Grape Colaspis In Corn and Aphids in Soybean: Will There Be a Repeat Performance in 2001?
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The year 2000 will be remembered by many in Illinois as a year of
unusual occurrences in the insect world. For the third consecutive
year, secondary insect pests of corn caused problems in various regions
of the state. The grape colaspis was particularly problematic throughout
the central third of Illinois, frustrating both growers and their advisers.
The damage was severe enough in some fields to warrant one or two replantings.
Our inability to predict their occurrence and the lack of reliable
control measures continue to hamper our ability to manage this pest.
The appearance of an insect pest of soybean formerly not known to occur
in North America posed another challenge for producers this year. The
soybean aphid, Aphis glycines, infested many soybean fields in the
upper Midwest, including northern Illinois, late in the 2000 growing
season, generating a great deal of activity and media coverage. By
late August, densities of soybean aphids began to decline dramatically,
leaving us with many unanswered questions. The stage is set for some
meaningful research in 2001.
Grape Colaspis
Situation in 2000
Grape colaspis was the focus of many growers' concerns by early May
and became the most notorious insect pest of corn in the central third
of Illinois for the next six weeks. Seedling corn plants infested with
grape colaspis larvae showed classic symptoms of injury-stunted, wilted
plants with purple stems and "burned" leaf edges. Most of the fields infested in 2000 had been planted to soybean
in 1999. In the past, I have written, "The grape colaspis is a
sporadic pest most often found in corn planted after red clover or
mammoth clover, and occasionally in corn planted after sweet clover,
alfalfa, or soybeans."
Information to support that quotation originated from old scientific
literature. Apparently, we need to reconsider the statement about "occasionally
in corn planted after ... soybeans." Very few acres of clover
currently are grown in Illinois, so that preferred host is no longer
readily available. The grape colaspis may have adapted to modern corn-soybean
rotation to survive.
Insecticide Efficacy Trial, 2000
We conducted an insecticide efficacy trial to compare products for
control of grape colaspis in Menard County in 2000. For each replicate
by treatment combination, we dug a trench 1 meter (3 feet) long and
6 inches deep and recorded the numbers of living corn seedlings. We
examined all of the soil from the trench for grape colaspis larvae
and recorded the numbers found. The plants in many of the plots were
stunted and purple, symptoms of injury attributable to both grape colaspis
and white grub larvae, which also occurred in the plot. We did not
attempt to distinguish which insects caused the injury.
We observed considerable variation in numbers of live plants and grape
colaspis among the plots. Only plots
treated with Regent 4SC had significantly more live plants (eight plants
per meter) than in the untreated control (about four plants per meter).
The numbers of grape colaspis larvae per meter of row in insecticide-treated
plots were not significantly different from the number of grape colaspis
larvae in the untreated control. This observation compares well with
past testimonials regarding lack of performance of soil insecticides
against grape colaspis.
One final note about this trial is in order. No soil insecticides currently
are registered for control of grape colaspis larvae. Results from this
experiment should not be used to justify the application of a product
for grape colaspis control.
Soybean Aphid
Situation in 2000
In late July, entomologists and plant pathologists at the University
of Wisconsin observed heavy infestations of aphids in soybean fields
in southern Wisconsin. They tentatively identified the aphid as the
cotton aphid, Aphis gossypii, and alerted us to their presence. We
verified the presence of aphids in soybean fields in northern Illinois
in early August. Some infestations were heavy, especially in fields
of late-planted soybean, and applications of insecticides were common
in some areas. After receiving some specimens in early August, David
Voegtlin, an aphid specialist at the Illinois Natural History Survey,
identified the aphids as Aphis glycines, a species heretofore known
to occur only in Asia, Australia (recent occurrence), and some Pacific
Islands. The soybean aphid is 2-4 mm long and pale green with black
cornicles ("tailpipes") on a pale rear end.
By the end of September, the soybean aphid had moved far and wide.
Although we did not examine soybean fields in all counties, the counties
in which we found soybean aphids include counties along Illinois' entire
perimeter. The heaviest infestations occurred north of I-80, but we
found small numbers (a female and a small number of young on some leaflets)
in most of the fields we sampled elsewhere. During August and September,
we found the soybean aphid in Illinois as far north as Winnebago County,
as far south as Pope County, as far west as Adams County, and as far
east as Vermilion County. Heavy populations of soybean aphids also
occurred in Wisconsin, Michigan, and Minnesota. By September, soybean
aphids also had been found in Indiana, Iowa, Kentucky, Missouri, Ohio,
and West Virginia.
By late August, densities of aphids began to decline dramatically.
Heavy rains and the activity of some natural enemies, especially the
multicolored Asian lady beetle and a fungal organism, reduced the numbers
of aphids. Within about six weeks, we experienced a significant rise
and subsequent fall of aphid populations, leaving us little time to
initiate much research.
Life Cycle and Injury Caused
The life cycle of the soybean aphid can be summarized as follows. Female
soybean aphids reproduce in soybean throughout the summer, giving birth
to living young and producing as many as 18 generations. In late summer
and fall, winged females leave soybean fields and fly to their overwintering
host, Rhamnus species, a woody perennial commonly called buckthorn.
On buckthorn, males are produced, and males and females mate. Females
lay eggs that overwinter on buckthorn. In the spring, nymphs hatch,
then become adults, and adult females give birth to living young on
buckthorn, completing two to three generations on this host. In late
spring, winged aphids leave buckthorn to colonize soybean fields.
Soybean aphids suck fluids from soybean plants, and injured leaves
turn yellow and crinkled. Sooty mold may grow on the "honeydew" that
is excreted by aphids. The Asian literature indicates that the aphids
can cause as much as 28 percent yield loss if they infest seedling
soybeans. However, little information about infestations later in the
summer exists. Preliminary information from the University of Wisconsin
suggests that 80-100 aphids per leaflet may cause an 8-bushel-per-acre
yield loss. The soybean aphid is known to transmit viruses in Asia;
its ability to transmit viruses in North American is not known.
Insecticide Efficacy Trial, 2000
John Shaw, Research Scientist at the Illinois Natural History Survey,
conducted an insecticide efficacy trial in Carroll County to determine
what insecticides might control soybean aphids. Nine of the insecticides
were labeled for use on soybean; six were not labeled.
John sampled the aphid populations in each plot before applying insecticides.
The average number of aphids in the untreated check plots was 53.9
aphids per leaflet. Three days after the treatments (DAT) were applied,
aphid densities in the untreated check plots had increased to an average
of 144.9 aphids per leaflet. However, aphid densities "crashed" (effects
of natural enemies and dispersal) to an average of 8.6 aphids per leaflet
in the untreated check plots 10 DAT. Therefore, measurement of the
residual efficacy of insecticides for 10 days was tenuous. Nevertheless,
several insecticides provided good control of soybean aphids for the
short duration of the study.
shows the percentage reductions in numbers of aphids
from pretreatment levels to levels on both three and 10 DAT, based
upon average numbers in the plots before treatments were applied and
average numbers in the same plots three and 10 DAT. Shows percentage control of soybean aphids three and 10 DAT,
based upon the average number of aphids in plots treated with a given
insecticide compared with the average number of aphids in the untreated
check plots on that same day.
At least five insecticides reduced numbers of aphids by 95 percent
or more by three DAT. By 10 DAT, numbers of aphids had been reduced
by at least 90 percent in seven plots treated with insecticides. All
but one registered chemical insecticide, Pounce 3.2EC, reduced numbers
of aphids by at least 75 percent by 10 DAT. By 10 DAT, control of aphids
by Pounce 3.2EC and Asana XL was less than 50 percent. Dimethoate,
Lannate, Lorsban, Penncap-M, and Warrior seemed to provide the best
control of soybean aphids.
Management of Grape Colaspis and Soybean Aphid in 2001
We do not know whether these two pests will occur in significant numbers
in 2001. Winter weather conditions may dictate their population levels
in 2001. We also do not know how successfully (or unsuccessfully) soybean
aphids found buckthorn plants on which to overwinter. Consequently,
we enter the 2001 growing season with considerable uncertainty about
both pests. Unfortunately, we still have very few options for controlling
grape colaspis; no soil insecticides or seed treatments are registered
for control of this pest.
We will monitor first occurrences of the soybean aphid in 2001 and
keep track of their population dynamics. If we detect escalating densities
and widespread occurrence of soybean aphid colonies, we will sound
the alert to prepare people for their onset. We also will conduct insecticide
efficacy studies and establish research trials to measure the impact
of soybean aphids on soybean yield and detect their potential for transmitting
viruses. Although we have much to learn, we will share the results
of our efforts as quickly as possible