MSU Extension Blueberries

A few cranberry fruitworm moths have been found at a few sites in the past week, and we expect more widespread captures in monitoring traps as the warm days and nights continue. This means it will soon be time to protect the newly forming blueberry fruit from cherry and cranberry fruitworms. Many fields do not have issues with these pests and some with light fruitworm pressure can be managed effectively with biological or cultural controls. Parasitic wasps can be used to attack their eggs, and tillage can disrupt the overwintering cranberry fruitworm. However, in fields that have a history of economic levels of fruitworm infestation, carefully-timed application of insecticides can greatly reduce infestation.

Cherry fruitworm larvae infest only one berry and these are generally active a week or so earlier than cranberry fruitworm, infesting early varieties soon after fruit set. Where this moth has caused crop losses previously, the fields can should be treated at 100 growing degree days (GDD) after biofix. Biofix for these fruitworms is considered the start of sustained catch (two consecutive checks of the traps). At most Michigan farms, we are generally not at the biofix point yet for cherry fruitworms or cranberry fruitworms.

As fruitworm emergence ramps up with the warm weather, check the traps to set the biofix and then count 100 GDD after the first sustained catch to determine when to protect fields. There is a degree-day model online at Michigan State University's Enviroweather for cranberry fruitworm to help identify the optimal timing for spraying to protect the small berries. Biofix and the target timing for initial egg hatch usually occur during bloom, so it is important to avoid insecticides while bees are working in your fields. The best way to prevent effects on bees is not to spray during bloom. However, if it is necessary for pest control, use the least toxic option and target sprays very early or late in the day when bees are not flying to reduce pollinator exposure to residues.

Insecticides registered for use during bloom or in the presence of pollinators have provided consistent control of fruitworms in our trials at the Trevor Nichols Research Center and at grower fields. These are the B.t. products (such as Dipel, Javelin, etc.), the bioinsecticides Grandevo and Venerate, and the insect growth regulators Intrepid and Confirm. The B.t. products have short residual activity, typically around five days, so they need regular reapplication. These are best applied when daily temperatures reach 70 degrees Fahrenheit so the larvae will actively feed and ingest the insecticide.

Grandevo and Venerate have demonstrated good efficacy against both fruitworm pests in our trials, and both can be used in organic production systems. Intrepid is more resistant to degradation by sunlight than these other insecticides and it is much more waterproof once residues are dry, giving between seven and 14 days activity. However, remember that new berries set after the application will likely not be well protected because they had a flower covering the calyx cup area and the fruit tissue is rapidly expanding, diluting the residue. Confirm is a similar insecticide to Intrepid, but with slightly less activity and residual activity than Intrepid.

Other options for controlling fruitworms during the egglaying period are the growth regulators Rimon and Esteem. These insecticides are ovicides and they also disrupt the adult moth's ability to lay viable eggs, hindering the development of larvae. As with all insecticides, follow the label restrictions if making applications while bees are foraging in the fields, and do whatever you can to reduce the risks to these pollinators, including application late in the evening after bee foraging has stopped. 

After 100% petal fall and removal of bee hives from the field, the range of options for fruitworm control increases, with Imidan, Asana, Danitol, Mustang Max, Cormoran, Lannate and Sevin being some of the available broad-spectrum contact insecticides. With all these products, maintaining good coverage is still important to get residue to the parts of the berry where fruitworms are found, especially as the leaf canopy is expanding at this time. Research trials in Michigan have demonstrated that Reduced-Risk insecticides Intrepid, Assail, Altacor, Verdepryn, Exirel, Entrust and Delegate applied after petal fall can also achieve excellent control of fruitworms, with reduced negative impact on natural enemies such as parasitic wasps, ladybeetles and lacewings.

For all these insecticides, correct timing and coverage are critically important, so regular scouting of fields, using sufficient spray volume to get good berry coverage and selecting appropriate spreader-stickers can increase activity of most insecticides applied for fruitworm control.

Also, be aware that this immediate post-bloom timing seems to also be the best window for controlling gall wasp infestations in susceptible blueberry fields. If that pest is present in your fields, work with your beekeeper to get the honey bee colonies removed from the farm as soon after bloom as possible. Select an insecticide that provides reduction of gall wasp (pyrethroids, Lannate, Imidan, Verdepryn, Exirel) and provides fruitworm control (see below), and plan to make an application as soon as the colonies are removed. Also, be aware of neighboring fields that may still be in bloom with honey bees in the fields. Sprays targeting gall wasp will also benefit greatly from much higher gallonage (60 gallons or more per acre) as well as the inclusion of a penetrating adjuvant (e.g., horticultural oil) to improve the insecticide’s control of this pest.

The table and figure below are designed to summarize several key factors that can help you select an insecticide for your integrated pest management program for fruitworm control in blueberries.