Manitoba Grasshopper Forecast for 2009

The 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

Although all stages of grasshoppers, except the egg stage, feed on plants, it is the older grasshoppers of particular species that later in the season can do the most damage to crops. 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 early season crops the previous year. 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 the following season alerts farmers and agronomists to the importance of monitoring field edges and vegetation surrounding the fields, as well as other areas favorable for emergence of high numbers of grasshoppers, in late-May and June for these younger grasshoppers.
Knowing the risk of grasshoppers being a problem is one of the factors that can help farmers choose crops and plan seeding practices for the following year, and also enables companies that produce insecticides to prepare appropriately for the coming season.

How the forecast is done

Knowing how many grasshopper eggs have been laid in the soil in late-summer can help predict what the population will be like the following year. Although grasshopper eggs can be counted by digging soil and shaking it through sieves, this is a very labour-intensive and time consuming process. Doing counts of the adults that are laying the eggs in late-summer can be done relatively quickly. This, combined with weather data during the egg-laying period, gives an estimate of the eggs that were laid and the risk level of grasshoppers 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 advisors, agronomists, and entomologists in August 2008. Grasshopper counts from 247 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 2009. White areas on the map are areas where data was not collected.

                        Figure 1.  Average density of grasshoppers in Manitoba during August 2008.

Recent trends in grasshopper populations
Grasshopper outbreaks usually develop after a few years of conditions that are favorable 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 table shows the percentage of surveyed area in Manitoba in each of the grasshopper population density categories for data collected from 1999 to 2008.

Table 1 - A 10 year comparison of grasshopper counts in Manitoba.

YEAR	Very Light (0-4/m2)	Light (>4-8/m2)	Moderate (>8-12/m2)	Severe (>12-24/m2)	Very Severe (>24/m2)
1999	74.16	23.07	2.77	0	0
2000	86.86	10.68	2.46	0	0
2001	76.37	14.66	1.50	4.07	3.41
2002	31.73	29.52	13.01	21.52	4.22
2003	15.69	25.39	21.97	28.29	8.67
2004	32.64	35.68	12.18	16.01	3.48
2005	96.15	3.85	0	0	0
2006	94.60	5.40	0	0	0
2007	56.30	38.26	5.21	0.23	0
2008	33.17	40.73	16.48	7.26	2.36

During the grasshopper surveys in August, grasshopper populations were high in 2003, a year which saw many grasshopper problems, decreased in 2004, and decreased to even lower levels in 2005 and 2006. Populations increased in 2007; placing a larger part of the province at a light risk, some areas at moderate risk, and a few count in the severe risk category. In 2008, there was a further increase in grasshopper numbers. Much higher areas of the province had grasshopper populations in the moderate and severe risk categories, and some of the southwest had populations that rated in the very severe category.

The Grasshopper Forecast for Manitoba for 2009

What the grasshopper population map shows

Although about ¾ of the surveyed area still rated as light or very light risk, overall the grasshopper numbers have increased for the second year in a row. The 247 counts done in August 2008 included 23 in the moderate risk category, 13 in the severe risk category, and 3 in the very severe category. The 3 counts in the very severe category were all in the southwest; counts of 30 grasshoppers/m² at 2 locations in the rural municipality of Authur, and 25/m² at 1 location in the rural municipality of Brenda. In addition, there were also 4 counts in the severe category in the southwest (rural municipalities of Winchester, Brenda, Sifton, and Glenwood). There were 5 counts in the severe category in the region to the west of the Red River; 19/m² at 2 locations in the rural municipality of Dufferin, 20/m² at 1 location in the rural municipality of Rhineland, and 15/m² at two locations in the rural municipality of Grey. In addition, counts of 20 grasshoppers/m² (in the severe category) were also reported from 1 location in each of the municipalities of Portage la Prairie, North Cypress, and Lorne.
The grasshopper population overall continues to increase. If conditions are favorable for grasshopper development in the spring and summer of 2009, it is recommended that preferred grasshopper overwintering areas be monitored regularly to determine if economic levels are being reached.

How weather affects grasshopper development and survival

Late-Summer weather: The potential for grasshopper populations to increase is to a very large extent determined by the number of eggs deposited, which in turn is greatly influenced by temperature during the egg-laying season. Warm and dry weather in late-summer will mean that there has been more opportunity for the potential pest species of grasshoppers to lay their maximum amount of eggs. Frequent rains during August and early September make it difficult for grasshoppers to mate and produce egg pods. As well, there is some embryonic development that will occur in these eggs before the colder weather sets in. Warmer conditions will mean that the embryonic development is further along before development ceases for the winter. The further along the development is going into winter, the earlier the eggs will hatch the next year. Earlier hatch means grasshoppers that are feeding on younger, less tolerant crops.

Winter temperatures: Very cold winter temperatures, with little snow cover, can result in higher mortality of grasshopper eggs as many will freeze. However, the species of grasshoppers that can potentially be pests of crops often lay their eggs in areas where snowfall tends to accumulate. So cold temperatures will likely only be a factor in years with exceptionally low snow cover. Temperatures of -15ºC or less at about 5 cm below the soil surface are needed to cause substantial mortality of eggs for these species of grasshoppers. So it is not likely that there will be many grasshopper eggs killed due to cold winter temperatures this year.

Spring and Summer Weather: Warm and dry spring and summer days favor early and rapid grasshopper development. As well as affecting their ability to cause more harm to crops, this also means they will become adults and start laying eggs for the previous year sooner. So it is easy to see how several years of hot, dry conditions can build a grasshopper population up to outbreak levels.
 

Potential affects of weather from August and September 2008 on the grasshopper forecast

The weather data used to forecast grasshopper abundance for 2009 is shown in Table 2. This data is from 15 sites located across the agricultural region of Manitoba.

Daily maximum temperatures were generally slightly above normal in August and September in the majority of the 15 sites. The warmer September temperatures may have extended the period of optimum egg laying.

Total rainfall was slightly below normal in many areas in August and September, with exceptions being in the East and Interlake areas of Manitoba. Dauphin and Pilot Mound areas also had substantially more rainfall then normal in August.

Weather conditions were generally favorable for those grasshoppers that were present to produce a level of eggs at or near their optimum level. Some areas with heavier rainfalls may have had egg laying reduced at times due to unfavorable weather.

Table 2 – Weather data for August and September 2008.

Location	Avg. Daily Maximum Temp.				Total Rainfall (mm)				Days with Rain
	August		September		August		September		August		September
	2008	Norm*	2008	Norm	2008	Norm	2008	Norm	2008	Norm	2008	Norm
Northwest
Roblin	23.8	23.9	17.0	17.4	35.8	70.6	29.2	53.7	14	10.6	6	9.2
Dauphin	25.8	24.0	18.1	17.5	119.5	60.4	45.5	66.1	15	11.0	8	11.8
Shoal Lake	24.4	24.5	17.0	17.6	71.0	65.3	33.6	49.1	8	13.9	11	11.8
Southwest
Brandon	25.7	24.8	18.7	18.3	42.3	69.2	27.7	49.9	16	9.9	13	8.9
Carberry	25.4	24.8	18.6	18.3	60.6	69.2	58.0	49.9	15	9.9	9	8.9
Melita	26.9	26.2	19.5	19.5	50.0	51.8	46.0	46.7	13	8.6	13	7.7
Boissevain	26.1	24.7	18.9	18.0	78.8	66.8	64.0	47.2	8	9.2	9	8.6
Central
Portage la Prairie	25.2	24.5	18.0	18.2	67.4	71.1	54.4	57.2	12	11.0	12	11.2
Pilot Mound	24.7	25.3	18.2	18.8	112.6	70.0	49.2	49.2	12	10.7	11	10.1
Carman	26.7	25.1	18.6	18.5	54.8	70.0	93.4	56.7	13	10.5	13	10.5
Gretna	26.9	25.5	19.5	19.2	21.8	72.9	101.6	56.1	5	10.0	11	9.0
East/Interlake
Oak Point	23.4	24.9	16.1	18.5	22.4	70.5	35.4	56.5	5	10.0	8	10.5
Gimli	24.2	23.3	17.2	16.9	80.0	64.2	105.5	65.6	21	11.6	16	10.9
Winnipeg	26.1	25.0	18.9	18.5	98.0	75.1	77.5	51.9	10	10.4	12	10.9
St. Pierre	26.5	24.7	19.8	18.4	96.6	68.5	105.8	59.6	12	10.1	10	9.5

*Normals are based on 1971-2000.
Values in Bold type indicate the 2008 reading was higher than the 30 year normal.

Summary

Weather conditions were generally quite favorable for grasshoppers during the egg-laying period, and populations of grasshoppers continue to increase in many areas. If spring and summer conditions in 2009 are favorable for grasshopper survival and development, some areas could have moderate or high levels of grasshoppers.

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. This monitoring should begin in late-May or June. Such monitoring is of increased importance in regions of Manitoba that are rated as a moderate, severe, or very severe risk.

For more information on the grasshopper forecast or monitoring for grasshoppers, please contact John Gavloski. The protocol for doing the grasshopper counts for this survey can be found at: http://www.gov.mb.ca/agriculture/crops/insects/fad95s00.html

A Fact sheet  on grasshoppers is available on line, or at your local GO Team Office or Centre.