May 19, 2016

Pruning key in managing blueberry stem gall wasp

Blueberry stem gall wasp is native to eastern North America, where it can infest new and actively-growing blueberry stems during bloom. It can be found in lowbush and highbush blueberries, and in managed and unmanaged locations. Levels of infestation of blueberry stem gall wasp have increased in recent years, and we are learning more about how best to control it.

The adult wasps emerge from the galls during bloom, and we have observed early emergence in Allegan County this week in blooming Jersey fields. Emergence will continue through bloom and extend until late bloom. Wasps lay eggs under the surface of the stems, and in susceptible varieties the developing larvae cause the shoots to swell after bloom, causing a gall. These are green and hard, kidney-shaped swellings that turn brown in the fall. Jersey and Liberty varieties are the most susceptible, Bluecrop is highly resistant. Some other common varieties may show low levels of infestation, but these are much lower than the susceptible ones. Galls can be up to 2 inches in diameter, and they contain many developing larvae that feed and grow during the summer, overwinter as larvae, pupate in the gall in spring then emerge during bloom.

Michigan State University has developed a blueberry stem gall wasp degree-day model on the Enviro-weather websitethat can help predict emergence of blueberry stem gall wasp from galls. As of May 18, the Fennville Enviro-weather station indicates first emergence was on June 10, and with the predicted temperatures over the next week, we expect strong emergence of gall wasps over the next week. Checking galls for emergence holes can show when emergence starts, though some of the emerging insects might be natural enemy insects. We are learning more about those, and have found three species that come out of these galls. Their role in regulating gall wasp populations is still under investigation.

MSU Extension advises the importance of including pruning to remove the galls as the foundation of your management program. If infestation is developing in your fields, pruning can be coupled with chemical control that is focused immediately post-bloom. Using effective insecticides at this timing kills wasps that are still actively flying, plus it penetrates the stems of the bush and prevents recently-initiated galls from growing any further. This timing is also best for avoiding honey bee kills – talk to your beekeeper about removing their colonies right after bloom of your Liberty and Jersey fields to give you the option of using insecticides that cannot be used during bloom.

Reducing the number of galls through pruning and keeping the galls small through effective sprays results in a direct reduction in the number of wasps emerging the next year. Managing blueberry stem gall wasp in susceptible varieties is a multi-year commitment. The best results are expected in fields that receive a combination of pruning in the dormant months and protection of young shoots in the immediate post-bloom timing.

Our recent research has shown minimal benefits of pre-bloom sprays against blueberry stem gall wasp. This timing is challenging to manage around beekeeper activities, it can pose a significant risk to honey bees, plus sprays at this timing have usually worn off before the wasp activity is very high. We have also not identified any insecticide that is effective against blueberry stem gall wasp but also safe to bees, so applications during bloom against this pest are not recommended. Instead, we have found better control with a focus on post-bloom applications of highly active insecticides.

Gall wasp spray program

• Make sure honey bees are removed from fields. Growers needing to treat fields for blueberry stem gall wasp should talk with their beekeeper and neighbors to plan for quick removal of honey bees that will allow immediate post-bloom application once those honey bees have been removed.
• Apply effective insecticide post-bloom. In 2015, the greatest reduction of gall numbers and size was seen in fields treated with an effective insecticide applied at high volume with a penetrant:
  • Water volume at 60-90 gallons of water per acre.
  • Using an effective insecticide. We found that Lannate was highly effective and so were the pyrethroids including Mustang Max, Brigade, Asana, Danitol, etc. One grower tested Exirel at 13.5 ounces per acre and saw good control of blueberry stem gall wasp too.
  • Penetrating adjuvants can help the insecticide move into the stem tissue to reach the eggs and larvae. This can be a low (0.5 percent by volume) rate of a refined horticultural oil such as JMS Stylet Oil or a specific penetrating adjuvant such as Wetcit, Exit or Dyne-Amic.
• Reapplication. In highly infested fields, reapplication after seven days is recommended to help ensure the full period of activity is covered. Product selection can be from the list above, keeping in mind the label restrictions and seasonal limits. Also consider what sprays will be needed for spotted wing Drosophila control later in the season.

— Rufus Isaacs, and Steve Van Timmeren, Michigan State University

Source: Michigan State University Extension