We spent a fair amount of time scouting wheat fields for cereal leaf beetles earlier in the season. Now it is time to focus attention on adjacent corn fields.

The adult cereal leaf beetles can now be found in adjacent corn fields. They are primarily found on the perimeter of the corn fields. Though I have encountered them throughout some corn fields. Adult cereal leaf beetle damage appears as longitudinal slits between the leaf veins. Corn plants usually outgrow the injury. Unlike larval damage in wheat, there are no firm thresholds for adult damage in corn. One threshold suggests treatment when 10 or more adults are found per plant and 50% of plants show feeding damage.

Cereal Leaf Beetle Adult
Courtesy Virginia Tech Cooperative Extension

By Mark VanGessel, Extension Weed Specialist; mjv@udel.edu

Working with Bill Bamka, County Extension Educator from Burlington County NJ, UD Weed Science Program confirmed that a population of common ragweed in New Jersey is resistant to glyphosate. The long-term history of the field is not known, but the past couple of years it was planted with soybeans. Last year, the preemergence herbicides did not provide adequate control and multiple applications of glyphosate were used postemergence. This is not the first confirmation of glyphosate-resistant common ragweed in the region; but the first in New Jersey. We have no reports of common ragweed resistant to glyphosate in Delaware.

In this situation, rotating to corn and use of atrazine is the best option. When rotating back to soybeans, use no-till because germination will be less than if tillage is done. The field will need to be treated with a preemergence application of cloransulam (i.e. Sonic or Gangster) or metribuzin (Tri-Cor, Boundary, or Canopy, generics). But there will definitely be a need for a postemergence herbicide and the best options would include Blazer Ultra, Cobra, Reflex, and Liberty (with Liberty Link soybeans). If cloransulam was not used at planting, FirstRate or Permit Plus (with STS soybeans) would also be options.

Source: Read More from Weekly Crop Update