Cranberry fruitworm management in blueberry

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

Cranberry fruitworm is one of the key insect pests of blueberry in Michigan, infesting the crop during and after bloom. Moths usually start flying during bloom, and this year is no exception. The first male moths have been trapped over the past week in Van Buren and Allegan counties, coinciding with peak Jersey flowering.

Once moths are flying and petals start falling off young fruit, growers should protect these blueberry fields to prevent crop infestation by the larvae that bore into berries and web them together. The aim of managing this pest is to minimize the number of larvae that bore into the fruit, but timing sprays for fruitworms can be challenging in some years. MSU entomologists have developed a simple degree day model to help growers know when to start protecting berries from fruitworm infestation. Using degree days to make sure you don’t miss the start of fruitworm flight is expected to improve the effectiveness of your insect management program. Implementing degree-day based management for cranberry fruitworm requires the following:

  • Monitoring traps to detect moth flight and biofix .
  • A method to track insect development.

Monitoring traps

We recommend the sturdy large plastic delta trap to monitor fruitworms, because these withstand rain and irrigation intact, plus they can be used for multiple years. Place one to two traps per field near historical fruitworm hot spots or near woods next to fields. Place the trap baited with a lure containing the fruitworm sex pheromone in the top third of the bush. Moths are predicted to start flying at 375 degree days (base 50) after March 1. Because of this timing, traps should be in place by the start of Bluecrop bloom, and checked twice each week until moths are trapped. Regular checking allows you to detect the first sustained catch of moths (biofix), the peak of moth activity, and how long moths are active. First sustained catch is when one or more moths are trapped in consecutive trap visits. The biofix (point at which degree days should start being counted) is the date immediately before this, i.e. the date at which zero moths were trapped, right before the start of the flight.

Degree day model

Our research has shown that egglaying by cranberry fruitworm starts between 80 and 100 GDD days after biofix. The MSU Enviroweather program now includes a cranberry fruitworm model page, accessible online at Degree days are tallied automatically for the numerous weather stations across Michigan, and this system also predicts degree days totals for the week ahead. This can allow growers and consultants to look at when the target degree day accumulation is expected to be reached, helping to plan sprays ahead of time.

If you have not used degree days in your pest management program before, there are some useful resources online to explain them. One is at: Your local MSU Extension educator will also be able to help with how to monitor degree days on your farm, or how to access information from the nearest weather station.

A short note on cherry fruitworm

There is no degree day model for this pest in blueberries, but we have been trapping this insect in the past few weeks. In fields that have experienced infestation in past years and where moths have been trapped this spring already, protection of the young fruit should be considered as petal fall starts to expose the fruit to egglaying.

Preventing frutworm infestation

Fields requiring protection against cranberry fruitworm should be treated using an insecticide applied to achieve excellent coverage of the berries. This will improve the chance that larvae are controlled. In high pressure fields, the first application is usually during bloom, so growers should use the bee-safe insecticide Intrepid (at 12 oz/ac) or aB.t. based product such as Dipel or Javelin starting at 50-100 GDD after biofix. Follow label directions regarding bee safety. A follow-up spray may be needed, with the timing of this depending on the residual control provided by the first spray, the amount of new fruit-set since the first application, whether bees are still in the field, and the amount of rain.

Once bees are removed from the fields, broad spectrum insecticides become an option that growers can consider for protecting their berries from fruitworm infestation. Guthion, Imidan, Lannate, Asana, Mustang Max, Danitol, and Sevin are effective broad-spectrum insecticide options available to blueberry growers. With all these products, maintaining good coverage of the fruit clusters is still important, to get residue to the parts of the berry where fruitworms are found, such as in the calyx cup where eggs are laid. The larvae of the two fruitworm species chew into the berries in this location, with cranberry fruitworm larvae preferring to enter berries at the stem end. Because these insects move over such a small distance, it is important to use sufficient water and to consider spray additives (spreader-stickers) that will help spread the material across the berry surface.

EPA’s phase-out of Guthion will remove this insecticide from blueberry production by the end of 2012. Given the current reliance on this chemical for fruitworm control, it would be wise for growers to test alternative programs on a few fields this season, so that an effective fruitworm control program is in place when Guthion is completely restricted. There are many options for chemical control of fruitworms, including some recently-registered products such as Assail and Delegate that have performed well in our recent trials at research stations and at commercial farms.

Research trials in Michigan have demonstrated that Intrepid or Confirm applied after bloom to fields with low or moderate fruitworm pressure can also achieve control of these pests. These insecticides have the benefit of minimal negative impact on natural enemies such as parasitic wasps, ladybeetles and lacewings, plus long residual activity because of resistance to wash-off and ultraviolet breakdown. In trials conducted at commercial blueberry farms over the past few years, a program that used Confirm or Intrepid during bloom followed by Asana post-bloom was similar in performance to a Confirm, then Delegate, then Assail program, and these were similar to performance of a Confirm then Guthion program. For organic growers, formulations of B.t. such as Dipel, Javelin, etc. and the spinosyn insecticide Entrust provide good control, but they must be reapplied every four to five days, and they are not resistant to wash-off.

The work of Dr. Isaacs and Dr. Wise is funded in part by MSU‘s AgBioResearch.

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