I have observed corn at the V1-V3 growth stage and soybeans ranging from just planted to V1 growth stage. Some soil crusting has been observed in recent scouting trips in soybean fields that were planted before the recent rainfall. In a few fields, I have noted replants of areas due to ponding or severe crusting. Grass and broadleaf weeds have been noted in recent scouting trips and I have noticed some early applications of corn pre herbicides in those cases. With the adverse weather conditions, scouting is important to identify field by field conditions.
Insects to scout for this spring include black cutworm in corn, and alfalfa weevil in alfalfa.
Black cutworm migrates into northwest Iowa via moth flights and with spring storms. The moths lay their eggs near the crop field and the larvae can cut corn seedlings when they reach the 4th instar stage. At the 4th instar, the black cutworm is large enough to cut corn plants and feed on leaves up until the V5 growth stage.
Moths are trapped in Iowa using pheromone traps to help understand when to start scouting for black cutworm. When a significant flight is trapped over a two-night period, calculating 300 GDD past that flight will give an indication of when black cutworm should start to be scouted in that area. This year in northwest Iowa the date of first possible cutting is after May 16th. Black cutworm moths are still migrating and being caught in pheromone traps thus scouting until V5 will be important as the flights are spread out and ongoing. Preventive insecticide treatments are not recommended for treatment of black cutworm but scouting and rescue treatments may be warranted if damage above thresholds have been discovered. Some Bt hybrids provide black cutworm suppression.
Alfalfa weevil is another pest that has been found in Iowa this spring. Reports are coming out of east central and south-central Iowa of heavy pressure in alfalfa fields. Larvae of alfalfa weevil are yellow-green in color with a white stripe along the middle of the back. Pinholes observed in alfalfa leaves can be an indication of first instar larvae feeding. Third and fourth instar larvae cause the most feeding injury. A heavily infested alfalfa field will appear frosted or silver due to dying leaf material. Typically, the first cutting suffers the greatest injury.
Additional information on black cutworm and alfalfa weevil can be found on the Iowa State University Extension and Outreach Integrated Pest Management website (https://www.ipm.iastate.edu/) or by contacting your local field agronomist.