Managing
Insects in Wheat: An Overview.
Douglas W. Johnson, Department of Entomology, University of Kentucky
2007 Illinois Crop Protection Technology
Conference
3-4 Jan. 2007, Champaign, IL
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 variation in date of first frost, fall onset of
continuous cold, and spring onset of general warm weather, makes it particularly
difficult to predict insect impact in any given year.
In Kentucky, wheat is grown as a “winter crop”
planted in the fall, usually following corn, and harvested the following summer.
Most often varieties which 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, and 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 find 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 Illinois farmers? Generally, the situation in southern
Illinois will be very similar to that in west
Kentucky. Conversely, as one moves further north
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 – 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, and 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 Kentucky
Integrated Pest Management (UK-IPM) scout manual for
small grains (2). In addition,
there are other methods of collecting data on important pests such as the use of
pheromone baited traps to capture adult moths (3). Whatever method you use, try to avoid
“selecting” your data by using randomly assigned data collection points (4).
Insect Pests
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-winter
in Kentucky. FAW migrates into Kentucky from the gulf
coast in mid-summer, 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,
reclaimed land, ditch banks, roadsides, etc., but may also infest small grains.
FAW can result in seedling death if it feeds on new plants before roots are
established. If plants are
established, FAW feeding is rather more like grazing. FAW can remain active until the first
killing frost and will survive longer where 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. Insecticidal control is relatively easy;
but, there are no established thresholds.
Damaged fields are sometimes replanted, 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 complex
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 (Sasaki), is very likely playing an important role, though
little is known of this aphid in Kentucky. It is interesting to note that these
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 other
hand, there are at least twenty five aphids that can move the yellows viruses
among plants. Regardless of the
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 (5).
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 insecticide use,
especially the systemic disulfoton (Di-Syston®), and then in the mid-90’s the
synthetic pyrethroid lambda-cyhalothrin (primarily Karate® and Warrior®) over the historic use
pattern were quite evident (5).
Moreover, there has been some historic use of systemic insecticide seed
treatments, primarily imidacloprid (Gaucho®), and now thiamethoxam (Cruiser®). Use of these latter seed
treatment products is likely to increase.
Control of the
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 cost
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 due out
shortly (6).
The Hessian fly, Mayetiola destructor (Say), is
another common insect pest infesting small grains in the fall (7). In general,
planting after the “fly free” date will provide adequate control in Kentucky. Agronomic and cultural factors favoring
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 (foliar applied insecticides), though use of systemic insecticides as
seed treatments and fall/ spring foliar applications of systemic insecticides
have been used in the deep south to combat this problem. Here, in the presence of good agronomic
practices, preapplied insecticide treatment is not warranted.
The wheat curl mite, Aceria tosichella Keifer,
is a common pest of wheat in Nebraska and other
plains states but is rarely a problem in the Midwest.
Wheat curl mite (WCM) was first noticed in Kentucky in 1987, with a
larger outbreak in 1988 (8). Since
that time, a significant infestation of WCM mite occurred in south central
Kentucky in
2000, with smaller scattered 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 because
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 than normal numbers. This is the typical cause of economic
problems with WCM in the western states. Typically in Kentucky there would be
no green bridge. Conversely, in
some poor soybean production years, weed control is reduced or abandoned and,
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
(9). 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.
Still, determining the need to control in a timely fashion is the more
important decision. Work done in the late 1990’s (10) produced scouting
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, the “True armyworm”,
is usually the first pest of wheat to appear in the spring (11). Armyworm (AW) makes its annual
appearance each spring in “flights” of the adult moths. These flights can be monitored by
capturing males using pheromone baited traps (3, 12). The numbers caught using this technique
can provide an advanced warning of
the insect, allow calculation of when the damaging stage (worm) will appear (13)
and can be compared to trap capture data from previous years (14).
AW is very common in Kentucky 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” (14, 15).
Very large populations 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. Conversely, effects on small grains 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 relatively 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 parasitoids in the genus Aphidius have been collected from the
grain aphids. In addition there are a plethora of predators, 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
Kentucky
wheat.
Pesticide
Strategies
Insecticide use is an important tool in managing insect pests
in small grains (16). Still, if, when, and how applications are made are often
more important than what insecticide
is used. This is especially true
when calculating economic returns.
Soil
Applied Insecticides
- Decision must be made before planting.
Treatments are very general, broadcast over a wide area, and affect many
animals other than the target pests. Few insecticides available, and most are
relatively toxic.
Seed
Applied Systemic Insecticides
- Like soil insecticides these must be chosen before planting, in fact when the
seed is purchased. Treatments 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 relatively more expensive than other
applications.
Foliar
Applied Insecticides
- Decisions can be made 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 targeted in time and area and based on pest threshold values;
they are probably the most cost effective.
If used correctly, they require the most management
skill.
References
You may find the references listed below to be of some use.
Nonetheless, you should recognize that entomologist and crop production
specialists in your 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 available on
the web at:
http://www.uky.edu/Ag/kpn/kpnhome.htm
- Sandell, L. [ed.]
2002. Crop Profile for Winter Wheat in Kentucky.http://pestdata.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.
- Johnson, D. Using
pheromone traps in field crops. Entfact – 112.
- Johnson, D. Procedures
for selecting random locations for sampling. Entfact - 118.
- 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. In Press.
- 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.
- Johnson, D. Cereal leaf
beetle in Kentucky wheat. Entfact – 107.
- Johnson, D. Armyworms
in small grains. Entfact – 111.
- Johnson, D., and S.
McNeill. 1993. Plans and Parts List for the “Texas” Style Cone Trap for Monitoring
Certain Insect Pests. Entfact-010. (HTML only).
- Johnson, D., R. Bessin
and L. Townsend. Predicting insect development using degree days. Entfact –
123.
- Lucas, P. Insect Trap
Data (Archive). http://www.uky.edu/Ag/IPM/trapdata/trapdata.htm
- Lucas, P. IPM Trap
Counts (Weekly). http://www.uky.edu/Ag/kpn/kpn_06/pn060925.htm#trap
- Johnson, D. Insecticide
Recommendations for Small Grains. ENT- 47. http://www.uky.edu/Ag/PAT/recs/rechome.htm