West central Michigan small fruit update – July 23, 2019

Spotted wing Drosophila catches increased this week in all fruit growing regions. There is not a threshold number of flies to start control, so initiate chemical control immediately upon detection.

Current weather conditions in west central Michigan are improving over those observed the past week. Average daily maximum temperatures ranged from 84 to 86 degrees Fahrenheit and the minimum temperatures ranged between 64 and 67 F. Grand Junction, Michigan, registered the highest temperatures and the Holland, Michigan, area registered the lowest.

Rains in the entire region contributed to improving weather conditions. In general, west central Michigan registered between 1.2 and 1.7 inches of precipitation over the past seven days. In the Grand Junction area, accumulated precipitation over the same period reached 4.1 inches. This precipitation broke the summer drought that persisted for more than one month in the region. It also provided the much-needed relief for small fruit crops, especially for blueberries. The extended weather forecast for the next seven day indicates daily maximum temperatures around the low 80s with some probability of rains by the end of the week.

Although improved weather conditions are welcomed by growers, it also brings a relief for spotted wing Drosophila (SWD). Up until two weeks ago, SWD captures were in the single digits in most traps around west Michigan. However, with cooler temperatures and high relative humidity, SWD populations are rapidly increasing all over the fruit growing areas, with captures in the past week at monitoring traps now in the triple digits, especially in Van Buren and south Allegan County. Monitor the presence-absence of this pest at your fields and initiate control immediately upon detection.

So far, the combination of weather (dry and hot) from the past several weeks and good pest management in most fields have prevented fruit infestations in blueberries. In raspberries, we are finding some SWD damage to fruit. However, growers are maintaining good SWD management by daily removing all ripe fruit and spraying when needed. With changes in weather conditions and high SWD pressure, it is important to keep an excellent monitoring and management program to prevent fruit infestations.

For this week, the following are recommendations for controlling SWD following the systems approach, which uses Michigan State University Enviroweather information, crop and pest conditions, and the characteristics of recommended insecticides.

Current and advanced weather forecast for the next seven days indicates daily temperatures in the low 80s and less than 50% of probability of rain. Based on this, the best insecticides for SWD control are Mustang Maxx, Brigade, Malathion and Assail. These insecticides have a preharvest interval of 24 hours. Between harvests, use Imidan, Lannate, Danitol, etc. Their preharvest interval is three days. For more insecticide options, consult the Michigan Fruit Management Guide (MSU Extension bulletin E154).

Remember, SWD monitoring traps are only devised to assess the presence-absence of SWD in fields. There is not a correlation between the number of flies caught in the trap and the percentage of fruit infestation observed in the field. In other words, it is not the number of flies (action threshold) at which insecticide applications should be initiated. Apply insecticides immediately upon SWD detection, even if it is only one fly per trap.

Following this advice is important for managing SWD in fruit crops where the industry has established a zero-tolerance for SWD larvae in fruit delivered for processing.

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