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Northwest Iowa Dairy Outlooks

A local discussion of current science and issues concerning dairying in northwest iowa

During field scouting on Wednesday I found two fields in the northern part of the county with greenbug populations at 20-30 insects per foot of row. 

According to Extension Agronomists the fall greenbug populations can hit a young stand of wheat hard.  Much of the damage often occurs, however, in late winter and into spring when the plants are still small.  Greenbugs numbers per foot of row that are generally considered damaging range from a low end of 25 to 50 on very young plants to 300 to 800 on plants 6 to 16 inches high.

Greenbugs are found primarily on wheat during the winter months but they also occur on Johnsongrass and other wild grasses. Adults and nymphs occur on the underside of leaves, particularly on the lower, older leaves of host plants. They damage plants in three ways: 1) they remove sap, stressing plants by removing water and nutrients; 2) they inject a toxic salivary secretion that causes cells, leading to a yellowing or reddening discoloration and eventually death on infested tissues; and 3) they can transmit plant disease such as the barley yellow dwarf virus in small grains and maize dwarf mosaic virus in sorghum. They infest plants of all growth stages and can kill young plants, reducing plant stand. Heavily injured older plants have stunted growth, delayed growth, reduced kernel size and quality. Several biotypes have evolved that have overcome plant resistance, tolerate temperature extremes and infest new host plants.

Mature females produce live young nymphs without mating. These nymphs develop into adults within 6 to 30 days, depending on temperature, and also produce live young at a rate of 2 to 3 per day over a 20 to 30 day period. Populations can increase quickly, about 5- to 6-fold per week under normal conditions. Many generations occur per year and they can be found year-round in the Texas wherever host plants occur. Infestations spread by winged forms flying and being blown many miles by the wind.

The average temperature must be below 20 degrees F for at least a week to kill 99 percent of the greenbugs. The population also must be without protection from snow cover. During the winter, infested fields may turn yellow, and spots in fields or entire fields can be affected.

Since greenbugs are a vector of barley yellow dwarf virus, cause more damage when small grains suffer from lack of moisture during a mild winter and cool spring. This damage may be confused with moisture stress, nitrogen deficiency or dryland root rot (foot rot).

I have found the “Glance ‘n Go” greenbug sampling system, developed by an Oklahoma State University, very helpful. It calculates a treatment threshold based on potential crop value, cost of control and time of year. For producers, sampling system is quicker and more simple than counting the actual number of greenbugs. Glance ‘n Go scouting forms and information can be obtained at http:/ entoplp.okstate.edu by selecting “Agriculture Models” and then “Cereal Aphids Pest Management.”

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