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IPM Stuff Newsletters: Issue
9 - Aug 4, 2005 SWROC Pages: Home Page Other Cropping Newsletters: Minnesota Crop eNews - Regional Newsletters Minnesota Fruit and Vegetable IPM News The Bulletin - Other States IPM Newsletters
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Integrated Pest Management Newsletters and Information IPM Stuff Print this newsletter using Acrobat Reader |
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Table of Contents:
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On the brighter side, 2005 corn emergence has started and those soybeans that have been planted are methodically working their way upwards to join corn in an uncertain above ground fate. Small grains in general look very good and the established alfalfa stands I have examined seem to have recovered nicely from frost.
A few weeks back, Dr. Seeley predicted below normal temperature and an above normal rainfall 90-day forecast. This is one case where the weather guys got it right. I sure hope Mark can call off the dogs. I don't know if I want to see another two months of this.
Cold, wet weather does not favor most insect populations, which is good unless you are trying to study them.
As planting is delayed by wet field conditions stress levels go up and the urge to multi-task increases. The urge to apply UAN fertilizer and weed control after the projected window is great. Read labels before assuming that a pre-emerge herbicide including UAN fertilizer is still ok after corn emergence. Some pre herbicides could function as an adjuvant for the UAN. Crop injury happens. It may be prudent to manage weeds and Nitrogen fertilizer separately where corn has emerged. This crop is already under enough stress.
As fields dry off enough to start scouting pay attention to stand problems. Before blaming insects (wireworms, seed corn maggot, cutworm) or a disease eliminate abiotic factors such as seed depth, compaction, drowning. Dead seedlings, regardless of the cause, will rot under high moisture conditions. Cold, slow growing conditions do however, favor seedling-attacking pests.
If it is a biological problem causing stand loss, proper identification is important in making a management decision. Black cutworm are much more likely to kill corn by cutting corn below the growing point. Dingy cutworm, perhaps our most common species, usually feeds at or above the soil surface and is seldom a threat for stand loss.
Seedling damping off fungi may also be a concern, although until recently soil temperatures have been too low for much fungal activity. http://swroc.cfans.umn.edu/SWMNPEST/diseases/beandiseases.htm lists several soil and seed borne pathogens that can cause seedling problems. I was hoping to have photos attached by this point in time but was much too busy feeling sorry for myself because of excess rain.
The winter of 2004-2005 may have killed off many of our pest problems but there are always reinforcements. Some of our 2005 insect problems may have arrived while you were transfixed to radar weather screens the past few weeks. While we didn't have systems that would have brought the embattled and elusive soybean rust out of Florida or Georgia, there were some systems that more than likely brought potato leafhopper from further west in the gulf. Some systems drew from the southwest and potentially brought in army cutworm, black cutworm, and painted lady. I would not be surprised if a large number fall armyworms were blown off course on their way to the Rockies and arrived in Minnesota again this year. Fall armyworm are not a crop threat and as always, they are going to be disappointed with the trout fishing in this part of the world.
Most of you heard about the "Puccinia pathway" during endless soybean rust talks this winter. We have had at least two classic cases of this long distance transport mechanism into Southwest Minnesota over the past few weeks. Low level jet streams moving unbroken from Texas as far as into Canada in one case. Rust spores can be picked up in the south transported on low-level jet streams and rained out. Look at continental radar maps for systems originating in the south and raining out in the north. This could be a big deal for wheat varieties susceptible to leaf rust and perhaps stripe rust. The wet weather should have been favorable for infection. Watch these fields, as we get closer to heading.
For those of you who don't grow wheat, watching radar is still good practice for soybean rust if it moves westward into a more favorable position to reach Minnesota.
Speaking of reinforcements, the soybean rust game might get more interesting. I invite you to watch the tropical storm development over central and northern South America. Atypically, this system is currently moving to the northeast. We may get to an orange alert yet.
The recent freezing temperatures have probably not been favorable for this insect. Unfortunately, it has been either much too windy or too wet to get a feel for populations waiting in alfalfa, etc. by sweeping
Wet weather wind and no soybeans have most probably hurt any soybean aphid in the Lamberton area. If we have a soybean aphid problem in 2005 it may have to come from an area with a more benign climate.
What is Integrated Pest Management (IPM)? Part 2a. The economics of pest induced yield loss In the last issue, I mentioned that economics are a component of an IPM program. I also mentioned that an economic threshold does not have to be part of an IPM strategy. However, where developed and used economic thresholds do help save money. There is only a limited amount of money to invest on insurance. The concept of economic threshold is a way to help determine where to spend management dollars to maximize returns. It also should help minimize some of the fear associated with pest management decisions. This issue discusses, in very general terms, how pest pressure causes yield loss.
The economics of crop production are defined in large part by yield. The economics of producing a crop surely include the unit value of the crop and cost of production in dollars/acre but it can be argued that these factors are influenced, at least in part, by yield.
Figure 1 shows a highly stylized representation of the relationship of yield loss and pest (insects, weeds or diseases) populations. The Y-axis shows yield loss with 100 % being the maximum yield that the combination of genetics and abiotic growing conditions allow. The X-axis shows increasing pest numbers and/or amount of time a pest is allowed to injure or to compete with the crop. The understanding that a combination of both pest numbers and duration drive yield loss is where concepts like insect-days (entomology), weed-free days (weed science) and area under the disease progress curve (plant pathology) have originated. The yield loss curve contains three portions.
There are two other points labeled on the yield loss graph. Economic injury level (EIL) is the point where pest losses are equal to or greater than the cost of controlling the pest. In crude and simplistic terms, if the yield loss from one hypothetical insect/plant costs $0.50/acre and it costs $10.00/acre to control the insect, the EIL would be 20 hypothetical insects/plant. Economic injury level for a pest can vary with crop price, susceptibility of the crop to damage and cost of control. The economic injury level for a soybean insect would be lower (further to the left or allow less damage) for $7.00/bushel soybeans than for $5.00/bushel soybeans. It would be considerably higher (further to the right or allow more damage) if insecticide + application were $15.00/acre than if you had a very benevolent agri-chemical dealer paying you to take inventory off his hands. The further you allow a pest population to develop past the economic injury level the more money you are losing. Even though I operate under the assumption that everyone is more organized than I am, time is usually a limiting factor, and EIL by itself is impractical to drive pest control decisions. With the scale of today's agriculture it is not possible to evaluate and treat all fields the same day. Another concept is needed. Economic threshold (ET) is the point where you should apply a control measure to avoid passing the economic injury level. At the minimum, they project pest population growth based on an understanding of life cycle and at least a rough guess at yield loss, both usually worse cases which may be derived from related species. An ET should allow for lead-time between identifying a pest population and the estimated economic injury level. ETa represents a slow increasing or less injurious pest and ETb represents a rapidly increasing or highly injurious pest. I hope the positions of these two thresholds illustrate the concept of lead-time and avoidance of yield loss. This is about economics. Remember the no yield loss portion of the curve? It is possible to obtain a very low economic threshold if crop value is high, control costs are relatively low, and pest population growth rates are very rapid. Therefore, many economic thresholds take into account biological factors, predation for example, which prevents treating too soon. It is not all that easy being a fungus or soybean aphid and pest populations often collapse without any help from the producer. In fact, premature applications can disrupt natural controls and exacerbate a pest problem. The good news is that through patient research these yield loss relationships can be quantified, if only crudely. The bad news is that this has not been done for many weeds insects or diseases. This lack of data is why initial economic thresholds are imprecise and hopefully overly conservative. Economic thresholds pest with a single or few generations/year and or relatively slow population increases. It may or may not be obvious from examining the yield loss graph that the closer an economic threshold is to the economic injury level the less chance of making an economic error in your decision. Additionally, the better the understanding of the relationship of a pest to the host crop and the environment, the more accurate the EIL and ET can be. Are economic thresholds perfect? No analysis involving biological organisms is certain. While individual fields are management units you are managing a whole farm. It is ok to lose an occasional battle and I can make the argument that if you never experience a trip across the EIL you have spent a significant amount on pest control you didn't need. In the next issue I'll discuss how economic thresholds get developed and why weed scientist's and plant pathologist's heads often start spinning when they are asked about economic thresholds. Experiments in reasonably real time.
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