MSU Extension

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.

With blueberry bloom complete in much of Michigan and beekeepers removing colonies from fields, grower insecticide options for fruitworm control expand. Blueberries are at risk from infestation by cherry and cranberry fruitworm, two moth pests whose larvae have the potential to infest fruit at harvest and which can cause reduced yield if populations are high.

During the past week, monitoring traps have detected increasing catches of cranberry fruitworm across southwest Michigan and cherry fruitworm moths continue to be trapped. Scouting of bushes has revealed fresh cherry fruitworm and cranberry fruitworm eggs on clusters in Van Buren, Allegan, and Ottawa counties, and the first entry holes of larvae into fruit have been found. This emphasizes the need to maintain active management of these pests and continue monitoring in the coming weeks after bloom, because fruitworm activity typically extends throughout June. 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, Danitol, and Sevin are effective broad-spectrum insecticide options available to blueberry growers. With all these products, maintaining good coverage of the 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 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 that has performed well in our recent trials and Delegate which we are testing this season for the first time. See our earlier article in the Fruit CAT Alert from May 20 for a description of these options and their performance characteristics .

Research trials in Michigan have demonstrated that Confirm applied at 16 oz/acre after bloom to fields with low or moderate fruitworm pressure can also achieve control of these pests. This insecticide has 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. We expect similar performance from the recently-registered Intrepid at 12 oz/acre. In trials conducted at commercial blueberry farms over the past few years, a program that used Confirm during bloom followed by Asana post-bloom was effective against fruitworms. We are also testing a Confirm, then Delegate, then Assail program this season. 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.

In fields with a history of high infestation by fruitworms and where traps continue to trap, an additional application of insecticide may be required to protect fruit. The residual activity of the previous insecticide and the amount of rain since the last spray will be critical determinants of the need for reapplication. Residual control under dry conditions ranges from a few days for B.t. up to a few weeks for Guthion, Confirm, and Intrepid. Few insecticide residues can withstand an inch of rain, although Confirm and Intrepid are the most rain-resistant of the current options. While decision-making during this wet time of year can be very challenging, it is important to maintain regular checking of fruitworm monitoring traps, to check the bushes in hot-spots for eggs or larval entry-holes into berries, and to think about the amount of rain since your last spray to protect the fruit.