Agronomic Crop Insects – April 6, 2012

Joanne Whalen, Extension IPM Specialist;

Economic levels of pea aphids and alfalfa weevil ( can be found in fields at this time. When sampling for aphids and weevils, collect a minimum of 30 random stems throughout a field and place them top first in a white bucket. For aphids, you want to count the number present per plant as well as any that have dislodged from the stem into the bucket. As a general guideline, you should consider a treatment in alfalfa less than 10 inches tall if you find 40-50 aphids per stem. The treatment threshold for alfalfa 10 inches or taller in height is 75- 100 per stem. Although beneficial insects can help to crash aphid populations, cooler temperatures will slow their activity. As a general rule, you need one beneficial insect per every 50-100 aphids to help crash populations. For alfalfa weevil, you will want to record the number of weevil larvae per stem. The following thresholds, based on the height of the alfalfa, should be used as a guideline when making a treatment decision: up to 11 inches tall – 0.7 per stem; 12 inches tall – 1.0 per stem; 13 to 15 inches tall – 1.5 per stem; 16 inches tall – 2.0 per stem and 17 to 18 inches tall – 2.5 per stem.

Field Corn
We recently received a 24(c) Special Local Needs Registration for Avipel Hopper Box (dry) Corn Seed Treatment for bird management on field corn in Delaware. It is my understanding that product should be available this week and the 24(c) label with use instructions and restrictions will be on the canisters. If you have any questions, you can call David Pyne at the Delaware Department of Agriculture: (302) 698-4570.

Small Grains
Aphids and cereal leaf beetle eggs and small larvae can be found in fields throughout the state. The next important time to consider aphid management in small grains is at grain head emergence. During heading, check 50 to 100 heads throughout a field. At grain head emergence, a treatment may be necessary once populations exceed 20‐25 per head. Since cereal leaf beetle populations are often unevenly distributed within the field, it is important to carefully sample fields so that you do not over or under estimate a potential problem. Eggs and small larvae should be sampled by examining 10 tillers from 10 evenly spaced locations in the field while avoiding field edges. This will result in 100 tillers (stems) per field being examined. Eggs and larvae may be found on leaves near the ground so careful examination is critical. You should also check stems at random while walking through a major portion of the field and sampling 100 stems. The treatment threshold is 25 or more eggs and/or small larvae per 100 tillers. If you are using this threshold, it is important that you wait until at least 50% are in the larval stage (i.e. after 50% egg hatch). The following links provide additional information on cereal leaf beetle in small grains:

Low levels of small true armyworm larvae have been found in an occasional small grain field in Kent and Sussex Counties. Our black light traps will not start running until mid-April so we do not have a record of moth activity at this point. However, pheromone traps being run as part of a research project at our Research and Education Center in Georgetown found the first moths two weeks ago and populations have increased this past week. So be sure to scout all fields for armyworms and grass sawflies. The following link provides information on these two insect pests:

Economic levels of the cereal rust mite ( can be found feeding on timothy. Symptoms can appear as retarded growth, leaf curling, stunting, and plant discoloration. Injured plants appear to be drought stressed even when adequate moisture is available for plant growth. There are no established economic thresholds for the pest; however, treatment is recommended in fields with a previous history of cereal rust mites and/or when 25% of the plant tillers exhibit curled tips of the new leaf blades within several weeks following green-up. The use of a 20x-magnifying lens is often necessary to find mites on leaves. The only effective and labeled material on timothy is Sevin XLR Plus. Be sure to read the label for information on the number of applications per season as well as the days to harvest. For effective rust mite control, the use of the higher labeled rate and at least 25 gal/acre of carrier to get good coverage of leaf surfaces generally results in better control.

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