Managing Insects in Wheat with Special Reference to C=
ereal
Aphids and Barley Yellow Dwarf1.
Douglas W. Johnson2
=
Introduction
Kentucky=
is located in a unique ecological situation between the cold winters of
northern prairie states and the very mild winters of the deep-south. Though
cold enough to stop most insect activity during mid-winter, the great varia=
tion
in date of first frost, fall onset of continuous cold, and spring onset of
general warm weather, makes it particularly difficult to predict insect imp=
act
in any given year.
In Kentu=
cky,
wheat is grown as a “winter crop” planted in the fall, usually
following corn, and harvested the following summer. Most often varieties wh=
ich
mature early enough to allow planting of “double-crop” soybeans=
are
used (1). This production system divides the insect pests into three groups:
those that infest in the fall, and that either, do or do not over-winter, a=
nd
those that infest in the spring. We will examine these pests in order of
appearance through the production year.
Insect pests are common in Kentucky wheat. Typically, one can fin=
d all
of these insects in almost every field every year, but rarely do their
populations grow to economically important numbers. Nonetheless, each pest =
has
the potential to cause significant damage under appropriate conditions.
What will this information mean to Indiana farmers? Generally, the situation in southe=
rn Indiana will be similar to that in west Kentucky. Conversely, as one moves further n=
orth
the likely hood of encountering these pests, especially in economically
important situations, is very much reduced. In any given year the importance of
several of these pests, particularly Hessian Fly and the cereal aphid ̵=
1;
barley yellows virus complex, will be dependent of the short term weather, =
most
importantly temperatures.
Scouting
In general, Kentucky
grown grain should be scouted weekly to determine pest activity and plant
growth stage from plant emergence to maturity when the temperature is above=
48
- 50oF. This will provide the best description of pest status, a=
nd plant
stage, and will allow important management decision to be made in a timely
manner. An outline for taking=
this
data in a standard format can be found in the University of Kentu=
cky
Integrated Pest Management (UK-IPM) scout man=
ual
for small grains (2). In addi=
tion,
there are other methods of collecting data on important pests such as the u=
se
of pheromone baited traps to capture adult moths (3). Whatever method you use, try to av=
oid “selecting”
your data by using randomly assigned data collection points (4).
Insect Pests3 in
General Order of Appearance
Fall Pests
The fall armyworm, Spodoptera frugiperda (J.E. Smith), is a common pest =
of
several late summer and early fall planted crops. Fall armyworm (FAW) cannot over-wi=
nter
in Kentucky. FAW migrates into Kentucky from the gulf coast in mid-su=
mmer,
initially infesting corn and grain sorghum as its primary crop hosts. In late summer/early fall, as corn=
/sorghum
begins to mature, it colonizes newly seeded grasses. Damage is most common in lawns, re=
claimed
land, ditch banks, roadsides, etc., but may also infest small grains. FAW c=
an result
in seedling death if it feeds on new plants before roots are established. If plants are established, FAW fee=
ding
is rather more like grazing. =
FAW
can remain active until the first killing frost and will survive longer whe=
re
crop residue provides shelter from the cold (conservation tillage).
Infestation typically results from early planting. Often, planting after the Hessian =
fly
free date will avoid this situation.
Occasionally frost and the onset of cold weather are late enough to =
allow
infestation of small grains even after this date. Additionally, late occurr=
ence
of a “Killing Frost” allows the survival of volunteer corn, whi=
ch
serves as a “green bridge” providing a food source between the =
time
of summer grain maturity and winter grain emergence. Insecticidal control is
relatively easy; but, there are no established thresholds. Damaged fields are sometimes repla=
nted,
but this is a risky technique. Many
damaged plants will survive, thus, replanting may result in a denser than
desirable stand. Very dense stands will, in turn, result in problems in the
spring such as lodging, and increased armyworm and disease pressure.Fall and Spring Pests
The cereal aphid com=
plex
primarily comprised of four common grain aphids; bird cherry-oat aphid, =
Rhopalosiphum
padi (Linnaeus), corn leaf aphid, Rhopalosiphum maidis (Fitch,)
greenbug, Schizaphis graminum (Rondani), and English grain aphid, Sitobion
avenae (Fabricius) are considered the most important aphid pests in our
region. In addition, the rice root aphid, Rhopalosiphum rufiabdominalis<=
/i>
(Sasaki), has been confirmed as occurring in Kentuck=
y
(5), and is very likely playing an important role, though little is known of
this aphid in Kentucky. It is interesting to note that the=
se
aphids are not considered important for their direct damage. Though some information on the
importance of direct damage, particularly from the bird cherry-oat aphid, is
coming to light, they are by far more notorious for their ability to spread=
the
viral pathogens that result in Barley Yellow Dwarf (BYD) disease. On the ot=
her
hand, there are at least twenty five aphids that can move the yellows virus=
es
among plants. Regardless of t=
he
aphids present, the risk, real and perceived, of Barley Yellow Dwarf and by
association the cereal aphid complex is, without doubt, the major driver of
insecticide use in Kentucky-grown wheat (6).
Though other insects may require in=
secticidal
control from time to time, only the cereal aphids are treated in a routine
manner. Beginning in the early 1990’s, increases in insecticide use,
especially the systemic disulfoton (Di-Syston®), and then in the
mid-90’s the synthetic pyrethroid lambda-cyhalothrin (primarily Karat=
e®
and Warrior®) over the
historic use pattern were quite evident (6). Moreover, there has been some hist=
oric use
of systemic insecticide seed treatments, primarily imidacloprid (Gaucho®=
;), and
now thiamethoxam (Cruiser&=
reg;).
Use of these latter seed treatment products is likely to increase.
<=
/o:p>
Control of t=
he
cereal aphid complex is relatively easy to obtain. As with most other insect pests of =
wheat,
it is hard to decide if treatment is warranted. The difficulty of finding aphids,
combined with the small number required for application, the relative low c=
ost
of insecticides and the fear of catastrophic loss to BYD probably result in
more insecticide use than is needed to mitigate the risk. The treatment of
these questions is dealt with in the southeast region in a new publication =
(7).
This publication was certainly developed for the southeastern states, but t=
he
driving force behind this problem remains fall and winter temperatures. To =
the
extent that local fall temperatures are warmer and frost /freeze is later t=
han
expected, this publication may serve as a guide.
The Hessian fly, Mayetiola destructor (S=
ay),
is another common insect pest infesting small grains in the fall (8). In
general, planting after the “fly free” date will provide adequa=
te
control in Kentucky. Traditionally resistant varieties =
have
been used in concert with the “fly free” date to provide contro=
l.
Most of these varieties originate from the northern breeding programs.
Certainly, agronomic and cultural factors favoring strong stems and stand-a=
bility
of the plant are preferred, but at last examination all resistance factors =
can
be overcome by the biotypes present in the state. There are no rescue treatments (fo=
liar
applied insecticides), though use of systemic insecticides as soil and seed
treatments and fall/ spring foliar applications of systemic insecticides ha=
ve
been used in the deep south to combat this problem. Many of these insectici=
des
are no longer available. The newer seed applied systemic insecticides may be
providing some control. In Ke=
ntucky,
in the presence of good agronomic practices, preplant soil insecticide
treatment is not warranted.
The wheat curl mite, Aceria tosichella K=
eifer,
is a common pest of wheat in Nebraska and
other plains states but is rarely a problem in the M=
idwest. Wheat curl mite (WCM) was first no=
ticed
in Kentucky
in 1987, with a larger outbreak in 1988 (9). Since that time, a significant
infestation of WCM mite occurred in south central Kentucky in 2000, with smaller scatter=
ed infestations
in more recent years.
It was first believed that outbreaks of this pest were=
the
result of mites carried in on winds from more western production areas. Though this is possible, and wind =
is a
method of dispersal, it appears more likely that this outbreak occurred bec=
ause
of the lack of weed control (thus increase in volunteer wheat) in soybeans
during the preceding summer.
Volunteer wheat provides a “green bridge” that may have
allowed the WCM to “over-summer” and build into much larger tha=
n normal
numbers. This is the typical =
cause
of economic problems with WCM in the western states. Typically in Kentucky there w=
ould be
no green bridge. Conversely, =
in
some poor soybean production years, weed control is reduced or abandoned an=
d,
thus, volunteer wheat remains in fields.
Spring Pests=
The cereal leaf beetle,=
Oulema
melanopus (Linnaeus), was first noted feeding in south central Kentucky in the
mid-1980s (10). Since that time it has moved generally westward to the Mississippi River counties. Cereal leaf beetle (CLB=
) is a
sporadic pest with a tendency to damage the later maturing varieties.
Control of CLB is comparatively easy.&n=
bsp;
Still, determining the need to control in a timely fashion is the mo=
re
important decision. Work done in the late 1990’s (11) produced scouti=
ng
procedures and thresholds that are currently in use. However, this insect is so rarely a
problem it is likely the “old” threshold of one CLB per head
bearing stem is most often used, except in the most highly managed wheat.
The armyworm, Pseudaletia
unipuncta (Haworth), also known as,
“True” armyworm, is usually the first pest of wheat to appear in
the spring (12). Armyworm (AW)
makes its annual appearance each spring in “flights” of the adu=
lt
moths. These flights can be
monitored by capturing males using pheromone baited traps (3, 13). The numbers caught using this tech=
nique
can provide an advanced warni=
ng of
the insect, allow calculation of when the damaging stage (worm) will appear=
(14)
and can be compared to trap capture data from previous years (15).
AW is very common in Kentu=
cky
but only rarely does sufficient damage to warrant control. In spite of this,
spectacular outbreaks of this pest do occur. One recent outbreak occurred in
2001. In this year our early
“peak” trap captures were more than three times the
“average” (15, 16). Very large populations appeared first in the
south, then progressively through the Midwest into Canada. Considerable damage w=
as
done to the first cutting of grass hay in Kentucky. Conversely, effects on small grain=
s are
debatable because of the late occurrence of the infestation.
AW is most often controlled by naturally occurring predators and
parasitoids. For example, eggs of tachinid
flies (Diptera: Tachinidae) are commonly found just behind the head =
on
armyworm larvae. Very dense plant stands and especially lodging, along with
cool cloudy springs, favor AW populations. Insecticidal control is relative=
ly
easy, if necessary.
Important Natural Enemies
There are many natural control agents operating in the=
small
grain fields of Kentucky.
As previously mentioned, caterpillars parasitized by tachinid flies, plus braconid wasps (Hymenoptera: Braconidae), =
and
infections by fungal and viral pathogens are often seen. Braconid parasitoi=
ds
in the genus Aphidius have been
collected from the grain aphids. In addition there are a plethora of predat=
ors,
e.g., ground beetles, (Coleptera: Carabidae) and syrphid flies (Diptera:
Syrphidae), easily observed. Though often given short shrift, these natural
controls, combined with good cultural practices, probably account for much =
of
the insect pest control in Ke=
ntucky
wheat.
Pesticide Strategies
Insecticide use is an important tool in managing insect
pests in small grains (17). Still, if,
when, and how applications are made are often more important than what insecticide is used. This is especially true when calcu=
lating
economic returns.
Soil Applied Insecticid=
es - Decision must be made
before planting. Treatments a=
re
very general, broadcast over a wide area, and affect many animals other than
the target pests. Few insecticides available, and most are relatively toxic.
This technique has most certainly decreased.
Seed Applied Systemic I=
nsecticides - Like soil insecticid=
es these
must be chosen before planting, in fact when the seed is purchased. Treatme=
nts
are more targeted affecting mostly insects that feed directly on the plant.=
They
are a good choice for crops at increased risk to BYD (early planted), but r=
elatively
more expensive than other applications.
Foliar Applied Insectic=
ides - Decisions can be mad=
e as
pests begin to appear. Like the soil insecticides, foliar applications are =
general
applications made over a broad area and affect many animals other than the
target pests. Though, if targ=
eted
in time and area and based on pest threshold values; they are probably the =
most
cost effective. If used corre=
ctly,
they require the most management skill.
References4
You may find the references listed below to be of some=
use. Nonetheless,
you should recognize that entomologist and crop production specialists in y=
our
state(s) will have similar publications tailored for your production area(s=
).
Kentucky Pest News newsletter. Weekly during field crop
season, biweekly in winter. Delivered by regular post, e-mail, and availabl=
e on
the web at:
http://www.uky.edu/Ag/kpn/kp=
nhome.htm
Sandell, L. [ed=
.]
2002. Crop Profile for Winter Wheat in Kentucky.http://pes=
tdata.ncsu.edu/cropprofiles/docs/KYwheat.html
Kentucky
Integrated Crop Manual for Field Crops: Small Grains. P. Lucas [ed.]. IPM-4=
. http://www.uky.edu/Ag/IPM/manuals.htm.
<=
o:p>
Johnson,
D. Using pheromone traps in field crops. Entfact – 112.
Johnson,
D. Procedures for selecting random locations for sampling. Entfact - 118.
Johnson,
D. and D. Voegtlin. 2006. Aphids captured in Kentucky aphid suction traps. Unpublis=
hed
Data.
Johnson,
D. and L. Townsend. Aphids and Barley yellow dwarf (BYD) in Kentucky grown wheat. Entfact – =
121.
Flanders, K., A. Herbert, D. Buntin, D. Johnson, K.
Bowen, J. Murphy, J. Chapin, and A. Hagan. Barley Yellow Dwarf in Small Grains in the Southeast. htt=
p://www.ca.uky.edu/entomology/entfacts/entfactpdf/ef150.pdf
Johnson,
D. Hessian fly in Kentucky.
Entfact – 101.
Townsend,
L., D. Johnson and D. Hershman. Wheat streak mosaic virus and the wheat curl
mite. Entfact - 117.
1=
0. Johns=
on, D.
Cereal leaf beetle in Kentuck=
y
wheat. Entfact – 107.
11.&=
nbsp; Her=
bert,
Jr., D. A. and J. W. VanDuyn 1999. Cer=
eal
Leaf Beetle Biology and Management. 199=
9.
VPI&SU No. 444-350. http://=
www.ext.vt.edu/pubs/entomology/444-350/444-350.html
&n=
bsp;
1=
2. Johns=
on, D.
Armyworms in small grains. Entfact – 111.
1=
3. Johns=
on,
D., and S. McNeill. 1993. Plans and Parts List for the “Texas” Style Cone Trap for Monit=
oring
Certain Insect Pests. Entfact-010. (HTML only).
&nb=
sp;
1=
4. Johns=
on,
D., R. Bessin and L. Townsend. Predicting insect development using degree d=
ays.
Entfact – 123.
1=
5. Lucas=
, P.
Insect Trap Data (Archive). http://www.uky.edu=
/Ag/IPM/trapdata/trapdata.htm
1=
6. Lucas=
, P.
IPM Trap Counts (Weekly). http://www.uky.=
edu/Ag/kpn/kpn_06/pn060925.htm#trap
17.
Johnson, D. Insecticide Recommendations for
Small Grains. ENT- 47. http://www.uky.edu/Ag/PAT/recs/rechome.htm
Footnotes=
1. Proceedings of the Indiana Certified Crop Advisors Confer=
ence. Indianapolis, IN. =
5 Dec
2007.
2. Extension Professor, Department of Entom=
ology,
University of Kentucky, UK-REC. 1205 Hopkinsville
St. Princeton, KY 42445-0469. doug.johnson@uky.edu
3. Common and
scientific names of insects from: Common Names of Insects and Related
Organisms. Entomological Society of America. http://www.entsoc.org/pubs/books/common_names/index.htm#About_=
this_Publication
4. The Entfact=
s may be
found at: htt=
p://www.uky.edu/Agriculture/Entomology/entfacts/eflists.htm.
Other on-line references =
contain
direct links.
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