| | Insect life cycle | Damage assessment | Management strategy
Schizaphis graminum
Insect Life Cycle
Host plants
The greenbug feeds on cereals and forage grasses. Late-sown oats, barley, fall rye and wheat are usually susceptible when this aphid is abundant, although in one year the greatest injury was to early-seeded fields of winter cereals. Timothy grown for forage is most susceptible because of succulent growth that appears after harvest.
Overwintering and spring appearance
The species is normally unable to survive the winter in Canada. Infestations are begun mainly by flying aphids that are carried into Canada on southerly winds. The species passes the winter on fall planted wheat and volunteer grains in Oklahoma and Texas. It migrates north in the spring, and passes through several generations during migration. The numbers that eventually reach Canada are presumably influenced by whether migrating individuals find suitable food plants, conditions suitable to successful colonization, and southerly winds when populations are flying. In 1986, infestations in southern Alberta did not arrive until early July.
Number of generations
The greenbug produces many generations each season. The number of generations that can be produced in Alberta depends on the time of initial infestation, crop condition, and temperature.
Natural enemies
Predators - Lady beetles (lady bugs), lacewings, big-eyed bugs.
Parasites - Parasitic wasps.
Pathogens - Various fungal pathogens.
Damage Assessment
Economic importance
The greenbug, an introduced species, has been on the Prairies since 1907. Greenbugs are not normally a problem because they do not overwinter in Alberta. Heavy migration and outbreaks do occur and large areas were infested in the 1930s and in 1986. This species of aphid injects a toxin into plants while feeding. The toxin causes brown spots at sites where aphids feed and causes the plant to turn yellow. Plants are set back from their normal maturity date. The aphid can carry barley yellow dwarf virus and maize dwarf mosaic virus.
Damage description
The condition of host plants may influence infestations; in one instance damage to late seeded barley and oats was highest in low lying land that had been flooded earlier. In another instance, serious outbreaks were attributed to the coincidence of aphid flight with a time when late crops were at a succulent stage of development. Late-seeded crops are more susceptible because they are less able to withstand attack, have more attractive succulent growth, and are less likely to produce a crop should plants recover.
Damage is caused primarily by the toxin that is injected into the plant. Aphid colonies are on the lower parts of the plant; necrosis (browning) of lower leaves occurred throughout infested fields during the 1986 outbreak. From a distance, however, fields took on a yellow appearance that was evident, on close inspection, in the top growth. In many barley fields, the unnatural yellow color was likely attributed to severe scald and net blotch, which were prevalent in 1986.
Economic threshold
From 5 to 25 aphids per stem warrants chemical control action, that is, the economic threshold is dependent on the stage of crop growth, the health of the crop and growing conditions. Abundant predators and parasites should enable greater populations to be tolerated.
Management Strategy
Effects of weather
The species has been abundant in hot dry weather. Cool weather may prevent population increases. Rain can wash migrating aphids out of the air.
Biological control
Predators, especially lady beetles, have prevented population increases and controlled outbreaks. |
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