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Friday, May 4, 2012

What should growers and homeowners who find SWD do?

I am getting reports of spotted wing drosophila SWD larval activity in some southeastern locations over the past two weeks, and we have observed SWD infestation in some of our research station plots in North Carolina.  Infestations in my plots are a good thing, because it allows us to study SWD here and provide you with up to date information, but infestations in grower fields are another matter. We have been catching adult SWD since January in North Carolina, but these are the earliest larval infestations we have recorded in the state.  This early activity has prompted several questions about what growers or homeowners should do if they find SWD infested fruit.  Here's a quick summary of the four key steps you should take in the event of SWD infestation:

1. Remove all ripe and ripening fruit and destroy it (by freezing, "baking" in the sun inside a clear plastic bag for a few days, or removing from the site).  Eggs or larvae may be present in otherwise sound appearing fruit. Leaving this fruit means that potentially infested fruit is present and could be harvested.

2. Begin an aggressive (once per week if it does not rain, reapplication in the event of rain) spray program.  Rotating between at least two modes of action will reduce the likelihood of resistance development.  See here for information about the probably efficacy of registered insecticides in southeastern small fruit crops.  Because these efficacy ratings are based on work done largely in the western US, where environmental conditions differ, they are not set in stone.  We are running several efficacy trials as I type and hope to have NC specific data by mid summer. 

3. Practice excellent sanitation.  Remove all ripe fruit and sell or destroy it.  Do not discard culls in the field, and clean up after rain and u-pickers. Unpicked fruit is a reservoir for SWD larvae.

4. Sample fruit regularly and consider trapping flies.  Traps tell you SWD presence or absence and are not perfect, but they are helpful.  Place traps in strawberry rows, near fruit for the best chance of catching flies.  Sample fruit each harvest by either cutting them open and looking for larvae, crushing them in salt water (1/4 cup salt per gallon), or freezing them.  Salt water and freezing will cause larvae to exit fruit.  Sample at least 30 berries per field.  See here and here for trapping information and here for larval sampling information.

All growers are strongly encouraged to monitor for SWD adults before larvae are found.  If adults are present and fruit are ripe, pesticide treatments are recommended through the end of harvest.  We hope that this strategy will prevent larvae from showing in fruit and triggering the actions listed here.

Update, June 22, 2012
I am aware that homeowners may be not be able or may prefer not to use insecticides on fruit in the gardens are yard.  Unfortunately, sanitation alone will not eliminate and SWD population.  However, if you can pick your fruit a bit earlier or a bit less ripe, it will be exposed to SWD for less time and will perhaps be at a lower risk of infestation.  It's also important to note that while SWD larvae cause fruit to decay quicker and are unpalatable if present in fruit, they are not poisonous or parasitic.  If you have inadvertently consumed fruit from your home garden that SWD larvae have been present in, they do not pose a heath risk.

Update, August 7, 2012
I have been reading LOTS of scientific papers about SWD in the last week.  A couple of Japanese references suggest that mesh cages or bags (specifically 0.98mm mesh) prevented SWD infestation in blueberries without impacting plant growth.  We use mesh bags a lot to either keep insects we are studying in or to keep insects we don't want out.

We have used mesh bags in blackberries to exclude thrips from developing fruit in order to determine if they impact fruit development.  They don't appear to.
This strategy is worth a shot for homeowners who can tolerate the sight of bags on their plants.  In order to exclude SWD, cages should be placed before fruit begins to change color and tightly sealed at the bottom.  We'll often place a piece of quilting foam between the cage and the stem of the plant to allow us to tightly tie the cage shut (with a zip tie or twine) but protect the stem from damage.  Cages should stay in place until you harvest your fruit.  We use organdy mesh from fabric store and sew our cages to size, but paint strainers also work in a pinch.  I plan to try this method with my backyard blueberries, blackberries, and figs next year and will be interested to hear if others also give it a shot.

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