7.08.13 Spotted Wing Drosophila (SWD)

It is no wonder with this HOT weather over the fourth and following weekend a few more SWD captures have surfaced since my update on July 2.  However, last week’s capture of a male SWD in Cherryfield was a misdiagnosis. When we looked at it, two spots were found on each wing and so we have NOT had a recent SWD capture Downeast yet. But, in the south…male SWD was captured in Dresden and Whitefield on July 3 in the southern coastal blueberry-growing region. On Saturday, July 6, I found a male SWD in a Winterport blueberry field. I have deployed 20 traps in this field and only ONE trap had an SWD. I also have 25 traps deployed in a Stockton Springs field, but as of yet no male SWD have been identified and I have not had a chance to go through all the traps for microscopic detection of females yet.  I am guessing that from the low number of captures that SWD has not completed its first generation, producing new adults.

An important and timely question…What shall I do if I detect male SWD in traps in my field at this time of year (early July)?  I think the thing to first state is that if you have only a slight proportion of ripe (blue) berries (< 0.5 % or so) then don’t panic. At this point, I believe ONLY the overwintered SWD are in our fields as of yet and SWD needs RIPE fruit to lay eggs in. However, they will attack these fruit and start to build in numbers over the next several weeks.

So, there is a trade-off between:

  1. starting to protect your crop too early with insecticides and having to spray many times prior to harvest and
  2. incurring damage and fruit loss. Each grower has to strategize to solve this dilemma of whether to take action or not.

ONE WAY of finding a solution to this problem is to determine at what percent of ripe fruit in your field is the trigger for action.  As an example…If you typically get 2,500 lbs./acre on a given field and guessing on past prices you can estimate what you will get for your crop…then you can calculate how much loss in revenue you will incur if SWD takes 1, 2, 5, 7%, etc., of your crop and you can determine what the economic threshold would be…or, in other words, the cost of crop loss that will equal the cost of controlling SWD (application + insecticide cost). So, in our example, if you normally get $0.75 / lb, then 1, 2, 5, and 7% infestation EARLY in the season will cost you… 25, 50, 125, or 175 lbs./acre or in dollars…$18.75, $37.50, $93.75, or $131.25…in this case if it costs you $100/acre to treat the crop then somewhere between 5 – 6% crop loss would be your decision making trigger to treat, meaning 5 – 6% of your crop being ripe.

The calculation above is ONLY a guide. Growers are different in their aversion to risk and price that they receive and so it is not possible to give a simple one answer that fits all.  BUT, this being said another thing you might want to consider prior to an insecticide application is to take a known sample of ripe fruit from your field (small collections from several locations throughout field). Place a quart of your fruit in a gallon zip-lock baggie and mix a salt solution of ¼ cup salt to four cups water.  Lightly crush the berries in a bag and add the salt solution.  Allow the fruit to sink to the bottom of the bag and wait approximately ten to fifteen minutes.  Several baggies can be easily set up if you want to sample more than one quart of fruit. If the fruit is infested, the larvae should float in the solution for you to visually inspect (See my SWD factsheet showing what larvae look like).  NOW you have a better idea of crop loss that exists at the time of sampling.

The last aspect of EARLY SWD management that I want to bring to your attention comes from my inspection of blueberry fruit fly traps this past Sunday in the field. I was at a field in Stockton Springs and the threshold (ten cumulative flies/trap) was already surpassed at 13 flies/trap. So, if this grower is thinking about a perimeter spray (a location where we also tend to capture lots of SWD)…then an insecticide that is ALSO effective against SWD would be a logical choice. An insecticide that would NOT a choice is imidacloprid (ADMIRETM, PROVADOTM, MONTANATM, PREYTM, etc.) since efficacy against SWD is low. Reduction of SWD adult numbers as a result of a perimeter blueberry fruit fly application might buy you some more time before you need to manage for SWD or maybe even allow you to harvest early making an insecticide treatment specifically for SWD not necessary. However, at this point, we do not know how this might play out and so it is no guarantee that spraying for blueberry fruit fly will have any effect on SWD.

*A NOTE…for organic growers the above points are also relevant. Insecticide options are EntrustTM or PyganicTM. The capture of SWD may also be a trigger for deploying exclusion netting over your crop or for deploying a grid of bait cups (every 25 ft) throughout exceptionally high yield areas of your field as a trapping out tactic.

NOTE: SWD updates will only be made when we have trap captures.