What are the thresholds used for controlling grasshoppers?
A general rule of thumb for thresholds with grasshoppers is 7-12/ square meter. However, grasshoppers have preferred crops that they will eat. Cereals are generally the preferred over broadleaved crops. Oats is an exception to this, and therefore oats will have less damage if other cereals are present. The two-striped grasshopper will also feed on broadleaved crops. Lentils are the most susceptible, as the grasshoppers like the developing pods. Canola is not preferred but will still be fed on in the young and pod stages. If the infestations of grasshoppers are high there can be damage in all stages of canola development. Peas are another crop that are not preferred food sources for grasshoppers.
Which grasshoppers are pest species and how do I identify them?
The two-striped and clear-winged grasshoppers are the most important species estimated for Alberta. The two-striped is most important in Saskatchewan and southern Alberta. The clear-winged is the dominant species in east central Alberta. At hatching they can be identified by color. The two-striped is light brown in color, and the clear-winged is black with a white band. The migratory grasshopper numbers may also be growing in Alberta, however it is not considered to be as large a threat this year. At hatching the migratory grasshopper has a black band across its thorax.
Ignore grasshoppers that are fairly large in early May, that have yellow or red wings in flight at any time of the year, and grasshoppers that make noises such as clicking or buzzing when flying at any time of the year.
Lifecycle
The pest species of grasshopper over winter as eggs. Hatching begins between early May and mid June. The newly hatched nymphs are about 5 mm long and are wingless. They begin feeding almost immediately but aren’t yet very mobile. They go through 5-6 instar stages before becoming winged. It usually takes 35-50 days to go through this change. Adults then lay eggs in areas of food supply. There is one generation of these species per year in Alberta.
Weather effects on population
Cold winter temperatures are not usually enough to kill the eggs. Spring temperatures can affect grasshoppers in two ways. A warm spring will speed hatching and development, as well as plant growth for feed availability. Cool spring temperatures will slow hatching and development. A small population of grasshoppers may inflict the same amount of damage under warm dry conditions as a large population under cool wet conditions. The chance of drowning the nymphs is very low as the eggs are laid in areas that are not likely to have water accumulation. Cool wet weather in the spring will affect grasshoppers by slowing their development and increasing the chance of fungal growth and disease that will decrease next year’s population.
Control
Control should happen around the third to fourth instar. Hatching will be winding down at this point and the older grasshoppers will still be young enough to be easily controlled. For the clear-winged grasshopper these instar stages range from 7-14 mm in length. The two-striped grasshopper will be 9-21 mm in length at these stages and the migratory grasshopper will be 8-16 mm in these stages. Once the grasshoppers have become winged they are much harder to control. There are several chemicals to control grasshoppers. They can be found in Crop Protection 2003 (the blue book), and under the insecticide calculator on Ropin the web.
Grasshopper management (includes pictures)
Prepared by Lisa Adam, Ag-Info Centre, Alberta Agriculture & Food |