Summer Pests

June 28, 2023

As we progress into July, we are seeing the development of Japanese beetle populations. They have hit midway on the GDD calculator for peak emergence, and with more uneven crop stages this year, they may find those early emerging corn silks quite appetizing.

Corn Threshold:

  • Three or more beetles are present per ear 
  • Silks have been clipped to less than ½ inch 
  • Pollination is less than 50% complete 

Soybean Threshold: 

  • % defoliation across the entire field and throughout the entire plant canopy 
  • 30% defoliation warrants insecticide application 
  • Insect defoliation tool >

It is not uncommon for Japanese beetles and/or grasshoppers to be noticed feeding in/around your farmstead, the roadside ditch, and, in our case, a corn or soybean field. Most of these foliar-feeding insects are approaching their adult stage of life and will continue to feed and thrive for about another 30 days. They are more prevalent this spring as their normal “hatch” mortality is low with the dry weather. When scouting, it is important to look at the whole field for injury, not just the field edges where populations are likely higher and do not represent the entire field.  With all vegetative feeding insects, (bean leaf beetles, grasshoppers, Japanese beetles, green cloverworm) thresholds for tissue and leaf loss will vary dependent on the cost of the treatment for control as well as the intended commodity price for the sale of the crop. Given the current commodity prices, treatments would only be justified in soybeans once you reach 20-25% defoliation in plants at/before bloom and 15-20% at full bloom and pod set. See below for a defoliation estimate guide. I have not seen anything close to that level yet. 

Dr. John McNamara // Wilbur-Ellis Agronomist