Greenbug: Schizaphis graminum (Rondani)

Description and Biology: Greenbugs are aphids and both adults and nymphs are light green, with a characteristic darker green stripe down the back. Adults are pear-shaped, approximately 1.6 mm long. Tips of cornicles and leg segments farthest from the body are black. Winged and wingless forms may be present in the same colony. Caution should be taken not to confuse the bluish-green corn leaf aphid with greenbug.
Females produce living young (nymphs) without mating. Under optimum conditions, newly born nymphs become adults in about seven days. Each female produces about eighty offspring during a 25-day period.

Symptoms and Damage: Greenbug is generally considered a key insect pest of sorghum. The aphid sucks juices from and injects toxin into plants. Small grains, primarily wheat, are the winter host. Where the growing season of wheat does not overlap that of sorghum, grasses such as johnsongrass, are interim hosts.
Greenbugs feed in colonies on the underside of leaves and produce much honeydew. The greenbug may be a pest during the seedling stage of growth but often does not reach damaging numbers until sorghum is in the panicle stage. Infestations may be detected by the appearance on leaves of reddish spots caused by the toxin greenbugs inject into plants. Reddened areas enlarge as the number of greenbugs and the injury they cause increase. Damaged leaves begin to die, turning yellow and then brown, from the outer edges. Damage to seedlings may result in stand loss. Larger sorghum plants will tolerate more greenbugs than will seedlings. Yield reductions during boot, flowering, and grain-development stages are dependent on greenbug numbers, length of time greenbugs have infested plants, and plant health. Many greenbugs on booting and older plants can cause yield reduction because of fewer and smaller kernels and weakened plants that may lodge later. Greenbugs also transmit maize dwarf mosaic virus and may predispose sorghum to charcoal rot. Yield of sorghum is affected when there are 500 to 1,000 greenbugs per plant; however, this number varies depending on stage of growth of sorghum.

Monitoring: A minimum of 40 randomly selected plants per 32-hectare field should be examined each week. Greenbugs seldom are distributed evenly in a field, so plants from all parts of the field should be inspected; avoid examining sorghum plants only along field borders. In fields larger than 32 hectares, or if making a control decision is difficult, examine more than 40 plants.
When making a decision to control greenbug, the amount of leaf damage, number of greenbugs per plant, percentage of greenbugs parasitized (mummies), number of greenbug predators per plant, moisture conditions, plant size, stage of plant growth, and overall condition of the crop should be considered. It is important to know from week to week whether greenbug abundance is increasing or decreasing and the extent of damage. For example, chemical treatment would not be justified if the recommended treatment level (based on leaf damage) had been reached but greenbug numbers had declined substantially from the previous time observations were made.
In seedling sorghum (less than 15 cm tall), greenbugs may be found on any part of the plant including in the whorl and under cool conditions, in the soil at the base of the plant. The entire sorghum plant and soil around the base of the plant should be examined when scouting seedling sorghum for greenbugs. The presence of greenbugs and any damage to plants (yellowing, death of tissue) should be noted. Seedling sorghum is very susceptible to greenbugs. Greenbugs should be controlled with insecticide when infested plants are commonly found on about 20% of plants, but before any plants are killed. Greenbugs on plants larger than seedlings to the boot stage should be controlled when greenbug colonies are causing red spotting or yellowing of leaves on 20% of plants, but before any full-sized leaves are killed. Greenbugs on boot- to heading-stage sorghum should be controlled when greenbug colonies are causing red spotting or yellowing of leaves and when one full leaf per plant is dying. At this growth stage, insecticide applications are suggested in some cases if greenbugs are colonizing upper leaves of booting sorghum and tissue is dying. Greenbug infestations after sorghum flowering and before the hard-dough stage should be controlled before they kill more than two full-sized leaves on 20% of plants. When estimating leaf damage, consider any leaf to be dead when more than 75% of its surface is red, yellow, or brown. Greenbug damage should not be mistaken for the natural senescence of small bottom "seed" leaves. Estimate an average leaf damage level for the entire field unless it is feasible to spot treat.
These guidelines are based on the assumption that greenbug abundance is increasing so rapidly that control by beneficial insects is not effective. Also, plants undergoing drought or other stress cannot tolerate as many greenbugs without suffering reductions in yield. However, when more than 20% of greenbugs are brown and swollen from being parasitized, application of an insecticide usually is not necessary.

Management: Greenbug abundance in a field can increase 20-fold per week, but the seasonal average is a five- to six-fold increase each week. Rain and predators suppress increase in greenbug abundance early in the season, although abundance of natural enemies has a lag time of one to two weeks. The parasite Lysiphlebus testaceipes (Cresson) usually is responsible for rapid decline in aphid abundance late in the season.
Sorghum hybrids resistant to greenbug should be used. However, four greenbug biotypes, C, E, I, and K are capable of infesting sorghum. Biotypes do not differ in appearance but differ in their effect on resistant hybrids. Greenbug biotypes E and I are currently the most common biotypes and they may occur together in some sorghum-growing areas. Biotype I greenbugs damage sorghums resistant to biotype E greenbugs, but biotype I resistant hybrids are resistant to biotype E. Greenbug resistant hybrids will not be free of greenbugs, but resistant hybrids are infested with fewer greenbugs and are more tolerate to damage than are susceptible hybrids. Treatment damage thresholds for resistant hybrids are the same as for susceptible hybrids because thresholds for both kinds of sorghum are based on plant damage.
The greenbug can be controlled with some organophosphate insecticides, but resistance to several organophosphates has been recorded. Most naturally occurring parasites and predators will be spared if extremely low dosage rates of organophosphates are used to control greenbug when abundance reaches the economic threshold. Some insecticides with systemic action applied to seed or in-furrow at planting for soil pests will control greenbugs.

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