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New Forecasting Model for Japanese Beetle in Minnesota

 Authors: Dominique Ebbenga, Eric Burkness, Adam Toninato, Bill Hutchison

Department of Entomology and UMN Extension

Edited by Annie Klodd, UMN Extension Educator - Fruit Production


Japanese beetle (JB) (Popillia japonica) is an invasive species first detected in 1916 in New Jersey, after an accidental introduction. Since this initial detection, JB continued to move south and west, reaching the Midwest region in the 1960s.

Although JB was first detected in Minnesota in 1968, it has only recently become a dominant pest since about 2010. The beetles are now a major insect pest of turf, residential ornamentals, and agricultural crops (Image 1).

The larval stage, which overwinters beneath the soil surface, feeds primarily on the root tissue of several grass species. Eventually, each summer, adults emerge from the soil and begin feeding on >300 different species of plants.


Image 1: Japanese beetles skeletonizing a grape leaf. Photo: Dominique Ebbenga.

New tool for predicting JB emergence

Because of their life cycle, it can be difficult to predict when JB adults will emerge and begin feeding. Current JB research at the University of Minnesota has focused on JB feeding damage and integrated pest management (IPM) strategies for apples, raspberry, and wine grapes. Given the extent of JB feeding damage, it is critical for growers to have new tools available to manage JB before feeding damage occurs. One of the objectives of a recent USDA Specialty Crop Block (SCB) grant was to develop a Degree-day model for JB as an early-warning forecasting system.

To improve advance notice of JB infestations, we developed a preliminary forecasting model based on the well-known "growing degree-days" concept, used for predicting insect and plant growth under field conditions. A degree-day model is a convenient way of integrating the daily variations in field temperatures, with the amount of “effective heat” that an insect can use for growth and development.

In this case, we recorded daily temperatures and accumulated Degree-days (DDs) per day that the pest can utilize for growth and development. For JB, both lower and upper thresholds for beetle development have been estimated, at 50°F and 88°F, respectively. In the model presented here, effective heat available for JB development is therefore accumulated each day, above 50°F and below 88°F. 


Degree-day forecast

Degree-days (DD) can be calculated manually or by computer models. Daily DDs for JB are calculated by using the average daily temperature (e.g., avg. of maximum and minimum for the day), then subtracting the lower developmental threshold of 50°F. For example, we calculate DDs here, using a high temperature of 80 and a low temperature of 60°F:

Daily degree days:

80°F+ 60°F = average daily temperature of 70°F

Then, 70-50 = 20 accumulated degree days for the day

Once daily degree days have been calculated, the next step is accumulating DDs for the field season, or period of interest. Using this information, we can begin to correlate pest phenology, or the timing of life events such as adult emergence, with DDs. In this case, we measured early, peak, and late season JB trap catches (Image 2). This will help determine at which DDs during the season Japanese beetles are active.


Image 2: Japanese beetle pheromone trap used for monitoring. Photo: Dominique Ebbenga.

Degree-days for JB

Looking at Figure 1, the Y-axis illustrates % total trap catch per season, in relation to cumulative DDs for the season (x-axis); 50% trap catch reflects the peak trap catch for the season, which correlates to 1610 cumulative DDs. This approach can also be used to predict the timing of early JB trap catch (10 % of total/season) and late trap catch (90% of total/season) for any given year.

Cumulative DDs provide a more consistent predicted timing for key insect events, such as peak emergence, compared to calendar dates. For example, in Table 1 we provide the predicted DD values for 10, 50, and 90% emergence, followed by the corresponding calendar dates in which we observed each key event in 2019 and 2020 at Rosemount, MN. For reference, for 50% emergence, the average DD value of 1610 reflects the cumulative degree-days accumulated since January 1st of each year.  Career

A final important point about insect forecast models developed using trap-catch data, is that the numbers of JB trapped are not limited to those that emerged from the local field site (where traps are located). JB trap catch numbers include both JB that emerged locally, and JB that flew in from other locations outside the field or farm. The commercial trap and lure system (Image 2) is known to be highly attractive to JB, and adults are attracted from relatively long distances, often beyond farm boundaries.


Figure 1. Degree-day model for Japanese beetle adults using commercial traps as a predictor of adult activity. This illustrates how the percentage of total JB trap catches for the season is related to cumulative degree-days (lower and upper thresholds of 50 and 88F, respectively). Red dashed line indicates 95% confidence intervals with an adjusted R2 of 0.98 (i.e., 98% of trap catch variability explained by model). Traps were deployed at Rosemount Research and Outreach Center (RROC) in Rosemount, MN, and the Horticultural Research Center (HRC) in Excelsior, MN in both 2019 and 2020.


Table 1. Forecasting key events for Japanese beetle emergence, based on trap catch, for data collected at RROC & HRC, for two field seasons (2019-2020)

Timing Event             Predicted Degree-days                     Calendar Date Comparison

                                                                                    Rosemount-2019            Rosemount-2020

Early (10% catch)                   1190                                        July 15th                         July 7th

Peak (50% catch)                    1610                                        July 25th                        July 28th

Late (90% catch)                    2110                                        August 19th                  August 18th

*Degree-day model based on best fit of a non-linear 4-parameter, sigmoid model.


Using the Automated Japanese Beetle Model for MN

For 2021, we developed an automated system for growers to predict JB activity based on growing degree days. Growers interested in using this tool can go to the UMN VegEdge site, at: to track DD accumulations for JB. The DDs for the JB model will be updated twice per week throughout the field season, and includes a 7-day forecast. Extension Educator, Anthony Hanson[WDH4] , maintains the DD model for JB and other key pest species in Minnesota.

Use of this model will allow for efficient tracking of JB populations throughout the season. For example, growers will know when JB adults are about to start emerging and can prepare accordingly. Knowing the forecast for peak adult activity is also useful for timing of in-field scouting activities like adult beetle sampling and estimating feed damage on susceptible crops or ornamental plants.

Although the current model was developed using two years of data at two locations, we are continuing to collect additional data this summer to refine and further validate the model. Additional updates to the model will be discussed in future Fruit & Vegetable News articles.



(MDA) Minnesota Department of Agriculture. 2018. Japanese beetle.

(USDA-APHIS) United States Department of Agriculture - _health/plant_pest_info/the Japanese beetle/downloads/the Japanese beetle-distribution-map.pdf

Davis, Terrance. 2018. Japanese Beetles (E0010TURF).

Ebbenga, D., S.J. Wold-Burkness, E.C. Burkness, & W.D. Hutchison. 2020. Japanese Beetle: An emerging pest of fruit crops. FruitEdge.

Kistner-Thomas, E.J. 2019. The potential global distribution and voltinism of the Japanese beetle (Coleoptera: Scarabaeidae) under current and future climates. J. Insect Sci. 19(2): 16 1-13.



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