By farmathand

This article originally appears on the Government of Saskatchewan, Ministry of Agriculture website. If you’re looking for information regarding recommended monitoring procedures, and for certain pests, alternatives to chemical control, see the full article on their website here.

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 question to ask is how many insects are too many? 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.

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. 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. The majority of the 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 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.

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. (See guides below)

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 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.

This fact sheet provides a quick reference to the currently accepted economic thresholds of the major insect pests in Saskatchewan.

Recommended Economic Thresholds for Insect Pests

alt
Download the pdf file here.

alt

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

Bertha Armyworm Economic Thresholds
alt

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 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 canola varieties tend to compensate for root maggot feeding better than rapa 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 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 an insecticide component (e.g. Cruiser – active ingredient – thiamethoxam).

This is a quick reference to the currently accepted economic thresholds of the major insect pests in Saskatchewan. If you’re looking for information regarding recommended monitoring procedures, and for certain pests, alternatives to chemical control, see the full article on Government of Saskatchewan, Ministry of Agriculture website here.






Request A Free Demo

Get a full demo of the tools available in Farm At Hand Agronomy and learn how they can make helping your clients easier. A member of farmily will reach out to schedule a time that works best for you.