 
Rangeland Grasshopper Forecast Maps For Wyoming
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The Outbreak Frequency Map is an historical (1944  1996) summary
of infestations >= 8 grasshoppers/yd^{2}. This map highlights areas that
have frequently suffered from grasshopper outbreaks.
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This map depicts the probability of a grasshopper infestation persisting
and answers the question, "If the rangeland was infested last year, what is the
probability that it will be infested this year?".
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This map depicts the expected duration that a site is likely to be infested and
answers the question, "If the rangeland is infested at present, how long will this
condition likely continue?".
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This map depicts the probability of a grasshopper infestation developing
and answers the question, "If the rangeland was not infested last
year, what is the probability that it will be infested this year?".
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This map depicts the expected duration that a site is likely to be uninfested and
answers the question, "If the rangeland is not infested at present,
how long will this condition likely continue?".
How to use these maps for Pest Management:
Survey:
The key to effective pest management is to focus the search for rangeland
grasshoppers on those lands most likely to be infested. Therefore, based on what
you know of last year's situation, the most effective strategy would be to survey: 1)
areas that were infested and have a high probability of continuing infestation and 2)
areas that were uninfested and have a high probability of becoming infested.
Control:
It may be necessary to focus management programs in areas that have the
greatest potential for economic return. One of the most important elements of
assuring a profitable treatment is to control populations that are likely to persist if
nothing is done, thereby accruing multipleyear benefits from this year's treatment.
Conversely, if an infestation has a low probability of persisting beyond the
current year, then the economic returns from a control program will be limited to the
forage that is protected in a single growing season. Therefore, with all other
factors being equal, management priority should be allocated to land that has the longest
expected duration of infestation and the greatest probability of continued infestation.
Limitations to this information:
As with financial investments, past performance is no guarantee of
future returns. In an ecological sense, our analysis assumes that all of the factors
that were responsible for grasshopper population dynamics over the last 50 years are
continuing to operate in the same manner at the present time. Any changes in the
inputs that accounted for the transitions in the past will affect the accuracy of our
current forecast.
The only grasshopper population density that was consistently recorded
since 1944 is > 8 grasshoppers/yd^{2}. We know that this density rarely
represents the economic threshold, although it may be close to the ecological carrying
capacity of the rangeland, and it may portend an increase in the population. However,
all of the probabilities and durations reflect this particular density. So, for
example, the chances of an infestation of 25 grasshoppers/yd^{2} persisting for
more than a single year may be much different than the probability indicated on these
maps.
Our analysis was conducted using a spatial unit of 250 acres.
That is, each of the grid "cells" on the map represents a fixed area of
rangeland, and it is possible that the analysis would have yielded a different results is
a substantially larger or smaller scale had been used. However, we chose a parcel
size of 250 acres as being small enough to provide an appropriate level of detail but
large enough to be of relevance to management practices.
Terms:
Infested = ³ 8 grasshoppers/yd^{2}
Uninfested = < 8 grasshoppers/yd^{2}
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Markov Chain Extension.
Copyright © 2000 Kiana Zimmerman, Jeffrey
Lockwood, and Scott
Schell.
Entomology Section
Department of Renewable Resources
College of Agriculture
University of Wyoming
