E. J. DIETRICK, E. I. SCHLINGER, AND M. J. GARBER

Vacuum cleaner principle applied in

Sampling Insect Populationsin alfalfa fields by new machine method

Accurate estimates of the total insectpopulation of an alfalfa field can be ob-tained by the use of newly developedequipment and sampling techniques. Allspecies of insects—each in relation tothe whole as well as one to another—andmeasurement of the separate life stages,and appraisal of the ratios of the bene-ficial biological control organisms to theharmful plant-feeders can be made.

The new method of sampling uses asuction machine that works on the prin-ciple of the vacuum cleaner and gathersall of the alfalfa-inhabiting insects intoa collecting bag, made of organdy cloth,so completely that reliable estimates ofthe true population levels per acre can bederived. A high-speed fan driven by agasoline motor was adapted to suck in-sects, from square-foot areas of an al-falfa field, into the collecting bag. Theinsects from five separate square-footareas constitute a single machine sampleunit.

Two persons are needed to obtain asample properly. With the motor fan

drawing air into a 15' x 14" flexible tube,the operator carries the collecting nozzleabove his head from one square-foot dropto another drop. An assistant keeps themotor fan away from the area to be sam-pled to avoid disturbing the insects. Theoperator lowers the nozzle over the hayand pushes it quickly to the ground. Theoperator cuts the alfalfa stems encirclingthe nozzle. The nozzle is then tiltedenough to allow the operator to cut thealfalfa stems within the nozzle and shakethem to help the air flow dislodge insectsheld by the leaves. The nozzle is againpressed to the ground for a few secondsover the exact outline of the square-footarea. Then, as the assistant raises thenozzle, the operator quickly fits a smalleradapter hose into the nozzle. The in-creased air suction in the smaller hosegathers the heavier insects and the trashleft in the square-foot area. Five sepa-rate square-foot areas are sampled beforethe collecting bag is removed, quicklytaped shut before the motor fan isstopped, and a fresh bag installed. A five-

square-foot sample can be obtained in10 minutes by an experienced team ofcollectors.

Gathered insects must be kept alive.Portable ice 'boxes provide refrigerationfrom the time of collection until the sam-ples are ready to be transferred to funnelseparators in the laboratory. A very shortexposure of the samples to carbon dioxideanesthetizes the insects so they can betransferred to the separators. In the labo-ratory the material is spread over trans-parent plastic-coated screen trash traysin the funnels and the covers are placedin position. The funnels are blacked outexcept for a beam of light directed upfrom below the alcohol jar into the centerof the funnel. Automatic timers turn thelights on and off every 15 minutes toavoid boiling of the alcohol. Seventy-five-watt glocoil heaters, controlled by power-stats, warm the samples slowly to 120°F.Thus, gradients of heat and light, as wellas any positive geotropic responses, causethe insects to leave the duff and become

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INSECT POPULATIONSContinued from preceding page

trapped in the alcohol. After 24 hours'separation time, every insect has beentrapped in the alcohol and the samplesare ready to count.

An experiment compared five netsweeps with five-square-foot machinesamples. The net sweeps were taken byone person using the standard sweep netwith the average sweep stroke measuredto cover approximately 10 square feet.Twenty replicates of paired sweep andmachine samples were taken. All insectswere counted and grouped by more than100 different species.

The sets of counts were analyzed in thelaboratory. The table on this page showssome of the highlights for 14 representa-tive categories. The means for the 20counts were adjusted for square footage,in the last two columns of the table, butthe statistical data are compared as meancounts for approximately 50 square feetby sweep collection and exactly five There was only one case—spotted al- sampled as easily as full-grown hay. Fur-square feet by machine sampling. falf a aphid adults—where the net count thermore, the collected insects can be

As a survey of this type called for an was significantly larger than that of the separated into preserved, easily countableanalysis of all of the kinds of organisms machine. In all other cases the machine units.present, the sample size could not be ad- counts were larger than or not signifi- For most insect species, the machinejusted for each species, but had to be cantly different from the five net sweep sampling technique appears to be the onlyarbitrarily set for all. The machine sam- counts. With adjustment to equal areas, way to gather enough units to make aple was set at 5 square feet because this all net counts were less than that of the quantitative study.represented the average amount of mate- machine.rial which could be spread evenly in the The major advantage of the machine E. J. Dietrick is Laboratory Technician in

Biological Control, University of California.funnel separators. method of sampling is that it permits the Riverside.

In general, where the sample means rapid collection of a nearly complete E. 1. Schlinger is Assistant Entomologist inwere 10 or above, the data were found quantitative measure of the insects from Biological Control, University of California,to fit the normal distribution. However, the ground surface to the top of the al- Riverside.where the means were below 10, other falfa plants, even under somewhat ad- J. Garber is Assistant Biometrician, Uni-distributions gave a better fit. verse conditions. Hay stubble can be versity of California, Riverside.

Portable machine for sampling field insect populations. Method of moving machine between sampling areas.

Cutting alfalfa encircling the nozzle of sampling machine. Clipped stems within nozzle shaken to loosen insects.

Placement of adapter hose for complete sampling. Increased suction through smaller hose lifts heavy insects.

Collection bag is sealed quickly to prevent escape of insects.

CALIFORNIA AGRICULTURE, JANUARY, 1960

Anesthetized insects put into separators for counting.