MSU Extension Cherries

As cherry harvest is underway, growers are balancing management decisions with pre-harvest intervals (PHIs) and the rainy forecast for the weekend and early next week. This season has been mostly dry and hot, and we hypothesize these conditions have been less conducive for spotted wing Drosophila (SWD) activity.

Previous data have shown SWD activity decreases at temperatures above about 86 degrees Fahrenheit, and egglaying is inhibited at about 91 F or more. In addition to temperature, SWD flies also prefer higher relative humidity. In a blueberry field study, they found that ambient temperature was not correlated to seasonal adult captures, but relative humidity played a significant role; higher humidity was directly correlated with higher trap catches, according to Tochen, et al. 2015.

They also found SWD survival is reduced and reproduction decreases at 20 percent relative humidity. Other previous research has shown that there is an interaction between temperature and humidity. For example, insect survival improves in hot conditions when humidity is high rather than low. This season’s weather events are likely influencing SWD population growth and activity.

Thus far this growing season, overall SWD trap catch is actually higher than in past years despite the slow start and multiple weeks with zero trap. Again, we hypothesize this hot and dry weather is affecting SWD activity in cherry orchards. As mentioned above, this week’s SWD trap catches are down substantially over last week’s catches (Table 1). Last week, we caught a total of 420 flies compared to this week’s total of 236 flies.

Growers and consultants may see this week’s lower numbers and think this decrease is a result of successful spray programs. While good SWD management is helping to delay fly population growth, the region’s recent weather is likely the important driving factor in decreasing trap counts rather than spray programs.

The 2018 trap counts appear to fluctuate whereas the 2016 and 2017 trap counts started to build slightly earlier but remained steady as the season progressed (Figure 1). The overall numbers of flies caught in 2016 and 2017 were also significantly lower than the 2018 trap counts.

At this same time in 2016 and 2017, we caught an average of about one fly compared to an average of around six flies in 2018. Because we have had many zeros in our traps until the past two weeks, we assumed SWD populations were relatively low in the region. However, the data indicates the SWD populations are high, especially compared with past seasons. This information is startling, and growers should be diligent about managing SWD this season.

Figure 1. Average number of SWD flies captured in traps in northwest Michigan, 2016-2018.

We reviewed rainfall events at the Northwest Michigan Horticultural Research Center’s Enviroweather station for this season, and we received 3 inches of rain in May, 2.06 inches in June and 0.47 inch thus far in July. These rainfall totals are similar to our 37-year averages: 2.75 inches in May, 3.09 inch in June and 2.35 inches in July, but hot temperatures have quickly dried this moisture in the region.

A combination of late cold spells followed by these dry and hot conditions likely contributed to the slow start but sudden rise of SWD populations as reflected in our 2018 trap count data. The flies almost appear to be “waiting” for moisture. We received 0.33 of an inch of rainfall July 1, and trap counts for the week of July 2 increased substantially: 420 flies the week of July 2 compared to five flies the week of June 25; this is an 84-times increase in fly catch. This rainfall event/increased moisture likely triggered this substantial increase in SWD trap catch. Hence, SWD fly activity has been depressed with the dry conditions, and even with a small amount of moisture, SWD have become significantly more active.

Since July 1-2, we have had little rain in much of the region (until last night’s rain event, July 12), and the SWD trap counts for the week of July 9 were back down to about less than half of the prior week’s counts. This week (July 9-13), daytime temperatures were in the mid-80s with low relative humidity and extremely dry orchard conditions. The lack of humidity and dry conditions likely decreased SWD activity during this past week, which is reflected in our trap count data. However, growers should note that even if the fly counts are down by almost half compared to last week, the numbers are still three times higher than at this time in 2016 and 2017.

The region received varied amounts of rainfall this week, and the current relative humidity at the Northwest Michigan Horticulture Research Center is 68 percent (July 13). There is more rain in the forecast for tonight, July 13, and again Monday, July 16. Based on our recent observation of the relationship of SWD trap catches and moisture, we hypothesize wet conditions will facilitate higher humidity and stimulate greater SWD activity over the coming days. We are particularly concerned about the combination of wetter conditions and fly activity as the SWD populations are substantially higher than they have been in past years.

In short, we have caught on average six times more flies at this time in the season compared with the last two years. Dry conditions may have kept the SWD trap catches seemingly low this season, but growers should not become complacent with management programs as this wet weather will likely increase SWD activity, and spray intervals should not be stretched.

For applications over the weekend, we suggest using a product rated excellent with a 14-D PHI in blocks where harvest will not occur for over 14 days, if this is an option. There is a 24(C) Special Local Need Label for Mustang Maxx that allows an application at a three-day PHI in tart cherry only. According to this label, there is a maximum of 24 fluid ounces per acre per year (0.15 pound active ingredient per acre per year) and a seven-day retreatment interval requirement.

This could be a long harvest season, particularly for tart cherries, and this label will be a helpful tool for tart cherry growers as they approach harvest. Please review the 24(C) label and other labels for additional information prior to applications.