Quick Facts...
- Grasshoppers often appear first in weedy areas of roadsides, fence
rows, irrigation ditches and other noncrop areas.
- After these food plants are gone, the Insects leave in search of other
food, often an irrigated crop or newly emerged winter wheat.
- Control grasshoppers in the weedy areas with low rates of insecticides.
Once they reach field margins, they may be larger and require higher
rates of insecticides for control.
- Two options are available to farmers once it has been determined that
Crops are threatened: poison baits and foliar or soil insecticides.
Grasshoppers are one of the most important insect pests in Colorado.
They follow a roughly 22-year cycle in Colorado. The last major outbreak
was in the late 1970s and early 1980s. Some problems occur even in years
of low numbers. Although they are most important on rangeland, they also
attack field Crops, often with economic losses to the farmer. An exception
is sorghum, which usually is not fed upon once it has reached about 10
inches in height.
Grasshoppers lay eggs in undisturbed areas, usually in late summer and
early fall. Small nymphs or "hoppers" hatch the following spring. Winged
adults appear five to six weeks after hatch. Eggs of a few Colorado species
hatch in late summer and pass the winter as nymphs. Winged adults of these
species usually appear early in the following summer, often causing undue
alarm about unusually early grasshopper activity. Some of these early-season
species are important on rangeland, but none are considered a threat to
field Crops. Most field crop damage is caused by the differential, redlegged,
two-striped and migratory grasshoppers, all species that follow the typical
grasshopper life cycle.
Control of Grasshoppers
The usual pattern of grasshopper damage in field Crops is for early development
to occur in weedy areas of roadsides, fence rows, irrigation ditches and
other noncrop areas. As these food plants are eaten or dry down, the grasshoppers
leave in search of other food, often an irrigated crop or newly-emerged
winter wheat. Here they first feed in the field margins and then, conditions
permitting, spread throughout the field.
Grasshoppers become more difficult and expensive to control as this pattern
develops. Grasshoppers in the weedy areas are concentrated in a small
area. They can therefore be controlled with low rates of insecticides
applied to a relatively few acres. Once they reach the field margins,
they may be larger and require higher insecticide rates for good control,
although the acreages involved will still be small. After they spread
throughout a field, high insecticide rates applied to larger acreages
are required to protect the crop.
Table 1 gives information useful in deciding if a grasshopper population
is enough of a threat to a crop to justify spending money on an insecticide
treatment. Modify these general guidelines according to grasshopper species,
crop conditions, and crop value. For example, use lower counts for a valuable
crop such as pinto beans and higher counts for a lower value crop such
as dryland winter wheat. Walk through the field and count the grasshoppers
that jump or move within a square foot area. Multiply that number by 9
to get a count per square yard. Take at least 20 counts per field. Consider
treatments when the average count reaches the threatening level.
Two options are available to farmers once it has been decided that Crops
are threatened: poison baits and foliar or soil insecticides.
Poison Baits
The main advantage to poison baits is that they can be applied to Crops
or weedy areas in which the plants are too small for good insecticide
spray coverage, such as newly-cut alfalfa or weeds that have dried or
been eaten down. Under other conditions, insecticide sprays are cheaper
and more effective.
Sevin insecticide-impregnated bran bait is available as a 2 or 5 percent
formulation. This method can provide good control when applied just before
winter wheat emergence, when crop plants are only a few inches tall, or
in areas with short, dry vegetation. The keys to success are uniform distribution
of bait and reapplication if the bait is no longer attractive to grasshoppers.
Attractiveness of the bait is reduced substantially by moisture (rain
or heavy dew).
Nosema locustae, a disease organism that attacks many grasshopper
species, is also available in bait form. This disease can reduce grasshopper
populations over a period of several years, but Nosema baits
will not protect a crop during the same growing season in which it is
applied. Nosema baits may be effective when applied early in
the season against small nymphs, but there are no data showing their effectiveness
in protecting field Crops.
Foliar insecticides
Foliar insecticides are the treatment of choice in most situations. The
High Plains Integrated Pest Management Guide lists the insecticides
currently approved for control of grasshoppers on Colorado field Crops
and noncrop areas that serve as infestation sources. This guide is available
at www.highplainsipm.org. These registrations are subject to change, so
check the current label. Be sure to follow all label instructions and
precautions.
Winter Wheat
Grasshoppers pose a significant threat to emerging winter wheat because
the plants are small and the grasshoppers are adults. Light to moderate
infestations, as defined in Table 1, can cause stand reductions in field
margins. Treatments can be limited to field borders.
Options for controlling low to moderate infestations include foliar insecticides
applied just prior to crop emergence or granular or liquid
systemic insecticides applied in the seed furrow at planting. In-furrow
treatments must be applied with appropriate equipment to ensure uniform
coverage and to avoid phytotoxicity. Be sure to read, understand
and follow all label instructions. If grasshopper infestations
are high, consider doubling the seeding rate for the outer one or two
passes.
| Table 1: Treatment guidelines based on number
of grasshoppers (nymphs and adults) per square yard. |
| Grasshopper Population |
Field |
Field
Margin |
Treatment necessary? |
| Non-economic |
0-2 |
5-10 |
No |
| Light |
3-7 |
11-20 |
Questionable, depends on size, species, type of crop |
| Moderate |
8-14 |
20-40 |
Probably |
| Abundant |
15+ |
41+ |
Yes |
The following websites provide a large amount of information on the biology,
indentification and management of grasshoppers in the West.
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