Agronomic Crop Insect Management – April 29, 2016

Joanne Whalen, Extension IPM Specialist;

Be sure to check for alfalfa weevil adults and larvae within a week of cutting, especially if populations were above threshold before cutting. Feeding from both stages can hold back re-growth. After cutting, there needs to be enough “stubble” heat to control the weevils with a cutting. A stubble treatment will be needed if you find 2 or more weevils per stem and the population levels remain steady.

Field Corn
Be sure to watch for both cutworms and slugs feeding in newly emerged corn fields. As a general guideline, a treatment is recommended for cutworms if you find 10% leaf feeding or 3% cut plants. If cutworms are feeding below the soil surface, it will be important to treat as late in the day as possible, direct sprays to the base of the plants and use at least 30 gallons of water per acre. For cutworms, fields should be sampled through the 5-leaf stage for damage. If slugs are damaging plants, you will be able to see “slime trails” on the leaves.

Small Grains
Similar to last week, we can find cereal leaf beetle, grass sawfly and true armyworm larvae in barley and wheat throughout the state.

Cereal Leaf Beetle – In areas where cereal leaf beetles have been a problem in the past, we are starting see an increase in populations and some fields are approaching the threshold level of 25 eggs and/or larvae per 100 tillers. Research from Virginia and North Carolina indicates that the greatest damage from cereal leaf beetle can occur between flowering and the soft dough stage.

True Armyworm – Although armyworm can attack both wheat and barley, they can quickly cause significant losses in barley. Armyworms begin head clipping only when all vegetation is consumed and the last succulent part of the plant is the stem just below the grain head. Larvae can feed on the kernel tips of wheat, resulting in premature ripening and lower test weight. On, barley, significant stem clipping can occur in a short time.

Grass Sawfly – Barley and wheat are both damaged by sawflies. During years of high population pressure, barley may experience more damage. Sawfly larvae prefer to feed on the stems and are potentially more damaging than armyworms. Larvae begin to climb and feed on stems when the larvae are half grown and the grain is in the tiller to head stage. Stem clipping often occurs before leaf feeding is complete and/or the grain reaches physiological maturity.

Stinkbugs – The first native brown stink bugs (not brown marmorated stink bugs) can also be found in barley and wheat. Information from states to our south indicates that wheat may be susceptible to native stink bug feeding at the milk and soft dough stages. Thresholds in the south for native stink bugs in wheat range from one per head to one per 5 to 10 heads. Currently, these detections are being found mainly along field edges.

Aphids – As barley heads are fully emerged and wheat is not far behind, be sure to watch for aphids moving into the heads. In the spring, direct damage to small grains is usually confined to English grain aphids feeding in the heads of small grains. The most significant damage occurs when large numbers of aphids feed on the grain head causing shriveled or blasted heads. The English grain aphid has been the predominant aphid found in fields this spring. At grain head emergence, a treatment may be necessary once populations exceed 15-25 per head. Lady beetle adults and larvae, syrphid fly maggots, lacewing larvae, damsel bugs, and parasitic wasps often help to keep aphid populations in check. A ratio of one predator to every 50 to 100 aphids is often sufficient to achieve biological control. However, if the weather remains cool, aphids reproduce rapidly whereas their natural enemies reproduce slowly and lag behind.