(Orthoptera: Acrididae)

Grasshoppers are one of the most destructive agricultural insect pests in Canada. Chewing insects that feed on vegetation, grasshoppers are a serious problem, especially for farmers. Infesting field crops, gardens, pastures, grasslands, and nurseries, they may cause millions of dollars of damage during outbreak years (Chapman and Joern, 1990).

In many old books and journals, grasshoppers were called locusts. The Latin equivalent of the word 'locust' means 'a burned place', an apt description of an area that has been devastated by grasshoppers (Parker, 1954). In many countries, the term 'locusts' is used for grasshoppers that march in bands (nymphs) or fly in swarms (adults). The individuals of the same species may be 'grasshoppers' when they are scarce and solitary and 'locusts' when abundant arid gregarious. In North America, most entomologists refer to them as 'grasshoppers', regardless of they are solitary or gregarious, scarce or abundant.

There is also some confusion about the terminology used for 'Mormon crickets'. Discovered by the Mormon settlers in Nevada in 1848, they are not crickets. Technically, they are long-homed grasshoppers. Unlike 'locusts', 'Mormon crickets' do not fly. However, they are avid walkers.

1. Species Described:

In the U.S.A. and Canada, there are about 600 species of grasshoppers, only about 12 of which occur frequently enough in high numbers to be of economic importance (Hewitt and Onsager, 1982). In Canada, there are only about a half-dozen species in this category.

In each region of Canada, different grasshopper species or groups of species are considered pests. In Ontario, for example, the spur-throated grasshopper, Schistocerca americana, is the major pest species. In the prairies, on the other hand, where choice habitat is available for grasshoppers, there is a complex of 4 species which are of major economic importance. These latter 4 species are covered in this integrated pest management (IPM) profile.

The members of the order Orthoptera include grasshoppers,.crickets, mantids, katydids, walkingsticks and cockroaches (Helfer,1953). Grasshoppers belong to this family Acrididae. The taxonomy of grasshoppers has been reviewed by several authors (e.g., Hebard, 1936; Handford, 1946).

Adult grasshoppers are identified by examining their body size, shape, color, stripes and patterns (Pfadt, 1988). The nymphal identifying characters include color patterns of the hind legs, variations in dark bands on the head and pronotum, and the shapes and sizes of certain anatomical structures.

There are 4 commonly-found grasshoppers in Manitoba: i.e.,

1.1 Clear-winged Grasshopper:

Although called the clear-winged grasshopper, this species belongs to the banded-wing group of grasshoppers. Camnula pellucida adults are yellow to brown and marked with large, dark-brown spots which are conspicuous on the forewings (Gibson, 1915). The hind-wings are clear. They are 2-2.5 cm long (Anon., 1981b). When concentrated together, these grasshoppers are much more yellow in color than the more solitary forms of this species (Criddle, 1933a).

1.2 Two-Striped Grasshopper:

A spur-throated grasshopper, Melanoplus bivittatus, is variable in color (reddish-brown to dark-green) with 2 distinguishing pals, light-yellow or cream stripes extending across the prothorax and forewings and forming a triangle on the tegmina (Webster, 1915; Herrick and Hadley, 1916; Anon., 1981a).

It also has small, rounded light spots on the brown, basal portion of the eye (Hanford, 1946) and the yellow on the hind femur has a conspicuous black dorsal band. The adults are relatively large, usually 2.5-3.7 cm long, the male being smaller than the female.

The adults are paler in color, more slender, and have longer wings when they are in their migratory phase (Criddle, 1933a).

1.3 Packard's Grasshopper:

Melanoplus packardii is another spur-throated grasshopper. It has green and fawn forms (Handford,1946). Males are about 2.5 cm long and females about 3.1 cm long. They, have long, feathery forewings (Anon., 1981c).

They have a lighter color and slightly longer Wing-length when abundant (Griddle, 1933a).

1.4 Migratory Grasshopper:

Melanoplus sanguinipes is also a spur-throated grasshopper. It is brown to yellowish-gray in color. Adults are 2-2.8 cm long (Anon., 1977). It has a hump on the underside, between the middle pair of legs, behind the spine.

2. Geographical Distribution:

Vickery (1989) reviewed the biogeography of Canadian grasshoppers: i.e.,

2.1 Clear-winged Grasshopper:

Camnula pellucida is widely distributed throughout the entire northern U.S.A. and southern Canada (Parker, 1924).

2.2 Two-Striped Grasshopper:

Melanoplus bivittatus is also widely distributed throughout be northern U.S.A. and southern Canada. Vickery (1989) provided a map of collection localities for this species in North America.

2.3 Packard's Grasshopper:

Melanoplus packardii, in Canada, is most common in Saskatchewan, having a much more restricted range in bordering provinces.

2.4 Migratory Grasshopper:

Melanoplus sanguinipes is the most widely distributed species in North America.

3. Habitat:

I

Quinn et al. (1991) described the habitat characteristics of rangeland grasshoppers: i.e.,

3.1 Clear-winged Grasshopper:

Camnula pellucida is found in the rich soils zones of the prairies (Criddle, 1933b). The Red River valley of Manitoba is a prime example. In addition to road allowances, it may also occur in meadows and closely-grazed pastures.

3.2 Two-Striped Grasshopper:

Melanoplus bivittatus is considered a semi-lowland grasshopper (Criddle, 1933b). It may often be found in moist, heavy-textured, weedy or grassy roadside ditches, weedy fields, clover fields, and fence rows.

3.3 Packard's Grasshopper:

Melanoplus packardii is considered a true prairie species (Criddle, 1933b). It prefers light-textured soils. It is a serious pest in Saskatchewan and, to some degree, small parts of Alberta and Manitoba.

3.4 Migratory Grasshopper:

Melanoplus sanguinipes normally selects well-drained, light soil and sparse vegetation in cropped fields or idle farm land. It may also oviposit in drift soil, weedy pastures, brome, and roadside ditches.

4. Hosts:

Grasshoppers usually feed on forbs and grasses (Pfadt and Lockwood, 1988) but often will feed on vegetables and field crops, especially wheat, oats, and barley. Most species show distinct food preferences but the economically important species are generally omnivorous. During outbreaks, grasshoppers tend to eat anything "green'.

When very abundant, they may also attack trees and shrubs in orchards and nurseries, feeding on leaves, fruits and even bark. Mitchener (1953), quoting Alexander Henry's journal of 25 July 1808, reported that "the very trees were stripped of their leaves".

4.1 Clear-winged Grasshopper:

Camnula pellucida is a grass-feeding species that also feeds on a few broad-leaved plants (Criddle,1933b; Harris, 1985).

4.2 Two-Striped Grasshopper:

Melanoplus bivittatus is a mixed feeding species, ingesting both forbs and grasses (Baily and Mukerjl,1976a,b). It also feeds on alfalfa and on lush weeds in ditches. This species often causes damage to young farm shelterbelts (Harris, 1985).

4.3 Packard's Grasshopper:

Melanoplus packardil is also a mixed feeding species, preferring forbs (Quinn et al., 1991).

4.4 Migratory Grasshopper:

Melanoplus sanguinipes prefers forbs and grasses (Hinks et al., 1990). It thrives in weedy grain fields, cultivated pastures and hay fields (Harris, 1985).

5. Life Cycle:

Grasshoppers have one generation per year in Canada. Oviposition usually begins in late July and continues into the fall. It may take 0.5-2 hours for a female to lay one egg pod (i.e., clusters of eggs in a frothy secretion that hardens), depending on ambient temperatures (Griddle, 1933b). The eggs are placed by the female in a cavity in the soil which is later covered, using the abdominal valves or the legs, depending on the species.

The number of eggs deposited varies with the species, weather and food supplies (Criddle, 1933b; Pfadt,1988). Those species depositing the fewest egg pods usually have the most eggs per pod (and vice versa). Depending on species, the number of egg pods per female varies from 4-25.

Most grasshoppers overwinter as eggs in the soil; a few species overwinter as nymphs or adults (Griddle, 1933b). In the prairies, grasshopper egg hatch usually begins in late April or early May, peaks about mid-June, and is complete by late-June (Randell and Mukerji, 1974; McBride, 1984). Soil temperature plays an important role in hatching which usually begins when the soil temperature is 15-16c for about 200 hours (Hewitt, 1979). 1

Grasshoppers undergo incomplete metamorphosis (Pfadt, 1988). The newly-hatched grasshoppers are about 5 mm in length. The nymphs resemble the adults but have wing pads instead of wings. There are usually 5 or 6 nymphal instars. Grasshopper nymphs usually mature in 35-55 days, depending on environmental conditions and species.

The nymphs and adults of a given species have food preferences. Their diet has a major impact on their growth, survival and fecundity (Hinks et al., 1990).

Adults may live for 4-6 weeks after mating and oviposition. Some of the surviving adults may continue feeding into the late fall, usually being killed off by a heavy frost.

Grasshopper dispersal is commonly associated with the flying adults, especially of the migratory grasshopper and, to a lessor extent, the clear-winged grasshopper (Parker, 1954). Some spectacular flights, up to 1000 km, were reported in the late 1930's in North America (Parker at al., 1955). However, the nymphs also can move short distances, in search of food. Drought conditions may stimulate grasshoppers to find areas with cooler temperatures, more moisture and greater food (Criddle, 1933b).

5.1 Clear-winged Grasshopper:

Eggs of Camnula pellucida are usually laid in pods of 25 eggs in the sod around fields, especially among the roots of western couch grass. Several thousand eggs may be laid per square foot.

5.2 Two-Striped Grasshopper:

Shorthorned grasshoppers, like Melanoplus bivittatus, overwinter as eggs, laid in weeds and grass, just below the soil surface. The eggs hatch in the spring, yielding nymphs that resemble the adults. The small, bright green, wingless nymphs emerge from the soil and feed on adjacent food plants. As they deplete their food, they disperse into adjoining areas.

Nymphs undergo 5 or 6 moults, maturing within 40-60 days. Growth and maturity may be delayed by cool, humid weather.

The adults mate and continue to feed on nearby plants. The females oviposit 200-400 eggs in pods. They lay their eggs almost anywhere in the soil. Each pod contains 15-120 eggs. They stick their ovipositor into the ground to lay their eggs just below the surface.

6. Seasonal Abundance:

Generally, grasshoppers increase in numbers during the season, peaking in July and August and then decreasing over time until they are destroyed by heavy fall frosts. Outbreaks tend to occur after several successive years of conditions favourable for survival and fecundity.

Sanchez and Onsager (1988) described the life history parameters of the migratory grasshopper in pastures.

Models, simulating the population dynamics of Camnula pellucida, Melanoplus packardii, and Melanoplus sanguinipes, were described by Hardman and Mukerji (1982) and Johnson and Worobec (1988). They considered the effects of weather conditions and food quality on development and survival of the eggs, nymphs and adult grasshoppers. Populations tended to decline in areas of above-average rainfall.

In recent years, there has been some interest in developing geographic information systems (taking into account habitat characteristics and weather conditions) to better understand the dynamics of grasshopper populations (e.g., Johnson, 1988, 1989; Kemp et al., 1989).

7. Responses to Environmental Factors:

Weather plays a major role in grasshopper population trends (Harris, 1985). Winter soil temperatures affect the mortality of grasshopper eggs. When the snow cover is abnormally light and allows the soil temperature (at about 5 cm) to reach -15' C, sortie mortality may occur (Mukerji and Braun, 1988).

Soil temperature has an affect on egg hatching. When the temperature reaches a certain point, the eggs are stimulated to begin hatching (Criddle, 1933b).

Ambient temperatures affect the oviposition behaviour of grasshoppers, most egg-laying occurring in the late afternoon (Criddle, 1933b).

Soil moisture affects egg hatch. If eggs are under water, hatching will be delayed until the water is gone (Criddle, 1933b).

Weather and food supplies will affect the rate of grasshopper development (Criddle, 1933b; Mukerji and Randell, 1975; Mukerji et al., 1977) and their population distributions (Gage and Mukerji, 1977; Mukerji et al., 1977). Hot, dry weather causes the grasshoppers to move quickly through their 5 nymphal instars, resulting in high adult populations early in the season. Cool, wet weather causes a late and erratic hatch which may extend into late June. Slow, erratic nymphal development results in low adult populations late in the summer. Given optimum conditions, most grasshoppers will mature in about 30-40 days.

Abnormally high rainfall may destroy many small grasshopper nymphs, injuring and beating them into the ground (Criddle, 1933b). Cold weather or prolonged damp spells may slow development.

Cold weather in the fall kills off the adult grasshoppers after they have laid their eggs.

Several workers have investigated the relationships between grasshopper population dynamics and various weather factors, including temperature, sunshine, rainfall (e.g., Riegert, 1968; Gage et al., 1976; Mukeril et al., 1977; Kemp and Dennis, 1989).

Holmes et al. (1979) found that grazing by cattle may affect the abundance of grasshoppers on grassland. Certain species may be found on lightly or mode irately grazed fields but not on heavily or very heavily grazed fields (and vice versa). Melanoplus bivittatus and Melanoplus sanguinipes were more abundant on lightly or moderately grazed fields. Heavier grazing usually resulted in higher populations of grasshoppers, perhaps reflecting their preferences for oviposition sites that were warmer and easier to use.

8. Importance:

Grasshoppers, when abundant, can cause serious economic loss to crops in western Canada, affecting crop yield and grade. Harris (1985) estimated that annual cop losses in Saskatchewan were about $5million and that, during outbreaks, costs could exceed $40 million. Mitchener (1953) considered grasshoppers to be the most destructive of all the field crop insects that occur in Manitoba. He reviewed the history of grasshopper outbreaks and their control in Manitoba, noting that wheat, oats and barley were most often injured.

Gage and Mukerji (1978) studied the losses in wheat, oats, and barley associated with grasshoppers. Results showed that losses caused by grasshoppers were dynamic but small relative to the dynamics of crop price and yield losses independent of grasshoppers. Economic losses caused by grasshoppers tended to occur when economic gains were otherwise depressed because of poor crop yields due to drought or low crop prices.

Metanoplus bivittatus was studied by Mukerji et al. (1976) to evaluate the damage that it can cause to spring wheat. They found that grasshoppers waste 6.2-times as much wheat as they consume. Foliage damage is reflected in lower grain yields.

Melanoplus sanguinipes may cause severe damage on the prairies, especially of wheat crops (Hardman et al., 1985). Melanoplus sanguinipes damage and losses to wheat have been studied (Pickford and Mukerji, 1974; Capinera and Roltsch, 1980). A significant inverse relationship between grasshopper density and grain yield was seen. They found that lower yields may be due to (1) early hatching of grasshoppers and complete destruction of newly-germinated seedlings; (2) gradual defoliation of the crop, reducing seed yield and grade; and/or (3) head clipping towards the end of the season.

 

Hardman et al. (1985) developed a model, simulating the effects of this species on the yield of spring wheat planted in infested stubble.

In areas of local abundance, they also may damage vegetables in home gardens and trees and shrubs in nurseries, orchards and recreational areas (Buckell, 1921 Mitchener, 1953). In fruit orchards, grasshoppers may completely strip the leaves and may kill young trees (Milliken, 1917; Anon., 1975).

Hewitt (1977) extensively reviewed the topic of forage losses caused by range land grasshoppers. Grasshoppers compete with livestock for forage, consuming about 20% of available vegetation in normal years (Hewitt and Onsager, 1982; Lockwood and Kemp, l987). They may damage plants permanently due to continuous feeding or the destruction of seed heads (Hardman and Smoliak, 1982).

Hardman and Smoliak (1980) investigated the potential economic impact of Melanoplus packardii and Melanoplus sanguinipes on forage rangeland in Alberta.

Some grasshoppers can transmit plant diseases (e.g., potato spindle tuber, turnip yellow mosaic, tobacco mosaic, tobacco ringspot, and verticillium wilt of alfalfa [Huang and Harper, 1985; Harper et al.,1988) and bird parasites (e.g., poultry tapeworm).

Grasshoppers are not just an agricultural pest. High populations around airports may attract feeding birds, increasing the chances of bird strikes to aircraft.

Criddis (1933a) and Mitchener (1953) reported that massive swarms of the two-striped grasshopper have resulted in the shores of Lake Winnipeg being covered in decaying grasshoppers, 5-20 cm deep. The stench was very unpleasant to vacationers.

Parker (1954) reported that grasshoppers may cut binder twine, eat holes in clothes hanging on the line, get inside homes where they damage clothing and curtains, eat holes in tents and parachutes in military camps, pollute wells, cisterns and reservoirs, smear windshields, plug radiators, and make railway tracks slippery for trains. He also pointed out that grasshoppers can bite, even draw blood, when molested by people.

Herbicide researchers, carrying out field evaluations of herbicides often complain about grasshoppers eating the plastic marker flags delimiting their field plots (J. Buth, pers. comm.). The grasshoppers climb up the wire stakes and nibble off the flag which then falls to the ground.

On the 'plus' side, grasshoppers are a source of food for praise nesting birds and small mammals (Hewftt and Onsager, 1982). They may also be considered litter producers in that they waste about half of what they consume. Fishermen sometimes use them for bait. Some people even eat them, considering them a delicacy (Madsen, 1989).

9. Natural Enemies:

Harris (1985) stated that natural enemies are, next to weather, the most important factor controlling grasshopper populations. Grasshoppers are attacked at all stages of their life cycle by pathogens, parasites and predators.

Pathogens:

Fungal diseases affecting grasshoppers have been known for many years (e.g., Webster, 1915; Criddle,1920; Parker, 1954; Chapman and Joern, 1990).

A fungus disease, Entomophagus grylli (a complex of more than one species), occurs naturally in the Canadian prairies, infecting grasshoppers, especially during seasons when the humidity is high (Erlandson et al., 1988). Typically, infected grasshoppers climb to the top of stems and wrap their legs around them and then die. The body is filled with fungal spores. As it disintegrates, the sticky spores transfer to other grasshoppers, killing them.

In recent years, there has been a renewal of interest in the use of protozoans for grasshopper control. Lockwood (1988, 1989) describes the biology and application of Nosema locustae, a fungus that infects the fat body of grasshoppers and is eventually lethal. A drawback is that it requires 3-5 weeks to kill the grasshopper. However, infected grasshoppers generally eat less, are more vulnerable to predation, and fail to reproduce.

Another protozoan, Malameba locustae, infects grasshoppers and is being investigated for the control of the migratory grasshopper (Hinks et al., 1987).

Other protozoans being investigated for grasshopper contra include Nosema cuneatum, Nosema acridophagus, Aspergillus parasiticus, Entomophaga grylli,Verticillium lecanii, and Beauveria bassiana (Erlandson et al., 1985, 1986, 1988; Johnson et al., 1988; Moore and Erlandson, 1988).

Beauveria bassiana is the fungus the nearly devastated the silkworm industry in 19th century France. A strain with a very narrow host range is being developed to control grasshoppers by Agriculture Canada, based on specimens collected from grasshoppers in Montana.

Apparently, there are grasshopper entomopox viruses and baculoviruses in North America but no information on these biocontrol agents was available for this review.

Parasites:

Grasshopper eggs are sometimes parasitized by wasps, Scelio calopteni (Mitchener, 1953; Mukerji,1987) but they are not considered to have much impact on Grasshopper populations.

Grasshopper nymphs and adults are sometimes parasitised by nemestrinid, sarcophagid and tachinid flies.

Hairworms (long, whitish and extremely slender) sometimes parasitise grasshoppers. Sometimes called 'horsehair worms', the eggs of this parasite are ingested by crickets and grasshoppers. After hatching, the worm penetrates the grasshopper's body cavity and grows in length. When the weakened host falls into water, the worm exits and swims away. Sometimes, these worms show up in bath tubs indoors, brought in by a cricket.

Predators:

Grasshopper eggs are fed on by blister beetle larvae, the larvae of carabid beetles, and the larvae of bee flies (Webster, 1915; Criddle, 1920; Parker, 1954). In Manitoba, the bee fly, Systoechus vularis is the only known species of bee flies involved. Three species of blister beetles have been reported in Manitoba: i.e., Macrobasis fabricii, Macrobasis subglabra and Lytta sphaericollis. The only carabid involved is Percosia obesa.

Red mites are also sometimes seen on grasshopper eggs and the nymphs and adults but their effects, if any, are poorly known (Milliken, 1915, 1917; Criddle, 1920). Eutrombidium locustarum is considered a significant predator of grasshopper eggs in Manitoba.

Nymphs and adults may be fed upon by robber files. Yellowjackets and spiders have also been reported feeding on grasshopper nymphs and adults (Milliken, 1915, 1917; Parker, 1954).

Toads, lizards and snakes feed on grasshoppers (Milliken, 1915, 1917).

Franklin's gull is often seen by the thousands in infested fields, feeding on grasshoppers. Other birds preying on these pests include terns, blackbirds, crows, killdeer, meadowlarks, horned lark, longspurs, grouse, quail, prairie chickens, shrikes, cuckoos, cowbirds, catbirds, kingbirds, woodpeckers and English sparrows (Webster, 1915; Criddle, 1920; Mitchener, 1953). Parker (1954) goes so far as to say all birds (except 'strictly vegetarian doves and pigeons') feed on grasshoppers.

Amongst the small mammals, mice, shrews, ground squirrels, males, cats, skunks, coyotes and foxes may dig through egg-infested land to feed on the egg pods or feed on nymphs and adults. Hogs will also search the soil for egg pods (Milliken, 1915, 1917).

Cannibalism has also been reported in some grasshoppers (e.g., Lockwood,, 1989).

II. MANAGEMENT

Each province that has a grasshopper problem publishes grasshopper control circulars or bulletins. These should be consulted before carrying out a control program. Usually, the publications include information on monitoring and control, including recommended insecticides.

In addition, a number of publications have been prepared, especially in the U.S.A., on certain types of grasshopper problems. For example, a number of publications have been written on IPM for rangeland grasshoppers, including a rational basis for control (Onsager 1985), between-season and within-season variation in grasshopper population dynamics (Onsager, 1983, 1986a; Onsager and Hewitt, 1982), insecticides for grasshopper control (Onsager, 1986b), and factors that influence the selection of control measures (Onsager, 1986c, 1987).

1. Population Monitoring Techniques:

Field or crop scouting should be the basis of a control program against grasshoppers. Grasshopper surveys and population estimates may be made in various ways. Larger nymphs and adults may be sampled by making semi-circular sweep with a 34cm diameter net at a rate of one sweep per step through the foliage. However, with smaller nymphs, cooler weather or higher and denser foliage, this technique is limited. The collected specimens may be preserved in 70% alcohol and later counted and identified.

Some workers use 'foot-square' counts to rate their fields. This is simply the number of grasshopper nymphs that jump from a square-foot area. When the nymphs are just hatched, one has to get down on his/her hands and knees to see the nymphs.

McBride (1984) suggests conducting a grasshopper survey, using foot-square counts, by starting at one corner of a field, walking diagonally past the center, then turning and walking straight out to one side of the field. This may be repeated at several locations within a field until the field has been adequately sampled. One should make at least 18 of these square-foot counts in a survey. Dividing the total grasshoppers counted by 2 will give the approximate number of grasshoppers per square yard or square meter. It is best to record these counts on a record form, along with the field number, crop stage, date, time, and temperature. Over time, such information provides a valuable reference for decision-making on control measures.

Smith and Stewart (1946) developed a bottomless cage for doing such counts. The cage is dropped on the ground and the grasshoppers trapped inside are counted. Researchers, carrying out efficacy studies, often use this sampling method.

Quinn et al. (1991) described grasshopper sampling methods used by researchers in studying grasshopper habitats and insecticide efficacy. Gangwere (1960) and Oma et al. (1990) described methods of rearing grasshoppers for research purposes. Pfadt (1988) described methods of collecting, mounting and identifying grasshoppers.

Aerial infra-red photography may be used for the detection and assessment of grasshopper damage to small grain crops (Olfert et al., 1980). It is generally less expensive to use than conventional ground surveys.

Traditionally, federal and/or provincial entomologists conduct annual grasshopper egg survey to determine infestation levels and prepare grasshopper forecast maps for the following year. Such surveys have been conducted since the 1930's (Gage and Mukerji, 1977; Mukerji and Hayhoe, 1988). These maps assist farmers in alerting them to expected arc as of infestation, enabling them to monitor their own fields in the spring and to make preparations for grasshopper control. The weather conditions occurring between egg deposition and egg hatching will, of course, affect such forecasts. Similarly, weather conditions during May and June will affect grasshopper damage.

Harris (1985) pointed out that the number of eggs in an area can be determined by taking a soil sample of about one meter square and about 5 cm deep and passing through a screen of 6 mesh hardware cloth.

Some workers are incorporating their survey results into computerized geographical information systems to generate colored maps, indicating infestation levels (Johnson, 1989).

Constant field monitoring is advised when the grasshoppers reach the adult stage. The winged adults ran move quickly into fields, causing serious damage within a few days.

2. Threshold/Action Population Level:

Economic injury levels (Stern, 1966, 1973; abbreviated EILs) have been developed for grasshoppers on some crops and range land (Hewitt and Onsager, 1980; Onsager, 1984; Torell et al., 1989). The economic threshold is the density at which control measures should be initiated to prevent an increasing pest population from reaching the economic injury level. The economic injury level is the lowest population density that will cause economic damage. Economic damage is the amount of injury which will justify the cost of artificial control measures. The cost control should be less than the value of the crop loss if no control is used (Ba-Angood and Stewart, 1980). Traditionally, the USDA has supported an EIL for grasshoppers on rangeland of 9.6 grasshoppers/m² (Onsager, 1984).

Riegert (1968) described the history of sampling and forecasting grasshopper outbreaks in Saskatchewan. He showed the evolution of rating methods as more and more was understood of the factors affecting grasshopper populations. He summarized the ratings based on adults per square yard and eggs per square feet.

For adults, the ratings (in grasshoppers/m²) were as follows: i.e.,

Category Rating Field Roadside

Normal 1- < 0.25 < 1

1 0.25-1 1-4

Light 2- 1-1.5 5-6

2- 1.5-2 7-8

Moderate 3- 2-3 9-12

3 3-4 13-16

Severe 4- 5- 6 17-24

4 7- 8 25-32

Very Severe 5- 9-12 33-48

5 > 13 > 49

Obviously, several other factors, besides grasshopper density, should be considered in making a decision on whether or not to spray (Torell et al., 1989). These include the stage of grasshopper development, the stage of crop maturity, crop vigour, insecticide label restrictions, the insecticide application rate required, the cost of application, the spray equipment available and its costs, and the possible coincidental existence and cumulative effects of other insect pests in the crop.

3. Management Alternatives - Non-Chemical:

Mitchener (1953), quoting George Dawson (1876), reported that early settlers in the prairies were encouraged to control grasshoppers by 'firing of prairie grass after hatching, fall ploughing where eggs have been laid and gathering eggs and receiving government bounty by measure for the eggs collected, the use of heavy rollers on the young, and driving young into straw which would be burned'. Similar advice was given in farmers' extension bulletins for many years (e.g., Criddle and Mitchener, undated; Milliken, 1917; Cooley et al., 1918; Criddle, 1920; Buckell, 19 21; Tullis and Vigor, 1921).

Burning grasslands in the fall is useless, having no effect on the eggs (Criddle, 1920). Burning may, in fact, benefit grasshoppers by providing the small nymphs with easier access to sunshine and warmth on colder days and by inhibiting the development of protozoan and fungal diseases of grasshoppers.

Some farmers used to raise hogs or chickens, turkeys or guinea fowl to protect their gardens from grasshoppers (Milliken, 1915).

Many farmers used an old device called the 'hopperdozer' to catch grasshoppers in their fields (e.g., Packard, 1915; Cooley et al., 1918). Drawn through the field by horses, the 'dozer used shallow trays (of water, with a thin film of kerosene, crude oil or tar), mounted on low wheels or runners, to trap the disturbed nymphs and adults. Criddle (1920) reported that farmers could collect 14 bushels of grasshoppers per day using a hopperdozer.

Some farmers used a variation of the hoppordozer to catch the grasshoppers alive (Buckell, 1921). They loaded them into sacks where they died. Once dead, the toppers were spread out in the sun to dry. After drying the hoppers were re-sacked and stored for winter poultry food.

Cultivation of fields in the fall may help control grasshoppers by exposing some of their eggs to predation, sun, wind and frost or, in the case of deep tilling, by burying the eggs so deep that then nymphs cannot reach the surface when the eggs hatch. However, the impact on the following years' population will be minimal and soil and moisture conservation also should be considered Anon., 1986).

'Zero till', 'no-till' or 'minimum-till' farmers should be aware of the increased potential for grasshopper problems when such management practices are followed (Hardman et al., 1985).

Many farmers, especially in Saskatchewan, practice summerfallowing - every second or third year, a crop field is left idle for a season. Grain stubble that is to be summerfallowed should be worked before the eggs hatch in the spring. Destroying all green growth on stubble in the spring at the time of grasshopper hatching may help to starve the young hoppers.

One should start tilling on the outer margins of the field, working inwards. Weedy trap strips may be left in the field (Harris, 1985). Grasshoppers will concentrate their feeding on these green strips. The nymphs can then be sprayed using an insecticide, minimizing the amount required.

Early spring seeding is important in reducing grasshopper damage (Anon., 1975). Early-seeded crops can make good, vigourous growth before the grasshopper eggs hatch. The crops can then stand feeding better than late-seeded crops.

Although the parasites and predators of grasshoppers may have some affect on populations, the pathogens have most potential for biological control. There is renewed interest in the use of protozoans against grasshoppers. The microsporidian pathogen, Nosema locustae, has a low but persistent virulence that allows it to infect grasshoppers of all ages (Johnson, 1989a,b; Johnson and Pavlikova,1986; Johnson and Henry, 1987). Lockwood (1988) and Lockwood and Debroy (1990) described methods of producing, storing and applying Nosema locustae spores as a bran bait. Although expensive, slow-acting (3-5 weeks) and providing limited control (10-50%), it has no adverse environmental impact. Ewen and Mukerji (1980) carried out an in-depth evaluation of this microsporidian against

several species of Canadian grasshoppers. This microbe is under review for registration as a biological pesticide in Canada. It may play a role in future integrated grasshopper management programs.

Other pathogens have been investigated as potential biocontrol agents for grasshoppers. Harper and Huang (1986) evaluated Verticillium lecanii against Melanoplus sanguinipes. (See also, section above on 'natural enemies').

Another non-chemical solution to grasshoppers may, some day, come in the form of grasshopper-resistant crops. Some plant breeders are searching for cultivars of wheat, oats and barley that reduce grasshopper development, survival and fecundity.

Mitchener (1953) reported that 'poison' began to be used against grasshoppers in Manitoba beginning in 1901. The provincial government supplied parts green free to those farmers who could use it. The poison was used in bran baits which were scattered on the ground where the grasshoppers occurred (e.g., Herrick and Hadley, 1916). Some farmers used fresh horse manure as a carrier instead of bran (known as the 'Criddle mixture'; Gibson, 1915). Subsequently, other poisons were used and various refinements were made to the bait recipes (Sherwood, 1918; Urbahns, 1920; Cooley et al., 1923, Parker, 1924).

The use of conventional insecticide sprays began in 1948 with the introduction of chlordane and toxaphene. The advantages of spraying chemicals include the availability of spray equipment and the relatively high rates of grasshopper mortality achieved. The disadvantages include mortality of non-target insects, environmental risks and the cost of product and application.

Onsager (1987) pointed out that insecticide bait provides a cheap, fast and selective method for reducing grasshopper infestations and stated that the technology has been grossly under-exploited. The value of using insecticide - bran baits was investigated by various authors (e.g., Ewen and Mukerji, 1982,1987; Mukerji et al., 1981; Mukerji and Ewen, 1984; Quinn et al, 1989), especially on rangeland and forage crops. Dimethoate baits seem to be most efficacious. The main benefits seem to be lower cost, less insecticide used, more specific to grasshoppers, and avoidance of insecticide drift.

Now, a variety of insecticides are registered for grasshopper control in Canada, reflecting the importance of grasshoppers as a serious agricultural pest.

Based on efficacy studies that have been carried out (e.g., Elliot and lyer, 1982; Ewen et al, 1984a,b; Mukerji and Ewen, 1984; Johnson et al., 1986; Hinks and Spurr, 1988), insecticides should be applied evenly, at the right time, and in the right place. Insecticides are most effective and the costs are lower when they are applied after the eggs hatch and while the nymphs are still concentrated in their breeding areas (Hardman and Mukerji, 1982).

Often, grasshopper nymphs can be residually sprayed in roadsides, headlands and field margins before they move into a field crop, vegetable garden or nursery. The low dosage given on the label should be used when the grasshoppers are small and the vegetative cover is low.

Higher rates may be necessary when the grasshoppers are older, the cover is thicker or the population is very high. Generally, it costs twice as much to treat adult grasshoppers in a field as it does to treat young grasshoppers along a ditch or field margin.

If spraying is necessary late in the season when vegetation is heavy, farmers should use adequate water in the applications: i.e., 12 1/a aerially and 30-40 I/a using ground equipment.

When grain is drilled into heavily-infested, unworked stubble, he field may later swarm with nymphs. Immediate spraying of the entire field is usually necessary.

Pfadt and Hardy (1987a,b), Hewitt and Onsager (19830 and Onsager (1981, 1987, 1988) described the value of and methods of controlling grasshoppers on rangeland in the U.S.A.

Harris (1985) gave the following pointers on insecticide use: i.e.,

• Most insecticides work best at temperatures between 15 and 20C.

• Malathion should not be applied when temperatures are < 20C.

• Carbaryl, cypermethrin and deltamethrin are most effective at lower temperatures.

In spraying any insecticide, one should carefully follow label precautions. The periods from application to harvest should be carefully followed to avoid any food Residues. For example, with Decis 5 EC, the 'pre-harvest interval' is 40 days for flax.

Follow directions on 'days to grazing'. For example, with Decis 5 EC, one must wait 1 day before allowing beef cattle to graze on treated pastures. Dairy cargo must not be fed or grazed on Decis-treated crops.

Any 'setbacks from sensitive zones' should be followed. For example, with deltamethrin (Decis 5 EC), there is a 100 m setback from environmentally-sensitive areas when this insecticide is applied by aircraft.

Carbaryl (Sevin XLR) @ 2300-3500 ml/ha (500-1400 ml/a). There is no waiting period for carbaryl on forage. It may be applied by air or ground equipment. It works as a contact and stomach poison. Use lower rates on young plants and early stages of insects and higher rates on mature plants and advanced stages of insects, or mature insects. Do not apply tank-mix combinations unless your previous experience indicates the mixture is effective and will not result in application problems, excessive residues or plant injury.

Dimethoate 40 @ 525-1000 ml/ha (210-405 ml/a), Furadan 480 F @ 275 ml/ha (110 ml/a), and Malathion 50 EC @ 1600 ml/ha (640 ml/a) are also registered for this use.

One should note that certain insecticides for grasshopper control are registered for use on some crops but not others. Dimethoate (Cygon), deltamethrin (Decis), malathion and carbaryl (Sevin) are examples of products registered for use on vegetable crops. Spraying both the garden and adjacent grassy areas may minimize any feeding damage and prevent reinvasion.

Naled (e.g., Ortho Dibrom Insecticide) is registered for grasshopper control in woodlands.