Cranberry Fruitworm (Acrobasis vaccinii) is a common pest on blueberries and cranberries in the northern United States. Moths usually emerge and begin to mate during flower bloom periods,  and females lay eggs on berries in their early growth stages. During bloom, bees are active and need to be protected which can limit options for insecticides. In fields with lower pest pressure, post-bloom insecticide applications may be successful in controlling the moth population, but in most cases, applying bee-safe products during bloom will have the most success. Insect growth regulators or Bacillus thuringiensis (Bt) based products are usually safe to apply when bees are present, however, they will have the most impact if the application is timed to coincide with egg laying, but before eggs begin to hatch. This is because the larvae will move into the fruit when feeding, which protects them from sprays. Using the Pest Prophet app and the Cranberry Fruitworm growing degree day model, developed by Michigan State University researchers, can help growers to best determine the optimal time for insecticide treatments.

Initiating the Biofix

The Cranberry Fruitworm growing degree day model will work best when used in conjunction with a monitoring and trapping program for moths. The biofix (point at which to start accumulating growing degree day units) can be adjusted to reflect the first consistent appearance of adult moths in a field. To determine this date, pheromone traps are placed in the field just before bloom starts. At this point in the season, traps should be checked twice a week. Once moths are trapped two consecutive weeks, the biofix date should be adjusted to the date before the first catch. In the Pest Prophet app, this date can be entered and the user should select, “Adult Moth Flight” as the growth stage.

Tracking Growing Degree Day Units

After the biofix is set, growing degree day units (base 50F) should be tracked. This can be done manually, using daily high and low temps or with the Pest Prophet app which can be used to track hourly temperature data for a given field and automatically calculate growing degree day accumulation. Researchers at MSU have determined that egg laying will begin at about 85 GDD (Fahrenheit) units after the biofix, (first sustained adult moth catch). Applying insect growth regulators, or B.t. around this time, followed by another application in 2 weeks have proven to be successful at controlling Cranberry Fruitworm populations. Waiting too long to after 85 GDD may result in some of the larvae being protected in the fruit, while spraying too early might result in poor results if not enough larvae and eggs are present in the crop.

Source:

Michigan State University Extension Newsletter. May 5, 2009.