Preparing for fruitworm management in blueberries

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. 

In the past week, monitoring traps in commercial blueberry farms in Van Buren County have caught their first cranberry fruitworm (CBFW), but only single moths have been seen in a few traps. Cherry fruitworm (CFW) moths have been trapped for a few weeks in the same region, but also at very low numbers (one or two moths per traps per week). These first captures indicate the flights of these important early-season insects are just beginning, but this is a little too early and the number of moths is too low to warrant any insecticide sprays in response. We have another series of cool, wet days ahead with predicted night-time lows in the 40s through the coming weekend. Under these conditions, temperature-based development of insects is slowed and moths are unlikely to fly when it is cold, windy and wet. This means that mating and egg-laying are also unlikely.

Monitoring traps should be checked again this week and again when it warms up next week. We should then expect the rest of the CBFW and CFW flights to start. Once a consistently increasing number of moths of either species are being caught in monitoring traps and early varieties are entering the susceptible stage when blooms start dropping and fruit set begins, these fields should be considered for protection against fruitworm larvae. Monitoring for the presence of fruitworm eggs on fruit can be used to better indicate the time when fruit protection is needed, though these are small and difficult to see.

Growers typically can manage both fruitworm pests together, but in recent years when there has been a cool period during blueberry bloom, the timing of CBFW and CFW have not overlapped. Instead, the earlier cherry fruitworm went unnoticed and the larvae were already inside fruit when CBFW control programs started. Monitoring for both insects in hotspots on the farm is a good idea, as insurance against missing one of these early pests. This is particularly important in early varieties where it is most likely that the fruit will be picked when CFW larvae are still developing inside the fruit.

There is an array of insecticides available for control of fruitworms, but their performance characteristics are not all the same, and only some of them can be used during bloom. During bloom, options for control are limited due to the need to protect bees. Two products registered for use during bloom and/or in the presence of pollinators have provided consistent control of fruitworms in trials at the Trevor Nichols Research Complex and at grower fields. These are the B.t. products (such as Dipel ® and Javelin ®) and the insect growth regulator Confirm ®. These products must be actively eaten by the larva to be effective, so they are best applied when daily temperatures are likely to reach 70 °F. This may be difficult during the predicted weather this year, so take note of this limitation when selecting when to use these products. B.t. products have short residual activity, providing up to five days active residue depending on the weather conditions. Confirm is more resistant to breakdown, giving between seven and 14 days activity, and it is quite rainfast, which can be a useful property in Michigan spring weather. These products are most effective when applied on top of fruitworm eggs (see table), before larvae hatch, so they are eaten as the larvae emerge from the egg. Another option for control of cranberry fruitworm is the growth regulator Esteem ®. This insecticide disrupts the adult moth’s ability to make eggs and disrupts hatching of eggs and molting of larvae and is most effective when applied just before egg-laying. When thinking about application timing during bloom, getting the most out of your insecticides will require close scouting of fields with high fruitworm pressure. As with all fruitworm control applications, excellent coverage of fruit clusters is required to ensure that eggs and/or larvae come in contact with the insecticide.

After bloom, the range of options for fruitworm control increases with Guthion ®, Imidan ®, Asana ® and Sevin ® being the most effective of the broad-spectrum insecticide options. With all these products, maintaining good coverage is still important to get residue to the parts of the berry where fruitworms are found. Recent research trials in Michigan have demonstrated that Confirm ® and SpinTor TMapplied after bloom to fields with low or moderate fruitworm pressure can also achieve control of fruitworms with minimal negative impact on natural enemies such as parasitic wasps, ladybeetles and lacewings.

Correct timing and coverage are critically important, so regular scouting of fields, use of sufficient spray volume to get good fruit coverage and selecting appropriate spreader-stickers can increase activity of most insecticides applied for fruitworm control.

The accompanying table and figure are designed to summarize several key factors that can help you select an insecticide for your Integrated Pest Management program.

Details of insecticide options and timing for fruitworm control in blueberry

Compound Trade Name

Chemical Class

Life-stage Activity

Optimal Spray Timing

Pollinator/Parasitoid Toxicity rating *

Guthion/Imidan

Organophosphate

Eggs, Larvae, Adults

100% Petal Fall

H

Lannate/Sevin

Carbamate

Eggs, Larvae, Adults

100% Petal Fall

H

Asana

Pyrethroid

Eggs, Larvae, Adults

100% Petal Fall

H

SpinTor/Entrust

Naturalyte

Eggs, Larvae

Early fruit set

over/under eggs

M

Dipel

B.t.

Larvae

Early fruit set

over eggs

S

Confirm

Growth regulator (MAC)

Eggs, Larvae

Early fruit set

over eggs

S

Esteem

Growth regulator (juvenoid)

Eggs, Larvae

Early fruit set

under eggs

S

* Pollinator/Parasitoid Toxicity rating; S – relatively safe, M – moderate toxicity, H – Highly Toxic

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