, cause damage by removing resources from the plants but cause relatively small amounts of physical damage. Under certain circumstances and with high enough numbers, the damage from any of these insects can be significant. Plants aren’t static objects, however and most can tolerate small amounts of damage without reductions in yield.

Plants are more vulnerable at some times than others. High populations of soybean aphids early in the season when soybeans are growing the most causes the biggest problems because in addition to the nutrients they remove, their waste material (honeydew) will culture fungus quite easily and slows plant growth by inhibiting photosynthesis in addition to removing nutrients. On the other hand, if you have the same population at the end of the season after the pods have already formed you can tolerate a much higher population of the same pest. In a similar manner, if you’re producing soybeans destined to become tofu a pest which removes amino acids will be more devastating than a pest which removes mostly sugars. In essence, at certain times in the season you can tolerate higher levels of pest just by virtue of where the plant is in it’s life cycle.

This codling moth caterpillar is an example of an insect which causes direct damage, which is an insect feeding on the useable product. Image courtesy of USDA-ARS from Wikipedia Commons.

One of the key points to IPM is that we can figure out how much damage insects do by measuring how populations damage crops in terms of the most important measure-the reduction in yield. If we know about how much a farmer will lose at the current pest population level, we can definitively say that ‘yes, treating is a good idea’ or ‘no, treating is a bad idea at this point’. There are two points which farmers take into consideration. The first is the economic threshold, and the second is an action threshold. The point at which a farmer takes an economic loss is the ‘economic threshold’ and the point at which treating a population of pests becomes cheaper than letting them be is called an ‘action threshold’. These will vary from pest to pest, crop to crop and the stage of the plant’s growth.

A great example of how economic thresholds are set was an article I hyperlinked in my last post about ladybug taint. In the paper, researchers added a bit of the chemical responsible for the ‘Ladybug taint’ in wine to wines and asked a panel of wine tasters to see if they could detect the taint. Given the data from that test, they calculated the concentration of ladybugs which would produce the minimum undetectable amount of ladybug taint during harvest and set a threshold much lower than the number which would cause the undesirable taste to give a bit of wiggle room to account for discrepancies in sampling. The numbers were also different for red and white wines, because these beverages have very different tastes and the chemical would be more noticeable in one over the other.

Of course for many other crops there are other things to consider; I chose the above example for simplicity’s sake. In soybeans, the action threshold for soybean aphids takes into account the stage of the plant, the cultivar (or type of plant), the cost of insecticides, what the properties of the desired product from the plant are and how they’re changed by the insects. The action threshold also takes into account the population of pest because to potentially cause economic damage, the pest population levels have to be increasing. Remember, farm fields aren’t completely barren except for pests…they have their own special ecology and pest populations are still regulated by predators, parasitoids and disease. It’s a complicated figure that takes many, many factors into account.

The soybean aphid is the largest pest of soybeans in the US. Photo courtesy of Robert J. O'Neil and Ho Jung Yoo from Wikipedia Commons

We need to be careful when treating because the way we treat pests is imperfect at best. There are all sorts of ecological control measures, like tilling corn stubble underground to prevent the emergence of corn borer moths as well as biological control measures such as biopesticides and natural enemy introduction (my area of study). The most common and most effective method at this point is chemical control, and this is why I’m making these posts. Pesticides are taken very seriously in IPM. We only use them when we have to, and we do a lot of time consuming and unglamorous research to figure out how and when to use them.

Despite the fact there are legitimate risks associated with pesticide use (which is why the USDA monitors pesticides in food), they still play an important role in agriculture and even medicine. The main reason you and I are alive today is because we have gotten so very good at killing insects. Pesticides are used to control malaria vectoring mosquitoes and largely because of pesticides we no longer have malaria in most of North America, although I’m also quick to point out that a thorough understanding of mosquito ecology helped us in furthering that goal as well. In America, we not only demand cheap food we also demand perfect food. The average consumer will quickly discard an entire ear of corn because they’re grossed out to find a giant corn earworm larva even though the rest of the ear is still quite edible. To prevent insects from eating our food, and to prevent insects in the final product we’ve got to spray pesticides. There are few, if any other viable options at this point in time.

Follow Joe Ballenger:

Joe Ballenger is an entomologist who works in the biotech industry as a contractor. In his spare time, he helps answer questions about bugs at Ask an Entomologist.https://askentomologists.wordpress.com/

  • IPM is just such a great tool. I learned about it when I was a “DoD Pest Controller” in the Army. We were taught to find mechanical or biological controls before using chemical controls as a last resort. This method keeps the troops safer and saves money. You can find an into to the military IPM strategy as well as some success statistics here.

    Corn earworm are so horrible. They eat my research, they eat my sweet corn… grr. I won’t throw the ear out, but I hate having to cut off a large part of the ear because of a gross caterpillar. You might be interested to read my post Sweet, sweet corn from last year where I talk a little about breeding for resistance to earworm. I’m guessing that a trait found in tropical corn varieties bred into US corn varieties would count as a biological control, but where does the Bt trait fit in? Is that biological control? I think Bt as a pesticide counts as biological control because we were pretty reliant on the briquettes in the Army to kill mosquitoes.

  • Richard L

    As a fellow entomologist, thank you for producing a clear introduction to the subject that I can send people to.

  • Thanks, Richard…it’s really cool to see fellow entomologists using my posts to educate folks about entomology. I’m hoping to eventually be able to discuss pest control policy on here, but first I have to discuss farmland ecology, pests, pesticides, population regulators, resistance…all that fun stuff.

    Anastasia…I’m actually pretty sure BT counts as biocontrol. It’s a biological entity which causes disease insects, and thus fits the definition. They also talk extensively about BT in biocontrol classes. The sticky part comes when we start parsing the bacterium from the toxin…and I’m not sure if the protein would be considered biocontrol or just a protein based gut toxin.

    As for antipest traits, I think host plant resistance falls under a separate category than biocontrol. The definition of biological control focuses more on insect population regulators like pathogens, parasitoids, parasites and predators.

    Corn earworm may be annoying, but the species is an incredible bug. It feeds on tomatoes, corn, cotton, grapes, sorguhm, stone and pome fruits. Hell, even tobbacco isn’t safe…their host range is incredible.

    http://edis.ifas.ufl.edu/in302

    • I see. So the whole Bt bacterium is biological control but Bt protein is not.
      I think host pest resistance would fall under biological control, though. It is non-chemical (well, plants are made of chemicals, as are we, but I mean externally applied chemicals) and is part of the living organism that is the crop. Perhaps we can call it a pre-control strategy, a preventive to needing control? Although even before we get to the point of needing Bt, the pre-pre-control strategy is crop rotation, right?

  • Clem Weidenbenner

    Joe:
    I have a few quibbles about your Soybean Aphid remarks – and since I’m not an entomologist (insects just bug me) I’ll try to set up my points so you can trash them if necessary.

    The first thing that caught my eye was the photo of the aphid with the legend stating “The soybean aphid is the largest pest of soybeans in the US.” For starters I think you meant ‘insect pest’ and then I think you were going for ‘most economically significant’ or words to that effect. The aphid is actually one of the physically smaller insect pests. The reason I want to specify ‘insect pest’ is so that the weed science and plant pathology folks won’t feel left out. Weeds (will confess I lump all the weeds together and this isn’t quite fair…), soybean cyst nematode, an oomycete, and at least one fungus cause more economic damage than the aphid (considering the whole US crop). Herbicides, nematicides, and fungicides are all pesticides too. And IPM can contribute to their control as well.

    Also – on the aphid side of the score card – the little buggers can vector some viruses. Virus spread, especially in food grade soy, can negatively impact seed quality. So those little aphids are a significant problem – just not the largest.

    Ragsdale et al. (2007) Economic threshold for soybean aphid (Hemiptera: Aphididae). J. of Econ. Ent. 100, 1258-1267.

  • Clem…I didn’t see where the Ragsdale paper discussed soybean root nematodes, but I wholeheartedly agree with your point. I’ve been blogging for awhile at another blog, but since writing for biofortified I’ve been trying to write more with the intention of talking about these topics for a popular audience and sometimes there are things I overlook as a result. In this post, I was paying more attention to the text in the main passage and in the captions of the codling moth and colorado potato beetle. Also, in entomology classes there’s a tendency to overlook agricultural diseases like viruses, non insect pests like mice, and those organisms which bridge the gap like nematodes. I’ll definitely remember this for future discussions.

    Fungicides, rodenticides, herbicides and nematicides are used in IPM in an identical manner to insecticides and that’s a great point. I tend to use the term ‘insecticide’ and ‘pesticide’ interchangeably even though they’re not interchangeable. I’m not sure it made a difference in this article, but there are other groups of animals other than insects which cause crop problems.

    Anastasia…biocontrol is concerned purely with the critters which regulate insect populations in the wild. It’s really a definition thing. Biocontrol specialists introduce new critters* to the wild, reintroduce ones which are already there or make the area an easier place to live by the introduction of new habitats or plants. As far as I know Host Plant Resistance (HPR) isn’t technically considered biocontrol.

    That’s not to say HPR isn’t an important part of agriculture, it just doesn’t meet the esoteric definition of a specific field of pest control**. .I’d agree with you that host plant resistance is important, and probably even the preferred mode of pest control.

    Although, evolution often does come with trade-offs. I know a bunch about insect molecular biology, but relatively little about plant biology other than the very basics. Are there any potential negative tradeoffs between resistance and say, fertilizer usage?

    *By critters, I’m referring to predators, parasitoids, parasites and diseases.

    **As far as I know…if you’ve heard differently, let me know because our fields intersect in very strange ways sometimes

  • Ewan R

    Are there any potential negative tradeoffs between resistance and say, fertilizer usage?

    The only one that springs immediately to mind is that with inherent resistance you may end up having increased fertilizer useage simply because it is now possible to fully exploit (or more fully exploit) the crops intrinsic yield by negating the impact of insect pressure pulling down the yield – although in situations where insect control is as effective as resistance (ie in US type systems) this may not be apparent.

    This is my guess as to why there are sometimes trends linking Bt cotton to increased fertilizer (although that said the one paper I saw this looked at it appeared to be driven entirely by a single region – would still be my main guess as to why there’d be an interaction of that type – unless the amount of protein required to confer resistance was unprecedented)

  • Clem Weidenbenner

    Joe:
    Sorry – Yes, the Ragsdale paper is a soybean aphid paper. Alan Wrather has a paper or two looking at estimates of soybean crop damage due to the various othre disease organisms. If you are interested I can get you a Wrather et al. citation.

  • Andre

    Ewan,

    Your suggestion rests on the assumption that the pest or disease reduces the plant’s ability to build up organic matter (and, as you rightly noted, that the pest or disease is not controlled by other means).

    Furthermore, I wouldn’t consider there is a trade off here, nor indeed a negative one. More fertiliser would obviously be applied to produce a larger crop and generate a larger revenue for the farmer. Moreover, resistance makes the crop more dependable and thus secures the investments made by the farmer in terms of not only input, but also labour, etc.

    Conventional wisdom has it that increased fertilisation (except ‘of course’ if it is organic…) is bad. But this is not so. The fossil energy that is put into the production of fertiliser and eventually agricultural produce is very wisely used. A higher yield per hectare has the potential to release agricultural land for other purposes (ask the Europeans who not so long ago had to leave a substantial proportion of land fallow). A higher yield will as a rule also come with a higher amount of organic matter that is left in the soil and enriches it. And pollution by nitrates is not a matter of dose, but of adequation of dose to yield (and here resistance provides a degree of security).

    As to Bt cotton, I do not know which paper you refer to. I suppose it is about India. My first guess would be that increased fertilisation is simply due to the fact that it pays. A factor here is that until recently Bt cotton has only come in the form of hybrids.

    • Clem Weidenbenner

      Andre:
      Great points. But pollution from improper fertilizer use/application can cause problems. Dead zones such as the one in the Gulf catch quite a bit of attention (some well deserved).

      Fertilization has the potential to be a great human activity, so long as its done wisely. For a comparison of US and Chinese N fertilization see: Vitousek et al. 2009. Nutrient Imbalances in Agricultural Development, Science 324, 1519-1520

    • Ewan R

      Andre –

      Your suggestion rests on the assumption that the pest or disease reduces the plant’s ability to build up organic matter

      I think this is a pretty fair assumption.

      Nitrogen fertilizer use in particular is a topic close to my heart – I agree mostly with your points about useage being a good thing in terms of increased yield meaning less land needs to be used – however I’m not convinced on the arguement that on all fronts increased fertilizer useage is a good thing. Nitrate leaching is a huge issue – when you increase Nitrogen on top of corn you see very nice increases in yield – however the more Nitrogen you apply the less is actually captured by the plant – and generally (at the moment) the economically ideal N application rates see N capture in the region of 20-30% as compared to perhaps 60-70% at lower rates – nitrate runoff is certainly a massive problem which needs to be addressed (either by buffer zones to protect waterways, by reducing application to balance profitability and environmental impact, or by altering plants such that either they can capture more N, or that they can hit max yields at lower N application rates (last two are precisely what my team is trying to do with corn)

      The cotton paper I was referring to is indeed India (aren’t they all?!) and is hopefully (if my html-fu works) linked below:-

      This paper

      Which was used by an opponent of GM tech as a scathing attack on GM, which reading the paper obviously requires a certain degree of quote mining.

      • Clem Weidenbenner

        Ewan:
        This thread is beginning to vary quite a bit off the original topic, but I am currently preparing some remarks about fertilizer use and abuse for another audience. With your comment about the work you and your team are into with corn N use efficiency I’m wondering if you might point me toward some current work in this area.
        Jerry Maranville (Nebraska) was doing some work on this subject back in the 80s, but it isn’t something I’ve kept up with over the years.

        • Ewan R

          Clem – I can see what I can dig up – if I remember some names offhand that are useful they’d probably be Hirel, Fred Below and Jonathan Lynch – I think Bruno Basso may also be relevant to this area although I forget how exactly (I know I have a graph of N uptake across development which I believe has his name in the citation at least)

          I’ll see if I have anything other than internal company slides to back up the %ages noted above – could be a perpetuated internal myth although I’m thinking probably not.

          (And yes, we’re veering horribly off topic… but if it weren’t for veering horribly off topic now and again things would get awfully stagnant!)

          • Clem Weidenbenner

            Thanks Ewan, will start with these names and wait for any other assistance you might find. The Vitousek article in Science that I mentioned to Andre above will be at one end of the piece I’m working on.

            To get us ‘on topic’, perhaps I’ll offer a version of what I’m working on for a guest post here at Biofortified. And I agree that getting off topic isn’t such a horrible outcome. So much of what one is interested in will usually dovetail with aspects of what others are into.