Japanese beetles began to be spotted late last week, and numbers are increasing. Japanese beetles will sometimes concentrate along the edges of fields, pivot tracks, and, this year, possibly mini-edges from drowned out spots. Japanese beetles damage corn by clipping silks prior to and during pollination. The pollination period is SHORT, so you need to be vigilant. Pollen shed for an individual tassel usually takes 2-7 days and 1-2 weeks for an entire field. As a general rule, treatment may be necessary if silks are clipped to less than ½ inch and less than 50% of the plants have been pollinated and 3 or more Japanese beetles per ear are actively feeding (information from Purdue University, https://extension.entm.purdue.edu/fieldcropsipm/insects/corn-japanese-beetles.php).
Japanese beetles typically do not move into the field interior, and border sprays may be all that is needed if you experience the pressure outlined above. Stink bugs can also sometimes be managed by border sprays. Please refer to our Field Corn Insecticide Recommendations if a control measure is necessary: https://cdn.extension.udel.edu/wp-content/uploads/2018/05/02095804/Insect-Management-In-Field-Corn-20181.pdf.
How can you tell when an ear has been pollinated? Here is an informative Youtube video by Dr. Bob Nielsen demonstrating how to perform an ear shake test to determine corn pollination progress: https://www.youtube.com/watch?v=K7DiwD4N0T0&feature=youtu.be.
If the ear has been pollinated, Japanese beetle silk feeding is of no consequence.