Research and IPM
Interactive Tools and Models: About the Citrus Thrips Scarring Prediction Model
This research is the work of Heinrich Schweizer and Joseph Morse, Department
of Entomology, University of California, Riverside, CA 92521. UC IPM Information
Systems staff prepared the model for use on this Web site.
|Summary of the research
|Scarring predictions ~ Potential explanations
for link between weather pattern and thrips scarring ~ Economic evaluation ~ Limitations of the model
|Use of the model predictions
|Current citrus thrips treatment decision-making strategy
|What to do with the predictions
|Check historical predictions ~ Help improve the model ~ Make decisions~ Disclaimer
- Schweizer, H. and J. G. Morse. 1999. Predicting citrus thrips damage on navel oranges in the San Joaquin Valley.
Citrograph 84(3): 3, 6-7.
- Heinrich Schweizer and J. G. Morse. 1997. Estimating the level of fruit scarring by citrus thrips from temperature conditions
prior to the end of bloom. Crop Protection 16(8): 743-752.
The following is a summary of the research from which citrus thrips damage
estimates were calculated on the UC IPM Web site. Complete methods, analysis,
and results are reported in the journal article in Crop Protection
Citrus thrips, Scirtothirps citri, cause scarring on fruit rinds
as they feed on small citrus fruitlets soon after petal fall. Because
scarred fruit may not meet cosmetic standards for fresh sales, the damage
can cause significant economic losses to California orange growers.
Schweizer and Morse developed and validated multiple regression models
to estimate the degree of fruit scarring by citrus thrips. The models
used heat and chill degree-days, and regression coefficients and indicated
that cool weather during early March (2 March-16 March) and warm weather
during bloom were associated with high levels of thrips scarring.
The study used data from:
- Annual pesticide evaluation trials in a Navel orange (Citrus sinensis)
block at the Lindcove Citrus Research and Extension Center near Exeter,
California. The trees were Early Atwood Navels on Rubidoux Trifoliate
rootstock planted in 1970.
- Fruit scarring levels from untreated trees were used to estimate citrus
thrips damage. Data for the years 1982-1996 were used to develop the
models, and data from 1972-1981, and 1994-1996 were used to validate
- From mid-1981 through 1996, daily minimum and maximum air temperatures
from a weather station located at Lindcove were used. Values from nearby
Lindsay were used to estimate temperatures for 1972 through mid 1981.
Different models using heat and chill degree-days during several periods
were developed and evaluated, based on the fact that:
- Relative rates of development for citrus thrips and the tree depend
- Overwintering eggs may require specific temperature patterns to enter,
remain in, and terminate diapause.
- Egg mortality may depend on temperature.
- Heat and chill degree-days were calculated using a double
- 14.6°C was used as a lower threshold for citrus thrips development.
- The fraction (p) of severely scarred fruit was transformed to z= ln(-ln(1-p)).
- The variable z was regressed against heat and chill degree-day sums
for selected periods.
- For model validation, observed percentages of severely scarred outside
fruit were compared with predictions made by the best three models.
The authors also studied the economic effect of using the scarring predictions
to make citrus thrips treatment decisions.
The most promising model suggests that:
- Cool weather in early March and warm weather during bloom are associated
with high levels of thrips scarring
- Relatively warm weather during early March and relatively cool weather
during bloom are associated with less scarring.
Models performed well in predicting fruit scarring for the specific field,
somewhat overestimating the percentage of scarred fruit, but followed
the observed trend.
Potential explanations for link between weather pattern and thrips scarring
- Eggs may require chilling in early March to ensure timely hatching
- Warm weather during bloom may be required for timely adult emergence
and high oviposition activity on suitable fruitlets, which leads to
high fruit scarring.
- Cool weather in late September may be crucial for eggs to enter an
appropriate overwintering state.
- Cold weather in late October may increase egg mortality.
- Using results from any of the tested models resulted in fewer treatments
and higher grower returns than was found with regular application of
prophylactic pesticide treatments.
- Fruit sampling for larvae, to determine the need for protective pesticide
treatments, was more expensive and left less time between data collection
and treatment decisions than using the model.
Limitations of the model
- The model is based on data from one field, but different levels of
thrips scarring have been observed in different fields in a single year.
- The model is intended to provide an estimate of citrus thrips damage
on Navel oranges in the San Joaquin Valley only.
- During the last 4 seasons (1998-2001), the model has considerably
overestimated the level of thrips scarring. Using its predictions without
additional information from field monitoring would have led to unnecessary
For more information, see the Crop Protection (1997, Vol. 16,
No. 8, pp. 743-752) article, or contact Joe
Use of the model predictions
Current citrus thrips treatment decision-making strategy
- UC recommends that citrus thrips treatment decisions be based upon
field sampling of thrips larvae on
- A major drawback with this method is that by the time economic thrips
levels are detected, treatments need to be applied almost immediately
to be effective. Also, accurate field sampling is laborious. Thus, sampling
is expensive and provides little advance warning of the need for treatment.
- This model attempts to estimate citrus thrips damage from weather
data that accumulates prior to petal fall.
- The model is a first step in developing a reliable citrus thrips damage
prediction with advance warning (e.g., at least one week in advance
of when thrips treatments would be applied, if needed).
What to do with the predictions
The primary purpose of posting the model on this Web site is to collect
feedback from citrus growers and PCAs. The model has not been fully tested
and will probably require substantial improvement before the authors can
recommend using its predictions without reservations.
Check historical predictions
- In case you have records of citrus thrips scarring or fruit infestation
levels, you can use historical model predictions from a nearby weather
station and compare those to your records.
- Develop a feeling of the accuracy or inaccuracy of the model predictions.
Contact Joe Morse to let
us know your opinion.
Help improve the model
Because of the limited validation of the model, we are interested in
acquiring further data for future improvement of the prediction model.
Two types of data may be useful for this purpose:
- Fruit infestation by immature citrus thrips before any insecticide
treatments have been applied.
- Levels of fruit scarring by citrus thrips on trees that have not
It is very important that the data have been collected before treatment
and from untreated trees, in the case of fruit infestation and fruit scarring,
respectively. There is no easy way to accurately estimate the potential
for citrus thrips damage if trees are treated.
If you are interested in evaluating the model, we suggest leaving a corner
of your orchard (e.g., 3 rows by 4 tree wide) untreated. Comparing thrips
damage from the untreated trees to the treated trees will enable you to
judge whether treatments have resulted in appreciable damage reduction.
Comparing fruit scarring on untreated trees to model predictions will
allow evaluation of the model performance.
In case you have such data or you are willing to cooperate in collecting
such data, please contact Joe Morse.
Make decisionsonly if you accept the risk; see disclaimer
Disclaimer: The University of California
and the Authors of this model do not assume any responsibility and cannot
be held liable for any loss that may result from using predictions from
this model. Validation of the model has been limited, and you are encouraged
to test the model for one or more seasons to see if it is helpful to
- Use the model as a supplement to the thrips scarring history in a particular field.
- In case your historical records of thrips damage are consistently
higher or lower than model predictions. adjust the predictions accordingly.
- Use model predictions to identify seasons and orchards with high fruit-scarring risk.
- Then optimize labor allocation for accurate field
- Optimize equipment allocation to high risk areas for eventual treatments.
- Use field sampling data to back up your decision on whether or not
to apply treatments against citrus thrips.
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