Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu
Alfalfa
Continue to scout fields for both alfalfa weevil and pea aphids. Both can be found in fields throughout the state. Fields should be scouted for these two insect pests until the first cutting. Examine 5‐10 stems for damage and weevil larvae 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 – 15 inches tall – 1.5 per stem; (d) 16 inches tall – 2.0 per stem and (e) 17 – 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.
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 Treatment for bird management in field corn is still available. The 24 (c) label and use directions should be on the pesticide canister.
Over the last week, we have started to see an increase in the number of marsh slugs encountered under shingle traps and slug eggs can now be found under last year’s crop residues. Marsh slugs can cause damage to newly emerging corn; however, the most damaging species is the grey garden slug. Although we generally see more 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.
Small Grains
In general, insect activity continues to be low in barley and wheat. Cereal leaf beetle adults, eggs and the first small larvae can now be found. Although we are seeing more egg laying compared to this same time last year, cool weather will slow egg laying and egg hatch. Cereal leaf beetle populations are often unevenly distributed within a field so it is important to carefully sample so that you do not over or under estimate a potential problem. Eggs and larvae are also found on leaves near the ground so careful examination of the entire plant is critical. The treatment threshold is 25 eggs and/or small larvae per 100 tillers, with more larvae than eggs present to justify a spray. For more information on timing sprays for cereal leaf beetle please check the following link from North Carolina http://entomology.ces.ncsu.edu/2015/04/should-you-spray-cereal-leaf-beetle/