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Economic Thresholds for Insect Pests

Last Updated May 2010

Introduction | Crop Monitoring Techniques | Scouting | Sampling | Cultural Control of Insect Pests |

Recommended Economic Thresholds | Resources | Literature Sources

Introduction

Keeping insect infestations below significantlevels through preventative measures is thecore of long term integrated pestmanagement. More immediate control isreactive and is warranted only when theinsects affect the producer financially. Acommon problem for most producers is

deciding whether or not to treat a crop for aspecific insect pest.

The question is how many insects are toomany insects? The initial response may be tospray as soon as insects are found in the crop.But implementing control measures is costlyand can require significant inputs / amounts /quantity of insecticides and fuel. In addition,the labour involved in control operations issignificant. Many insecticides have broad spectrum activity affecting target and non-target

(including beneficial) organisms. Therefore, unnecessary applications can have undesirableenvironmental 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 guidanceas 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 valueof 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.

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 determineif it the insect is truly 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 inSaskatchewan leaves some gaps in knowledge as to how insects will affect some of the "newer"crops.

Figure 1: Flea beetle feeding on canola seedling. Notethe “action threshold” for flea beetle on canola is 25% of cotyledon surface damage.Source: Saskatchewan Agriculture.



Economic thresholds can fluctuate depending on a combination of factors including the pest, thecrop type, growth stage, expected market value and cost of control. The economic threshold mayalso 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,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 cropalready under stress. Economic thresholds serve as a guideline to the producer, and to be effective,the plant’s growth stage and growing conditions must also be considered.

The wheat midge on wheat provides an excellent example of how the damage potential of an insectis related to plant growth stage. Spring wheat is highly susceptible to wheat midge damage from thetime the boot swells and splits and any part of the head is visible until flowering (anthesis). Controlmeasures 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 inother crops. Other examples can be found in the comprehensive economic threshold charts for theBertha Armyworm and Lygus bug. These charts illustrate a range of economic thresholds in relationto 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 bemade objectively. First there should be evidence of damage. Second, an estimate of potential cropdamage should be made and then compared to the cost of applying an insecticide. If crop damage iswidespread, control measures may be required for the whole field. However if damage appears to bemore isolated, a "spot" spray concentrating on the infected area(s) may suffice. Thorough monitoringand 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 alsoworthy of consideration. Certain pest species, by virtue of their life cycles, can be greatly affected byagronomic practices, sometimes without the use of chemicals.

In some cases cultural control of insect pests can coincidentally be achieved by implementingagronomic 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 tomanage 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.



Crop Monitoring Techniques

Every field should be monitored on a regular basis to detectspecific insect pests and to

determine densities within thecrop (e.g. Pattern 1). With anadequate monitoring program toestablish presence of pestspecies and to monitor changesin population densities, producers are more likely to beaware of potential problems.

The first step is to determine the potential insect pests. Producers

unfamiliar with the possibleinsect pests of a crop should acquire a production guide for the crop. The second step is to identifythe 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 tothe 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 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 insectsare not readily visible. These insects may remove material from leaves, stems, or other plant partsgiving it a ragged or chewed look. Injured roots will often show sign of bored holes or lesions whileabove ground the plant may appear wilted or stunted. Examples of insects with chewing mouthpartsare 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 injecta 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 sapextruded on pods and stems.

More advanced symptoms of severe injury include shrivelled stems and seeds and a reduction innumber of seeds set. Extreme cases in canaryseed have been observed where aphid feeding has

Figure 2: Pattern 1 – Used for insects that often are uniformly distributed:including aphids, bertha armyworm, diamondback moth and lygus bugs.Source: Saskatchewan Agriculture.



resulted in empty, whitened tips of heads. Examples of insects with sucking mouthparts areleafhoppers, plant bugs (e.g. Lygus), and aphids.

There are many other signs of insect infestations: lodged plants, silken webs, discolouration of planttissue, 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 localenvironmental 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 soilmoisture 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 more moist soils found inlow-lying areas.

Many insects tend to be "edgefeeders" as a result of migrationfrom ditches and adjacent fieldswith damage more prevalentaround the margins. Therefore,field scouting can be mosteffective using Pattern 2.

Concentrating control in affected areas can reduce input costs whilekeeping insect populations belowthe economic threshold. Scoutingfor signs of infestation where theyare most likely to occur will lead to early detection.

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 stagesare not considered to be a 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 Armyworm pupae or Wheat Midge cocoons in the soil).

Even larvae, which are more susceptible to insecticides, can be difficult, or not economicallyfeasible 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.

Figure 3: Pattern 2 – Used when pests are at the edges of fields.Including flea beetles, Colorado potato beetle and grasshoppers.Source: Saskatchewan Agriculture.



Once the presence of a pest has been confirmed, its identification must be verified. Correctidentification 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 1-866-457-2377, or directly 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 areseveral important points to consider while sampling.

First - It is important to utilize a sampling technique that is appropriate for the type of insect beingmonitored. The monitoring method is largely related to specific insect behaviour. Highly mobileinsects 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 meter. However, samplingsuch mobile insects by counting the number within a measured area is difficult.

The following procedure for estimating grasshopper densities is relatively easy and reliable:1) Before counting grasshoppers in a field or roadside, measure a distance of 50 metres on areasonably level surface. Usually this will be adjacent to the actual area to be sampled, such as aroad. 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.

2) 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 somedisturbance with your feet to encourage any grasshoppers to jump. Any grasshoppers that jumpthrough 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 canoften visualize the required width and a metre stick may not be required.

3) At the end of the 50 metres, the total number of grasshoppers counted is divided by 50 to give anaverage per square metre. A hand-held counter can be useful to count the number of insects whilethe observer measures off the required distance. This tool may not be practical under high insect populations.

Second - It is important to sample randomly and gather numerous samples. The samples mustrepresent, 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. Collectingnumerous 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 notspray, 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 anadjacent 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 populationestimates, they may indicate a potential problem before it becomes serious. Be sure to samplethroughout 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 tableswithin this document to determine the preferred sampling method.

If the chart says:

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

2) # of plants showing damage,

or

3) # adults or larvae/stem / plant

Walk through the crop to obtain or observe the required sample units (i.e. leaves, stems, whole plantsor insect counts) every few steps. To get an accurate population estimate, sample randomly atreasonably 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) # adult insects or larvae / m2

Use a meter-stick or pre-measured piece of string to mark off a square meter of the crop. Examinethis 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) # adult insects or larvae / sweep

Obtain or make a sweep net

(Figure 1), a tool used byentomologists to sample insects. Itconsists of a muslin bag or somesimilar material held open by ahoop attached to a long handle. Anangler’s net lined with a pillowcase will work as well.

The size of the net opening isimportant, however, as this willaffect the number of insects

caught. The standard net size is 38centimetres (15 inches) indiameter.

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 thresholdsspecify 90º 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 incanola, this can be 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 thanonce. Keep track of the number of sweeps, and count the number of pests in each sweep or take anaverage of the number of insects divided by the number of sweeps.

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

Cultural or Non-Chemical Control of Insect Pests

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 cutearly.

Aphids: Early seeding can help to avoid infestations because the crops mature before the pest levelsexceed economic thresholds. As plants mature they are less attractive to aphids.

Figure 4: Scouting for insects using a sweep net.

Source: Dan Johnson, University of Lethbridge.



Beet webworm: Weeds in and around susceptible crops should be removed to reduce theattractiveness 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 seedinguntil late May if damage is evident.

Grasshoppers: Control of annual weeds before grasshopper emergence will help reducegrasshopper 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 beforeweed control is carried out, or in forage used for animal feed, traps strips can be maintained in whichgrasshoppers 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 tothe main crop.

Wheat Midge: In areas where wheat midge is expected to be abundant, one should consider notseeding spring wheat or at least avoid planting it in or near fields that were infested the previousyear. 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 levelsincrease 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% blend of midge tolerant and conventional wheat. Midge tolerant wheat contains a gene that results insignificantly less damage from feeding by midge larvae, and consistently grade better under midgeinfestations. It may still be necessary to apply an insecticide to these varieties if there is very highmidge 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, fieldsthat were infested the previous year.

Root maggots: (Canola) Increase seeding rates. Napus canola varieties tend to compensate for rootmaggot feeding better than rapa varieties.

Sunflower beetle: Crop damage in the year following infestation can be minimized with late fallcultivation 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 cutfor 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. Producersare recommended to implement management strategies if 10 to 15 per cent of wheat stems were cutthe 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 thefollowing 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 insecticidecomponent (e.g. Cruiser – active ingredient - thiamethoxam).

Remember: It is critical to know the insect pest you are dealing with. Have it properlyidentified to determine best management options.

Recommended Economic Thresholds for Insect Pests

Insect Crop Economic ThresholdAlfalfa weevil Alfalfa Hay: 25 – 50 % of leaves on upper 1/3 of stem, or

50 - 70 % of foliage tips show injurySeed: 35 - 50% of plants show damage or

20 - 25 larvae / sweep (180o)

Aphids: Birdcherry-oat aphid

CerealsCanaryseed

12 - 15 aphids / stem prior to soft dough10 - 20 aphids on 50 % of the stems prior to soft dough

English grain aphid CerealsCanaryseed

12 - 15 aphids / stem prior to soft dough10 - 20 aphids on 50 % of the stems prior to soft dough

Cabbage aphid Canola 10-20 % of the stemTurnip aphid Canola 10 – 20 % of the stems have aphid clustersCorn leaf aphid Cereals 12 - 15 aphids / stem prior to soft doughRussian wheat aphid Cereals 10 % of the plants infested with at least 1aphid when first node

visible10 % of the tillers infested with 1 aphid when tip of flag leaf just

visibleGreen bug (aphid) Cereals 12 - 15 aphids / stem prior to soft doughPotato aphid Flax

Flax2 - 3 aphids / main stem at full bloom *8 aphids / main stem at green boll stage *

Pea aphid Pea 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 CerealsFlaxMustard

> 10 larvae/m2 Economic threshold not yet established < 5 / m2 at seedling stage

Thrips Barley, OatsRed Clover

7 - 8 thrips / stem prior to head emergence50 - 80 thrips per flower head

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

Bertha Armyworm Canola (Napus) See chart below for variationsClover cutworm Canola, Mustard,

Flax20 - 30 larvae/m2

Cutworms CerealsOilseeds

Pea

3 - 4 larvae/m2 Economic thresholds not yet established – nominal threshold of 25 to 30 per cent stand reduction2-3 larvae /m2 in the top 7 cm (3 inches) of soil

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 standsaveraging 150 to 200 plants / m2. In areas where stands arethinner 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

Flax, Lentil

Canola (Napus)

8 - 12 grasshoppers /m2 2 grasshoppers /m2 - depending on crop stage (i.e. lentil pods arefar more prone to attack than is the foliage)> 14 grasshoppers /m2

Wheat midge Wheat 1 midge / 4 - 5 wheat heads

Painted lady butterfly Sunflowers 25% defoliation when larvae are less than30 mm (1.25 inch) inlength. Larger larvae have completed most of there destructivefeeding and treatment is not required.

Plant bugsAlfalfa plant bug

Alfalfa Seed: 4 bugs/sweepHay: control not recommended

Pea leaf weevil Pea 1 in 3 plants showing feeding damage on the clam leaf or 30 %of the plants showing feeding damage

Lygus bug Alfalfa

Canola

Seed: 8 bugs/sweepHay: control not recommended 1.5 bugs / sweep (see chart below for variations)

Red sunflower seed weevil Sunflower Oil crop: 12 - 14 weevils/head at 85 - 100% bloomConfectionery: 1 - 2 weevils/head at 85 - 100% bloom

Red turnip beetle Canola As soon as beetles become numerousSunflower beetle Sunflowers 1 adult beetle / 2 - 3 seedlings at the 2 - 6 leaf stage or > 10

larvae per plant during summer

Sunflower moth Sunflowers As soon as moths are present and > 10% of blooms openSweetclover weevil Clover 1st year stands: 1 weevil / 3 seedlings (1 / 5 seedlings under dryconditions)2nd year stands: 9 - 12 weevils / plant

Wheat Stem Sawfly Cereals Economic threshold not established – see “Cultural / non-chemical control”

Wireworms Cereals Economic threshold not established. Nominal threshold:32/m2or greater - seed treatment is required the following year



Economic Thresholds for Lygus Bugs in Canola

Stage * / Canola Crop # Lygus bugs per 10 sweepsApplication Cost

$/ha $/ac Bud End of Flowering* Pod Ripening*

22 8.90 14 12 10 20 17 15

24 9.70 16 13 11 22 18 16

26 10.50 17 14 12 24 20 17

28 11.35 18 15 13 25 22 19

30 12.15 19 16 14 27 26 20

32 12.95

No yield responseshown withinsecticidesapplied at thisstage.

21 17 15 29 25 21

Canola Price$/tonne 220 260 300 220 260 300

$/bushel 5.00 5.90 6.80 5.00 5.90 6.80

*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.

Expected Value of Canolato Grower ($ per bushel)

6 7 8 9 10 11 12 13 14 15 16

Cost of Spraying($/acre) # of larvae / square metre

617 15 13 11 11 10 9 8 7 7 6

7 20 17 15 13 12 11 10 9 9 8 8 8 23 20 17 15 14 13 12 10 10 9 9 9 26 22 19 17 16 14 13 12 11 10 10

10 29 25 22 19 17 16 15 13 12 11 11 11

32 27 24 21 19 17 16 14 13 13 12 12 34 30 26 23 21 19 17 16 15 14 13



Resources:

Pest Identification, Threshold Information

Crop Protection Laboratory

346 MacDonald StreetRegina, SaskatchewanS4N 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.

Literature Sources

The Guide to Crop Protection is an annual publication of Saskatchewan Agriculture and providesinformation on registered pesticides (Insecticides, Fungicides, Seed Treatments and Herbicides) for use in field crops.

Specific insect fact sheets and other publications are available on the Saskatchewan Agriculturewebsite; or by contacting the Agriculture Knowledge Center (1 866-457-2377); or from the CropsBranch of Saskatchewan Ministry of Agriculture, Room 125 - 3085 Albert St., Regina, SK. S4S0B1. Practical Crop Protection Alberta Agriculture and Rural Developmente-mail [email protected]

Publishing Branch7000-113 StEdmonton, AlbertaT6H 5T6Contains detailed information on crop monitoring, as well as specific management of pests in field crops.

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