| ||Insect life cycle | Damage assessment
Insect Life Cycle
- Host plants
The migratory grasshopper is one of the most destructive pests in western Canada. Outbreaks can lead to costly losses for grain growers. This species attacks both field and garden crops, especially cereals, tomato, celery, onion and carrot.
Females lay pods of about 25 eggs in stubble and wheat fields, between clumps of grass or in other patches of dry soil during August and September. The eggs of this species and others in the genus Melanoplus can complete up to 85 per cent of their embryonic development before winter.
- Spring appearance
Eggs hatch between early May and mid July, although the date depends upon temperature and moisture conditions in both spring and the preceding fall. Watch for signs of hatching in stubble fields and along roadsides and pastures where adults were seen in August and September.
- Number of generations
As with all of Alberta's pest grasshopper species, migratory grasshoppers have one generation per year.
- Natural enemies
Next to weather, natural enemies are the grasshopper's most important population control factor. In some localized areas natural enemies may cause even more mortality than the weather.
Some enemies attack when grasshoppers are still in the soil awaiting spring. Others attack nymph and adult stages.
Predators - Among the most important egg predators are bee flies, blister beetles, ground beetles and crickets. Common field crickets eat the eggs and may destroy up to 50 per cent of the eggs in some areas. Bee flies and blister beetles deposit their eggs in the soil near grasshopper eggs. When the larvae of these egg predators hatch, they locate the egg pods and feed upon the eggs. If bee flies and blister beetles are abundant, they may destroy up to 80 per cent of eggs in a localized area.
Spiders, some wasps and many birds feed on grasshoppers and consume large numbers of nymphs and adults. Their effect on the total grasshopper population is not known.
Parasites - A few other insects, such as the tiny wasps of the genus Scelio, parasitize eggs just after they have been laid. The young parasitic larvae complete their development within the eggs in time to emerge as adults and parasitize the eggs of the next generation of grasshoppers. They may destroy from 5 to 50 per cent of the grasshopper eggs.
Parasites of nymphs and adults include flesh flies, tachinid flies and tangled-veined flies. Fly larvae burrow into the grasshopper when they contact it on the ground. Other fly larvae are deposited on or into the grasshopper's body by the female fly. The maggot then feeds inside the grasshopper and eventually kills its host as the maggot leaves the body. This group of insects may parasitize up to 60 per cent of the nymphs and adults.
Threadworms attack grasshoppers if the young worms encounter a grasshopper or if grasshoppers eat threadworm eggs. Threadworms overwinter in soil and lay their eggs on the soil or on vegetation.
Pathogens - The fungus, Entomophthora grylli, can effectively control grasshoppers under warm, humid conditions. This fungus may occasionally reach epidemic proportions. The disease leaves the corpses of its victims clinging to the stems of plants.
The microsporidian parasite, Nosema locustae, is an effective enemy of grasshoppers. A grasshopper becomes infected if it eats contaminated vegetation or a diseased grasshopper. A grasshopper population infected with this organism may be reduced by as much as 60 per cent in one year. It also reduces the number of eggs laid and restricts the movement of individuals and thus affects grasshopper populations. Perhaps this organism's greatest potential as a biological control agent, however, is for reducing food consumption.
Since most of the natural enemies of grasshoppers are already widespread, it is unlikely that they could be used to prevent grasshopper outbreaks over extensive areas. Nevertheless, natural enemies do control localized grasshopper infestations and hasten the decline of grasshopper outbreaks.
Poultry - During the depression, some farmers successfully used turkeys to control grasshoppers. In years when grasshoppers were plentiful, the turkeys were simply released into fields. This resulted in a secondary benefit: the turkeys required little supplemental food since grasshoppers provided a plentiful high quality protein diet.
Other birds - Gulls, hawks, crows, meadowlarks, crowned larks, lark buntings, desert horned larks, shrikes, curlews, killdeer, partridges and cranes are all predators of grasshoppers. Many birds scratch up the egg cases, and have been credited with clearing from 5 to 150 acres of grasshopper pods. Birds, especially gulls and meadowlarks, are credited with stopping some infestations in the early part of the century. This is more likely to happen if a habitat provides food and refuge for the birds. Farm habitat can be made more attractive to birds and so encourage predation of insect pests.
Vertebrates other than birds - Most animals are opportunists; they will eat what is nourishing and available to them. Mice, rats, shrews, gophers and badgers all eat grasshoppers and their egg pods. Coyotes, skunks, lizards, snakes, toads, bobcats and kit foxes eat nymphs and adults.
- Economic importance
The migratory grasshopper is normally the most numerous pest grasshopper species in Western Canada and is often injurious to cereals.