Plan to change when dealing with spotted wing Drosophila

Growers will need a fresh approach to managing spotted wing Drosophila.

Ripe Montmorency tart cherries are attractive to female spotted wing Drosophila. Protect your fruit before harvest. The longer ripe fruit hangs on the tree, the greater the risk. All photos by Mark Longstroth, MSU Extension.
Ripe Montmorency tart cherries are attractive to female spotted wing Drosophila. Protect your fruit before harvest. The longer ripe fruit hangs on the tree, the greater the risk. All photos by Mark Longstroth, MSU Extension.

In 2017, we are seeing an early start to spotted wing Drosophila (SWD) and there is a good crop of berries on host plants in the woods. To me, this means SWD will arrive earlier than normal and be a bigger problem than in recent years. I am concerned SWD will be an issue for fruit growers who have escaped SWD in the past because in previous years growers were harvesting earlier than the SWD surge. It looks like the surge will be early this year.

Ripe fruit in the field is an incredible SWD magnet. Leaving ripe fruit in the field means SWD has many opportunities to attack that fruit and find any weakness in your spray program. Growers who fail to protect their fruit may be in for an unpleasant surprise.

Fruit growers have a hard job. They just want to harvest their crop with high quality, high yields and get it to market to be sold as quickly as possible. Having a pest at harvest makes this much more challenging, but growers are up to the task. In this new world of SWD, the management challenges really complicate harvest. To control SWD, you need to pay close attention to the ripeness of fruit in the field, protect that fruit, and watch the pre-harvest interval of the material used and your harvest schedule.

How SWD has changed growing fruit

SWD is a small, vinegar fly from Asia that has become a major fruit pest around the world. It first appeared in the United States in 2008. In 2009, it exploded up the West Coast attacking one of the major berry production areas and destroying millions of dollars of fruit. It appeared in Michigan in late 2010.

In 2011, SWD caused significant damage to Michigan blueberries. Growers who did not control this pest lost the crop from some of their most profitable fields. Since then SWD has become a major fruit pest in Michigan. It causes millions of dollars worth of damage to Michigan blueberries and cherries as well as peaches and plums. It has virtually destroyed the Michigan fall raspberry industry. Homeowners who grow berries know it well.

Even more importantly, SWD has increased the cost of growing an acre of fruit by hundreds of dollars. Many growers simply cannot afford to protect their fruit because the SWD has destroyed the profit margin.

I want to share my thoughts on what people need to do to control this pest. To me, it is not a fly; it is the spotted weapon of destruction. I think it has earned that nickname. Many of the habits and practices that have served fruit growers well for years do not work against SWD. Doing things the way you would have done them in the past will not work.

SWD is nothing like any insect pest you managed in the past

SWD is different than most of the other insect pests that attack fruit. If you think of SWD as just another pest that can be controlled with a properly timed spray or two, you are doomed to fail. If you think of it as something that will destroy your fruit if you make a mistake, you have the proper mindset. If you think of SWD as a disease with “spores” that attacks your fruit from ripening through harvest, you will grasp the situation. SWD is always there.

The first thing to understand is SWD is not a pest of any particular fruit—it is a pest of all soft fruit. It overwinters as adults under the snow or in leaf litter protected from winter cold. The adults emerge in spring and begin to lay their eggs in the first fruit that ripen such as strawberries and other wild berries. It takes about a day or two for the eggs to hatch and two weeks for the maggots to mature and pupate into adults. In two weeks, 200 or 300 more SWD are looking for a place to lay their eggs.

SWD is not particular and the fly moves to many different fruit as they ripen through the season. As long as there are ripe fruit in your fields or in the wild areas around your fields, there are probably SWD there to lay their eggs. This means the population builds continuously over the season.

Denial is a big part of the SWD problem. “It can’t happen to me.” “I don’t have that pest.” No one wants to talk about SWD. Growers do not want people to know they have a problem, marketers do not want people to know we have a pest that requires sprays and the public does not want worms in their fruit.

When MSU first established our trapping network, there were growers who wanted to keep the location of the traps a secret. My biggest fear was that a grower in Township A would assume there were no SWD in the area because they caught them in Township B, so they must not be in Township A.

Unless you are trapping for SWD on your farm and have not caught any flies, you have to assume you have them. Let me say that again. If you are not trapping for them, you have to assume SWD are there. If you have ripe fruit, you need to protect it.

ripe blueberries

Blueberries are quickly turning blue. Hand harvest in blueberries has begun and we are catching SWD in many blueberry fields with ripening fruit.

Yes, you have to spray weekly and thoroughly

I am often asked, “How many days after I catch SWD should I spray?” My response is that if you caught SWD, they are laying eggs in your fruit. You should spray right now. This is not a pest where the capture of the first adult means the egglaying will begin several days later. This is not cherry fruit fly or blueberry maggot. Capturing an adult SWD means they are active and laying eggs.

Since SWD lays its eggs in the fruit, pesticide sprays target the adults. The goal is to kill the fly when it lands, before it lays its eggs. If you miss any SWD, their offspring will cause a lot of damage in a couple of weeks.

The fly does not sit out in the open where it would be easy to kill. SWD likes the shade, so it is inside the bush or the tree, or over in the cool, dark woods. SWD is active during the cooler parts of the day in the early morning and late afternoon.

In order to get good control, you need to cover the bush or tree. Penetrate sprays deep in the canopy and cover the backsides of the fruit. This requires changing the way you spray your fruit.

Growers do not like to spray. It takes a lot of time, pesticides are expensive, labor is expensive, the equipment is expensive and time is expensive. Many spraying tricks like speeding up, spraying multiple rows and reducing gallonages to make the work go faster and reduce the time to spray do not work with SWD. You need excellent coverage with contact materials that will kill the fly on contact. SWD does not spend a lot of time crawling around and systemic materials in the plant do not work against this pest.

Slow down, spray every row or two and double or triple your gallons per acre. I feel I cannot say this enough; all the tricks to reduce sprays to control insects and diseases work the wrong way when it comes to SWD.

You need excellent control, so you’ll need to increase the gallons per acre and spray from both sides to get into the interior of the plant where the shade is. Think ahead to next winter too, and invest in pruning to open the bush canopy and help those sprays get into the deepest part of the bush where SWD likes to hide.

Michigan State University Extension’s blanket recommendation for SWD control is to apply effective insecticides every week and reapply after a rain. Growers are used to materials that work for about two weeks; unfortunately, you’ll need to reapply insecticides every week because there is not enough residue there to kill the SWD.

I have been telling blueberry growers for five years they need to spray once a week. Many tell me at first they did not believe me, but now they say seven days is too long, it needs to be every five or six days. A week after making a mistake, your fruit will be soft and the buyer will reject it.

Plan to avoid pesticide resistance

With a tight insecticide schedule of weekly sprays and needing to reapply after any significant rain, pesticide costs quickly become an issue. Growers look to the cheapest material that will do the job, however, it will take a variety of materials to do the job.

Because SWD reproduce so quickly in such large numbers, there is a real risk of pesticide resistance if many growers use the same insecticide repeatedly because it is the cheapest material. Since there are several effective classes of insecticides, MSU recommends that no class of insecticide be used twice in a row and that growers rotate between chemical classes on every spray.

Give careful thought and make a schedule for what you are going to use and when you are going to use it. What material will you use if it rains? Can you apply that material or do you need another material?

MSU has a website dedicated to Spotted Wing Drosophila (SWD) and its control in many crops. There are factsheets on SWD and specific recommendations for several important Michigan crops.

Crop recommendations for blueberries (in English and Spanish), cherries and raspberries are available as well as organic blueberry production. These crop recommendations include the insecticides MSU entomologists think will be the most effective with all the different restrictions. The insecticides are also listed by chemical class so growers can quickly see which materials are in the same class with the same mode of action.

Resources to help you track SWD

MSU has a statewide SWD trapping network and posts the trap catch numbers on the SWD website and at the MSU Extension Fruit and Nuts page, usually on Tuesday afternoon. This helps growers and consultants keep track of where SWD populations are in the state. You can even sign up for the MSU Extension Fruit and Nuts newsletter and get weekly updates emailed to you.

The spotted weapon of destruction can be controlled and your fruit protected, but not if you assume you do not have it or think your traditional pest control program will be enough.

See also

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