| ||Insect life cycle | Damage assessment | Management strategy
Insect Life Cycle
- Host plants
Two-striped grasshoppers feed on grasses and broad-leaved plants. The broad-leaved plants are necessary for maximum growth. They prefer the lush growth around edges of streams, marshes and cultivated fields. Hosts include weeds and most crops, especially alfalfa and vegetables, and occasionally trees and shrubs. They were first noted in large numbers in 1932 after broad-leaved weeds became common on the Prairies.
This species overwinters in the egg stage. Drift ridges of soil in abandoned fields that suffered severe wind erosion are favored egg laying sites. Other areas include heavier textured soils along roadsides, closely cropped pastures, fence rows, ditch banks, prairie sod and field margins, but not cropped fields.
Air temperature must be above 20ºC and soil moisture between 10 and 20 per cent for egg deposition to occur. Forty to 100 eggs are laid per pod; only two or three pods are laid by each female during August and September.
- Spring appearance
First instar nymphs appear in late May to early June.
- Number of generations
There is one generation per year.
- Natural enemies
The two-striped grasshopper has a greater number of natural enemies of the egg stage than have the clear-winged or migratory grasshoppers. The nymphs and adults are attacked by at least twelve species of insect parasites, two mermithid nematode species, three microbial pathogens and by various birds, small mammals and parasitic mites. Insect parasites are most abundant two or three years after an outbreak. There are various records of heavy mortality from fungus disease to which the two-striped grasshopper seems particularly susceptible. Red mites have infested up to 100 per cent of a population; however, about eight per cent is a more normal figure. Their effect on these grasshoppers is unknown.
- Damage description
Damage to cereal crops is generally concentrated near field margins and is caused when hatchling grasshoppers move out of egg beds into field edges; damage to grasslands tends to be more evenly distributed. Damage to cereals includes leaf notching and stripping but is most costly when stems are severed just below the heads of maturing or mature crops. When grasshopper numbers are extremely high and natural plant hosts in short supply, grasshoppers will consume or attempt to consume any plants or plant products that they come upon during their migrations in search of food.
- Sampling and monitoring methods
Walk through the infested area and estimate the number of grasshoppers per square metre as they jump in front of you. A sampling 'T' will likely improve your estimate. The 'T' consists of a metre-long measuring stick, carried by a handle so that a square metre can be visualized at crop height. Walk and carry the 'T' just above the crop.
Late summer and fall surveys of grasshopper adults have been carried out by agriculture fieldmen in Alberta since 1932. Grasshopper forecast maps are produced yearly from data collected in about 1,700 townships.
Spring surveys of grasshoppers and grasshopper eggs are also conducted in years when high grasshopper numbers are expected. In this way, improved estimates are obtained for time of hatch, population density and the effects of predators and parasites.
Economic thresholds for grasshoppers:
Number of grasshoppers/m2
Control not usually required
0 - 6
0 - 12
Control may be required
7 - 12
13 - 24
- Effects of weather
Population size is primarily determined by weather. Outbreaks are usually preceded by two to three years with hot, dry summers and open falls. Dry weather increases the probability of egg survival, hastens spring hatch, and promotes nymphal development and adult feeding. Open falls allow grasshoppers more time to feed and lay eggs. Cool, wet weather increases egg mortality by promoting fungal diseases, retards nymphal development, reduces the numbers of eggs laid by delaying sexual maturity and reduces the activity of grasshoppers at all stages.
- Cultural practices
Tillage - Cultivation is probably the most effective cultural practice available to farmers for the reduction of grasshopper populations. Tillage controls grasshoppers primarily by eliminating the green plants on which grasshoppers feed.
However, tillage is of little value for the sole purpose of physically destroying grasshopper eggs or exposing them so that they dry out or are eaten by birds and other insects. Excessive tillage may also increase the risk of soil erosion.
Tillage to eliminate weeds from summer fallow fields during late summer and early fall will discourage female grasshoppers from depositing their eggs in these fields. Grasshoppers seldom lay eggs in clean summer fallow even when it has a heavy cover of trash. Similarly, thorough cultivation of fields immediately after harvest will help discourage grasshoppers from laying all their eggs within the field.
Complete spring tillage before grasshoppers hatch to eliminate all green growth on stubble fields that are to be in summer fallow. If no food is available when grasshoppers hatch, they will starve to death because they are unable to move long distances to find food. Early tillage will also give good weed control and conserve moisture at no extra cost.
Trap Strips - If grasshoppers are present when tillage operations begin, elimination of all green plant material in a field will probably not achieve adequate control. Once grasshoppers have fed and developed to the second stage of growth (second instar), they usually are mobile enough to move to adjacent crops when their food supply becomes exhausted. Use trap strips in these fields to collect grasshoppers into a relatively small area where quick and economical control will be possible with a minimum of insecticide.
To make strips, cultivate a black guard strip 10 m wide around the outside of a field. Leave an uncultivated green strip at least 10 m wide before resuming cultivation. Repeat the process as often as necessary to produce additional trap strips. All green vegetation must be eliminated between the trap strips if they are to be effective. The black guard strip will ensure that grasshoppers promptly move into the trap strips to feed.
The effectiveness of trap strips can be improved considerably by seeding them to wheat, barley or oats several weeks before tillage begins. Trap strips should have adequate vegetation to feed even the largest of grasshopper populations for three to five days.
Migration of young grasshoppers from the cultivated guard strips to the trap strips may take several days. Once the migration is complete, the trap strips and a 10 m strip of adjacent crop should be treated with insecticide. Apply the highest recommended rate of insecticide to ensure adequate control.
Before cultivating the trap strips, allow three days to assess the effectiveness of the insecticide. If adequate control is not achieved after three days, treat the trap strip again. When grasshoppers have been eliminated from the trap strip, it should be possible to complete tillage without fear of displacing large numbers of grasshoppers into adjacent crops.
Summer fallow that is not properly managed can be a major source of grasshoppers. Many cases of growers applying insecticides six or seven times to a field border are a direct consequence of improper grasshopper control on summer fallow.
Early seeding - Seed crops as early as possible. Older plants can withstand more grasshopper damage than younger plants that are not well established. Although early seeding may not totally prevent crop damage, damage will be reduced and more time will be available to apply insecticides. Also, crops that are seeded earlier will mature earlier and migrating grasshoppers are not as likely to be attracted to them.
Crop rotation - Whenever possible, avoid seeding cereals on stubble fields heavily infested with grasshoppers. Seed cereals only on stubble fields where soil moisture is adequate and where one or more applications of an insecticide over the entire field is economical.
Canola seeded with an in-furrow application of a granular insecticide may be a suitable alternative to cereals where grasshopper infestations are light.
Roadside vegetation management - Certain of our common roadside weeds, such as stinkweed, alfalfa and dandelion, are nutritious food plants for nymphs and adults. Such plants promote high survival and egg laying. Western wheatgrass on the other hand, is one of the poorest food plants.