Manitoba Grasshopper Forecast for 2008

The grasshopper forecast is based on a survey of grasshopper populations in August (which estimates the egg-laying population), weather data (which helps estimates 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.

Purpose of a grasshopper forecast

Although all stages of grasshoppers, except the egg stage, feed on plants, it is the older grasshoppers later in the season that do the most damage to crops. The young stages are often highly concentrated around field edges early in the season. 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 important of monitoring field edges and vegetation surrounding the fields 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 2007. Grasshopper counts from 267 locations in Manitoba were used to produce the map. The legend on the map shows the average grasshopper counts in an area, and relate these to risk for many of our crops. Factors affecting grasshopper development, survival and behavior 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 2008. White areas on the map are areas where data was not collected.

Figure 1. Average density of grasshoppers in Manitoba during August 2007.

The Grasshopper Forecast for Manitoba for 2008

What the grasshopper population map shows

The map of grasshopper densities (Figure 1) shows large areas of Manitoba in either the very light or light risk categories. Some areas are rated as moderate risk, and there were a few counts in the high risk category. To interpret this data, it is helpful to compare this data to that of past years (Table 1). Although much of the area is still rated as light or very light risk, overall the grasshopper numbers have increased. In the previous 2 years, no counts were recorded in the moderate, severe or very severe risk categories. The 267 counts done in August 2007 included 15 in the moderate risk category and 12 in the severe risk category (although many of these areas show as a lower risk on the forecast map due to these higher counts being averaged with lower counts nearby in a region). This shows the grasshopper population overall has started to increase. If conditions are favourable for grasshopper development in the spring and summer of 2008, it is recommended that preferred grasshopper overwintering areas be monitored regularly to determine if economic levels are being reached. The highest individual counts from August 2007 were 18, south of Austin in the Rural Municipality of North Norfolk, and 17 near Mariapolis in the Rural Municipality of Lorne. The only area to appear on the map as severe risk was in the southwest corner of the Rural Municipality of North Norfolk.

Recent trends in grasshopper populations

Grasshopper outbreaks usually develop after a few years of favorable conditions lead to a steady increase in grasshopper numbers. 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 1998 to 2007.

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)
1998	64.81	25.25	5.11	3.27	1.55
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

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, and some areas at moderate risk. A few count in the current survey were in the severe risk category.

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 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 the egg 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, grasshoppers tend to lay their eggs in areas where snowfall will accumulate. So this 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 grasshopper eggs. 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 favour 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 2007 on the grasshopper forecast

Some of the weather data used to forecast grasshopper abundance for 2008 is shown in Table 2.

Daily maximum temperatures were generally slightly below normal in August, and slightly above normal in September. Of the 15 sites that weather records were used from, 13 of the 15 sites (all except Oak Point and Roblin) has September temperatures above the 30 year normal. The warmer September temperatures may have extended the period of optimum egg laying.

Total rainfall was below normal in all locations except Dauphin during August, and in all locations during September. In the central region total rainfall was less than half the normal amount in August and September (except for Portage in September). Although total rainfall was generally less than normal, the number of days with rain was often near or above normal.

Weather conditions were generally favorable for those grasshoppers that were present to produce a level of eggs at or near their optimum level.

Table 2 – Weather data for August and September 2007.

Location	Avg. Daily Maximum Temp.				Total Rainfall (mm)				Days with Rain
	August		September		August		September		August		September
	2007	Norm*	2007	Norm	2007	Norm	2007	Norm	2007	Norm	2007	Norm
Northwest
Roblin	21.0	23.9	17.2	17.4	60.4	70.6	43.6	53.7	14	10.6	14	9.2
Dauphin	22.6	24.0	17.9	17.5	60.5	60.4	46.5	66.1	20	11.0	22	11.8
Shoal Lake	21.2	24.5	17.9	17.6	63.6	65.3	28.6	49.1	10	13.9	11	11.8
Southwest
Brandon	23.5	24.8	19.8	18.3	49.4	69.2	20.2	49.9	11	9.9	17	8.9
Carberry	23.9	24.8	19.9	18.3	35.2	69.2	20.6	49.9	9	9.9	10	8.9
Melita	21.3	26.2	21.4	19.5	18.2	51.8	6.6	46.7	9	8.6	9	7.7
Boissevain	23.7	24.7	20.5	18.0	32.5	66.8	21.7	47.2	9	9.2	9	8.6
Central
Portage la Priaire	23.7	24.5	19.1	18.2	23.6	71.1	40.0	57.2	10	11.0	15	11.2
Pilot Mound	23.2	25.3	19.7	18.8	12.2	70.0	15.4	49.2	9	10.7	9	10.1
Carman	24.2	25.1	20.1	18.5	23.6	70.0	19.8	56.7	9	10.5	7	10.5
Gretna	23.8	25.5	20.2	19.2	23.8	72.9	16.4	56.1	8	10.0	7	9.0
East/Interlake
Oak Point	21.8	24.9	17.2	18.5	31.2	70.5	28.0	56.5	5	10.0	11	10.5
Gimli	22.6	23.3	17.7	16.9	27.0	64.2	54.0	65.6	9	11.6	19	10.9
Winnipeg	24.5	25.0	129.8	18.5	31.0	75.1	30.5	51.9	19	10.4	10	10.9
St. Pierre	24.6	24.7	20.3	18.4	58.2	68.5	20.9	59.6	14	10.1	11	9.5

*Normals are based on 1971-2000.
Values in Bold type indicate the 2007 reading was higher than the 30 year normal.

Summary

Weather conditions were quite favorable for grasshoppers during the egg-laying period, and populations of grasshoppers have started to increase in many areas. If spring and summer conditions in 2008 are favorable for grasshopper survival and development, some areas could have moderate to 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. 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 or severe risk.

For more information on the grasshopper forecast or monitoring for grasshoppers, please contact John Gavloski (email) or phone at (204) 745-5668. A Fact sheet  on grasshoppers is available on line, or at your local GO Team Office or Centre.