Arkansas Fruit, Vegetable and Nut Update

Spotted Wing Drosophila Management in Blackberry

by Aaron Cato - June 8, 2020

Provided here are factors to consider when developing a management plan for spotted wing drosophila.

Blackberry harvest is beginning across the state and our #1 insect pest, spotted wing drosophila (SWD), is out and about in high numbers. Growers across much of Arkansas should have already initiated insecticide applications to manage SWD. There are several factors that can influence when you should start spraying and which insecticides you should use, and we will discuss these below. Be sure to check out the Southeast Caneberry IPM guide from the Southern Small Fruit Consortium for more information.

How does SWD damage berries?

Damage from SWD is caused only by the larvae (immatures) and control should focus on preventing adults from laying eggs in berries. The larvae of this species feed on drupelets leading to the berry becoming soft and sunken making it unmarketable (Photo 1). More importantly these larvae are visible to consumers, and detection of infestations can lead to loss of part of your market. Considering it is hard to determine which berries are infested, egg laying by the adults must be prevented to assure a low risk of selling infested berries to consumers.

Blackberry close-up of druplets with spotted wing larvae Photo 1 – Spotted wing drosophila (SWD) larvae feeding on a blackberry. Photo by Dr. Rick Bessin, Univ. of Kentucky.

When is my blackberry at risk of infestation?

Drosophila fruit flies can generally only lay eggs in overripe or damaged fruit which aren’t considered for harvest. SWD are a pest with major impact on berries because they can lay eggs in ripening or ripe fruit that are previously undamaged and marketable. When berries are beginning to turn color, from red to black, their skin begins to soften enough for SWD to successfully lay eggs. This means that blackberries are at risk of infection around 7-10 days before harvest.

When should insecticide management programs begin?

Insecticide management programs should begin no later than 7-10 days before your first harvest. Use traps baited with a yeast+sugar solution or apple cider vinegar to determine whether SWD adults are present in your planting prior to this timing. If adult SWD are present more than 10 days before harvest, begin weekly insecticide applications a week or so early. Be sure not to confuse SWD adults with other fruit fly species that will also be present (Photo 2).

Male and female spotted wing drosophila with key identifying anatomical parts labeled. Photo 2 – Photo of spotted wing drosophila (SWD) adult flies. Males are identifiable by black spots on wings and females by a serrated ovipositor. Other fruit flies that look similar will have neither of these features. Photo and illustrations by John Obermeyer, Purdue University.

Insecticide options for SWD

Insecticides that effectively control SWD are listed in Table 1 below. Four main factors should be considered along with economics when choosing insecticides:

Rotation of Insecticide Classes (IRAC) to Prevent Resistance – Considering the limited number of available products and the risk that SWD infestations pose, it is imperative that producers do everything possible to prevent losing effective insecticides to resistance. Insecticides with the same class (IRAC) should not be used on two consecutive applications.
Conventional vs. Organic production - Entrust is the only effective OMRI approved insecticide that will control SWD. Pyganic can be rotated in to offer some control of adults from a different insecticide class, but due to its lack of residual activity, it should not be considered alone in a 7-day rotation.
Pre-Harvest Interval (PHI) – When considering insecticides in the pyrethroid chemical class, both Danitol and Brigade have a 3-day PHI. Three days between harvests is often hard to achieve and relegates these products to before harvest begins or special cases throughout the season.
Product Use Limits – Many of these products only have a limited number of uses that are allowed throughout the season. This is especially important when considering rotation of effective insecticide classes. Additionally, this is where it will be important to find times where products like Danitol or Brigade could be used, especially if you have many cultivars with an extended harvest period.

Table 1 – Effective Insecticides for Control of Spotted Wing Drosophila (SWD).

Trade Name	Active Ingredient	Class (IRAC)	OMRI	PHI
Mustang Maxx	zeta-cypermethrin	Pyrethroid (3A)	no	1 day
Danitol	fenpropathrin	Pyrethroid (3A)	no	3 days
Brigade	bifenthrin	Pyrethroid (3A)	no	3 days
Malathion	malathion	Organophosphate (1B)	no	1 day
Delegate	spinetoram	Spinosyns (5)	no	1 day
Entrust	spinosad	Spinosyns (5)	yes	1 day
Exirel	cyantraniliprole	Diamides (28)	no	1 day
Pyganic**	pyrethrin	Pyrethrins (3A)	yes	0

** Pyganic offers no residual activity and berries are potentially at risk the day after application.

How to determine if Control Efforts are Effective

It’s important to understand how effective your management efforts have been because failures that slip through can negatively impact market sources. Take a representative sample of your planting consisting of 50-75 berries. Put these berries in gallon sized bags or a metal pan that contains a salt water solution (1 cup of salt per gallon of water). After 15 minutes larvae will float to the top and infestation levels can be assessed (Photo 3).

Raspberry close up with spotted wing drosophila circled in yellow. Photo 3 – Spotted wing drosophila assessment technique using a salt water solution with raspberry. Larvae floating to the top after 15 minutes in the solution are indicated by yellow circles. Photo by Jim Jasinski, OSU Extension.

Spotted wing drosophila (SWD) continues to be one of the biggest issues blackberry growers in Arkansas face each year. Although there is a lot of thought that has to go in to devising an effective and efficient management plan, most growers will find that a high level of control is achievable. If you have any questions about the factors mentioned here please give me a call or shoot me a text at 479-249-7352 or email me at acato@uada.edu. Also be sure check out the Southeast Caneberry IPM guidefrom the Southern Small Fruit Consortium for more information.