Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu
Alfalfa
Continue to scout fields for both alfalfa weevil and pea aphids. 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 60 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. Although beneficial insects can help to crash aphid populations, the recent cooler temperatures have slowed their activity.
Field Corn
As soon plants emerge, be sure to check for cutworm feeding, even if an at planting insecticide or a Bt corn was used for cutworm control. In Delaware, a number of cutworm species may be present at planting, including the dingy cutworm. In Delaware, black cutworm populations result from local overwintering populations as well as moths migrating to the area, generally in early March. Factors that favor black cutworm outbreaks include late planting, heavy infestations of winter annual weeds before tillage and planting, reduced tillage, and corn grown after soybean. Fields with a combination of these factors are more attractive to migrating moths and are likely candidates for egg laying. They should be monitored closely as corn emerges. Young larvae will feed on plants, resulting in small, irregular shaped holes. Black cutworms generally begin cutting plants at the fourth instar. One cutworm larvae can cut an average of three to four plants during its lifetime. In certain years and locations we can also find cutworms damaging very early emerging corn. In most cases, this damage was probably caused by the clay backed cutworm or the dingy cutworm. These species overwinter as half-grown larvae in the soil so they can get a “jump” on black cutworms. Regardless of species, as a general guideline, a treatment should considered in 1-2 leaf stage corn if you can find plants with 10% leaf feeding or 3% cut plants.
Small Grains
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 and cereal leaf beetles.