Agronomic Crop Insect Management

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

Alfalfa
All fields should be sampled for alfalfa weevil by examining 5-10 stems for damage and larvae. A full stem sample will be needed once damage and/or larvae are found on the plants. If leaf feeding is present, randomly collect 30 stems from throughout the field. Grasp stems at the base and place each stem upside down in a bucket. After collecting the stems, separate them into 3 or 4 bundles and beat them against the inside of the bucket to dislodge larvae from the stems. Count and record all larvae found per 30 stems. You will also need to measure 10 of the 30 stems and record the average stem height. 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. More information on alfalfa weevil biology, damage and treatment options can be found at the following links:

http://extension.udel.edu/factsheets/alfalfa-weevil-control-in-alfalfa-2/

https://cdn.extension.udel.edu/wp-content/uploads/2012/05/31152336/InsectControlinAlfalfa2016.pdf

Small Grains
It is time to start checking fields for cereal leaf beetles. Adult beetles begin to leave overwintering sites when daily high temperatures are consistently above 60°F. During the past week, we started to find adult feeding and the first egg masses. Treatment decisions are based on the number of eggs and/or small larvae per 100 tillers. More information on insect identification, sampling, treatment thresholds and management options can be found at the following links:

http://extension.udel.edu/factsheets/cereal-leaf-beetle-control-in-small-grains/

https://cdn.extension.udel.edu/wp-content/uploads/2012/05/31152336/Insect-Control-in-Small-Grains2016.pdf

Although aphid populations have increased over the past couple of weeks in some fields, overall populations still remain low. Aphid outbreaks in the spring are favored by a mild winter followed by a cool, dry spring. Under these conditions, aphids reproduce rapidly whereas their natural enemies reproduce slowly. The predominant species being found is the English grain aphid which is the only aphid species known to move into grain heads, causing shrunken kernels and reduced test weights. Once grain heads emerge, it will be important to sample for aphids and treat if you find 15-25 aphids per head and low beneficial insect activity.

http://extension.udel.edu/factsheets/aphid-control-in-small-grains-in-the-spring/

Timothy
Continue to watch for an increase in cereal rust mites which are favored by cool temperatures. 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. As a general guideline, 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/A of carrier to get good coverage of leaf surfaces generally results in better control.

http://extension.udel.edu/factsheets/cereal-rust-mite-in-timothy/

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