Agronomic Crop Insects – May 7, 2010

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

Alfalfa
If economic levels of alfalfa weevil are present before harvest and you cut instead of spray, be sure to check fields within one week of cutting for damage to the regrowth. If temperatures remain cool after cutting, there is often not 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 weevils per stem and the population levels remain steady. The first potato leafhoppers have also migrated to our area so be sure to sample within a week of first cutting. In addition, new seedings should be watched carefully since leafhoppers can quickly damage these plantings. Once the damage is found, yield loss has already occurred. The treatment thresholds for leafhoppers are 20 per 100 sweeps on alfalfa 3 inches or less in height, 50 per 100 sweeps in 4-6 inch tall alfalfa and 100 per 100 sweeps in 7-11 inch tall alfalfa.

Field Corn
Be sure to watch for cutworms and slug damage on newly emerged plants. Even though it has been warmer and dry compared to last season, the cooler, wet weather predicted this weekend in combination with the presence of slugs under no-till residue can still result in economic slug damage on newly emerging seedlings. So far, we only know of one field that was treated last week for slugs. Deadline M-Ps should be considered if the weather remains cool and wet and damage is present and/or increasing. The following link provides good information on slug biology, scouting and management of slugs on field crops. http://ohioline.osu.edu/ent-fact/pdf/0020.pdf

Small Grains
Be sure to watch for the movement of aphids into grain heads. In many cases, beneficial activity is still not high enough to take care of populations moving from the lower canopy of the plants into the grain heads.

Economic levels of Cereal leaf beetles continue to found in fields throughout the state. Since there were 2 periods of egg laying, we can often find larger larvae up on the flags leaves and in many cases higher numbers of smaller larvae still in the lower plant canopy. Depending on the temperature, newly hatched larvae will feed for up to 3 weeks. Research from Virginia and North Carolina indicates that the greatest damage can occur between flowering and the soft dough stage so continue to sample carefully for this insect. The treatment threshold is 25 eggs and/or small larvae per 100 tillers. Treatment is suggested when the egg threshold is reached and more than 50% of the sample consists of larvae, i.e. 50% egg hatch.

Wheat and barley should also be sampled for sawfly and armyworm larvae. Armyworm larvae are nocturnal so larvae are generally found at the base of the plants during the day. However, during cool, cloudy weather, you may also see them feeding on the stems 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.

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