Management of Aphids: Barley Yellow Dwarf Transmission and Direct Aphid Damage in the Spring

Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu, Bob Mulrooney, Extension Plant Pathologist; bobmul@udel.edu and Gordon Johnson, Extension Ag Agent, Kent Co.; gcjohn@udel.edu

We are starting to receive questions regarding the need to spray for aphids in late winter and early spring in regards to both barley yellow dwarf virus (BYDV) transmission and direct aphid damage. Although fall conditions were favorable for aphids (warm and dry), we did not see an increase in populations in many wheat fields until late November and early December. In addition, there have been reports of a second period of population increase in late January and early February.

All of the aphids found in Delaware wheat fields are capable of transmitting BYDV. However, the virus strains that cause barley yellow dwarf are generally transmitted to the wheat in the fall or early spring before growth stage 4. For aphids to successfully transmit the virus in the most efficient manner, they normally need between 12 and 30 hours feeding to acquire the virus, and then 4 or more hours of feeding to transmit it. However, aphids are capable of acquiring the virus after feeding on infected plants for only 30 minutes. Once they acquire the virus and it is allowed to incubate for one to four days, they can transmit it to healthy plants for the rest of their life.

Although we have seen isolated sections of fields with high incidences of BYDV in the past, overall it has not been a widespread problem in Delaware. Typical symptoms of infection include stunting, plant yellowing and erect leaves with yellowish to reddish-purple tips. Early spring infections can result in purpling of flag leaves that resembles phosphorus deficiency. Differences in BYDV incidence from one field to another may be due to differences in efficiency of aphid transmission of the virus among fields, the source and strain of the virus being transmitted, difference in aphid mobility and feeding habits, the age and susceptibility of plants when infected, and differences in weather conditions from one field to the next.

Yield reduction due to BYDV is generally greater when infections occur in the fall compared to spring infections; therefore, the greatest concern is past. BYDV tends to be most severe in fields planted before the fly-free date at a time when aphid populations are high and aphids are still actively feeding. There is no high level of resistance to BYDV in wheat cultivars. Recent information from Alabama, Georgia, Kentucky and Virginia regarding foliar sprays has indicated that: (1) planting time and late spring (March) sprays for aphids had no effect on BYDV incidence and wheat yield and (2) well timed fall aphid sprays can provide almost complete control.

Although aphids can cause direct damage to wheat (in addition to BYDV transmission), this type of damage generally occurs when: (1) green bug aphid populations are heavy in the fall, (2) plants are drought or nutrient stressed, or (3) spring conditions are cool and dry, resulting in economic levels of aphids in wheat heads. Since the green bug aphid secretes a toxic substance into the plants, extensive feeding in the fall and early spring may result in circular yellow to brown spots with dead spots in the center. A lack of moisture in midwinter and a cool, dry spring can result in more significant damage from aphids in the spring. The most significant damage occurs when large numbers of aphids feed on the grain head causing shriveled or blasted heads.

The most common species in late spring is the English grain aphid. Heavy head infestations can reduce yields by 13%. Scouting is especially valuable in predicting problem infestations because English grain aphids are usually found in heads of plants that were infested in the lower canopy earlier in the season. Although natural enemies can keep aphids under control, cool dry weather in the spring often allows aphids to reproduce rapidly whereas their natural enemies reproduce slowly. Beneficial insects that attack aphids reproduce slowly at temperatures below 65°F, whereas aphids can rapidly increase when temperatures exceed 50°F.

Begin checking for aphids on a weekly basis in mid-March. On tillering grain, examine 5 linear foot of row in at least 10 areas of a field. Examine areas that exhibit plant stress. At each site, count or estimate the number of aphids per linear foot of row. During heading, check 50 to 100 heads throughout a field. While counting aphid populations, be sure to check for natural enemies.

The following thresholds should only be used as a general guideline. With the recent increase in wheat prices, these numbers may need to be reduced. Also, these thresholds do not apply if you are trying to manage aphids for BYDV transmission. Since almost undetectable levels can transmit virus, entomologists in the South feel that there really is no threshold for aphids when you are dealing with BYDV transmission.

After spring growth resumes until hard-dough stage:
100/row-foot, plants 3-6 inches tall
200/row-foot, plants 7-10 inches tall
300/row-foot, plants 11+ inches tall

Heading – At grain head emergence, a treatment may be necessary once populations exceed 20-25 per head. If the crop is approaching the hard-dough stage and there is good beneficial insect activity, no control should be needed. Often a ratio of one predator to every 50 to 100 aphids is sufficient to achieve biological control

A number of insecticides are labeled for aphid control in wheat including: Baythroid, Baythroid XL, Dimethoate 4E, Lannate LV, Mustang MAX, Penncap-M, Proaxis, and Warrior. Check the labels for restrictions and harvest intervals. Note that these are materials for wheat, several of theses insecticides do not have barley or other small grains on their labels.

Information extracted from “Barley Yellow Dwarf Virus and Aphids on Wheat” by Ron Hammond, Pierce Paul, Andy Michel, and Bruce Eisley in the February 5, 2008 – February 20, 2008 edition of the Crop Observation and Recommendation Network newsletter from the Ohio State University Extension and Factsheets on aphids in small grains from Virginia Tech and the University of Delaware http://ag.udel.edu/extension/IPM/fs/pdf/ipm4.pdf http://www.ext.vt.edu/pubs/entomology/444-018/444-018.html

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