Pest Management & Crop Development Bulletin • No. 13 / June 18, 1999
INSECTS
Corn Rootworm Larvae Found in Many Fields
During the past week, we’ve received a number of calls that indicate corn
rootworm larvae can be found in some cornfields. Joe Spencer, an entomolo-
gist with the Illinois Natural History Survey, indicated that corn rootworm
larvae could be found with relative ease in experimental plots near
Champaign. Although it’s too early to assess the full extent of root injury,
it’s not too early to begin examining some root systems for potential prob-
lems that may be just around the corner. By mid-July, corn rootworm larvae
are typically finished with their root-feeding activities. In 1996 and 1997,
egg hatch was delayed, and rootworm development was slowed consider-
ably, resulting in larval injury through the end of July. We don’t anticipate a
prolonged feeding period this season. Bottom line—a month from now,
rootworm larval feeding should be nearly over for central Illinois and near-
FOR IMMEDIATE RELEASE ing completion in northern counties.
No. 13 / June 18, 1999
Each year at about this point in the season, we begin to receive a number of
inquiries concerning the performance of soil insecticides for corn root-
worms. Evaluating soil-insecticide performance for corn rootworm larval
Executive editor: Kevin Steffey,
control requires some effort! Of utmost importance is the fundamental use of
Extension Entomologist
check strips (untreated rows) in a producer’s field. Doing this means shut-
Available on the Web at ting off some insecticide boxes at planting in a few areas of a cornfield.
http://www.ag.uiuc.edu/cespubs/pest/ Although this requires very little extra time during planting, many producers
For subscription information, phone fail to perform this important step. Comparing root injury in treated and
217.244.5166, or e-mail untreated strips throughout a field is the only way to determine adequately
acesnews@uiuc.edu the value of your soil-insecticide investment for rootworm control. The
following questions and answers are designed to provide some additional
input on this topic.
In This Issue I find very few rootworm adults in my cornfield. Doesn’t this suggest that
my soil insecticide worked pretty well?
t Watch Alfalfa Fields for Lack
of Regrowth: Adult Weevils No. Soil insecticides were designed to provide only one important func-
Are Busy, 126 tion—root protection. Research in Illinois and other states clearly indicates
t Reports of Potato Leafhopper that, in certain years, more corn rootworm adults may emerge from treated
Injury Are Numerous, 127 than untreated areas of cornfields. If you don’t find many corn rootworm
adults in your cornfield, it may simply mean that you didn’t have much of an
t Grape Colaspis Injury
infestation to begin with. Also, corn rootworm adults are very mobile (par-
Finished for the Season, 127
ticularly the females) and may have left your field and flown to a nearby
t Few Reports on Corn Borer later-planted and more generously pollinating field.
Infestations, 128
If I don’t have any severe lodging in my field, is it safe to assume that my soil
t Touchdown Labeled for
insecticide performed adequately?
Postemergence Applications
in Glyphosate-Resistant No. Corn plants lodge for a variety of reasons that may have nothing to do
Soybeans, 128 with corn rootworm larval injury. Plants that are top heavy (tall with large
t Turning Down the Heat, 128 ears) may topple over in severe thunderstorms. Saturated soils may predis-
pose plants, even those with no root injury, to lodging due to the weakened
t Hail and Flooding Damage
soil structure around the root system. Conversely, if soils are extremely dry
in Corn, 128
and hardened, plants with severe root pruning may not lodge because they
“concrete-like” growing conditions.
are held in rigid 125
Pest Management & Crop Development Bulletin • No. 13 / June 18, 1999
Aren’t corn rootworm larvae almost
impossible to find?
Locating corn rootworm grubs in early
to mid-June requires some effort. First
instars are quite small, and they tend
to burrow into root tissue and feed
internally. As rootworms reach the
third and last larval instar, they be-
come easier to find; however, a full-
grown grub will not grow much be-
yond 1/2 inch in length. Figure 1 pro-
vides some information that can be
used to determine larval instars for
Figure 1. Body lengths and head-capsule widths for instars of the western corn
corn rootworms.
rootworm.
If I find corn rootworm larvae, how
concerned should I be? ments at cultivation is increasingly no nately, many producers are unwilling
longer an option for many producers to leave some rows untreated at plant-
Don’t be alarmed. Soil insecticides
—the plants are too tall. ing. If you don’t leave some untreated
will not kill 100 percent of corn root-
areas in your cornfield, you’ll never be
worm larvae within a field. Remember How do I grade the performance of my able to estimate the real value of your
that soil insecticides are placed in- soil insecticide? soil insecticide purchase.—Mike Gray
furrow or in a narrow band during
planting. Untreated areas between To get an accurate picture of how well
rows effectively serve as a rootworm your soil insecticide worked, you
refuge. Because the entire rootworm should dig several (5 to 10) plants Watch Alfalfa Fields for Lack
larval population within a treated field from about 10 different areas of your of Regrowth: Adult Weevils Are
is not exposed to a soil-insecticide field. Following the digging, wash off Busy
application, resistance to commonly the soil from the roots and look for
any general feeding (brown scars); or, Most of the alfalfa in the state has
used products has not occurred for
more importantly, examine the plants been harvested, and the first crop has
decades. Corn rootworm resistance to
for pruned roots. Most entomologists been removed. Just yesterday (June
methyl parathion (*Penncap-M) has
across the Corn Belt suggest that a soil 15), I received a report that a stand
been confirmed in Nebraska. How-
insecticide has done its job if it keeps was failing to “green up” properly.
ever, this resistance developed due to
root injury below a rating of 3.0 (sev- Closer examination of plants within
the widespread and long-term use of
eral roots pruned to within 1.5 inches the field revealed alfalfa weevil adults
broadcast applications of *Penncap-M
of the plant; never an entire node busily removing any green tissue
for egg-laying suppression programs.
pruned) on the Iowa State root-rating evident. Fields of alfalfa are now at a
Those products preceded by an aster-
scale. The economic root-injury index point in the season where weevils and
isk are restricted-use products and
is static and varies according to the potato leafhoppers can jointly “work
may be applied only by certified
hybrid selected and the amount of over” a regrowing stand. Rescue treat-
applicators.
precipitation that occurs throughout ments may be required for weevils
When should I be concerned if I find the growing season. Rainfall after the after a cutting when larvae and adults
corn rootworm larvae? larval-feeding period is very impor- are feeding on more than 50 percent of
tant. The level of root regeneration the crowns and regrowth is prevented
If you find three or more larvae per for 3 to 6 days. Products suggested for
from mid-July through mid-August
plant (7-inch cube of soil and roots adult weevil control include *Furadan
may significantly affect yield.
from base of plant) and root injury is 4F (1 to 2 pints of product per acre),
evident, a rescue cultivation treatment Let’s assume that I follow your sug- Imidan 70-W (1-1/3 pounds of product
may be warranted. An insecticide gestions and find very little root in- per acre), Lorsban 4E (1 to 2 pints of
applied during cultivation does not jury. Can I now assume that I got my product per acre), and *Penncap-M (2
generally perform as well as a plant- money’s worth for using a soil to 3 pints of product per acre). Those
ing-time treatment. This difference is insecticide? products preceded by an asterisk are
especially true under very dry soil restricted-use products and may be
conditions. Given the earlier and ear- Perhaps, but only if roots also were applied only by certified applica-
lier planting we’ve witnessed in recent removed from a check strip(s) and you tors.—Mike Gray
years, the application of rescue treat- found they were injured. Unfortu-
126
Pest Management & Crop Development Bulletin • No. 13 / June 18, 1999
sweep. Take 20 sweeps per location in
Reports of Potato Leafhopper 5 to 10 sites within the field if the
Injury Are Numerous alfalfa is taller than 4 inches. If the
Duane Frederking, field sales agrono- alfalfa is shorter than 4 inches, more
mist with Pioneer Hi-Bred, has ob- sweeps are required for a precise
served very impressive densities of sample estimate. Count the number of
potato leafhoppers in many alfalfa leafhoppers caught in the net and
Figure 2. Potato leafhopper adult.
fields across central Illinois. One par- divide by the number of sweeps taken.
ticular field in Sangamon County had If the number exceeds suggested treat-
days. Multiple generations are accom-
as many as six to eight leafhoppers per ment thresholds, an insecticide appli-
plished throughout the summer, with
sweep! During the critical regrowing cation may be warranted.
leafhoppers persisting until cooler fall
stage, alfalfa plants are extremely temperatures return. As reported last year, entomologists
susceptible to feeding injury by potato with Iowa State University suggested
leafhoppers. In fact, it takes only 0.2 How do leafhoppers injure plants?
that economic thresholds need not
leafhopper per sweep on 3-inch depend on plant height. Table 1 pro-
Potato leafhoppers suck fluids from
stubble and 0.5 leafhopper per sweep vides some economic thresholds for
alfalfa plants with their piercing–
on 3- to 6-inch plants to cause eco- your consideration based on crop
sucking mouthparts. They inject saliva
nomic problems. value, control costs, and leafhopper
that contains a compound toxic to
Because potato leafhoppers will re- plants. In addition, their feeding clogs densities. In general, these thresholds
main for the duration of the growing the conductive tissue of plants, result- are less conservative than those (based
season, they have a longer period of ing in an accumulation of starches. on plant height) mentioned previously
opportunity to create challenges for This accumulation causes a relative for shorter plants, and more conserva-
alfalfa producers. Potato leafhoppers nitrogen deficiency, resulting in yel- tive for taller stands (12 inches or
not only reduce yields but also may lowing or injured leaves. Most people more).
have a significant negative effect on notice the first symptom of potato
Insecticides suggested for control of
the nutritional quality of hay and im- leafhopper injury as a V-shaped yel-
potato leafhoppers include the follow-
pair the vigor of a stand. Growers lowing at the tips of the leaflets, com-
ing: *Ambush 2E (3.2 to 12.8 ounces
should begin scouting alfalfa fields monly referred to as “hopperburn” or
of product per acre), *Baythroid 2 (0.8
now and at least on a weekly basis “tipburn.” As injury progresses, the
to 1.6 ounces of product per acre),
throughout the growing season. Potato leaves may turn purple or brown and
dimethoate (see product label), Imidan
leafhoppers will be with us through then die. Severely injured plants also
70-W (1-1/3 pounds of product per
the first several hard frosts. are stunted and bushy in appearance
acre), Lorsban 4E (1/2 to 1 pint of
because the internodes stop growing
What do potato leafhoppers look like? product per acre), *Penncap-M (2 to 3
normally.
pints of product per acre), *Pounce
Adult potato leafhoppers are lime What’s the best way to scout for potato 3.2EC (4 to 8 ounces of product per
green, wedge-shaped insects about 1/8 leafhoppers? acre), Sevin XLR Plus (2 pints of
of an inch in length (Figure 2). Adults product per acre), and *Warrior T or
have fully developed wings and are Invest in a sweep net. The purchase of 1E (1.92 to 3.2 ounces of product per
very active fliers. The nymphs, a 15-inch (diameter) sweep net is a acre). Those products preceded by an
smaller versions of the adults, are valuable insect sampling tool for al- asterisk may be applied only by a
yellowish green and lack wings. When falfa producers. Static treatment certified applicator. Please follow all
disturbed, the nymphs move sideways thresholds are based on the number of label directions and precautions.—
or backward (“crablike” movements). leafhoppers per sweep of the net. The Mike Gray
threshold increases as alfalfa grows
What kind of life cycle do leafhoppers and becomes more tolerant to leafhop-
have? per feeding: 0.2 per sweep on stubble
Grape Colaspis Injury Finished
(up to 3 inches), 0.5 per sweep on 3-
Potato leafhoppers do not overwinter for the Season
to 6-inch alfalfa, 1.0 per sweep on 6-
in Illinois. Instead, they migrate north-
to 12-inch alfalfa, and 2.0 or more per Many of you this past week indicated
ward from southern states, assisted by
sweep on plants 12 inches or taller. that you’d found grape colaspis pupae
wind currents. Soon after they arrive
The sweep net should be swung like a in cornfields. Textbooks suggest that
in the Midwest, the females begin to
broom through the top 4 to 6 inches of larval development should be com-
lay eggs in stems and larger leaf veins.
growth. Each thrust with the net is a pleted by mid-June for central Illinois,
The eggs hatch into nymphs in 6 to 9
127
Pest Management & Crop Development Bulletin • No. 13 / June 18, 1999
Table 1. Economic thresholds for potato leafhoppers (numbers per sweep) at different crop values and
control costs (from Iowa State University).
Crop value ($ per ton) Insecticide cost, $6 per acre Insecticide cost, $8 per acre Insecticide cost, $10 per acre
$50 1.3 1.5 1.8
$75 1.2 1.1 1.3
$100 0.8 0.9 1.1
$125 0.7 0.8 0.9
so your observations are right on tions can be made up to and including first to average temperature and rela-
schedule. Later this month and the full-bloom stage of soybeans, at a tive humidity to indicate the “heat” of
through early July, grape colaspis maximum of 3.2 pints per acre. For contact herbicides. The current Blazer
adults will become noticeable in corn many annual weed species, apply 1.2 label states that “if the temperature
and soybean fields throughout Illinois. to 3.2 pints per acre. An application and relative humidity exceed 150 (add
The adults are not considered of eco- rate of 1.6 pints per acre for weeds up temperature + relative humidity) then
nomic importance, only that they lay to 6 inches in height is commonly use the lower additive rates.” PPG/
eggs later in the season to begin next recommended by the manufacturer. Ortho/Valent (Cobra) went one step
year’s problems. Please refer to Kevin Including a spray-grade ammonium farther and made adjuvant recommen-
Steffey’s article in last week’s issue sulfate at 8.5 to 17 pounds per 100 dations (NIS vs. COC) based on tem-
(no. 12) for more information on the gallons of water may improve control perature and relative humidity to mini-
life cycle of this intriguing insect pest. of annual and perennial weed species. mize crop injury/maintain weed
The good news—larval injury is over Repeat applications are allowed up to control.
for this season.—Mike Gray a maximum per acre application rate
of 12.8 pints. Several tank-mix options The Buctril (bromoxynil) label indi-
are included on the new label. cates NIS or COC is allowed if you
can stand the injury. The Aim
Few Reports on Corn Borer Additionally, Touchdown 5 also may (carfentrazone) label allows the use of
Infestations be used as a preharvest treatment in NIS only (no oil) to minimize crop
We have received very few reports of glyphosate-resistant soybean varieties injury. Pinnacle recommends adding
corn borer activity so far this season. to aid in soybean harvest. Make oil when conditions are dry or cool.
As indicated in last week’s issue, preharvest applications at least 7 days Reflex allowed NIS or COC, while
scouting efforts should not be delayed. before harvest, with no more than 1.6 Flexstar actively promotes the use of
If you’re finding significant problems pints per acre. Do not graze or harvest COC or MSO.
with borers in certain areas of the for hay following a harvest-aid
application. Translocated herbicides are more
state, please pass along your observa- likely to cause crop injury when crops
tions to us. Kevin and I welcome your The active ingredients (acid) in Touch- are growing rapidly and leaf cuticles
input and would be glad to share your down 5 and Roundup Ultra are the are thin, particularly if “hotter” addi-
field experiences with our readers. same, but these two herbicides are tives are used. Adjuvant ratings for
E-mail correspondence works particu- formulated as the trimesium and heat are MSO > COC > NIS > AMS =
larly well for this purpose.—Mike isopropylamine salts, respectively. UAN.—Marshal McGlamery and
Gray Touchdown 5 contains 5 pounds active Aaron Hager
ingredient/3.43 pounds acid equivalent
WEEDS per gallon, whereas Roundup Ultra CROP DEVELOPMENT
contains 4 pounds active ingredient/
2.95 pounds acid equivalent per gal-
Touchdown Labeled for lon.—Aaron Hager, Marshal Hail and Flooding Damage
Postemergence Applications in McGlamery, and Christy Sprague in Corn
Glyphosate-Resistant Soybeans
Corn planted the first week of May is
Last week, a federal label was issued in about the 8-leaf stage at Urbana,
that permits Touchdown 5 (glyphosate Turning Down the Heat and color and stand uniformity are
trimesium or sulfosate) to be applied very good in most fields. The return of
postemergence to glyphosate-resistant Contact herbicides generally cause
more crop injury when temperatures cooler weather this week will slow
(Roundup Ready) soybean varieties down the rapid rate of growth, but the
for control of broadleaf and grass and relative humidity levels are high
during and shortly after application. crop is still ahead of average in its
weed species. Postemergence applica- development. With the warm tempera-
BASF (with Blazer) was one of the
128
Pest Management & Crop Development Bulletin • No. 13 / June 18, 1999
tures that held through the first 2 The key to assessing hail damage is to
weeks of June, we were accumulating see if the growing point (the tip of the Contributing Authors
the 65 or so growing degree-days stem, visible by splitting the leaf tissue Mike Gray (m-gray4@uiuc.edu),
required to put a new leaf on corn down through the stem) has a healthy, Extension Entomology,
plants in less than 3 days. whitish color. If so, then the plant (217)333-6652
likely will survive and grow back. If
Although the corn crop appears to be the growing point is brown or mushy, Aaron Hager (hager@uiuc.edu),
in good shape in most fields, there are then the plant probably will not sur- Extension Weed Science, (217)333-
places in Illinois where thunderstorms vive. A more certain way to assess hail 4424
last week caused flooding and hail injury is to note whether plants start to
injury, sometimes in the same field. show new leaf growth. To assist in Marshal McGlamery
Plants with six leaves or more are seeing such growth, you may wish to (mcglamerym@ces.aces.uiuc.edu),
better able to withstand flooding than cut some plants off “clean” above the Extension Weed Science, (217)333-
are smaller plants, but saturation of the growing point where the hail has in- 4424
soil results in loss of oxygen and will jured the plant, and then see whether
quickly affect root function. If tem- Emerson Nafziger (ednaf@uiuc.edu),
new leaf growth appears within 1 to 2 Crop Sciences, (217)333-4424
peratures are warm and soils are satu- days. If so, then the signs are good
rated for more than 3 to 4 days, plants that the crop will grow back. Research Christy Sprague
will likely not survive. Cooler tem- has shown that, if the crop is capable (lsprague@staff.uiuc.edu), Extension
peratures mean that water dissolves of growing back, there is little benefit Weed Science, (217)333-4424
more oxygen, so plants usually sur- to mowing damaged leaf area off to
vive longer. Even short periods of try to “help” the crop grow back.
standing water may, however, affect
the ability of the roots (and hence the Previous research has given us a way
plants) to recover and grow normally. to estimate yield loss to be expected
Furthermore, nitrogen can be lost from loss of leaf area. Most of the
when soils are flooded, and this can corn that lost leaf area to hail last
affect the yield potential of the crop. week was probably in the 6- to 8-leaf
The fungus that causes crazy top in stage at the most, with some smaller
corn also is favored by flooded condi- than that. Corn with fewer than six
tions and may affect whole areas that leaves suffers minimal loss from loss
have been under water. of leaf area, providing the growing
point stays healthy. Corn in the 7-leaf
While there isn’t much we can do to stage is expected to lose only 2 per-
prevent losses in flooded parts of a cent yield when it loses half its leaf
field, any steps that can be taken to area, and 9 percent from losing all its
drain water away from ponded areas leaf area. Moderate leaf loss—25
will help. Where the crop has been percent—reduces yield by only
killed by standing water, corn can be 1 percent, even at the 10-leaf stage.
planted back, particularly if there is a Corn that loses all its leaf area at the
way to use those areas for forage. 9-leaf stage is expected to lose about
Soybeans were also planted success- 13 percent of its yield. These numbers
fully in many flooded-out areas in can vary considerably with the
1998, though herbicide labels would weather but should be close on aver-
not in most cases permit this. age. The other effect of leaf loss is a
Hail injury on young corn plants usu- delay in crop development. This delay,
ally looks much worse than it is. In which could total one to two leaf
cases of very heavy hail injury, stages, will delay pollination, in some
though, especially when plants are in cases to worse weather conditions, but
the 6-leaf stage or older, the growing sometimes to better conditions. This
point can be killed outright by hail or delay is responsible for part of the
may be bruised and infected by dis- effect on yield.—Emerson D. Nafziger
eases that eventually kill the plant.
129
Pest Management & Crop Development Bulletin • No. 13 / June 18, 1999
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