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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. The sampler is operated by plunging the cylinder from
a maximum distance of 2 m into the water, 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 was 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
which 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. References Cited 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. Illustrations Use of water
column sampler: Placement of
Plexiglas cylinder into sample site; Insertion of
suction apparatus into cylinder; Extraction of
insect fauna with suction |
