Insects are the most numerous group of animals on Earth with more than 750,000 different kinds known. This is more than all the other animal groups combined. Fortunately, 95% of all insects are harmless or helpful to humans.
Insect pests in field crops are few and diverse. Some overwinter in crop fields, field borders or nearby woodlands. Others migrate far distances to fields during the spring or summer. Some insects are tracked using pheromone traps, others by calculating accumulative degree days, and others only with timely visual observation while scouting fields.
Regarding corn, soybeans and alfalfa, my Insect Pest Crop Scout Calendar is only concerned with about 24 insect pests, and most of these being infrequent problems. Whether working with field crops or a home’s lawn and garden, its horrible management to apply insecticides to any perceived insect threat without first identifying the problem.
Helpful resources for making decisions
Research has developed economic thresholds for most insect pests. Some are specific to insect numbers per plant or per sweep (using a sweep net), while others are based on percent leaf defoliation or amount of root injury.
Iowa State University’s Integrated Pest Management web page provides threshold calculators for four common insect pests. Other thresholds are defined in other resources including the excellent pest management publication CSI 14, Field Crop Insects, which can be ordered from your local Extension office or the ISU Extension Store.
As mentioned earlier, my Insect Pest Crop Scout Calendar addresses about 24 insect pests as to general scouting windows, degree day guidelines, when the insects may be active, and other scouting tips.
Scouting tips and tools
To take crop scouting seriously, there are a few important tools of the trade:
• basic scouting tools for plant inspection include a knife, spade, hand lens (10x to 20x works well), tape measure and camera
• leaf defoliation photos to estimate defoliation damage (This is available in many publications including on Page 6 in Extension publication CSI 14, “Field Crop Insects.”)
• 15-inch-diameter sweep net with a strong handle (This is the only way to determine thresholds of certain insects, such as potato leafhopper in alfalfa, but it can also be used to quickly survey for other insects in forages or soybeans to determine if closer inspection is needed.)
• calculation method for degree days (For those insects with known degree-day relationships, a weather website allows for inputs of location, start and end date, and degree-day base temperature to track development of those insects.
Schedule adequate time to scout your fields yourself, or contract with a crop consultant or an input supplier to do it or use some combination of all three. Check with local agronomists and Extension specialists and get on the list to receive their e-newsletters.
Also, attend local field days to be aware of developing pest issues and adapt your scouting plan accordingly. Always scout fields first as part of an integrated pest management plan and treat only when needed to protect beneficial insects and to avoid resistance developing to pesticides from over-application.
Lang is an ISU Extension field agronomist at Decorah in northeast Iowa. Contact him at email@example.com.