Manitoba Grasshopper Forecast for 2021

 
                   
                                  Twostriped (left) and migratory (right) grasshoppers

Grasshopper surveys have been conducted in Manitoba in various degrees of detail since 1931. The current grasshopper forecast is based on counts of grasshopper populations in August (which estimates the egg-laying population), weather data (which helps estimate whether those female grasshoppers present are capable of laying their optimum level of eggs), and recent trends in grasshopper populations. In some years, natural enemy populations may significantly affect the number of grasshopper eggs that survive and hatch, and such data may be pertinent to the forecast as well. Counts are generally done in or alongside crop fields in Manitoba. The goal is to estimate levels of the 4 species of grasshoppers that have potential to be pests of crops in Manitoba.
 
Purpose of a grasshopper forecast
 
All stages of grasshoppers, except the egg stage, feed on plants. Some species will feed on crops, while other species do not, or rarely will. Older grasshoppers of these crop feeding species can do the most damage to crops, particularly later in the season as these grasshoppers can move greater distances. In annual cropping systems, the young stages of these species are often highly concentrated around field edges early in the season, particularly around fields that had sparse green vegetation late in the previous summer. If grasshopper populations get quite high, these younger, concentrated populations of grasshoppers are much easier to control than older and more dispersed populations later in the season.
 
Knowing the risk of grasshoppers being a problem alerts farmers and agronomists to the importance of monitoring field edges, vegetation surrounding the fields, and other preferred egg laying areas in late-May and June for these younger grasshoppers. This information can also help farmers choose crops and plan seeding practices for the following year.
 
Interpreting the grasshopper population map
 
The grasshopper population map for Manitoba is based on counts of adult grasshoppers per m2 done by farm production extension specialists, agronomists, and entomologists in August 2020. Grasshopper counts from 132 locations in Manitoba were used to produce the map. The legend on the map shows the average grasshopper counts in an area, and relates these to risk for many of our crops. Factors affecting grasshopper development, survival and behaviour will determine whether these August populations are likely to increase, decrease, or remain fairly stable for the next year and are also important factors in the overall forecast for 2021. The small circles on the map show where data was collected. White areas on the map are areas where data was not collected. Note that the averaging of counts in a region will result in a density category for a region representing the cumulative data, not the value from a specific count.
Figure 1. Average density of grasshoppers in Manitoba during August 2020.
  
 
The Grasshopper Forecast for Manitoba for 2021
 
What the grasshopper population map shows
 
About two thirds of the counts (89 out of 132 counts) were in the very light risk category, from 0 to 4 grasshoppers /m2. Twenty-seven count were in the light risk category (>4-8/m2), twelve counts were in the moderate risk category (>8-12/m2), four counts were in the severe risk category (>12-24/m2), two in the Central region and two in the southern Interlake. There were no counts in the very severe risk category.
 
Table 1. Total grasshopper counts, counts in the higher risk categories, and dominant grasshopper species for each agricultural regions surveyed.  

Counts in Risk Category
Region Total Counts Severe Moderate Light Dominant Species
Northwest 8 0 0 0 Twostriped
Southwest 25 0 0 5 Twostriped
Central 48 2 5 14 Twostriped
Eastern 21 0 1 4 Twostriped
Interlake 30 2 6 4 Twostriped
 
The highest count in the survey was an average of 19 grasshoppers per square metre in the Rural Municipality of Rockwood in the Interlake region. The Interlake and Central Region generally had the highest counts.
 
Dominant species: Twostriped grasshopper (Melanoplus bivittatus) was generally the most common species of grasshopper at most of the count sites where species was determined.  Migratory grasshopper (Melanoplus sanguinipes) was noted as one of the dominant species in at least one count site in each region, and clearwinged grasshopper (Camnula pellucida) was noted as one of the most common species at 1 site in the Central region and 2 sites in the Interlake. Both migratory and twostriped grasshoppers feed on a variety of types of plants (both crops and non-crop). Clearwinged grasshopper is primarily a grass feeder, and seldom feeds on broad-leaved plants. Carolina grasshopper (Dissosteira carolina), a non-pest species of grasshopper was noted to be one of the dominant species at 1 site in each of the Southwest and Central regions, and 3 sites in the Interlake.
 
Recent trends in grasshopper populations
 
Grasshopper outbreaks usually develop after a few years of conditions that are favourable for a steady increase in numbers of those species of grasshopper that can become pests of crops. Comparing the current August grasshopper counts with those of previous years can determine if the populations tend to be rising or falling.
 
The following figures show the area in Manitoba survey maps in each of the grasshopper population density categories for data collected for the 2017 to 2020 grasshopper forecasts.
 

  

Grasshopper counts were quite low in surveys for the 2017 and 2018 grasshopper forecasts. The data collected in the past three grasshopper surveys (for the 2019 to 2021 forecasts) show an overall increased grasshopper population compared to surveys from the previous 2 years.
 
Weather for 2020 Growing Season
 
Report from: Timi Ojo, Agricultural Meteorology Specialist, Manitoba Agriculture and Resource Development.
 
Air temperature was similar to historical average throughout the growing season. Areas around Brandon ended the season with growing degree days (base 5) of 6-10% above historical average.
Precipitation: Almost all areas were drier than normal except areas south of the Riding Mountain National Park and in the far southeast corner of the province. These two areas had above normal precipitation mainly due to the weather systems that both areas received  in June. Menisino in the southeast received 160 mm of precipitation between June 4 -10. Brandon received 152 mm on June 28.
Summary
 
Grasshopper levels have increased over the past few years. Whether populations continue to increase will depend on factors such as weather and natural enemies. Grasshopper levels should be monitored carefully, beginning in late-May or early-June in 2021.
 
Grasshopper populations have more successful development in dry years and generally increase more over a series of dry years. Conditions for egg laying in late-summer were generally  good.  Our pest species of grasshoppers all overwinter in the egg stage. Populations of bee flies, field crickets, and Epicauta species of blister beetles, all of which feed on grasshopper eggs, were quite noticeable in some locations of Manitoba in 2020.
 
The risk of economical populations of grasshoppers developing in 2021 varies, depending on location. Overall the risk is generally low to moderate in most areas, but there has been an increase in higher counts in surveys over the past three years. If weather is favourable for grasshopper survival and development there may be areas where grasshoppers are a concern to crops in 2021. 
 
When they have the opportunity, farmers and agronomists are encouraged to monitor grasshopper populations along roadsides, field edges, and other areas where populations tend to be concentrated or at high levels early in the season.
 
For more information on the grasshopper forecast or monitoring for grasshoppers, please contact John Gavloski at (204) 750-0594.
 
The protocol for doing the grasshopper counts for this survey can be found at: http://www.gov.mb.ca/agriculture/crops/insects/mb-grasshopper-survey.html 
 
More information on grasshopper biology, species identification, monitoring and management is available at: 
 
Produced by: John Gavloski; Entomologist; Manitoba Agriculture and Resource Development, Box 1149, 65-3rd Ave. NE, Carman, Manitoba, Canada, R0G 0J0                               February 2021