Agronomic Crop Insect Management – April 15, 2016

Joanne Whalen, Extension IPM Specialist;

Continue to scout fields for both alfalfa weevil and pea aphids. The first weevil larvae have been found at very low levels. Fields should be scouted for these two insect pests until the first cutting. Examine 5‐10 stems for damage and weevil larvae. A full stem sample is not needed until damage or larvae are found on the plants. Once larvae are found, a decision to treat should be based on collecting a minimum of 30 stems throughout a field and checking for the number of larvae per stem. The following thresholds, based on the height of the alfalfa, should be used as a guideline when making a treatment decision for alfalfa weevil: (a) up to 11 inches tall – 0.7 per stem; (b) 12 inches tall – 1.0 per stem; (c) 13 to 15 inches tall – 1.5 per stem; (d) 16 inches tall – 2.0 per stem and (e) 17 to 18 inches tall – 2.5 per stem. The following thresholds should be used as a guideline when making a treatment decision for aphids : (a) alfalfa less than 10 inches tall treat if you find 40-50 aphids per stem, and (b) alfalfa 10 inches or taller in height treat if you find 75- 100 per stem. Information on identification, damage symptoms, treatment thresholds and options for insect management in alfalfa can be found at the following links:

Field Corn
As soon as plants emerge, be sure to watch for cutworm, slug and bird damage. You can distinguish bird damage from cutworm damage by the pattern in the field. With bird damage, you generally see longer strips of damaged plants, plants pulled out of the ground, and/or plants cut high that are compressed at the base of the stems. Although birds can cut plants off at the soil surface, they tend to pull plants out of the ground. In addition, if you look closely you should see “bird prints” near the missing plants or holes where birds have pulled plants out of the ground. In Delaware, the 24(c) Special Local Needs Registration for Avipel Hopper Box (dry) Corn Seed is still available for the 2016 season and expires on July 1, 2016. A copy of the 24C label must be in your possession to use this product. In past years, the 24 (c) label and use directions were on the pesticide canister. However, if this is not case you will need to contact Chris Wade at the Delaware Department of Agriculture for a copy of the label and/or for additional questions (

Although we generally see more slug damage on seedling corn when conditions remain cooler and soil remains wet, significant egg hatch of grey garden slugs generally occurs during warmer days in April and early May. Although no thresholds are available, in the past levels of five or more grey garden slugs per square foot have indicated the potential for a problem

You will also need to scout for cutworm feeding as soon as plants emerge, even if an at-planting insecticide, seed treatment or Bt corn was used for cutworm control. Depending on when you plant, a number of cutworm species may be present at planting, including the black cutworm, dingy cutworm and clay backed cutworm. In Delaware, black cutworm populations result from local overwintering populations as well as moths migrating from areas in the south. Populations in Kentucky are slightly higher than this time last year ( Remember, this should be used as just an early warning sign since spring temperatures and weather conditions also have an impact on the size of the population and time of egg hatch.

Small Grains
In general, insect activity remains light in fields throughout the state. As we see a return to warmer temperatures, be sure to scout fields on a weekly basis for aphids, cereal leaf beetles, armyworms and grass sawfly. Low levels of cereal leaf beetle adults, eggs and larvae have been found in an occasional field throughout the state. As far as armyworm, a combination of local overwintering and migratory populations can cause potential problems in small grains. The following link to the University of Kentucky’s website provides an idea of the potential size of the migratory population

Trap counts in Kentucky at the end of last week for true armyworm were lower than their rolling 5-year averages.

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