West central Michigan small fruit regional report – August 9, 2016

Spotted wing Drosophila (SWD) high population density, overripe fruit and adverse weather conditions are causing SWD control failures and an increment of fruit infestations in blueberries.

Environmental conditions in west central Michigan remain without much change, with moderate drought and daily high temperatures reaching around 90 degrees Fahrenheit. On average, daily minimum temperatures during the past seven days were 60 F, and the daily maximum averaged 85 F. No substantial rain was reported during the past week, and the area remains under moderate drought conditions. Daily high temperatures and lack of precipitation are creating substantial problems for blueberry growers unable to provide the crop with supplemental irrigation.

Summer raspberries are approaching the end of the harvest season, but there are still some fields that continue harvesting good quality fruit. However, given the increase in the spotted wing Drosophila (SWD) population, it is becoming increasingly difficult to keep the fruit free from SWD infestations. Clean harvesting combined with insecticide applications is becoming the most effective SWD control practice.

In blueberries, the situation is more complex. There was a two-fold increase in the size of SWD flies trapped in our monitoring network. Some traps showed up to 90 flies. That is almost double the number of flies trapped last week. That increase in the SWD population density had an impact on the number of complaints from growers requesting assistance with SWD management. Also, packing facilities reported a substantial number of fruit loads with high numbers of SWD larvae at the moment the fruit was tested SWD infestation. Field observations and information provided by growers seem to indicate that, in addition to the unfavorable weather conditions, the reason behind those control failures include insecticides that do not work as expected, tall bushes with dense canopy and, most important of all, not enough water volume applied per acre.

As discussed in the Michigan State University Extension "West central Michigan small fruit regional report for July 12, 2016," daily high temperatures tend to inactivate, or reduce, the toxicity of insecticides such as Malathion or pyrethroid insecticides such as Mustang Maxx. Therefore, relying on a seven-day protection that some of these insecticides provide under tempered weather conditions increases the risk of SWD fruit infestations. Thus, shorter spray intervals are required when daily temperatures are above the upper 80s.

Also, most growers are applying recommended insecticides in water volumes of no more than 30 gallons per acre. That volume works well during spring time when daily temperatures remain below 80 F. This summer, temperatures have been around the upper 80s under moderate drought conditions that also affect the relative humidity. Under these weather conditions, a considerable portion of the volume sprayed onto the bush evaporates, or the low relative humidity reduces the size and mass of droplets carrying the insecticide, preventing them from reaching the inner portion of the bush. For example, in relatively dry, warm air (e.g., 30 percent relative humidity and 78 F), a 100-micron droplet quickly loses water by evaporation and becomes less than half its original diameter (one-eighth of its original volume) while falling only 2.5 feet, according to Ohio State University Extension Bulletin 816-00, "Reducing Spray Drift." Therefore, it is recommended to increase the spray volume to more than 30 gallons per acre to provide enough droplets carrying the insecticide into the bush, and compensate for the volume lost to evaporation or drift.

Finally, during the past week, temperatures in southern counties reached the mid- and upper 90s, and most insecticides were affected by high temperatures and sunlight. Therefore, before making the decision to spray, check the current and extended weather forecast for the next 24-72 hours via MSU Enviro-weather and select the appropriate insecticide accordingly.

For a complete list of recommended insecticides and doses for SWD control, check the “2016 Fruit Management Guide” or call your local MSU Extension office for assistance. You may also contact me at 616-260-0671 or garcias4@msu.edu.

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