Grasshoppers

There are about 180 species of grasshoppers in Canada, and of these there are four common grasshoppers found in the Canadian Prairies that can at times get to high levels and become pests of crops. These are the twostriped [Melanoplus bivittatus (Say)], migratory [Melanoplus sanguinipes (Fabricius)], clearwinged (Camnula pellucida Scudder) and Packard (Melanoplus packardii Scudder) species.

There is one generation each year, beginning with eggs that overwinter in the soil. Around late-May, once the soil temperature is warm enough, the nymphs emerge and can be present as late as early July. Adults can be found between late June until about mid-September¹.

Colour varies between and sometimes within species, as does size. All the potential pest species have relatively short antennae compared to some non-pest grasshoppers, such as katydids, and belong to a family of grasshoppers called the short-horned grasshoppers²,³.

Grasshoppers can feed on a range of plants, with different preferences between species. The species which are more likely to be found in canola fields are the twostriped and migratory grasshoppers, although canola is not generally viewed as a preferred feeding target. Clearwinged grasshopper prefers grasses and is unlikely to feed on canola¹, however it can cause feeding damage in safflower⁴.

Both nymphThe juvenile or immature stage (ex. of an insect) which goes through incomplete metamorphosis. More and adult grasshoppers feed on canola leaves, stems and pods, but the most significant damage is done when feeding on pods².

Grasshoppers can be more of an issue in hot, dry conditions. This is partially because insects, as cold-blooded creatures, eat more and grow faster in hot weather. Another reason is that hot, dry weather limits fungal infection, which can significantly impact some species of grasshoppers. Also, in dry weather, reduced natural vegetation can force grasshoppers to move to cultivated crops⁵.

Since grasshopper development is impacted by temperatures, their development can be estimated with model simulations, such as those provided by the Prairie Pest Monitoring Network on the migratory grasshopper [Melanoplus sanguinipes (Fabricius)].

However, note that there are limitations to the degree-day grasshopper development predictions (as they are estimations, often based on air temperature), and conditions vary from site to site based on soil, crop, landscape, and previous year’s weather during breeding season, so they should be considered more of a general guideline.

Improvements continue to be made to temperature-dependent insect development models⁶. Still, variance in developmental timing is a challenge for forecasting. For example, even after many days with very hot June and July temperatures (which would accumulate a large number of GDDs) many grasshopper species will still be maturing by mid-July. As well, in similar conditions (when a large number of GDDs have been accumulated) some species may still be found in multiple development stages (such as instars 2 through 5 plus adult stage)⁷.

Monitoring

For population distribution information, see the Prairie Pest Monitoring Network’s historical grasshopper risk and grasshopper forecast maps.

Monitoring for grasshopper can begin at the canola rosette stage and continue into ripening².

Counting grasshoppers can be a challenge because as soon as you walk into an area, grasshoppers take off (and do not cooperate with attempts to count them). So entomologists have come up with a more practical scouting technique. It involves three steps:

1. Walk into a few areas of the field. If grasshoppers jump around with every step you take, and seem to be general throughout the field, try to estimate approximate levels, but don’t worry if you can’t be too accurate.
2. Look for feeding and identify the species. Lots of clearwinged grasshoppers which are not feeding on the canola won’t warrant considering control (while migratory or twostriped grasshoppers may). If there is more feeding damage than there was the last time you checked, and the problem appears to be grasshopper feeding, spraying may be warranted, particularly if pods are being targeted.
3. Control grasshoppers, if needed, when wings only partially cover the abdomen. At this stage, they’re hopping but not flying, and much easier to control. Note: Some insecticides are not particularly effective against winged grasshoppers⁵.

For a precise method to monitor adult grasshopper populations, see the Prairie Pest Monitoring Network’s protocol. This includes:

1. Measure off 50 metres on the level road surface and mark both starting and finishing points using markers or specific posts on the field margin.
2. Starting at one end, either in the field or the roadside, walk toward the other end of the 50m making some disturbance with your feet to encourage any grasshoppers to jump.
3. Grasshoppers that jump/fly through the field of view within a one metre width in front of the observer are counted. (A metre stick can be carried as a visual tool to give perspective for a one metre width. However, after a few stops one can often visualize the necessary width and a metre stick may not be required. Also, a hand-held counter can be useful in counting while the observer counts off the required distance.)
4. At the end point the total number of grasshoppers is divided by 50 to give an average per square metre.
5. Compare counts to the following damage levels associated with pest species of grasshoppers:

• 0-2 per m² – None to very light damage
• 2 per m² – Lentil pod and flax boll feeding economic thresholdAn economic threshold is the level of infestation (ex. pest insect density) at which lost yield (ex. due to feeding/insect pest damage) exceeds the cost of the chemical and its application. More
• 2-4 per m² – Very light damage
• 4-8 per m² – Light damage
• 7-12 per m² – Action thresholds in cereals and canola
• 12-24 per m² – Severe damage
• >24 per m² – Very severe damage

Research into insect surveillance, such as the ‘Detection, surveillance and management of weed, insect and disease pests that threaten the economic viability of crop production and the environmental health of Prairie agro-ecosystems’ project continues to be an area of interest.

Thresholds

Action threshold is likely >12 (or 7-12) grasshoppers/m² and it is better to control younger instars, as the damage grasshoppers cause continues to accumulate from nymphThe juvenile or immature stage (ex. of an insect) which goes through incomplete metamorphosis. More through adult stages². The higher end of that range may be more appropriate in a typical canola crop as, given a choice, most grasshopper species prefer grass and cereals to canola⁵.

University of Lethbridge entomologist Dan Johnson suggests that the low end of the grasshopper economic thresholdAn economic threshold is the level of infestation (ex. pest insect density) at which lost yield (ex. due to feeding/insect pest damage) exceeds the cost of the chemical and its application. More in canola is more applicable in dry conditions, with the potential for it to be even slightly lower than seven grasshoppers per square metre, under very specific situations (hot, very dry)⁷.

When calculating economic thresholds be sure to:

• Start from a corner of the field and work your way along a line to the field centre, then to one side
• Sample at least 20 sites along this transect
• Count the number of nymphs that jump in a 1m² area as you approach each site (e.g. every 100 steps)
• Divide the total number of grasshoppers counted by the number of sites sampled to obtain the number/m²³.

Cultural options

According to the Western Committee on Crop Pests Guide to Integrated Control of Insect Pests of Crops’ Insect Management in oilseed crops in Western Canada guidelines, some practices which may impact grasshoppers include:

• destroying green growth on stubble in the spring (when grasshoppers are hatching) to potentially starve young grasshoppers.
• Growing barrier strips of a non-preferred crop like oats or peas next to an infested area at the margin of fields may act as a bit of a barrier to delay young grasshoppers from feeding on susceptible crops⁸.

If populations are significant and crop feeding has begun, a spray or bran bait application around field edges may be enough to reduce the threat. Grasshoppers are easier to manage when they’re small — when the majority are in the third to fourth instarA developmental stage within one life stage (ex. of an insect) More (as there would be too many unhatched eggs when the majority are still in the earliest instars). Except in cases where there are extremely high populations feeding on crop along the edges, ideal timing for grasshopper management is when you are starting to see wing buds on some of the older grasshoppers. Hatch of grasshopper eggs can often be quite prolonged, and applying controls too early can result in some of the later part of the hatch being missed⁹.

If canola is the only green material nearby, grasshoppers can cause significant damage as they move in from drying pasture, cut hay or a nearby crop that is becoming unpalatable. Grasshoppers are often at higher numbers at field margins, making a targeted spray possible. If the insecticide of choice for grasshopper control is registered and within pre-harvest interval for the neighbouring crop, consider spraying into the crop or crop stubble that has been hosting the grasshoppers to provide a bit of a buffer area where the insecticide can begin to work as they move towards your crop of concern. A bran bait, EcoBran, is particularly effective on younger grasshoppers and field margins. Scout to see where the grasshopper migration margin exists if using this approach.

Insecticide options

There are several insecticide control options for grasshoppers in canola in the Prairies, if damage, timing and grasshopper numbers warrant it. It is best to target spraying for nymphs to use the lowest recommended rates and to reduce the area requiring treatment. Grasshoppers present before late-May are not likely pest species and insecticides are much less effective in protecting crops once grasshoppers are adults¹.

Also note that there are grasshopper species that are not pests, but at the nymphThe juvenile or immature stage (ex. of an insect) which goes through incomplete metamorphosis. More stage look very similar to pest species. As a general rule, control is not needed for these non-pest grasshoppers, which can be seen flying before June, have coloured (red, yellow, orange or black) hindwings which can be seen when they are flying, or make sounds (e.g. singing, calls, clacks) on the ground or in flight¹⁰.

For a complete list of insecticide control options for grasshoppers in canola in the Prairies, see Table 1.

Table 1. Grasshopper control option in canola

When applying an insecticide, always check the Spray to Swath calculator to make note of and follow the pre-harvest intervals (the minimum number of days that must pass between product application and cutting your crop by swathing or straight-cutting)¹¹.

For best results, also avoid spraying on hot days. At temperatures above 25°C, grasshoppers can metabolize insecticide more quickly, and it can be less effective. Moderately warm days result in slower insecticide metabolization and better control⁵.