West central Michigan small fruit update – May 18, 2021

Weather conditions are improving and warm temperatures are arriving, but drought conditions remain unchanged.

After going through a series of freeze/frost events, finally the warm weather has arrived in west central Michigan. From May 11 to 14, overnight temperatures dropped below freezing at most locations in west Michigan including Ottawa, Allegan and Van Buren counties. These counties comprise the bulk of the blueberry and small fruit industry. At those locations, blueberries were at pink and late pink bloom stages, which made them susceptible to frost damage by temperatures below 28 degrees Fahrenheit.

Regarding the prevailing weather conditions, for the past seven days average daily minimum and maximum temperatures in the region were 38 F and 70 F, respectively. On the other hand, the drought conditions continued unchanged. During the same period, only traces of rain (0.01 to 0.09 inches) occurred in the region, increasing the water deficit in the region.

So far, accumulated precipitation since Jan. 1 remains at 4.4 inches, well below the needs of most small fruit crops. Growers have been compelled to provide supplemental irrigation to avoid crop losses due to water stress during the bloom period. One factor that ameliorates this water deficit is that prevailing low temperatures decreased the daily evaporation rate, preventing a more drastic impact of a lack of significative precipitation on plant growth and development.

With the arrival of good weather, insect problems are also arriving. Cherry fruitworm and cranberry fruitworm are emerging from their overwintering sites. Weather conditions in the region are allowing for a rapid growing degree day (GDD) accumulation. As of May 18, we have accumulated 368 GDD base 50 F in Ottawa County and 490 GDD base 50 F in Grand Junction, Michigan, since March 1. These GDD accumulations are accelerating the emergence of both insects and the need to prepare for the initiation of their chemical control.

Below is the current phenology forecast for this week based on our fruitworm phenology models. Check Michigan State University Enviroweather for more precise forecast for your location.


First adults

First eggs

Current GDD accumulation (May 18, 2021)

Grand Junction


West Olive

Cherry fruitworm

238 ± 30

432 ± 15




Cranberry fruitworm

375 ± 20

460 ± 20




(*) Adults are already emerging. Check your traps to establish your biofix to initiate the GDD accumulation for applying the first insecticide application against the first batch of fruitworm eggs.

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