Using MSU’s cranberry fruitworm degree day model

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

Cranberry fruitworm is an early season insect pest of blueberry with one generation per year in Michigan. Moths emerge during bloom, mate, and the female moths lay eggs on berries as the calyx cup becomes visible when the petals fall off. The activity of this insect during bloom means that growers are restricted in their control options because of the need to protect bees. In fields with low pest pressure, growers may be able to achieve sufficient control by relying only on post-bloom insecticides. However, in fields with high fruitworm pressure, use of a bee-safe insect growth regulator such as Intrepid or a Bacillus thuringiensis (B.t.) insecticide such as Dipel or Javelin applied during bloom can help reduce infestation by this pest. Because these bee-safe insecticides work best when timed for egglaying, i.e., before egg hatch, it is important to know when this pest starts laying eggs. It is particularly important to avoid being too late because once the larvae are in the fruit, they are much harder to control.

To help predict when cranberry fruitworm egglaying starts, we have developed a degree day model for this pest in Michigan blueberry. This requires that monitoring traps are placed in the field and checked regularly, along with tracking of degree days that can be provided by the MSU Enviroweather website.

Monitoring to identify the biofix

In fields with a history of fruitworm infestation, deploy a large plastic delta trap baited with a pheromone lure. Traps should be placed in blueberry bushes near woodlots and previous hot spots, in the upper fruiting zone, and placed just before bloom starts. Moth flight starts during bloom, so put traps up in early bloom to ensure you don’t miss the start of the flight. Keep records of the date and the number of new moths trapped, clearing the traps each time you check them. In large farms, a separate trap per field is recommended to detect differences in timing across the farm.

The first consistent appearance of moths is used to set the biofix, or start of the model, and this usually starts at about 350 GDD base 50oF after March 1. To set biofix, check traps twice a week during bloom and keep careful records of cranberry fruitworm captures. Once you trap moths on two consecutive checks, the biofix is set as the date before this sustained catch, which is the “zero” catch before.

Tracking degree days after biofix

Once the biofix is set, keep track of degree days using base 50oF. The Enviroweather system can automatically track the predicted start of egglaying and its progression using weather data from MSU’s weather station network. Go to and click on the weather station nearest to your farm. Next, click on the “Fruit” pages and then scroll down to “Cranberry Fruitworm” in the “Small Fruit” insect models section. This will bring up a page that provides the percent of cranberry fruitworm egglaying predicted to have occurred on today’s date. Early in the season, this number will be zero (as it is today, on May 6). Once the degree days accumulate beyond 300 GDD after March 1, the page will include a table with dates along the left side and the date of biofix along the top. Look along the top of the table to select the date of biofix you set using moth catches in the traps. Then look down the column for the date when the accumulated degree days exceed 85. The table cells will turn red when this point is reached and they stay red for 400 more GDD to indicate the period of fruitworm egglaying.

The image included in this article displays what the model showed during blueberry bloom in 2008. Note the red sections that indicate the predicted start of egglaying by cranberry fruitworm. This illustrates how the timing of egglaying would be predicted to be later when the biofix is set later. In 2009, once the degree day accumulation progresses a little further, the table will be shown on the cranberry fruitworm model page.

Improved control of cranberry fruitworm using the model to time the first spray

Recent studies at a blueberry planting with high fruitworm pressure at the Trevor Nichols Research Complex in Fennville, Michigan have shown that a first application of Intrepid (12 oz/ac) close to 85 GDD after biofix (applied during bloom), followed by a reapplication two weeks later, provided equivalent control to Guthion applied twice after bloom. Delayed applications were less effective, showing the importance of getting spray timing correct. Similar results have been found in trials at commercial blueberry farms in recent years.

Figure 1. Example of the cranberry fruitworm model page from spring 2008, showing degree days along the left of the table and with the dates of biofix across the top of the table. Look down the table from the biofix date to see when the start of egglaying is predicted (when the column turns red). This table will be posted once the selected site reaches 300DD after March 1. Fruitworm model

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