Use a disease management approach when thinking about an SWD management plan

Cherry growers should think of spotted wing Drosophila (SWD) as a disease rather than an insect pest as this will help in controlling this pest.

Yellow tart cherry fruit. Tart cherry growers need to get in the mindset of beginning their seven-day programs immediately when fruits reach this developmental stage. Photo by Dave Jones, MSU Extension.

Yellow tart cherry fruit. Tart cherry growers need to get in the mindset of beginning their seven-day programs immediately when fruits reach this developmental stage. Photo by Dave Jones, MSU Extension.

I have had numerous discussions on spotted wing Drosophila (SWD) with growers in the past few weeks. The analogy that has worked best for them in understanding the new reality of SWD management in west central Michigan has been an apple scab analogy.

Every year in apples, we set up our spore rods to monitor apple scab ascospores. We observe spore numbers after each rain to determine the start and end of primary risk period for this disease. Nevertheless, as soon as green tissue is exposed on apple trees, we spray. Why? Because we always catch primary apple scab spores ahead of green tip. This is a basic assumption in the apple business.

As a result, we know that our crop is in danger and we take appropriate action. We spray ahead of rain events and we spray regardless of the number of spores caught on the monitoring rods until primary scab is over. We know that spore rod counts are not a tool to gamble on with apple scab sprays. Whether we catch one or 100 ascospores, we know we are at risk until primary scab has ended. Spore catch means only one thing at the beginning of every year: The trees have exposed leaves. We’re at risk. We spray. Period.

Let’s apply this same train of thought to SWD.

Every year in cherries, we set out traps to monitor SWD. We begin to catch a few flies and monitor the building population. We observe SWD numbers each week to determine the official start of the risk period. Nevertheless, as soon as we have yellow fruit on trees, we need to spray. Why? Because we always catch SWD ahead of yellow cherry fruit. This has to be a basic assumption in the cherry business. We need to realize our crop is in danger from the moment it turns yellow and we need to take appropriate action. We need to spray every seven days and we need to spray regardless of the number of flies caught in the traps until harvest is over.

We need to realize that trap catch numbers are not a tool to base our sprays on. Whether we catch one or 100 SWD, our cherries are at risk until harvest has ended. Catching SWD means only one thing if the cherries are yellow or later. We’re at risk. We spray. Period.

Growers ask about weekly trap numbers because they assume the block with the most SWD in a trap is most likely to have SWD larvae in the fruit. Unfortunately, this is not necessarily true. This highlights the risk of treating trap counts as spray guides. In the three conventionally managed tart cherry blocks where we detected larvae in fruit this season, none had the highest SWD counts in traps the weeks that larvae were found in the fruit. One site was actually the lowest the week larvae were detected. The other two sites were in the middle of the pack.

Watching the SWD population increase each season still gives us valuable insight on where the pest is and what it is doing, but it does not help us determine relative risk in the orchard. The sad reality is that SWD is so-well established in Michigan at this time that all blocks are at risk once yellow fruit is present, regardless of population counts.

This July, SWD numbers are higher than anything previously recorded at this point in the growing season. There is a good chance this may be the new “normal” as this pest becomes established in the region. Battling our way through this high-pressure year has taught us several important lessons as an industry in west central Michigan.

Programs that use products rated “excellent” for SWD at seven-day intervals are generally getting very good management. Challenging as it is for growers to meet the high demands of this aggressive spray schedule, those who have risen to the challenge are generally seeing very good control and high grades at the processing plant. It is evident you should either be using this level of aggression towards SWD or not bother to spray at all. Anything less than an outstanding management program will result in contaminated fruit.

Products rated “good” for SWD that get pushed past four to five days consistently result in larval contamination of fruit. Every report of sweet cherry contamination and the majority of tart cherry contamination reports we received at the Michigan State University Extension office this season all had one thing in common: Every spray schedule included a product that was not rated “excellent,” and these applications were used as four to five day stopgaps in the program. We cannot emphasize enough at this time that this simply will not work in a management program targeting this pest. Even with an “excellent” product, seven days is pushing the limit.

Trap counts in a block are not an indication of the relative risk for fruit contamination. We still do not know if “high” versus “low” pressure means anything regarding risk to the crop. The risk to an orchard is not necessarily proportional to the number of SWD adults caught in a trap. I have seen SWD larvae in fruit from blocks where as few as three adult SWD were caught in five traps in a week and I have seen clean fruit in blocks with over 200. Trap counts are nice. They tell us when the insect starts flying in the spring and allow us to watch populations ebb and flow, but we do not know if there is a difference between 50 and 500 SWD adults in terms of relative risk. For all intents and purposes, we cannot assume there is a marked difference at this time in terms of management considerations.

Growers using only a single cup trap in a block to determine if they need to spray for SWD risk being burned with bad information. SWD catches are highly inconsistent between traps in a single block. Furthermore, scouts who are only looking for males are not going to be able to give an accurate read on SWD populations. To demonstrate this, look at this example of the male and female counts from one week at a site in west central Michigan.

Trap/Fly gender

Males

Females

Trap 1

0

2

Trap 2

0

8

Trap 3

0

0

Trap 4

0

0

Trap 5

0

8

There are two points here.

  • There were two traps that caught no flies out of the five traps. This means there was a 40 percent chance of catching no flies in a trap. This type of result is common, particularly early in the season when fruit first begin to change color. It is not hard to imagine that a single trap in an orchard might catch no flies in a week, particularly early in the season before populations build. In this example, a grower using trap 3 or 4 would assume no flies were present in their block, and would not feel the need to spray.
  • Additionally, no males were caught in any of the traps. A scout looking for “spotted winged” males in the trap with their naked eye would assume the count is zero. The count is actually 18 flies. This is common, particularly early in the season, and demonstrates all flies, both male and female, need to be counted.

Rotating insecticides is critical to maintaining our ability to manage this pest when we look at the “long game” of SWD management. We are getting excellent control of this pest right now because the cheaper pyrethroid insecticides such as Mustang Max are working extremely well for us. We know from experience with other insect pests such as oriental fruit moth that pyrethroids can quickly become ineffective if they are over-applied. The scary thing from a resistance management standpoint is that oriental fruit moth’s reproduction rate is miniscule compared to SWD.

We need to be extremely conscious of rotating our insecticides each season, mixing multiple modes of action into a program to ensure adequate management. Diamides (Exirel, Harvanta), pyrethroids (Mustang Max, Warrior) and organophosphates (Imidan) should all be used in rotation going forward to help preserve the efficacy of our best products.

Take the time to read the MSU Extension article, “Plan to change when dealing with spotted wing Drosophila” by Mark Longstroth. This season has been a big wakeup call for all of us in west central Michigan and it demands adjustments from all of us. This is a manageable problem, but we need to be highly methodical in how we proceed if we want to maintain our industry as we know it.

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