Common Insects of Wheat in Kentucky
Doug Johnson
Department of Entomology, University of Kentu=
cky
Proc. Eastern Wheat Workers and Southern Small Grain W=
orkers
Conf. 9-12 May 2005. Bowling Green, KY.
Introduction<=
o:p>
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 impa=
ct 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. This production system divides the insect pests into three groups: th=
ose
that infest in the fall, and that either do or do not over-winter, and those
that infest in the spring. We will examine these pests in order of appearan=
ce
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. However, each pest has =
the
potential to cause significant damage under appropriate conditions.
Insect Pests =
in
General Order of Appearance
Fall Only
The fall Armyworm, Spodoptera frugiperda ( J.E. Smith), is a common pest=
of several
late planted summer and early planted fall crops. Fall armyworm (FAW) cannot over-win=
ter in
Kentucky. FAW migrates into Kentucky from the gulf coast in mid-su=
mmer, initially
infesting corn as its primary crop host.&n=
bsp;
In late summer / early fall, as corn begins to mature, it colonizes
newly seeded grasses. Damage =
is most
common in lawns, reclaimed land, ditch banks, and roadsides, etc., but may =
also
infest small grains. FAW can damage small grains (in fact any host) if it f=
eeds
on seedlings before roots are established, resulting in seedling death. If plants are established FAW feed=
ing is
rather more like grazing. FAW=
can
remain active until the first killing frost and will survive longer where c=
rop
residue provides shelter from the cold.
Infestation usually results from early planting. Often, planting after the Hessian =
fly
free date (Johnson1993a) w=
ill
avoid this situation. However,
occasionally frost and onset of cold weather are late enough to allow
infestation of small grains. =
Insecticidal
control is relatively easy; however, there are no established thresholds. Damaged fields are sometimes repla=
nted, but
this is a risky technique. Ma=
ny
damaged plants will survive, thus, replanting may result in a denser than
desirable stand.
&nbs=
p; &=
nbsp;
Fall and Spring
The Hessian Fly, Mayetiola destructor (Say), is
another common insect pest infesting small grains in the fall (Johnson1993a). See Cambron’s treatment else=
where
in these proceedings. In general, planting after the “fly free”
date will provide adequate control in Kentucky. Agronomic and cultural factors fav=
oring strong
stems and stand ability 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 seed treatmen=
ts
and fall / spring applications of the systemic insecticide disulfoton, (e.g=
., Di-Syston® 8 and generics),
targeted at aphids may have some effect.
There is little doubt that the most important insects =
in
Kentucky-grown wheat are a complex of cereal aphids, HOMOPTERA: Aphididae (=
Johnson
and Townsend 1999). In Kentucky, this c=
omplex
is primarily composed of the bird
cherry-oat aphid, Rhopalosiphum padi (Linnaeus); English grain aphid, Sitobion
avenae (Fabricius); corn leaf=
aphid,
Rhopalosiphum maidis (Fitch) and greenbug, Schizaphis gram=
inum
(Rondani). In addition, the rice root aphid, Rhopalosiphum rufiabdominal=
is
(Sasaki), 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 important for their direct damage, but rather as vectors of =
the
viral pathogens that result in Barley Yellow Dwarf (BYD).
The risk, real and perceived, of Barley Yellow Dwarf, =
and by
association, the cereal aphid complex is without doubt the driver of
insecticide use in Kentucky-grown wheat. Though other insects may require
insecticidal control from time to time, only the cereal aphids are treated =
in a
routine manner. Beginning in the early 1990’s, increases in insectici=
de
use, especially disulfoton, and then in the mid-90’s the synthetic
pyrethroid lambda-cyhalothrin (primarily Karate® and Warrior®) over the historic =
use
pattern were quite evident (Sandell=
2002). Additionally, there has been some =
use of
systemic insecticide seed treatments, primarily imidacloprid (Gaucho®),=
but
this use is very hard to measure.
<=
/o:p>
Control of t=
he
cereal aphid complex is relatively easy to obtain. It is, as with most other insect p=
ests
of wheat, IF treatment is warranted that is hard to decide. The difficulty of finding aphids,
combined with the small number required for application, the relative
inexpensiveness of insecticides and the fear of catastrophic loss to BYD pr=
obably
results in more insecticide use than is needed to mitigate the risk.=
Spring Only
The armyworm, Pseuda=
letia
unipuncta (Haworth), also know as the
“True armyworm”, is usually the first pest of wheat to appear in
the spring (Johnson 1994a). Armyworm (AW) makes its annual appe=
arance
each spring in “flights” of the adult moths. These flights can be monitored by =
capturing
males using pheromone baited traps (Johnson1994b, Johnson and McNeill 1993)=
. 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
(Johnson, Bessin and Townsend1998=
)
and can be compared to trap capture data from previous years (Lucas 2004).
AW is very common in Kentu=
cky
but only rarely does sufficient damage to warrant control. However, spectac=
ular
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” (Lucas 2004). Very large populati=
ons
appeared first in the south, then progressively through the Midwest into Canada.=
Considerable
damage was done to the first cutting of grass hay in Kentucky. However, effects on small grains a=
re 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 stands and especially lodging, along with cool
cloudy springs, favor AW populations. Insecticidal control is relatively ea=
sy,
if necessary.
The cereal leaf beetle, Oulema
melanopus (Linnaeus), was first noted feeding in south central Kentucky in the
mid-1980s (Johnson 1993b). Since that time it has moved generally westward =
to
the Mississippi River counties. Cereal l=
eaf
beetle (CLB) is a sporadic pest with a tendency to damage the later maturing
varieties.
Control of CLB is relatively easy.
However, determining the need to control in a timely fashion is the =
more
important decision. Work done in the late 1990’s on thresholds for th=
is
insect (Herbert and VanDuyn 1999) produced scouting procedures and threshol=
ds
that are currently in use. Ho=
wever,
this insect is so rarely a problem it is likely the “old” thres=
hold
of one CLB per head bearing stem is most often used, except in the most hig=
hly
managed wheat.
Pests Associate with Particular Events
The wheat curl mite, Aceria tosichella Keifer, =
is a
common pest of wheat in Nebraska and oth=
er
plains states but is rarely a problem in Kentucky
(Townsend, Johnson and Hershman 1995).&nbs=
p;
Wheat curl mite (WCM) was first noticed in Kentucky in 1987 with a larger outbrea=
k in
1988. Since that time signifi=
cant infestation
of WCM mite occurred in south central Kentucky
in 2000.
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 t=
o “over-summer”
and build into much larger than normal numbers. This is the normal cause of econom=
ic
problems with WCM in the western states. Normally in Kentucky there would be no green
bridge. However, in some poor
soybean production years, weed control is reduced or abandoned and, thus,
volunteer wheat remains in fields.
Natural Controls
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: Sy=
rphidae),
easily observed. Though often given short shrift, these natural controls,
combined with good cultural practices, probably account for much of the ins=
ect
pest control in Kentucky
wheat.
&nbs=
p;
References<=
o:p>
Herbert, Jr., D. A. and J. W.
VanDuyn 1999. Cereal Leaf Beetle Biology and Management. 1999. VPI&SU No. 444-350. http://=
www.ext.vt.edu/pubs/entomology/444-350/444-350.html
Johnson,
D. 1993a. Hessi=
an Fly
in Kentucky.
Entfact-101.
Johnson,
D. 1993b. Cereal Leaf Beetle in <=
st1:place
w:st=3D"on">Kentucky Wheat. =
Entfact-107.
Johnson,
D. 1994a. Armyworms in Small Grai=
ns. Entfact-111.
Johnson, D. 1994b.Using Pheromone Traps in Field Crops
(A practical cheat sheet). Entfact-112.
<=
/o:p>
Johnson, =
D. 2005. Small Grain Insecticide
Recommendations. ENT-47. htt=
p://www.uky.edu/Agriculture/PAT/recs/crop/recgrn/recgrn.htm
Johnson, 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).
Johnson, D. and L. Townsend. 1999. Aphids and Barley Yellow Dwarf (BYD) in=
Kentucky Grown W=
heat.
Entfact-121.
Johnson, D., R. Bessin and L. Townsend. 1998. Predicting Insect Developm=
ent
Using Degree Days. Entfact-123.
Lucas, P. 2004. KY Inse=
ct
Trap Data. http://ww=
w.uky.edu/Agriculture/IPM/trapdata/trapdata.htm.
[In] Dillon, P. KY IPM web pages. http://www.uky.edu/Agri=
culture/IPM/ipm.htm
Sandell, L. [ed.] 2002. Crop Profile for Winter Wheat in Kentucky. http://pes=
tdata.ncsu.edu/cropprofiles/docs/KYwheat.html
Townsend, L., D. Johnson and D. Hershman. 1995. Wheat Streak Mosaic Virus and the Wheat Curl Mite. Entfact-117.