Economic Thresholds of Insect Pests

Keeping insect infestations below significant levels through preventative measures is the core of long-term integrated pest management. More immediate control is reactive and warranted only when the insects 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 initial response may be to spray as soon as insects are found in the crop. But implementing control measures is costly and can require significant inputs/amounts/quantity of insecticides and fuel. In addition, the labour involved in control operations is significant. Many insecticides have broad-spectrum activity affecting target and non-target (including beneficial) organisms. Therefore, unnecessary applications can have undesirable environmental effects.

When does an infestation become economically viable to control

Ultimately, the decision is made by the producer. Economic thresholds help with these management decisions by providing guidance as to whether insect control will have an economic benefit.

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 the 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 educated estimates – nominal thresholds.

It is important to note that just the presence of insects in a crop does not suggest that there is the potential for damage and subsequent crop loss. There will always be a diversity of insect species in any crop. It is important to identify the insect and to determine if it is really a pest, a beneficial species or is inconsequential to the crop. Proper identification is essential to making sound control decisions. Most economically important insects have been studied. However, the growing diversity of crops in Saskatchewan leaves some gaps in knowledge as to how insects will affect some of the newer crops.

Economic Fluctuation

Economic thresholds can fluctuate depending on a combination of factors including the pest, the crop type, growth stage, expected market value and cost of control. The economic threshold may also vary with growing conditions. A vigorously growing crop may be able to withstand higher insect pest populations with little yield loss, depending on the stage of the plant. Conversely, relatively few insects may significantly damage reproductive or yield components (pods, bolls and heads) or 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 a crop already under stress. Economic thresholds serve as a guideline to the producer. To be effective, the plant's growth stage and growing conditions must also be considered.

On wheat, the wheat midge provides an excellent example of how the damage potential of an insect is related to plant growth stage. Spring wheat is highly susceptible to wheat midge damage from the time the boot swells and splits and any part of the head is visible until flowering (anthesis). Control measures are not recommended for wheat midge if the wheat is not in this susceptible stage. Grasshoppers do not do serious damage to lentil foliage, but preferentially feed on developing pods, having a greater impact on yield and consequently a lower economic threshold than recommended in other crops. 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. 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 the 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 to control or suppress 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 early season alternative food source for some pest insects. Other practices, such as crop rotation, adjusting seeding depth and soil packing will help to manage certain insect species, as well as reduce the levels of some disease organisms.

Crop Monitoring Techniques

Every field should be monitored on a regular basis to detect specific insect pests and to determine densities within the crop (e.g. Pattern 1). 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 is to assess potential insect pests. Producers unfamiliar with the possible insect pests of a crop should acquire a production guide for the crop. The second step is to identify the insects, their life stages and to detect their presence by the effect they have on the crop.

Signs of Potential Problems

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, examine individual plants to determine 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 help to indicate the presence of an insect pest and help in its 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 when the insects are not readily visible. These insects may remove material from leaves, stems, or other plant parts giving it a ragged or chewed look. Injured roots will often show sign of bored holes or lesions, while above ground 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).

It is more difficult to discern damage caused by insects with sucking mouthparts as the symptoms are often not readily visible. Insects with sucking mouthparts pierce the plant and feed on sap and juices. 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 insect infestations: lodged plants; silken webs; discolouration of plant tissue; cocoons or pupae found on leaves; insect frass (faeces) on and around plants; and of course direct observation of insect adults and/or larvae. These signs should arouse suspicion of a potential problem and help determine what insect(s) could be causing the damage.

Scouting

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; 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 may not be noticed in low-lying areas until the insects become larger and more numerous. Conversely, wireworms will be less abundant on hilltops, preferring the moister soils found in low-lying areas.

Many insects tend to be edge feeders because of migration from ditches and adjacent fields with damage more prevalent around the margins. Therefore, field scouting can be most effective using Pattern 2. Concentrating control in affected areas can reduce input costs while keeping insect populations below the economic threshold. Scouting for signs of infestation where they are most likely to occur will lead to early detection.

Control Measures

The life stage of an insect is an important factor to determine the best timing for control measures. For example, egg and pupal stages are usually difficult to control. These are non-feeding life stages and are not considered an immediate threat to the crop. 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 larvae of some blister beetle are 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 an agronomist. To facilitate this process, collect samples of the damage and a few specimens of the pest, including as many life stages as possible. The insect and associated damage should be compared with good reference material. If uncertainties remain, contact the Agriculture Knowledge Centre at 1-866-457-2377, or contact the Crop Protection Laboratory (address below). These resources will help to ensure a proper identification.

Sampling

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.

• 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 per cent plant damage threshold is used. For grasshoppers, the economic threshold is measured in insects per square metre. However, sampling such mobile insects by counting the number within a measured area is difficult.

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

  • Before counting grasshoppers in a field or roadside, measure a distance of 50 metres on a reasonably level surface. Usually, this will be adjacent to the actual area to be sampled, such as a road. Flag both ends using markers or specific fence posts on the field margin. These points should be easily visible for the observer because they will be used as starting and end points.

  • To begin the count, start in the area to be sampled, aligned with one of the markers. Walking parallel to the measured distance, move through the crop toward the other marker making some disturbance with your feet to encourage any grasshoppers to jump. Any grasshoppers that jump through 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. 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 counted is divided by 50 to give an average per square metre. A hand-held counter can be useful to count the number of insects while the observer measures off the required distance. This tool may not be practical under high insect populations.

• 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.

  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.

• 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 prefer the edges of a field because of light, temperature or moisture factors. 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 within this document to determine the preferred sampling method.

  If the chart says:

  • % damage to leaves, plants, foliage, or
  • # of plants showing damage; or
  • # adults or larvae/stem / plant.

  Walk through the crop to obtain or observe the required sample units (i.e. leaves, stems, whole plants or insect counts) every few steps. To get an accurate population estimate, sample randomly at reasonably 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:

  • # adult insects or larvae / m²

    Use a metre-stick or pre-measured piece of string to mark off a square metre 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.

  • # adult insects or larvae/sweep

    Obtain or make a sweep net (Figure 1), 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 degrees. However, some economic thresholds specify 90 degree 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 can be difficult.

    As a general rule, 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.

Caution: A sweep net catches all insects, including wasps, bees and other stinging and biting insects. Therefore, be careful when examining the contents of the sweep net.

Cultural or Non-Chemical Control of Insect Pest

Cultural practices can be used to manage insect populations. The pest species affected to some degree by cultural practices and the steps that can be taken are listed below.

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

• Alfalfa weevil - In alfalfa grown for hay, reduce populations of new adults by harvesting the first cut early.

• 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 (conventional or chemical) fields are kept free of weeds that can be attractive for egg-laying females 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, trap 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.

• Wheat 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. 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 may complete flowering before midge levels increase to harmful levels. Susceptibility to wheat midge damage decreases dramatically after flowering (anthesis). 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. Midge tolerant hard red spring wheat, durum and extra strong varieties are available. These are composed of a 90:10 per cent blend of midge tolerant and conventional wheat. Midge tolerant wheat contains a gene that results in significantly less damage from feeding by midge larvae, and consistently grade better under midge infestations. It may still be necessary to apply an insecticide to these varieties if there is very high midge pressure.

• 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. Napus (Argentine) canola varieties tend to compensate for root maggot feeding better than rapa (Polish) varieties.

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

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

• Wheat Stem Sawfly - There are no established economic thresholds for wheat stem sawfly. There are no insecticides registered for wheat stem sawfly and research trials have not shown any insecticides to be cost effective. The best option to manage wheat stem sawfly, if spring wheat is to be grown as part of a rotation, is to seed a solid-stemmed wheat variety. Early swathing of infested fields once the crop drops below 40 per cent moisture content. Producers are recommended to implement management strategies if 10 to 15 per cent of wheat stems were cut the previous year. In conditions conducive to successful over-wintering, a field with this level of damage could produce enough adults to increase cutting levels to 70 per cent or greater in the following year.

• Wireworms - Crop damage may 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 an insecticide component (e.g. Cruiser - active ingredient - thiamethoxam).

It is critical to know the insect pest you are dealing with. Have it properly identified to determine best management options.

Pest Identification, Threshold Information

Crop Protection Laboratory
346 MacDonald Street
Regina, SK  S4N 6P6
306-787-8130

The laboratory is operated by the Production Technology Section of the Crops Branch, Saskatchewan Ministry of Agriculture. There is a fee for identification services. Contact the lab for details.

The Guide to Crop Protection is an annual publication of Saskatchewan Ministry of Agriculture and provides information on registered pesticides (insecticides, fungicides, seed treatments and herbicides) for use in field crops.

For information on economic thresholds for insect pests, please view the Insect Control Section of the Guide to Crop Protection.