Manitoba Grasshopper Forecast for 2020

 
                   
                                  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 2019. Grasshopper counts from 106 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 2020. 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 2019.
  
 
The Grasshopper Forecast for Manitoba for 2020
 
What the grasshopper population map shows
 
Almost two thirds of the counts (67 out of 106 counts) were in the very light risk category, from 0 to 4 grasshoppers /m2. Twenty-three count were in the light risk category (>4-8/m2), seven counts were in the moderate risk category (>8-12/m2), eight counts were in the severe risk category (>12-24/m2), and one count was 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 Very Severe Severe Moderate Dominant Species
Southwest 30 0 6 2 Twostriped (25 sites)
Central 46 1 1 4 Twostriped (36 sites)
Eastern 13 0 0 0
Interlake 17 0 1 1 Twostriped (15 sites)
 
The highest count in the survey, and the only count in the very severe risk category, was an average of 27.5 grasshoppers per square metre in the Rural Municipality of Lorne in the Central region. The Southwest and Central Region generally had the highest counts, although some high counts also occurred in the Interlake.
 
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 the most common species at 5 sites in the Southwest and 7 sites in the Central region. Both migratory and twostriped grasshoppers feed on a variety of types of plants (both crops and non-crop). Carolina grasshopper (Dissosteira carolina), a non-pest species of grasshopper was noted to be common at a couple of locations 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 2016 to 2019 grasshopper forecasts.
 

  

Grasshopper populations decreased to quite low levels in surveys for the 2016 to 2018 grasshopper forecasts . The data collected in the past two grasshopper surveys (for the 2019 and 2020 forecasts) show an overall increasing grasshopper population compared to surveys from the previous 3 years.
 
Weather for 2019 Growing Season
 
Report from: Timi Ojo, Agricultural Meteorology Specialist, Manitoba Agriculture and Resource Development.
 
Spring conditions for 2019 were drier and cooler than normal following below normal winter precipitation. Most areas south of the Trans-Canada highway had between 30 – 50% of normal precipitation from November 01, 2018 to April 30, 2019. As the season progressed, the dryness became more pronounced especially at the Interlake and Northwest regions, which received less than half of the historical average in May and June. Most areas in the southwest and eastern regions had close to normal precipitation due to a couple of systems that brought heavy showers. However, the 4 to 5 weeks between the systems resulted in crop stress.
 
As the season was wrapping up, the six weeks of September and early October had up to 3 times the amount of normal precipitation, coupled with a snowstorm in many areas, which hampered harvest operations. 
Summary
 
Grasshopper levels have been increasing over the past couple of 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 2020.
 
Grasshopper populations have more successful development in dry years and generally increase more over a series of dry years. The rainy weather in September and early-October may have somewhat reduced the number of eggs laid, although conditions for egg laying in August were good. The early snowfall in October will not likely significantly decrease overwintering success. Our pest species of grasshoppers all overwinter in the egg stage, which is quite resilient to excess moisture. 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 2019.
 
The risk of economical populations of grasshoppers developing in 2020 varies, depending on location. Overall the risk is generally low to moderate in most areas, but there has been a progressive increase in higher counts in the past two surveys. If weather is favourable for grasshopper survival and development there may be areas where grasshoppers are a concern to crops in 2020. 
 
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                               December 2019