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Section 5: Sampling andmonitoring arthropods

• To be able to estimate insect abundance isimportant in decision making relative topest management.

• Approach to sampling varies with goals:– Approximate, quick and simple, versus accurate,

time-consuming, complex.

– Cost (available time) may determine approachselected.

Methods of sampling

• Absolute methods - estimate density fromprecise area; used to compare densitiesbased on area.

• Relative methods - estimate density withoutstrict regard to area sampled; used tocompare among efforts based on effort.

• Population index - measures product oreffect of a population.

Examples of absolute methods

• Visual examination

• Quadrate sampling

• Suction sampling

• Enclosure devices

• Extraction

• Emergence

• Aerial nets

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Visual examination is often an effective, if time consuming,way to sample arthropods. For insects that are likely to takeflight, an aspirator (shown here) is a handy way to captureand hold them for examination.

Visual inspection is commonly used for detecting householdand structural pests, and important where the insects thatmight be encountered are unknown, cryptic, or variable.

Cryptic pests, such as termites, are difficult to detect visually. Dogs andelectronic detector are sometimes used to find invisible tunneling likethat which occurred in this wall baseboard, followed by visualconfirmation.

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Quadrate samplingdelineates an exact area ofsampling, though the areaneed not really be 4-sided.

An aphid sampler (left) samples a known volume of air by suction,while the enclosure (right) samples a known area of ground.

A mite brushingmachine brushes itemsfrom the surface ofleaves, propellingthem downward ontoa thin film of oil thatcoats a glass plate(shown below). Thus,it is possible todetermine the numberof mites per leaf.

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Live traps (above) are usedto capture small animalssuch as rodents, which thencan be examined for bothtick or flea vectors. Bysampling blood (below) thedisease incidence in theanimal population can bedetermined. It is thecombination of animalpopulation, diseaseincidence and insect vectorpopulation that determinesthe threat to humans.

Emergence traps often are used to capture insects that live below-ground or under water. Usually such traps take advantage of theinsects tendency to move to light, and have a capturing or killingmechanism at the top of the trap.

Some insects occupy cryptichabitats, which makes samplingdifficult. In the case of grasshopperegg pods, for example, which aredeposited beneath the surface of thesoil (left, see grasshopper depositingeggs) you can dig up definedvolumes of soil, sift through the soilfor egg pods, and obtain an estimateof potential population size. This is agreat deal of work, however, andusually it is easier to watch for theirhatch to begin populationassessment.

How would you know when to begincounting newly hatchedgrasshoppers? Do all species hatchsimultaneously?

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Examples of relative methods

• Sweep net

• Vacuum trap

• Timed counts

• Traps

- Visual

- Chemical

- Interception

A pheromone-baited trap like this capturesmoths when they fly and crawl upwards andare captured in the container on top.

Sweeping with a net is a verycommon and inexpensivemethod of sampling. Flighttraps (right) cost substantiallymore, and are limited to flyinginsects.

Various vacuumsamplers are used, andare often called “D-vacs” after the originalmodel. They areefficient, but costlyand noisy to operate.

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Here a vacuum sampler is used to vacuum thecontents of a box placed over shrubs. Thiseffectively converts its operation from a relative toan absolute sampling procedure.

Dippers are commonlyused to sample formosquito larvae inwater, whether it is avery artificial breedingarea such as anautomobile scrapyard(above), or a morenatural water source(below).

Traps are widely used forinsect sampling andmonitoring. Light traps(shown here) were morepopular before the advent ofpheromones. They have theadvantage, and disadvantage,of being fairly non-selective.They can be operated offbattery or household current.Here the battery is being re-charged by a solar panel.

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Sticky adhesive,combined with color-based (below) or odorbased attraction (right),can be used for sampling

A fruit model makes agood visual trap, such asthis grapefruit model.When covered withadhesive, it allowsestimation of adultnumbers.

Urban pest control companies often use sticky traps to sample for insectpests in commercial establishments and schools. They are not visuallyattractive (some would describe them as disgusting), so they must beplaced in out-of-the-way places, such as store rooms.

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Insects stuck in adhesive are messy, and can be difficult toremove and identify. Also, such traps may not be selective.

The bucket trap (left) andwing trap (below)introduce selectivity byusing pheromones aslures.

Pheromone traps are popularbecause they providespecificity, are commerciallyavailable, and can be usedunder a variety of circumstances

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Though pheromone trapsusually rely on adhesivefor insect retention, this isnot always optimal.Bucket traps, water-pantraps, and other designssometimes are moreeffective than sticky traps.

Shown here is acomparison of stickywing and water pan trapsfor European corn borermonitoring: water is moreeffective in this case.

There are an infinite number of trapdesigns, and a rich literaturesurrounding their development anduse. It is important to investigate theliterature prior to sampling. Here is anexample of an unusual method: ablack, swinging, sticky-covered ballfor deer flies.

This is a food-based trap,called a McPhail trap. This,or a variation of thisdesign, is widely used fortephritid fruit flies, and canbe used for yellowjacketsas well. A liquid bait (oftenprotein hydrolyzate) isplaced inside. The insectsfly in from below, throughthe circular opening in thebottom. Once inside, theyhave difficulty escaping,and usually drown.

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Mosquito traps are quitevaried, but often involve theuse of dry ice as a source ofcarbon dioxide, and a smallfan to blow the mosquitoesinto a receptacle.

This flight trap, called amalaise trap, is an exampleof an interception trap. Thereis no active element to thetrap, they flying insectsinsects are intercepted, crawlupwards, and are captured ina jar. The other commoninterception technique is atransparent pane of plastic orglass which interrupts flight.Insects are captured onadhesive or fall into areservoir or receptacle.

Termite-sniffing dogshave become popularfor detection of thesehidden pests.

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Sampling with traps of any design is not without itsproblems. Here you see the effect of the phase ofthe moon on capture of flies by a light trap. Thelight trap does not compete well with a full moon.(after Bowden and Morris 1975)

Similarly, wind amplifies pheromone trap captures, asshown by the positive relationship of wind speed andSpodoptera capture (after Campion et al. 1974).

Examples of population indexes

• Defoliation• Frass• Tents, nests, or webs• Emergence holes• Tunnels• Sounds• Biting counts

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The level of insect feeding (damage) on foliage, asshown on these bean plants, is a good index of insect(Mexican bean beetle) abundance.

The number ofpeppers that drop tothe ground (in thiscase in the irrigationfurrow) is a goodindex of pepperweevil activity.

The amount of hopperburn, caused by the toxic feedingsecretions of leafhoppers, is an index of their abundance.

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Though the leaf miners (in this case, birch leafminer) maybe difficult to see, the mines (tunnels) they form are easy tosee, and long-lasting.

A drop cloth is a good means to collect insect fecalmaterial (frass) produced by tree-feeding insects.

Sometimes frass or emergence holes are the best wayto assess the occurrence of boring insects. Shown ispickleworm frass extruded from a pumpkin.

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Cicada abundancecan be assessed bythe presence of shedcuticles but also bytheir level of calling.

Freshly molted cicada (above) andshed nymphal cuticle (below)

Tents caused by fallwebworms, tentcaterpillars, ugly-nestcaterpillar, and othernest-building insectsare a good index oftheir abundance.

Fall webworm nests

Remote sensing, whetherusing satellite (above) oraircraft (below) imagery,depends on the responseof vegetation to insectinfestation. Both visibleand nonvisible portions ofreflected lightwavelengths can be usedin assessing plant coverand vigor.

Infra-red imagery (above)and normal visual (below)

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Numerous electronicdetectors are useful todetect the tunneling oftermites. At right is anacoustic detector; at left isan x-ray unit. Moisturedetectors also are common.

This is what an X-ray image of termite tunneling looks like using acommercial termite sensor; there is no surface damage.

Development of a sampling plan

• Sampling “plan” (protocol) not the same as“method” (approach/technique)

• Should assess density of insects

• Should be independent of sampler

• Should be relevant to questions asked

• Should be practical

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Components of a sampling plan

• Sample universe (area of concern)• Timing (seasonality and developmental

stage)• Sample unit (actual area sampled)• Spatial distribution (aggregated or clumped,

random, uniform)• Sample size (accurate vs. practical;

calculated?)

Representative spatial distributions

Four common types ofsampling plans

• Fixed sample size

• Sequential

• Variable-intensity

• Binomial

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Fixed sample size

• Most common approach (e.g., “10”) thoughoptimal size can be calculated.

• Optimal sample size decreases withpopulation density.

• Should be able to get by with fewer samplesat high density.

Optimal sample size

• There are various means to calculate the optimalsample size if you elect to use a fixed sample. Youwant to be sure that you can be sure that youobtain an accurate assessment of the population.

• Calculation of optimal size is done tooinfrequently, probably because the results areunfavorable.

• One technique follows:

Calculating optimal sample size

• Within a homogeneous habitat, the number ofsamples needed to estimate the mean accurately,assuming a standard error of 5% of the mean, iscalculated as follows:

n = s2 / E x

Where n is the number of samples, s is the standarddeviation, E is the predetermined standard error asa decimal of the mean (in this case 0.05), x is themean.

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Calculating optimal sample size

• Several calculations are required, including the mean,variance, and standard deviation.

• Mean (x): the numerical average of the counts (x)from each sample. This is calculated by summingthe individual counts and dividing the summedcounts by the number of counts.

• Example: 6 counts of leaf tissue reveal 17, 15, 10, 18,25 and 20 insects per leaf, or 105/6, or a mean (x) of17.5

Variance: a measure of the variability among sample counts. This is calculated bysubtracting the mean from each individual sample count, squaring the differencesbetween these two values, summing the squared values, and then dividing the summedsquared values by the number of samples taken minus 1.

Example: x (count in x-x (x-x)2

each sample) (count-mean) (differences squared)

17 -.5 .2515 -2.5 6.2510 -7.5 56.2518 .5 .2525 7.5 56.2520 2.5 6.25

105 0 125.50

(x-x)2 = 125.5 = 25.1n-1 5

More calculations

More calculations

• Standard deviation (s)= a commonly used measureof variability among samples, and is calculated bydetermining the square root of the variance

sq root of 25.1=5.01

• Note that the standard deviation decreases as thesample number increases, so variability decreaseswhen you increase sample number. This isimportant when you are trying to determine if thereis a difference between treatments or fields.

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Recall that the optimal sample size is calculated asfollows:

n = s2 / E xWhere n is the number of samples, s is the standard deviation, E isthe predetermined standard error as a decimal of the mean (in thiscase 0.05), x is the mean. So for our example:

n = 5.012 = 5.01 2 = 33(0.05)(17.5) 0.87

Note that the optimal sample size increases as the standard deviationincreases.Also, if you desire greater confidence in your assessment (e.g.,standard error of less than 5%) you can use a smaller predeterminedstandard error, which will require more sampling.

Remember that everything depends on your preliminarysampling where you determine mean values andvariance among samples. Therefore, you should samplemore thoroughly than you can imagine sampling on aroutine basis.

Note that you will need to recalculate if mean andvariance values change.

If in doubt, check the literature for acceptable samplingprotocols, or consult a statistician.

Sequential sampling

• Variable number of samples taken

• Sample until the population can beclassified (e.g., no threat or control needed)

• More sampling at intermediate levels

• Efficient

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More sequential samplingThe chart at the right is agraphical display of the processof sequential sampling. In thiscase, we have elected to define 3categories:- a population requiring treatment- a population not requiringtreatment- a population that is intermediate,and in need of close scrutiny, soif the population is determined tobe intermediate, we will returnand sample again in 3 days

More sequential samplingAlong the bottom you seedesignation of sample number.As you continue to takesamples you accumulate insectcounts. So after taking 4samples, if you have observedonly 100 insects, you categorizethe population as not requiringtreatment. On the other hand, ifyou have observed 700 insectsafter 4 samples, you have apopulation requiring treatment.

More sequential sampling

• The location of the sets of parallellines is determined mathematically,based on knowledge of theeconomic impact of the insects onthe crop.

• If your counts fall between theparallel lines, you continue tosample.

• The width of the parallel lines isdetermined by your need forconfidence in classifying thepopulation accurately.

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More sequential sampling

• In most cases, sequential sampling plans simplyclassify the population into 2 categories: needingtreatment or not.

• More information on calculating sequentialsampling plans, including how to calculate thedividing lines, can be found in most texts onsampling. For beginners, I generally recommendthe following:– J.A. Onsager. 1976. The rationale of sequential

sampling, with emphasis on its use in pest management.USDA Technical Bulletin 1526.

Variable intensity sampling

• Modification of sequential sampling

• Assures that sampling is broadly distributedover the sampling universe

• Spreads out sampling

Binomial sampling

• Records presence or absence; proportioninfested or damaged

• Does not tabulate number of insects

With insects such as theseaphids, actual tabulation ofinsect numbers is a hugetask. It may be better toapproach such sampling ona presence-absence basis,simply recording whether ornot plants (or portions ofplants, such as individualleaves) are infested.

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Allocation of sampling units

• How to allocate sampling effort from thesampling universe– Random sampling (unbiased, each unit has equal

chance of being selected)

– Stratified sampling (variation of randomsampling based on knowledge of insect ordistribution; assures collection)

– Systematic sampling (predetermined pattern, e.g.,“X, W or Z”-shaped transects are common)

Sampling ofindividual plants inthis field (circled) isstratified, ensuringthat both the marginsand center aresampled. For speciesthat enter from theedges, it may be mostuseful to concentratesampling at themargins.

Sampling of this field(the path of thesampler is shown, asare the plantssampled) issystematic, and strivesfor a balance of goodcoverage and minimaleffort. This is acommon samplingplan.

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Questions

• Can you describe the 3 basic methods ofsampling, and provide examples of each?

• Can you describe the components of asampling plan?

• Can you describe the types of samplingplans?

• How are sampling units allocated?

Questions from supplementaryreading

• Reading 5, sampling– What is the difference between the sampling plan and

sampling method?

– How does the goal of a sampling plan affect the methodselection process?

– What is spatial dispersion?

– What is the purpose of random sampling? How does itdiffer from stratified sampling? Systematic sampling?

View a Short Video on Sampling

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View a Short Video on Scouting