This presentation is about using
a tool, called degree-days, to help you better time insecticide
treatments and other pest management practices in your
orchards. Use degree-days to predict when susceptible pest
stages are present so you can maximize control.
For example, you need to apply an insecticide for peach twig
borer after the majority of larvae hatch but before they
bore into the ends of twigs where insecticide won’t reach
them. Use degree-days to pinpoint the best time to kill
the most peach twig borer larvae with a single application. Although degree-days are used to
manage pests in many crops, we will focus on its application in
stone fruits and nuts. In these crops, degree-days are useful
for the management of several insects and for this
presentation we chose different insect examples to show how
degree-days can be used. At the end of this presentation
you will know what a degree-day is, what information you need to
collect to calculate degree-days, where to find resources for
calculating degree-days, and how degree-days can optimize pest
management in your crop. Let’s define a few terms
important to understanding degree-days. The first is
phenology, the study of an organism’s development and how
it’s influenced by weather. Ambient temperature influences
the development times of cold-blooded animals and plants.
For example, as temperature warms in the early spring, trees
start to bloom. Being cold- blooded, insects and mites do
not regulate their internal temperature; instead their
development is dependent on the temperatures they are exposed
to. Because of this, temperature affects when an insect will go
through its various life stages: in other words, when it
becomes an egg, caterpillar, or adult moth.
Humans are warm-blooded and so regulate their internal
temperature. Human development doesn’t change based on ambient
temperature. For example, a human pregnancy is 9 months
whether a woman is living in the Arctic Circle
or at the equator. So let’s take a little quiz.
How long does it take for a peach twig borer egg to hatch?
Is it 5 days, 10 days, 11 days, or all of the above? That’s right, all of the above,
because how long it takes for the caterpillar to get ready to
hatch out of the egg depends on the temperature it
is exposed to. This graph shows the average
development time from egg to adult for different batches of
obliquebanded leafroller also called (OBLR) raised at different temperatures.
As the temperature increases, the development time decreases.
At fifteen degrees Celsius it takes approximately one hundred
days for an egg to develop to an adult. But at twenty-five
degrees Celsius it takes only about forty days. In the previous graph, it seems
that if the temperature kept on increasing, the development time
would continue to get shorter and shorter. But this doesn’t
happen. Below a certain temperature insects cannot
develop. And above a certain temperature their development
slows and will eventually stop. These critical temperatures are
referred to as the lower threshold, or base threshold,
and upper threshold, and represent temperature limits
on insect development. When temperatures fall between
the upper and lower thresholds, the insect will develop.
Depending on how long these temperatures occur we get so
many units called degree-days, which can be used to
measure development. A degree-day is a unit combining
temperature and time used to measure development
of an organism. One degree-day is a single
degree of temperature above an insect’s lower temperature
threshold, maintained for twenty-four hours, or in other
words, one degree above the lower threshold
maintained for one day. For example the lower threshold
for navel orangeworm is fifty-five degrees Fahrenheit.
If the temperature is maintained at fifty-six degrees Fahrenheit
for twenty-four hours, a single degree-day accumulates. But temperatures are not
constant in the real world. So how do we measure degree-days
with temperature fluctuations? Here is a graph of degree-days
accumulated over two twenty- four-hour periods. The red solid
line shows the temperatures over two days and the dashed lines
mark the upper and lower thresholds. The yellow area,
showing when the temperatures are between the upper and lower
thresholds, represents the accumulated degree-days for
each day. Adding the areas under each day’s curve gives the
total number of degree-days accumulated for the two days.
Once we know the number of degree-days required for each
development stage of an insect we have a phenology model. Recall the example of
maintaining navel orangeworm at fifty-six degrees Fahrenheit, or
one degree Fahrenheit above its lower threshold, for twenty-four
hours. We said that one degree-day would accumulate.
Then the next question is: At a constant temperature of
fifty-six degrees Fahrenheit, how many days would it take for
navel orangeworm to develop from an egg to an adult? The navel
orangeworm phenology model says that they require, on average,
one thousand fifty degree-days to complete their life cycle.
Therefore, if maintained at fifty-six degrees Fahrenheit,
meaning an accumulation of one degree-day per day, navel
orangeworms would take one thousand fifty days to develop. Now let’s say that navel
orangeworms are placed at fifty-seven degrees Fahrenheit,
or two degrees Fahrenheit above the lower threshold. Development
would be twice as fast as compared to rearing them at
fifty-six degrees Fahrenheit. So it takes the navel
orangeworms only five hundred and twenty five days, or half
the time, to develop. As the temperature increases,
more degree-days accumulate in a day, and less time is required
for development, until the upper threshold is reached.
When temperatures are above the upper threshold,
no degree-days accumulate. Also remember that development
time represents an average for a population! So the majority of
navel orangeworms will develop in one thousand fifty
degree-days, while some individuals take less
and others take more. Now for an important question:
How do we know when to start accumulating degree-days?
Accumulation starts either on a particular date, for instance,
January first or on the biofix date. A biofix is an observable,
biological event such as the first pheromone trap
catch or egg laying. The biofix for omnivorous
leafroller is the date when traps start consistently
catching moths. For navel orangeworm it is when
fifty percent of the egg traps have eggs. So now that you’ve got a good
understanding of the terms commonly used and the basics of
how degree-days are calculated, let’s take a closer look at what
it takes to use them in the field to manage your pests. You can effectively predict
population trends of an insect if the insect has
discrete generations, you know the developmental thresholds
(upper and lower), and the degree-day accumulation needed
for a particular event. You know when to begin
accumulating degree-days, and if it’s a biofix that
signals the start, you have the tools to detect
when this occurs. If all these are known, a
phenology model can be used to manage your pests. If a phenology model has been
developed in a location far from yours, you should test the model
for one or more seasons in your area to verify it will
predict biological events accurately. A different location
presents different conditions an insect is exposed to, which may
influence the development of the insect. This also applies for a
phenology model developed for an insect in one crop that you’d
like to use for the same insect in another crop. For example, at one time we
didn’t have a phenology model for obliquebanded leafroller in
California, but one was developed in New York.
We performed a field test to verify if the New York model
could predict timing of OBLR’s growth stages in California. According to the New York model,
one generation of OBLR takes two thousand degree-days using lower
and upper thresholds of forty-three degrees Fahrenheit,
and ninety degrees Fahrenheit, respectively. The biofix is
the date the first moth is caught in a trap. To see how well the New York
model would predict California populations researchers recorded
the number of obliquebanded leafroller males caught in traps
throughout the season in a California pistachio orchard.
This graph represents the results of a field test.
Obliquebanded leafroller is an easy insect to follow because it
has distinct generations that don’t overlap. In other words,
at the end of the first flight there is a period of time with
no trap catches until the second flight begins. You can see that the first moth
of the first flight was trapped in the orchard on May eighth.
The first moth of the second flight was trapped around July
fifteenth, and the degree-day accumulation between these two
dates is one thousand, nine hundred and eighteen degree-days.
According to the New York model we saw that one generation of
obliquebanded leafroller takes two thousand degree-days
for one generation. Although there is a discrepancy
of eighty-two degree-days between the New York model and
our California results, it is not uncommon in June and July to
accumulate twenty to twenty-five degree-days each day.
This means the difference between the values is only about three
calendar days. A three-day discrepancy will not influence
whether an insecticide application is successful or not.
So, after several years of similar results, we accepted the
New York model for predicting obliquebanded leafroller
generations in California. In addition to monitoring traps,
it’s important to keep records of other production and
management activities in the field. Here is an example
of why: see this dip in the population on August seventh?
Without records of orchard practices or events, the
population drop could lead to unnecessary speculation. As it
happened, this particular orchard was treated with an
insecticide targeting another pest on that day. So, what do you need to
accumulate degree-days in the field to predict treatment
application dates? You need the minimum and maximum field
temperatures for each day. Weather stations or data loggers
like this one record rainfall, temperature, relative humidity,
solar radiation, wind speed, or soil moisture and send
information directly to computers—including yours! You may not need your own
weather station. There are many located
throughout the state and there may be one in your area.
To find out go to the UC IPM website, the California
Irrigation Management System, (or CIMIS) website, or to
a commercial network. On the UC IPM website, when you
select the county of interest, you will be linked to the
information about sites that are recording temperatures.
Choose the site that is closest to your farm. Now that you know how to get
weather data, let’s look at another bit of information you
need to have—when to start accumulating degree-days
if there’s a biofix. Grower Bill Chandler is looking
for obliquebanded leafroller. Traps are checked twice weekly. When he gets multiple moth
catches, meaning more than one moth within one week, he will
designate that date as the biofix and start
accumulating degree-days. It is critical to correctly
identify the insects in your traps because they can look very
similar to each other. The big moth, is an obliquebanded
leafroller. You may think, “But I’m using a
pheromone trap that will attract only OBLR.” Although pheromones
are pretty selective to species, in the case of some tortricids,
like obliquebanded leafroller, this is not always so. A very similar, but smaller
moth, the garden tortrix, has very similar wing patterns.
People have mistaken garden tortrix for obliquebanded
leafroller and begun accumulating degree-days based
on the wrong insect; garden tortrix emerges almost one
and a half months before OBLR! The advantage of using
degree-days to time treatments is that they allow you to reduce
insecticide use by targeting the most susceptible insect stage,
attaining maximum control and reducing costs. Monitoring and
using degree-days allows for the correct application timing of
reduced-risk products preserving many of the parasites and
predators that control other orchard pests. There are many benefits to
using phenology models. The highest level of control is
achieved, pesticide applications are minimized, and beneficial
insects are preserved. But getting a workable phenology
model takes a lot of research. Because insects and mites are
cold-blooded, the temperature around them influences their
development and so their developmental times vary as
daily temperatures vary. Phenology models are developed
in the lab for pests by determining their upper and
lower development threshold and the rate of growth at
different temperatures. However, temperatures aren’t
constant in the real world, so phenology models are refined in
the field to estimate developmental times. One
degree-day accumulates when the average temperature is over the
minimum threshold for twenty-four hours. The
accumulation of degree-days is used to predict when an insect
will reach a certain stage in its life cycle. Phenology models for
obliquebanded leafroller, omnivorous leafroller, peach
twig borer and others, plus more information on calculating
degree-days and using degree-days in your orchard can
be found on the UC IPM website. [No audio]