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.
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.
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.
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.