GRASSHOPPERS OF COLORADO

MANAGEMENT OF GRASSHOPPERS IN DIFFERENT ENVIRONMENTS

Rangeland

Rangeland in Colorado, on an area basis, is characterized by low productivity. A cow and calf may require 40 to 60 acres of short-grass prairie grazing land. Nevertheless, since over 70% of Colorado's land is used for grazing, and livestock production accounts for the greatest dollar value in Colorado's agricultural economy, rangeland is a valuable resource. Protection of this resource is vital to Colorado's economy, but any investment in protection should be followed by an increase in productivity that compensates for the protection cost. With current technology, insecticide application is the most economic pest suppression tactic. In the nearfuture, however, use of Nosema locustae may prove to be more economic, if it proves to be more residual, even though initial application costs are higher. Long-term protection cannot be guaranteed following chemical suppression (see discussions of chemical and biological control).

Mormon cricket, which is actually a grasshopper, can be damaging to rangeland in northwest Colorado. Since this species is flightless and tends to aggregate in bands when numerous, unique management measures have been employed. In the past, barriers and ditches usually in combination with poison have been used to trap immigrating bands of crickets (fig. 373, fig. 374, and fig. 375). This practice is now obsolete. Also, since mormon crickets aggregate, applying poisoned bait in the path of the migrating crickets is relatively easy, resulting in high levels of mortality. In recent years there has been a shift away from bait applications for cricket control, paralleling changes in grasshopper control. This unfortunate change in policy should be reversed. Mormon crickets are susceptible to Nosema locustae, and bait application would be required to infect these crickets.

The most economic future strategy may be prevention of grasshopper population buildup, which would alleviate the need for direct suppression costs. In a study conducted in northeast Colorado, total grasshopper numbers were highly correlated with forage biomass. Grasshopper numbers were lower in moderately to heavily grazed pastures, relative to ungrazed or lightly grazed pastures. This suggests that cattle grazing can be used to regulate grasshopper numbers. However, range depletion must be prevented, and excessive weed growth should be avoided because it favors increase in grasshopper populations; overgrazed land often supports high grasshopper numbers. Also, while total grasshopper numbers were lower in more heavily grazed pastures, grasshoppers in the subfamily Oedopodinae were more numerous (Capinera and Sechrist, unpublished). Oedopodines are less susceptible to Nosema infection than many other grasshoppers, so certain management options may be lost as a result of grasshopper species-complex shifts resulting from grazing management.

Irrespective of the management tactic chosen for rangeland grasshopper sulypression, an economic basis for action should be ascertained. Grasshoppers often are numerous on Colorado's rangeland, but number should not be the sole basis for decision making (see economic basis for grasshopper suppression). The grasshopper species generally associated with rangeland grasshopper outbreaks in Colorado are listed in table 5. However, any species can become numerous enough to be damaging under certain circumstances; conversely, the presence of a pest species does not necessarily signify a problem.

Cropland

Cropland in Colorado usually is not severely damaged by grasshoppers. Most damage occurs to alfalfa and small grains. Grasshoppers rarely develop in crops; characteristically they develop in weeds growing along roadsides, fence rows and irrigation ditches and disperse to crops as the weeds become less palatable. Thus, damage to crops generally is restricted to field edges. Treatment of crop field margins with insecticide usually will be sufficient to prevent damage to the rest of the crop. If grasshoppers are extremely abundant, however, crop loss can ensue (fig. 376).

Damage to crops often can be prevented by treatment of adjacent weedy areas. It can be much more economic to treat localized grasshopper populations, especially while they are young and more susceptible; however, all too often grasshoppers in weeds are ignored until crop damage commences. Nosema locustae treatments would seem suitable for this application.

Grasshopper species associated with rangeland usually are not found in crops (table 5). The principal exception is Melanoplus sanguinipes, which feeds primarily on rangeland forbs but also attacks numerous crops. Weedy areas, but not well-managed rangeland, are the source of most crop feeding grasshoppers. However, if rangeland is over grazed and weeds become numerous, rangeland can provide a large inoculum of grasshoppers for crop infestation.

Roadsides in Colorado often are planted with crested wheatgrass. This is a vigorous bunchgrass that requires little maintenance and is effective for erosion control. Unfortunately, however, it is a highly favored food source of many grasshopper species, including many that attack crops. Farmers should be aware that the traffic on public highways also includes insect traff ic, and that highway planners sometimes are not cognizant of agricultural concerns.



 
Table 5. Grasshopper species generally considered to be the primary pests in rangeland, cropland and suburban environments in Colorado.
Rangeland Cropland Suburban
Ageneotettix deorum Melanoplus bivittatus Melanoplus bivittatus
Amphitornus coloradus Melanoplus differentialis Melanoplus differentialis
Anabrus simplex Melanoplus femurrubrum Melanoplus femurrubrum
Aulocara elliotti Melanoplus packardii Melanoplus sanguinipes
Aulocara femoratum Melanoplus sanguinipes
Cordillacris occipitalis
Eritettix simplex
Melanoplus sanguinipes
Mermiria bivittata
Opeia obscura
Phlibostroma quadrimaculatum
Trachyrhachys kiowa

Suburban

The nature of grasshopper problems confronting the homeowner and home gardener are similar to those facing farmers. Grasshoppers do not build to large numbers in yards and gardens unless an abundance of weedy material exists. Rather, homeowners are confronted with dispersal of grasshoppers from undeveloped lots to vegetable and flower gardens and to ornamental shrubs. The species associated with suburban areas (table 5) are practically the same as those associated with commercial crop production. Homeowners also face the difficult task of attempting to control mature grasshoppers, but they are handicapped because access to the more toxic insecticides used by commercial agriculture is restricted.

Tactics used for protection of cropland are useful in suburban areas. For effective suppression with insecticides, infested areas should be treated while grasshoppers are still young and more susceptible. This may require treatment of vacant lots or other grasshopper sources. Landowners generally are cooperative and welcome attempts at grasshopper control. Suppression of grasshopper numbers before dispersal occurs is more effective than attempts at killing adults. Insecticide contamination is restricted to waste areas, where less contact by people and pets occurs. Bait formulations are very useful around the home. Since no formulation is required, there is less danger of accidental insecticide contamination or incorrect mixing. Also, Melanoplus spp. readily feed on bait, and most grasshoppers encountered by homeowners will be Melanoplus spp.

Many of the urban counties in Colorado have appointed pest inspectorsorwill appointthem upon request. County pest inspectors are authorized to examine grasshopper-infested property following written complaint. Landowners are responsible for suppression of grasshoppers on their property, if the grasshoppers pose a threat to adjacent property. Grasshoppers not controlled by landowners following proper notification may be controlled by the county at landowner expense, up to a maximum of $500 annually. Further information regarding the "Pest Control Act" can be obtained from the Board of County Commissioners and Colorado Department of Agriculture.

Alternatives to chemical control are limited. Valuable plants can be screened to prevent defoliation. Organic gardening enthusiasts sometimes recommend that marigolds or other "companion" plants be used to ward off insects; this will provide only temporary relief since grasshoppers will move to "protected" plants as soon as the companion plants are consumed. Nosema locustae will impose a relatively low level of mortality and, therefore, is limited in potential (see biological control of grasshoppers). Similarly, the insect parasitic nematode Neoaplectana carpocapsae, which kills a wide variety of insects, does not seem to be especially well suited for grasshopper control because nematode survival is poor in the hot, dry environments inhabited by grasshoppers.

Probably the most important aspect for homeowners to consider is that plants are very tolerant to defoliation. Perennials usually produce new foliage following grasshopper consumption and although growth is slowed, death is unusual. Annual crops such as peas, tomatoes and squash are less favored by grasshoppers than lettuce, spinach and onions. Gardeners may want to be selective in their planting during grasshopper outbreak years. It is not possible to have cosmetically perfect plants in the face of grasshopper invasion.

Grasshoppers of Colorado Contents