Absolute Density Estimates Research Articles

Comparison of Sweep-net and Ground-cloth Sampling Methods for Estimating Arthropod Densities in Different Soybean Cropping Systems

Methods involving a sweep net, ground cloth, and ground cloth plus visual observation were evaluated for sampling arthropod pests and predators in three soybean cropping systems. The ground-cloth technique provided a good estimate of absolute density for most of the species sampled. In conventionally planted soybeans with rows spaced 81 cm apart, the ground cloth alone sampled from 82 to 97% of the true arthropod density as measured by the ground cloth plus visual observation method. In drill-planted soybeans and soybeans planted in small grain stubble, these ranges in efficiency were 75 to 96% and 73 to 90%, respectively. The lower percentages in ground-cloth sampling were primarily due to individuals remaining on the soybean foliage after the plants had been shaken. A power model was used to transform sweep-net counts to equivalent ground-cloth counts using regression analyses on mean densities of nine arthropod species encountered in 150 fields. In 11 of the 27 species/cropping system combinations, a linear model adequately described the relationship between the two methods. In those regressions that showed nonlinear relationships, the sweep net was more efficient than the ground cloth at high pest densities. Our equations provide estimates of absolute densities of arthropods in different cropping systems using different sampling methods and can be used to convert action thresholds between systems and methods.

The range of attraction of the biconical trap for Glossina pallidipes and Glossina brevipalpis

Although biconical traps are now widely used for the sampling of tsetse populations, they provide only relative population estimates determined by the fly density, activity and trap response. Studies were therefore undertaken to convert such biconical trap indices into unbiased absolute population density estimates. The first step was to determine the range of attraction of such traps for the main trypanosome vector in Kenya, Glossina pallidipes, although some data were also obtained for G. brevipalpis. Two different techniques were used. Firstly traps were placed at varying distances apart, resulting in competition and consequent reduction of catch sizes per trap when ranges of attraction overlapped. Secondly, the catches in grassland at varying distances from a forest edge were compared with two predicted functions relating catch size to distance. Results from the competition experiments showed a marked decline in catch per trap when the intertrap distance was less than 30–40 m. The resultant estimate of maximum range of attraction of 15–20 m for G. pallidipes was compatible with the observed pattern of decline of catches at increasing distances from the forest edge. The range for G. brevipalpis was tentatively estimated as up to 10–15 m.

Analysis of Sampling Procedures for Corn Earworm and Fall Armyworm (Lepidoptera: Noctuidae) in Peanuts

Relative-density estimates from two insect-sampling devices (sweep-net and shake cloth) were compared with absolute-density estimates for three size classes of the corn earworm, Heliothis zea (Boddie) (Lepidoptera: Noctuidae), and the fall armyworm, Spodaptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae), in peanuts. Linear regression analysis indicated that medium and large larvae of corn earworm and all size classes of fall armyworm were consistently recovered by both devices (r2 ≥ 0.80). Small corn earworm estimates were poor for both devices (r2 ≤ 0.38).

Evaluation of Methods for Sampling Predatory Arthropods in Soybeans 1

Sweep-net, plant shake, and vacuum net procedures were evaluated for sampling Orius insidiosus (Say) (Hemiptera: Anthocoridae), Nabis spp., (Hemiptera: Nabidae), Araneida, adult Chrysopa spp. (Neuroptera: Chrysopidae), adult Coccinellidae (Coleoptera), and adult Anthicus cervinus La Ferte-Senectere (Coleoptera: Anthicidae) in Iowa soybeans. Absolute-density estimates, taken by whole-plant removal procedures, provided standards for comparison. Plant shaking generally produced the most precise and cost-efficient estimates and is recommended for sampling the majority of predators. Sweep-net sampling was superior in terms of cost and in variability analyses for adult Nabis spp., Chrysopa spp., and Coccinellidae. The vacuum net was least satisfactory and is not recommended. Correlation of relative estimates with absolute population trends varied between years and seemed to be influenced by soybean plant growth factors.

Absolute-Density Estimation from Sweep Sampling, with a Comparison of Absolute-Density Sampling Techniques for Adult Potato Leafhoppper in Alfalfa1

A study of sampling techniques for adults of Empoasca (abae (Harris) in alfalfa was undertaken to derive ahsolute-density estimates from sweep-net sampling. A comparison of three absolute-density techniques showed that either the D-VacR or a modified EchoR suction device, operated with a small drop trap, yielded accurate estimates upon which the relationship of sweep-net catch and absolute density suhsequently was hased. Large drop trap sampling may be highly influenced by the density and uniformity of the alfalfa stand. Sampling several meters in front of the individual to avoid disturhing potato leafhoppers, compared with sampling immediately in front of the individual, gave no measurable advantage. Sweep-net sampling in fields that had been cleanly harvested yielded precise estimates of absolute density, using the equation x = IX.5y, where x = adult potato leafhoppers per m2 and y = catch per sweep. Distribution of five-sweep samples in cleanly harvested fields followed a Poisson series, and optimum sample size curves were determined. Harvesting of a portion of an alfalfa field altered the spatial distribution of potato leafhopper adults.

Estimation of Absolute Larval Densities and Calibration of Relative Sampling Methods for Velvetbean Caterpillar, Anticarsia gemmatalis (Hübner) in Soybean

Two methods for estimating absolute densities of three size classes of velvetbean caterpillar, Anticarsia gemmatalis (Hubner) (Lepidoptera: Noctuidae), larvae in soybean fields are described and evaluated. These absolute-density estimation methods, sweep-net and ground-cloth, were used to sample field populations of velvetbean caterpillar larvae in soybean. These data were used to derive equations converting sweep-net and ground-cloth larval density estimates to absolute-density estimates. Calibration equations for converting sweep-net estimates to ground-cloth estimates are also presented.

Szacowanie zagęszczenia drobnych ssaków: porównanie technik stosujących wyłów

Kaufman D. W., Gentry J. B., Kaufman G. A., Smith M. H. & Wiener J. G., 1978: Density estimation of small mammals: comparison of techniques utilizing removal trapping. Acta theriol., 23, 7: 147—171 [With 10 Tables]. Removal trapping was conducted on 12X12 grids with assessment lines to test the relative effectiveness of the Standard Minimum, Inner Square and Assessment Line methods for small mammal density estimation. Twelve study sites were trapped in desert, chaparral and woodland habitats in California and Nevada to provide a wide array of species and densities for the comparison of estimation techniques. The procedure used was to trap each grid for 10 days followed by 4 days of trapping on the associated assessment lines. Field sites were quite variable with a range of 12—167 rodents per grid representing 2—10 species. Density estimates ranged from 3.7—51.5 animals/ha for the Standard Minimum method, 4.0—36.9 animals/ha for the Inner Square method and 0.7—114.4 animals/ha for the Assessment Line method. All methods provide restive density estimates for within method comparisons but their usefulness for estimating absolute density is limited. The Assessment Line technique probably yields the best absolute density estimates (DAL) when sufficient numbers of assessment lines are trapped; however, under most conditions a single 12X12 grid with 8 assessment lines is not sufficient. To obtain reliable Dal values, captures on assessment lines need to be combined for two or more grids from the same habitat to delineate the area of effect around a grid. Problems associated with each of the methods are discussed in detail. Savannah River Ecol. Lab., SROO, Bldg. 772-G, Aiken, South Ca

Emergence Trap and Sweep Net Sampling for Adults of the Potato Leafhopper from Alfalfa12

Adults of Empoasca fabae (Harris) were sampled from alfalfa under varying field conditions by using a newly designed emergence trap, absolute density determinations, and the sweep net. Efficiency of the emergence trap in collecting adults was not significantly affected by wind, temperatures, or alfalfa height. However, decreasing solar intensity resulted in a significantly decreased efficiency as predicted by the equation, E = 0.87S0.36 (E = efficiency, S = cal/cm2/min). At solar intensities greater than 0.8 cal/cm2/min, the emergence trap gave a good estimate (80%) of absolute density. Wind velocity and, to a much lesser extent, temperature were the 2 important variables affecting sweep net catches. When pendular sweeps were used, the relationship of sweep net catches to wind velocity was predicted by the equation, M = 0.075 (1.205)w (M = sweep net conversion factor, W = wind speed (m/sec)). The ratio for pendular vs. 180° sweeps for adult E. fabae was 0.48:1. Absolute density ranged from 17.6–66.0 per m2 and the average sex ratio of males to females was 1.5:1. Female leafhoppers were caught more frequently in sweeps and less frequently in emergence traps than expected, probably due to lessened flight activity of females relative to males.

Epizootiology of Massospora levispora in an Isolated Population of Okanagana rimosa1,2,3

The Massospora pathogen occurs only in the aboveground population, i.e., adults. Cicadas become infected just prior to emergence by resting spores which then produce conidia; initial infection usually results in production of conidia. Adult cicadas infected by the conidia in turn normally produce only resting spores. Prevalence of conidia infected males remains constant, whereas a slight increase was noticed in females in 1967 but was constant in 1969. In populations of 17-yr cicadas, Magicicada septendecim, where Massospora cicadina occurs, conidial prevalence is relatively constant and equal between the sexes. The majority of individuals harboring resting spores are females indicating a higher degree of secondary transmission to them. Two major emergences of O. rimosa were studied. Mark-recapture analysis and a technique for determining age distribution were utilized to estimate absolute population density. The sex ratio of adult cicadas did not differ from a 1:1 ratio at anytime. The cicadas normally emerged in the morning but emergence was prolonged or deferred by inclement weather. Total emergence for the 20 ha study area was between 1.07 × 108 and 1.37 × 106 in 1967 and 0.96 × 106 to 1.57 × 106 in 1969 at a 90% level of confidence. Bailey's triple catch analysis gave the best estimate of absolute density from mark and recapture data. Comparison of emergence with the total population shows that even this analysis grossly underestimated the population. The age distribution analysis method gave a better estimate of absolute population density.

Estimating Populations of the Hessian Fly 123

Estimates of populations of Race B Mayetiola destructor (Say) produced by sampling 3 Indiana wheat fields during 1970–72 ranged from about 0.1–1.5 million pupae (flaxseeds) per acre. The spatial distribution of the counts of the flaxseed stage was determined to be consistent with the negative binomial distribution. The relative variability (RV) of the estimates of the flaxseeds and of plant culms per acre were below 10, well within a level of acceptable variability. As few as 15 and a maximum of 30 samples per field would be required to yield an estimate of absolute population density of the insect with an error of 10% or less of the mean. Costs and relative net precision were determined based on effort expended and the level of accuracy obtained.

Population Densities of Birds Derived from Transect Counts

GoOD estimates of absolute population density as distinct from indices of relative abundance have been virtually unavailable for nonflocking land birds except in the breeding season when singing males, representing mated pairs, restrict themselves to more or less fixed territories where they or their nests can be counted. The lack of efficient and reasonably accurate census methods applicable at any season has seriously hampered the progress of quantitative studies of avian population ecology. This paper, after reviewing the potentialities and limitations of currently available methods, describes a new method that is, 1) applicable at all seasons, 2) more efficient in terms of area covered per unit of effort than the nest or territory count methods, and 3) comparable in accuracy. The method uses the lateral distribution pattern of all detection points for each species to derive coefficients of detectability with which trail counts may be converted directly to density values in units of birds per 100 acres. The method was developed over a period of 3 years while the author was gathering data on the ecological distribution of birds in mesquite grasslands in southern Texas, pine forests in Florida and the Bahamas, and mixed woodlands in Wisconsin and Michigan.

The use of insect suction traps for sampling mosquitoes

“Johnson-Taylor” suction traps 9 inches in diameter were operated during 1967 and 1968 on Brownsea Island, Dorset, and in Monks Wood, Huntingdonshire, to sample local mosquito populations. The unbiased catches obtained were segregated into 24 samples of 1 hour each to enable the flight periodicities of Mansonia richiardii (Ficalbi), Aedes detritus (Haliday) and Ae. cantans (Meigen) to be obtained. The numbers caught each hour were used to give estimates of absolute aerial density at the height of the traps.