Agronomic Crop Insects

Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu

Alfalfa
Economic levels of alfalfa weevil continue to be found in many fields throughout the state so be sure to sample all fields for larvae. If alfalfa is in the full-bud stage and economic levels are present, early harvest is an alternative to spraying. However, if harvest is not possible within 3 days and populations are increasing, use a short residual insecticide. If economic levels were present before cutting, be sure to check re-growth for larvae and feeding damage within a week of cutting. In recent years, cool conditions after first harvest have not produced enough “stubble heat” to control populations with early cutting. In some cases, damage to re-growth can be significant. A stubble treatment will be needed if you find 2 or more larvae per stem and the population levels remain steady.

Field Corn
As corn emerges from the ground, you will want to watch for cutworm activity as well as slugs. Last week’s cutworm traps catches (the last of the season) were high in many locations throughout the state (http://ag.udel.edu/extension/IPM/traps/currentbcwtraps.html). Due to the warm winter, slugs can be easily found under surface residue in no-till situations. With the recent rains, you should watch for newly hatched juveniles under surface trash, especially where corn stalk residue is present. Knowledge of slug biology, conditions favoring outbreaks, scouting practices and potential management options can help reduce slug problems.  Please see the accompanying article “Slugs – Scouting and Control in Field Corn.”

Small Grains
Fields should be sampled on a weekly basis for sawfly and armyworm larvae. We continue to find low levels of both in fields in Kent and Sussex counties – however, we are starting to see an increase in armyworm moth activity. Remember, armyworm larvae are nocturnal so look for larvae at the base of the plants during the day. As a general guideline, a treatment should be considered if you find one armyworm per foot of row for barley and 1-2 per foot of row for wheat. Since sawflies feed on the plants during the day, small sawfly larvae can often be detected early using a sweep net. However, there is no threshold for sweep net samples. Once sawfly larvae are detected, sample for larvae in 5 foot of row innerspace in 5-10 locations in a field to make a treatment decision. You will need to shake the plants to dislodge sawfly larvae that feed on the plants during the day. As a guideline, a treatment should be applied when you find 2 larvae per 5 foot of row innerspace or 0.4 larvae per foot of row. If armyworms and sawflies are present in the same field, the threshold for each should be reduced by one-half. We can also find cereal leaf beetle larvae in fields that have had historical problems. When daytime temperatures are cooler, insects will feed lower in the canopy so be sure to sample the entire plant for small larvae. As temperatures increase, larvae can quickly damage fields so early detection is important. The treatment threshold is 25 eggs and/or small larvae total per 100 tillers. You should also continue to watch for aphids feeding in small grain heads. As a general guideline, the treatment threshold is 20 aphids per head with low beneficial insect activity. Although beneficial insect activity has increased, fluctuating temperatures may limit their ability to reduce aphid populations.

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