Gall wasp management guidelines in blueberries for 2017
Gall wasp emergence in Michigan blueberries is starting.
In the past few years, blueberry stem gall wasp has become more common in some Michigan blueberry fields, and Michigan State University researchers have been working to find effective controls for this pest. With emergence of the wasps starting this week in southwest and west-central Michigan, it is a good time to review what tactics can provide gall suppression.
The main varieties affected are Jersey and Liberty, though there are low levels of galls showing up on some other cultivars. Focus control on fields with galls, but keep monitoring through your farm to see if other fields are infected. Planting a non-susceptible cultivar and pruning galls in infected fields are both important approaches to managing this pest, but for growers that already have many galls in their fields, a chemical control approach can help reduce the infestation.
Our trials indicate there are four keys to reducing gall density and size through the chemical control approach.
First, if blueberry fields are infested with gall wasp, use pyrethroids Mustang Maxx, Asana, Brigade or Danitol, the organophosphates Imidan or Diazinon, the carbamate Lannate or the diamide Exirel. Those insecticides also have efficacy against cherry and cranberry fruitworms. Apply these insecticides at petal fall, immediately after the honey bee colonies are removed from the fields. The most effective insecticides we have tested are the pyrethroids Mustang Maxx, Asana, Brigade and Danitol, the organophosphates Imidan and Diazinon, and the diamide Exirel. These are all highly toxic to honey bees, so they can only be applied after bloom and when the bee colonies have been removed from the area. Remember that honey bees forage over many miles, so make sure these insecticides do not drift onto nearby fields that might still be in bloom.
Second, because gall wasp lays its eggs into the young shoots, it is important the insecticide penetrate the plant tissues. Use a penetrant such as a light summer oil or other product to help improve control. A low rate (0.25 percent by volume, or 1 quart per 100 gallons) should be sufficient to have the desired effect.
Third, use higher gallonage than usual. In all our gall wasp trials on farms, we see smaller galls and fewer galls when growers use more than 50 gallons per acre.
Fourth, if you are managing a highly infested field, retreating seven days after petal fall will be important to cover the span of gall wasp pest activity. Walk the fields and see whether wasps are still active and if there are galls starting to develop (reddish swellings on the new growth). Applications at this timing have been able to stop these galls from growing, indicating they are killing the wasp eggs and young larvae before they can fully form a gall.
This post-bloom timing lines up well with timing for cranberry and cherry fruitworm control, and each of the insecticides mentioned above are effective on fruitworms in addition to gall wasp. The selective insecticides Intrepid, Confirm and B.t., which are used during bloom to control fruitworms, will have no effect on gall wasp.
Gall wasps are emerging this week in southwest and west-central Michigan during the warm weather. The recent rainfall is also likely to have triggered emergence because the galls become softer after rain. This new flush of wasps will be active through bloom while bees are still active, and as mentioned above, protecting bees should be a priority of all blueberry growers since pollination is so important for your crop yields.
There are very few options for gall wasp control during bloom, but this season we will be testing Sivanto, a new butenolide insecticide that has received a 2ee label for suppression of gall wasp. This has low toxicity to honey bees, but we still recommend application late in the evening to reduce exposure, and do not tank-mix with azole fungicides. The label allows for two applications of 14 ounces each, which must be made seven days or more apart.
The MSU Enviroweather system contains a degree-day model for gall wasp that is predicting first emergence in West Olive, Michigan, last Friday, May 12, but taking the time to check your fields is the best way to see if emergence from galls has started in your farm (small round holes on last years’ galls are the tell-tale sign). For growers planning one application of Sivanto, timing it to cover the tail end of bloom a week before you expect the bees to be removed should provide the greatest benefit. If two applications are planned, the first could be made a few days after the wasps have started to emerge, followed by the second application seven days later to cover the rest of bloom.
Controlling gall wasp effectively will be a multi-year effort as none of the treatments we have tested have shown 100 percent control under field conditions. As MSU research develops more insights into how to prevent this pest, we will alert growers through the MSU Extension programs.
Dr. Isaacs work is funded in part by MSU’s AgBioResearch.