QUANTITATIVE WATER COLUMN SAMPLER FOR INSECTS
IN SHALLOW AQUATIC HABITATS
E. F. Legncr (2), R. A. Medved (2) and R. D. Sjogren (3)
A water column sampler was developed to quantitatively sample. insect predators in shallow aquatic habitats. The sampler eliminated or greatly reduced problems of location and scattering of specimens in the water, sampling time, escape flight of adult individuals, etc. Increased attention to the value of insect predators of mosquito control in aquatic habitat~ (Legner et al. 1974) made the development of an accurate sampler imperative for quantitative studies. Problems of location and scattering of specimens in the water, sampling time, escape flight of adult specimens, differential photo taxes, and excessive weight of equipment and samples have been heretofore unresolved. Trials with numerous devices eventually produced an accurate quantitative water column sampler described herein.
METHODS & MATERIALS
Description and Operation of Sampler .-A graduated hollow Plexiglas cylinder, 9.53 cm diam. and 50 cm. long with a ca. 2 m aluminum handle was constructed (Figure 2) to isolate a column of water within. Plunging the cylinder from a maximum distance of 2 m into the water operates the sampler, fixing its base slightly in the benthic mud. This distance causes minimal disturbance of insects at the sample site. Following fixation the insect fauna contained within the cylinder, including benthos inhabitants, are removed by suction into a perforated polyethylene wash bottle attached to a grease extractor syringe which is manually operated (Figure I ). The wash bottle is removed from the Plexiglas cylinder and back-flushed onto a suitable nylon screen which removes the water and retains the insects sampled. Three to five extractions, depending on water depth, remove all the water and contained larval and adult insects in the cylinder, ttle whole process requiring a maximum of 2 minutes. Photographs of the entire sampling. procedure are shown in Figure I. A prototype of this sampler developed by T. Yamaguchi is shown on page 26 of Usinger (1971). The nylon screen containing living wet arthropods is then placed into a polyethylene bag and either stored living in an ice chest or killed with the addition of a piece of ethanol-soaked cellu cotton . The weights of equipment and samples are light enough that an operator may easily carry 50 samples at one time. An aperture of 2 cm cut from the perimeter of the wash bottle base is sufficient for most large insects and small enough to minimize water loss during the transfer from cylinder to nylon screen. The aperture being positioned off center as shown in Figure I produces a swirling motion that aids in flushing insects from vegetation that may be located within the cylinder at the time of sampling. One back-flush within the cylinder will dislodge chironomids from the benthic mud. The depth of the column of water being sampled may also be measured by reading off the graduated Plexiglas cylinder, with a suitable adjustment made for benthic penetration. Sampling Accuracy .-Separate water column samples were taken in square, shallow 4-acre duck club ponds (10- 20 cm depth) near Wasco, California on 19 September, 9 and 16 October and 12 November, 1973. There were a total of 16 adjacent ponds in the area totaling 64 acres of water surface. The number of insect species, their density and sample variability was compared in weedy (largely emergent grasses~ versus open water habitats in the same ponds, and at different times of the day. Comparisons were also made with the standard 400-ml mosquito dipper and a series of immersed, side-darkened light traps similar to those used by Washino (1969).
The bulk of collected specimens were identified t<' the nearest accurate taxon, and sample specimens were mounted and sent to specialists in the Systematic Entomology Laboratory of the U. S. Department of Agriculture for species identification.
RESULTS & DISCUSSION
Insect species identified from the duck club near Wasco are listed in Table I. Further discussion of these species will be in groups that could be practically discerned during the data counting process. Table 2 shows the average density at different times of day and sample variability of insects secured in one 4-acre pond on September 19. Comparisons are shown with immersed light traps operated during the two hours before midnight. It is immediately apparent that the column sampler secured more species at all sampled intervals of the day than the light traps did after dark (Table 2). However, the light traps attracted two groups of Hemiptera in greater numbers than the column sampler, the Corixidae and Notonectidae. This probably was due to attraction of these species from variable distances and thus a greater sample area. The sample variability as measured by the coefficient of variability indicates that use of the column sampler often reduced variability and in any case estimated most accurately the true population distribution per volume of water. Direct observation of high densities of Notonectidae during daylight hours showed that the placement of the transparent Plexiglas cylinder did not scatter specimens that seemed largely unaware of the cylinder being placed around them. A comparison of the water column sampler with the standard 4O0-ml mosquito dipper in 4, 4-acre ponds on October 9 is shown in Table 3. Although dip samples were taken rapidly, there were no predators collected with this method. However, mosquito larvae were adequately sampled. Mosquito pupae appeared to be most accurately sampled with the column sampler (Table 3). The column sampler was further tested in two ponds on October 16 to determine differences between rapid and slow placement of the cylinder both in open water and grass covered habitat (Table 4). The results show that quick placement was more efficient than slow in trapping specimens in both open and grass covered habitats. Also, the grass covered habitat contained the greatest insect biomass. It is interesting to note that variability was relatively constant regardless of habitat or rapidity of insertion (Coefficient of Variability = ca. 200%) (Table 4).Final comparisons between the column sampler, 400-rnl dipper and immersed light traps were made in two ponds on October 16 (Table 5). Results were similar to those secured previously (Tables 2 and 3) even though the average density of insects showed a seasonal decline (Table 5). The last general sample in the area was made in 4 ponds, 25 samples per pond, on November 12, 1973. Most species showed a further seasonal decline (Table 6), which may have involved both a lack of mosquito prey and a lower water temperature.
Legner, E. F., R. D. Sjogren and I. M. Hall. 1974. The biological control of medically important arthropods. Critical Rev. in Environ.ControI 4(1):85-113.
Usinger, R. L. 1971. Aquatic Insects of California. University of California Press, Berkeley. 508 p.
Washino, R. K. 1969. Predator prey studies in relation to an integrated mosquito control program. A progress report. Proc. Calif. Mosq. Control Assoc. 36:33-34.
1 Research performed with the assistance of personnel in the Kern Mosquito Abatement District, Post Office Box 9428, Bakersfield, California 93309.
2 Division of Biological Control, University of California, Riverside, California 92502.
3 Metropolitan Mos4uito Control District, 797 Raymond Avenue, St. Paul, Minnesota 55114.
Use of water column sampler: Placement of Plexiglas cylinder into sample site;