Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu
Alfalfa
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 – 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
Although corn planting is delayed this year, you will 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 currently higher than their rolling 5 year average. (http://www.uky.edu/Ag/IPMPrinceton/counts/bcw/bcwgraph.htm). 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.
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 is often 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, cereal leaf beetles, armyworms and grass sawfly. Low levels of cereal leaf beetle adults and the first egg laying has 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: http://www.uky.edu/Ag/IPMPrinceton/counts/taw/tawgraph.htm
Trap counts in Kentucky at the end of last week for true armyworm were significantly greater than their rolling 5-year averages. If the armyworm counts continue to increase at the current rate, the population will approach the outbreak levels seen in 2006 and 2008 in Kentucky.