Insecticide options for spotted wing drosophila control in blueberry
There are many options available for spotted wing drosophila (SWD) control in blueberry, and some new ones.
Spotted wing drosophila (SWD) remains a key pest of blueberry, but it can be effectively controlled through implementing a pest management program that includes effective insecticides. Over the past decade, we have learned a lot about approaches that result in control of this pest, and growers are able to prevent infestation through adoption of some key tactics that help ensure harvest of insect-free berries.
1. Monitoring fields
Monitoring traps can help identify the start of SWD flight (already being trapped in southwest Michigan at low levels and expected to pick up in the coming few weeks). Traps in wild habitat or in regions with a history of high pressure can provide an early warning of SWD activity. For most early ripening varieties, pressure from this pest remains low with the potential to harvest Weymouth, Duke, and early Bluecrop berries with minimal pest pressure. One way to have confidence in this is to run fruit samples through a salt test to confirm that the fields are free of infestation. Adding fruit samples to your monitoring will provide an added layer of information that informs your IPM program.
2. Selecting effective insecticides
To protect ripening berries from SWD infestation, apply effective insecticides when the fruit are ripening or ripe and when there is SWD activity. SWD females can lay their eggs into the berries as soon as they start to turn color, with the survival of larvae increasing as the berries turn blue and become easier to feed in. The table below provides an overview of the main options for SWD control. Also, consult Michigan State University Extension’s E154 Michigan Fruit Management Guide for information on the listed insecticides.
The table shows multiple insecticides ranked as “excellent,” and conventional growers have been highly successful with maintaining control through a program that rotates among the most effective pesticides. Rotating among the chemical classes is important for minimizing the risk of resistance development. While price and longevity are important considerations, using products that are ranked with 3 or 4 stars in this table is important for preventing infestation and maintaining control.
For organic growers, field monitoring and Entrust are the foundations of an SWD program, with another insecticide used in rotation according to the Entrust label direction. For this, growers have some options including Pyganic, Grandevo and Spear-T. Pyganic has very short residual activity so it should be followed within three to four days with another Entrust to maintain control. The Entrust-Grandevo rotation has been working well for many growers. We are gaining more experience with Spear-T, and this should be applied with high pressure and a surfactant to generate a fine mist that coats the insects as it if more effective when delivered this way.
|Key insecticides for SWD control in blueberry|
|Trade name||Class||Active ingredient||Rate||Season max.||Max apps.||Days between spray||PHI* (d)||REI** (h)||Residual (d)||Rank|
|Imidan*||Org. phos.||phosmet||1.33 lb 2.5 pt||7.13 lb||5||0||3||24||10-Jul||****|
|Malathion 8Fa||Org. phos.||malathion||4 oz||5 pt||2||5||1||12||5||***|
|Mustang Maxx||Pyr.||z-cypermeth.||10.6-16 oz||24 oz||6||7||1||12||7-May||****|
|Danitol||Pyr.||fenpropathrin||5.3-16 oz||32 oz||2||14||3||12||7-May||***|
|Lannate SP*||Carb.||methomyl||0.5-1 lb||4 lb||4||3||3||48||7||****|
|Exirel*||Diamide||cyazypyr||13-20.5 oz||60 oz||-||5||3||12||7||****|
|Verdepryn*||Diamide||cyclaniliprole||11 oz||33 oz||-||5||1||12||14||****|
|Delegate*||Spin.||spinetoram||3-6 oz||19.5 oz||6||3||3||4||7||****|
|Entrust WP*||Spin.||spinosad||1.25-2 oz||9 oz||6||6||3||4||5-Mar||**|
|Entrust 2SC*||Spin.||spinosad||4-6 oz||29 oz||6||6||3||4||5-Mar||**|
|Assail*||Neonic.||acetamiprid||5.3 oz||26.6 oz||5||7||1||12||7-May||**|
|Grandevo||Biologic||C. subtsugae||3 lb||-||-||-||0||4||5||**|
a Malathion 8F (Gowan) has a 24c label for Michigan blueberries.
* Retains relative efficacy in high heat.
Weather effect on recommended insecticides
In selecting the appropriate insecticide, consider the prevailing temperatures at the target site during and after the spray application. This is especially important under Michigan’s summer conditions when most berry crops are harvested and SWD control is critical.
Some of the recommended insecticides for SWD management are positively affected by high summer temperatures; they are more toxic at high temperatures. That is the case of OP insecticides like Imidan. However, Malathion is adversely affected by high temperatures and very rapidly loses its effectiveness with temperatures above 80 degrees Fahrenheit. Similarly, most pyrethroid insecticides are affected by summer’s high daily temperatures. Above 90 F, their toxicity decreases substantially. There is one exception to this, the insecticide bifenthrin (Brigade, Bifenture, etc.). This pyrethroid responds to high temperatures in the same manner as OP insecticides, retaining activity at high temperatures. Therefore, check your daily weather forecast and MSU Enviroweather to select the best insecticide for the predicted weather conditions.
3. Making the most of sprays
For selected pesticides to do their job most effectively, it is important they reach the target. Since SWD prefers to inhabit shady places where humidity is highest, it is critical that applications reach through the whole canopy. Adjust ground speed, spray volume and nozzles to ensure that the sprays get to the target. Water sensitive paper can help with this, and there is some good information on how to improve coverage in the article, “Crop-Adapted Spraying – A Multiyear Study in Highbush Blueberry (2021 Update).”
Typically in August the pressure from SWD increases and later-harvested blueberry cultivars face more risk from this pest. Combined with rainfall and more humid conditions the SWD populations can increase quickly over a few weeks. It is critical at this time to maintain control, through reapplication after rainfall. To help determine how much rainfall requires reapplication of different products, MSU research has tested this question for various insecticides registered in blueberries. This information is summarized in a table presented in the MSU Extension article, “Rainfast characteristics of insecticides on fruit.” You will need to scroll down to find the table with blueberry and SWD information, showing for six key insecticides how well they can withstand 0.5, 1 or 2 inches of rain. For Imidan, Mustang Maxx and Lannate, they will still provide protection for a day (but not a week) if there is an inch or less of rain, but none of the insecticides will last for a week if there is rain. The take-home message is that if there is high SWD pressure and a recently-treated field gets rain with multiple days remaining until harvest, then it should be retreated to maintain protection.
Growers have recently increased the use of Jet-Ag and other sterilants in harvest-time applications, providing pathogen control and also reducing some of the microbes that SWD is attracted to. This is also acidic so it can provide the pH buffer needed for some insecticides. Growers also report erosion of their spray hardware using this, so that’s a good reminder to check and maintain sprayer tips and to flush sprayers out after use.
4. Combining strategies
Effective SWD control requires close attention to the pest population, the weather, and the crop stage and planned days to harvest. It is a logistical challenge to manage this aspect of blueberry production through to harvest, but with planning and selection of effective strategies for control of this pest blueberry growers will be able to harvest berries that meet the market requirements.