Keeping insect infestations below significant levels through preventative measures is at the core of long term integrated pest management. More immediate control is reactive and is warranted only when the insects begin to affect the producer financially. A common problem for most producers is deciding whether or not to treat a crop for a specific insect pest.

The question is, how many insects are too many insects? The initial response may be to spray as soon as insects are found in the crop. But implementing control measures is costly, sometimes involving large amounts of chemical and fuel. Also, the labour involved in control operations is not insignificant. Many insecticides have broad spectrum activity affecting non-target organisms and therefore, unnecessary applications can have undesirable environmental effects.

So when does an infestation become economically viable to control? Ultimately the decision will have to be made by the producer. Economic thresholds attempt to resolve this dilemma by providing guidance in making the decision as to whether insect control has an economic benefit.

Understanding economic thresholds of insect infestations is central to sound pest management. Knowing whether or not it is necessary to take action against an insect pest, especially when insecticides are involved, enables the producer to make financially and ecologically sound decisions. Properly used, this knowledge can reduce crop losses, production costs, and potential impacts on non-target organisms and the general environment.

An economic threshold is the insect’s population level or extent of crop damage at which the value of the crop destroyed exceeds the cost of controlling the pest. Economic thresholds can be expressed in a variety of ways including, number of insects per plant or per square metre, the amount of leaf surface damage, etc. In many cases thresholds have been established through scientific research. Unfortunately, not all combinations of pests and crops have been studied, and some reported thresholds are merely educated estimates.

It is important to note that the mere presence of insects in a crop does not suggest that there is the potential for damage and subsequent crop loss. It is important to identify the insect and to determine its status as a pest. That is, is the insect truly a pest, is it a beneficial species or is it inconsequential to the crop. The majority of the economically important insects have been studied to some extent. However, the growing diversity of crops in Saskatchewan leaves gaps in knowledge as to how insects will affect some of the "newer" crops.

Economic thresholds can fluctuate depending on a combination of factors including the pest, crop, stage of the crop, cost of control and the final market prospects for the product. The economic threshold may also vary with growing conditions. When conditions are ideal, a vigorously growing crop may be able to withstand a higher pest population with little yield loss, depending on the stage of the plant. Conversely, relatively fewer insects may significantly damage a stressed crop. Furthermore, researchers have suggested that with some sucking insects, such as aphids on flax, a higher yielding crop will suffer a greater percentage yield loss than will a crop already under stress. Economic thresholds serve merely as a guideline to the producer, and to be effective, the plant’s growth stage and growing conditions must be considered along with these other factors.

The Orange Wheat Blossom Midge on wheat provides an excellent example of how the damage potential of an insect is related to plant growth stage. Wheat is highly susceptible to wheat midge only during the period from when the wheat heads begin to emerge from the boot until the onset of flowering (anthesis). Control measures are not recommended for wheat midge if the wheat is not in this susceptible stage. Other examples can be found in the comprehensive economic threshold charts for the Bertha Armyworm and Lygus bug. These charts illustrate a range of economic thresholds in relation to other variables - spray costs, varying commodity prices and canola plant growth stages.

If an economic threshold is not available for a specific insect in a crop, control decisions should be made objectively. First there should be evidence of damage. Aside from pest species, insect presence may be inconsequential to the crop and some species may be of actual benefit to the producer. Second, an estimate of potential crop damage should be made and then compared to the cost of applying an insecticide. If crop damage is widespread, control measures may be required for a whole field. However if damage appears to be more isolated, a "spot" spray concentrating on the infected area(s) may suffice. Thorough monitoring and insect identification are essential elements in helping reduce input costs and crop loss even if an economic threshold is unknown.

The extent to which cultural practices can help in controlling or suppressing insect infestations is also worthy of consideration. Certain pest species, by virtue of their life cycles, can be greatly affected by agronomic practices, sometimes without the use of chemicals.

In some cases cultural control of insect pests can coincidentally be achieved by implementing agronomic practices that are already recommended for other reasons. For example, effective weed control will help conserve soil moisture and, at the same time, destroy an alternative food source for insects. Other practices such as rotating crops, adjusting seeding depth and packing will help to manage certain insect species as well as reduce the levels of some disease organisms.

This fact sheet provides a quick reference to the currently accepted economic thresholds of the major insect pests in Saskatchewan. Also included is information regarding recommended monitoring procedures, and for certain pests, alternatives to chemical control.

Every field should be monitored on a regular basis to detect specific insect pests and to determine densities within the crop. With an adequate monitoring program to establish presence of pest species and to monitor changes in population densities, producers are more likely to be aware of potential problems.

The first step in confirming the presence of a pest in the crop is determining what insects are potential pests. Producers unfamiliar with the potential insect pests of a crop should acquire a production guide for the crop, or a more general publication, such as "Insect Pests of the Prairies" (University of Alberta). The second step is being able to identify these insects, their life stages, and to detect their presence by the effect they have on the crop.

Know the signs of a potential problem. The most obvious sign of a problem is physical damage to the crop. Stands that show patches of thinning, stunting, or dying off, may be the first indication of an infestation, as they are usually visible from a distance. If the problem is due to insect damage, examining the plants within these areas may reveal chewing or sucking damage to leaves, stems, flowers and buds, and possibly, the insects themselves.

The most obvious sign of a problem is physical damage to the crop. Stands that show patches of thinning, stunting, or dying off, may be the first indication of an infestation, as they are usually visible from a distance. If the problem is due to insect damage, examining the plants within these areas may reveal chewing or sucking damage to leaves, stems, flowers and buds, and possibly, the insects themselves.

Being able to recognize the symptoms of damage within the crop and on individual plants, can assist not only in indicating the presence of a pest, but also in identification.

Symptoms of insect damage will vary, depending on the type of mouthparts of the insect pest. Damage caused by insects with chewing mouthparts is often easy to identify even if the insects are not always visible themselves. These insects may actually remove material from leaves, stems, or whatever portion of the plant they attack, giving it a ragged or chewed look. Injured roots will often show sign of bored holes or lesions while the above ground portions of the plant may appear wilted or stunted.

Examples of insects with chewing mouthparts are grasshoppers, larval and adult beetles, larvae of moths and butterflies (caterpillars), and larvae of flies (maggots). Conversely, it is more difficult to discern damage caused by insects with sucking mouthparts as the symptoms are often not readily visible externally. Insects with sucking mouthparts simply pierce the plant and feed on sap and juices. In this case, the damage may appear as tiny dots where the mouthparts have pierced the plant tissues. Eventually symptoms may include dead plant tissue in leaf tips, heads, etc. Since these insects inject a chemical to prevent the sap from coagulating while feeding, plant juices will continue to flow after the insect has moved on. Therefore, evidence of sucking insects may be seen as glistening sap extruded on pods and stems.

More advanced symptoms of severe injury include shriveled stems and seeds and a reduction in number of seeds set. Extreme cases in canaryseed have been observed where aphid feeding has resulted in empty, whitened tips of heads. Examples of insects with sucking mouthparts are leafhoppers, plant bugs (e.g. Lygus), and aphids.

There are many other signs of infestation: lodged plants, silken webs, discolouration of plant tissue, cocoons or pupae found on leaves, frass (feces) on and around plants, and of course direct observations of adults and/or larvae. Any and all of these should arouse suspicion of a potential problem and help determine what insect(s) could be causing the damage.

Where does one look for these signs? Insects are rarely uniformly distributed throughout a field. They are simply too dependent on local environmental conditions, and often, terrain is variable, even within a single field. Hills and depressions within a field dictate the local pattern of soil moisture, and insects sensitive to soil moisture conditions will distribute themselves accordingly.

Cutworms, for example, can be found first on the tops of hills, because of the warmer, drier soil, and will not be noticed in low-lying areas until the insects become larger and more numerous. Conversely, wireworms prefer moist soils and will be less abundant on hilltops, preferring the more moist soils found in low-lying areas.

Many insects tend to be "edge feeders" often as a result of migration from ditches and adjacent fields and therefore, damage is more prevalent around the margins. Again, in these cases control concentrated in these areas can reduce input costs while keeping insect populations below an economic threshold. Looking for signs of infestation where they are most likely to occur will lead to early detection.

The life stage of an insect is an important factor in determining the timing of control measures. For example, egg and pupal stages are usually difficult to control. In addition to the fact that these life stages are not considered to be a threat to the crop, they are often the natural biological stage which enable the insects to overcome harsher environmental conditions and therefore are in a hardier state. Because they are immobile in these stages they are often in locations that are more difficult to access by predators and control measures (e.g. Bertha Army worm pupae or Wheat Midge cocoons in the soil).

Even larvae, which are more susceptible to insecticides, can be difficult, or not economically feasible to manage when they are below the soil surface. In a few cases, insects (e.g. blister beetles) may exhibit both destructive and beneficial behaviour depending on life stage. As adults, blister beetles can cause serious damage to portions of canola fields. However the larval blister beetle is predatory on grasshopper eggs.

Once the presence of a pest has been confirmed, its identification must be verified. Correct identification may require consulting a reference guide or a specialist. To facilitate this process, collect samples of the damage and a few specimens of the pest, including as many life stages as possible. Then compare the insect and associated damage with good reference material. If uncertainties remain, contact your local extension agrologist, or directly contact the Crop Protection Laboratory (address below). These resources will help to ensure a proper identification.

Once the pest has been identified, the level of infestation in the crop must be established.

There are several important points to consider while sampling.

First, it is important to utilize a sampling technique that is appropriate for the type of insect being monitored. The monitoring method is largely related to specific insect behaviour. Highly mobile insects like flea beetles and grasshoppers provide two different examples of monitoring techniques.

Rather than attempt to count flea beetles, a plant damage threshold is used. For grasshoppers, the economic threshold is measured in insects per square meter. However, sampling such mobile insects by actually measuring a square metre area and then counting the number of individuals within the area may be difficult since the act of measuring the area will scare away the grasshoppers.

The following procedure for estimating grasshopper densities is relatively easy and reliable:

A hand-held counter can be useful in counting the number of insects while the observer counts off the required distance.

However if one is not available the following method is recommended: Measure off a distance of 50 metres on a reasonably level surface such as a road and mark both ends using markers or specific posts on the field margin. These points should be easily visible for the observer.

Begin sampling aligned with one of the markers as a starting point and walk toward the other end (through the applicable area) of the fifty metres making some disturbance with your feet to encourage any grasshoppers to jump. Any grasshoppers that jump through the a one metre field of view in front of the observer are counted.

A metre stick can be carried as a visual guide to give perspective for a one metre width. However after doing this a few times one can often visualize the required width and a metre stick may not be required.  At the end of the 50 metres, the total number of grasshoppers is divided by 50 to give an average per square metre.

Secondly, it is important to sample randomly and gather numerous samples. The samples must represent, as much as possible, the entire field being monitored. Random sampling reduces the risk of biased estimates that could result from uneven distribution of insect populations. Collecting numerous samples will also increase the accuracy of an overall field estimate.

Certain areas of a field may have insect numbers that are in excess of economic thresholds. However other areas may be very low in pest densities. In these situations a decision could be made to either not spray, due to the overall average density being below economic threshold or to concentrate control measures on the more highly infested areas. Either choice would actually benefit the producer financially while reducing environmental impacts.

Third, keep in mind the "edge-effect". In situations where insects migrate into a field from an adjacent field or ditch, the population density is likely to be highest at field margins. Some pest species will prefer the edges of a field because of factors such as light, temperature or moisture preferences.

Edge effects can also be important for other reasons. Although they may distort true population estimates, they may indicate a potential problem before it becomes serious. Be sure to sample throughout the field, not only the field margin, to avoid overestimating population densities.

Sampling methods can vary according to the particular pest. Consult the economic threshold tables to determine what to do.

If the chart says:

1) % damage to leaves/plants/foliage, or

2) # of plants show damage,
or

3) # adults or larvae/stem/plant

Do this:

• Walk through the crop obtaining or observing suitable sample units (i.e. leaves, stems, whole plants or insect counts) every few steps. To get an accurate population estimate, sample randomly at fairly widely spaced intervals.

As previously discussed, the best estimate of a population or damage will be achieved with adequate, representative samples taken over a well-distributed pattern. A "zigzag" route through the field sampling approximately every 10 metres is a commonly used pattern.

If the chart says:

1) # adults or larvae/m2

Do this:

• Use a meter-stick or pre-measured piece of string to mark off a square meter of the crop. Examine this area, counting the numbers of pests seen. Do this at several randomly chosen and widely spaced sites. Average your results.

If the chart says:

1) # adults or larvae/sweep

Do this:

• Obtain or make a sweep net, a tool used by entomologists to sample insects. It consists of a muslin bag or some similar material held open by a hoop attached to a long handle. An angler’s net lined with a pillowcase will work as well.

The size of the net opening is important, however, as this will affect the number of insects caught. The standard net size is 38 centimetres (15 inches) in diameter.

Walk through the crop sweeping the net, from side to side, in front of you, through the crop canopy. Generally the arc of the sweep will cover approximately 180 o. However, some economic thresholds specify 90 o sweeps. The main point is to keep the sweeps consistent. Ideally the entire open end or "mouth" of the net should pass through the crop. In some crop stages, such as the pod stage in canola, this is extremely difficult.

As a rule of thumb try to keep as much of the net as possible within the crop canopy. Flying insects and those on the plants will be knocked into the bag. Do not sweep through the same pass more than once. Keep track of the number of sweeps, and count the number of pests in each sweep or take an average of the number of insects divided by the number of sweeps.

Alfalfa plant bug: Crop damage can be minimized by burning alfalfa stubble in spring.

Alfalfa weevil: In alfalfa grown for hay, it is sometimes possible to reduce populations of new adults by harvesting the first cut early. Infestations of this pest have, so far, been restricted to southern Saskatchewan, specifically irrigated fields in the southwest, near Maple Creek.Aphids: Early seeding can help to avoid infestations because the crops mature before the pest levels exceed economic thresholds. As plants mature they are less attractive to aphids.Beet webworm: Weeds in and around susceptible crops should be removed to reduce the attractiveness of the field to egg-laying females.

Cutworms: Ensure fallow fields are kept free of weeds and the soil is allowed to form a crust between mid-August and mid-September, making it difficult for moths to lay eggs. This should only be done under threat of serious infestation, however, since this type of practice may encourage soil erosion. In the case of army cutworms, check for damage on volunteer cereals and weeds before seeding. Delay seeding until late May if damage is evident.Grasshoppers: Control of annual weeds before grasshopper emergence will help reduce grasshopper populations by eliminating alternative food sources for young grasshoppers. Disturbed soil is less attractive to egg-laying female grasshoppers. In areas where they have emerged before weed control is carried out, or in forage used for animal feed, traps strips can be maintained in which grasshoppers will be concentrated before application of insecticide. Research has shown that "barrier strips" of less preferred crops such as oats or peas around the perimeter will help reduce damage to the main crop.Orange Wheat Blossom Midge: In areas where wheat midge is expected to be abundant, one should consider not seeding spring wheat or at least avoid planting it in or near fields that were infested the previous year. However, if planting in infested areas, increase seeding rate from 1.5 to 2.0 bushels of viable seed per acre. This encourages a more uniform stand that will complete flowering before midge levels increase to harmful levels. Susceptibility to wheat midge damage decreases dramatically after flowering (anthesis). Also consider growing early-maturing varieties.Hard red spring wheat varieties may benefit from early seeding (late April to early May) in most years. However durum and CPS varieties do not reflect this same trend.

Red turnip beetle: Damage can be minimized by destroying volunteer mustards and other cruciferous weeds in the spring before seeding, and by not seeding to canola in or adjacent to fields that were infested the previous year.

Root maggots: (Canola) Increase seeding rates. Argentine canola varieties compensate for root maggot feeding better than do Polish varieties.

Sunflower beetle: Crop damage in the year following infestation can be minimized with late fall cultivation which will expose beetles to the elements and increase winter mortality.

Sweetclover weevil: Infestations can be reduced by establishing new stands of sweetclover as far as possible from second-year stands using high-quality scarified seed, planted no deeper than 2.5 cm to ensure rapid germination. Also, cultivating second-year sweetclover immediately after it has been cut for hay or silage will help destroy larval and pupal stages.

Wireworms: Crop damage can be minimized by including less preferred crops such as flax or canola in rotation with cereals, cultivating summerfallow fields as shallow as possible, seeding cereal crops shallow to induce quick germination, and using seed treated with a dual-purpose pesticide.

Remember: it is critical that you know exactly which pest you are dealing with! Have it properly identified!

Currently Recommended Economic Thresholds for Insect Pests

Alfalfa weevil Alfalfa Hay: 25 - 50% of leaves on upper 1/3 of stem, or 50 - 70% of foliage tips show injury

Seed: 35 - 50% of plants show damage or 20 - 25 larvae/sweep (180o)

Aphids:

Birdcherry-oat aphid Cereals 12 - 15 aphids / stem prior to soft dough; Canaryseed 10 - 20 aphids on 50% of the stems prior to soft dough

English grain aphid Cereals 12 - 15 aphids / stem prior to soft dough; Canaryseed 10 - 20 aphids on 50% of the stems prior to soft dough

Cabbage aphid Canola Economic threshold not yet established

Turnip aphid Canola Economic threshold not yet established

Corn leaf aphid Cereals 12 - 15 aphids / stem prior to soft dough

Russian wheat aphid Cereals 10% of the plants infested with at least 1aphid when first node visible 10% of the tillers infested with 1 aphid when tip of flag leaf just visible

Green bug Cereals 12 - 15 aphids / stem prior to soft dough

Potato aphid Flax 2 - 3 aphids / main stem at full bloom *  Flax 8 aphids / main stem at green boll stage

*Pea aphid Peas 2 - 3 aphids on top 20 cm of plant tip (Trapper peas can withstand considerably higher levels)

* main stem is considered to be the main yield component of the plant (usually the primary stem)

Armyworm Cereals

Thrips Barley, Oats 7 - 8 thrips / stem prior to head emergence; Red Clover 50 - 80 thrips per flower head

Beet Webworm Canola 20 - 30 larvae/m2; Flax > 10 larvae/m2

Bertha Armyworm Canola (Argentine) See chart below for variations

Clover cutworm Canola, Mustard 20 - 30 larvae/m2; Flax Economic thresholds not yet established but expected to be lower than cereals

Cutworms Cereals 3 - 4 larvae/m2; Oilseeds Economic thresholds not yet established but expected to be lower than cereals

Diamondback moth Canola, Mustard 100 - 150 larvae/m2 in immature and flowering fields **; 200 - 300 larvae/m2 in podded canola fields **

** Note that these threshold numbers are based on stands averaging 150 to 200 plants / m2.  In areas where stands are thinner the economic threshold should be lowered accordingly.

Flax bollworm Flax 3% of flax bolls infested

Flea beetles Canola 25% of cotyledon surface destroyed

Grasshoppers Cereals 8 - 12 grasshoppers /m2; Flax, Lentil 2 grasshoppers /m2 - depending on crop stage (i.e. lentil pods are far more prone to attack than is the foliage); Canola (Argentine) > 14 grasshoppers /m2

Orange wheat blossom midge Wheat 1 midge / 4 - 5 wheat heads

Painted lady butterfly Sunflowers 25% defoliation

Plant bugs

*Crop Staging: End of flowering to early pod development in the upper canopy is stage 4.4 - 5.1. Pod Ripening is stage 5.2.

Note: Each sweep is 90 degrees

Pest Identification, Threshold Information
Crop Protection Laboratory
125 - 3085 Albert St
Regina, Saskatchewan
S4S 0B1
(306) 787-8130
The laboratory is operated by the Pest Management Unit of the Sustainable Production Branch, Saskatchewan Agriculture and Food. There is a fee for identification services. Contact the lab for details.

Literature Sources

Crop Protection Guide:  An annual publication of Saskatchewan Agriculture and Food. Contains information on pesticides and their registration and use on field crops. May be obtained from the Agriculture Knowledge Centre 1-866-457-2377.

Farm Facts:  An ongoing series of fact sheets dealing with life-cycles and control measures of specific insects. The following are currently available:

• Bertha Armyworm
• Diamondack Moth
• Flea Beetles
• Forest Tent Caterpillar
• Grasshoppers
• Orange Wheat Blossom Midge
• Russian Wheat Aphid

Others may become available in the future. The above Saskatchewan publications are available on our Website or by contacting the Crop Protection Laboratory, the Agriculture Knowledge Centre, 1-866-457-2377, or from the Sustainable Production Branch of Saskatchewan Agriculture and Food, Room 125 - 3085 Albert St., Regina, SK. S4S 0B1.

Practical Crop Protection
Alberta Agriculture, Food and Rural Development
Publishing Branch
7000-113 St
Edmonton, Alberta
T6H 5T6
Contains detailed information on crop monitoring, as well as specific management of pests in field crops.

Insect Pests of the Prairies
Hugh Philip and Ernest Mengersen, 1989. A University of Alberta, Faculty of Extension Publication. Corbett Hall, Edmonton, Alberta, T6G 2G4.