| 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
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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