International Journal of Marine Science 2015, Vol.5, No.57: 1-5
2
for the assessment of growth and contamination. The
successful axenic cultures were diluted and subcultured
to 100 ml of culture media in 250 ml conical flasks.
The cultures of diatoms and planktonic green alga
were maintained in Walne’s medium at 20±2˚C,
whereas cyanobacteria were cultured in f/2 medium at
28±2˚C and incubated under illumination of 1000 lux
with 8 : 16 h light and dark regime.
2.2 Harvesting of cultures
The cultures were harvested at stationary growth
phase by centrifugation at 3500 rpm for 10 min,
whereas filamentous cyanobacteria cultures were
recovered by filtration. The harvested cells were
diluted with distilled water to obtain a concentration
of OD at 620 nm = 0.6 (Ahmad et al., 2001). For
filamentous cyanobacteria, the homogenized cultures
to a similar cell density were used.
2.3 Evaluation of phytoplankton isolates as larvicides
against mosquito larvae
Larvicidal activity of the phytoplankton suspensions
against mosquito larvae was determined by transferring
200 ml suspension to a glass beaker containing 25
numbers of second-instar larvae of
Culex quinquefasciatus
.
Three replicates were used each time and test was
repeated two times. The control consisted of larvae in
distilled water fed with 50:50 finely ground brewer’s
yeast and egg albumin (Rey et al., 2009). The food
was added to the containers at a rate of approximately
0.5 mg/larva/day.
The larval mortality was assessed after every 24 h and
Lethal Time (LT) values were calculated using Probit
analysis (Finney, 1971). The percentage of mortality
for each test was calculated. Daily observations on
larval and pupal mortality were continued through
adult emergence or until termination of the test. Adult
body size was determined by measuring the wing
length (distance from axial incision to the apical
margin, excluding fringe of scales) of each individual
(Rey et al., 2009) and compared with the control by
Analysis of Variance.
The larval digestibility was tested by transferring 25
larvae into
the 100 ml of phytoplankton suspensions
.
The larvae were allowed to feed for one hour and
removed from the suspension. The phytoplankton
cells attached to larvae were washed with distilled
water and placed in distilled water to allow for further
digestion. The gut contents were then teased from the
membrane into a vial containing sterile distilled water
and observed under phase contrast microscope. Cell
counts were carried out to determine the percentage
digestion of the phytoplankton cells.
3 Results
Among the 12 phytoplankton isolates, mortality of the
Culex quinquefasciatus
larvae was observed in the
suspensions of cyanobacteria species namely,
Nostoc
commune
,
Phormidium corium
,
P
.
tenue
and planktonic
green alga
Nannochloropsis oceanica
. No mortality of
larvae was observed up to 3 days except in
Nannochloropsis oceanica
suspension which showed
32% of larval mortality. After 7 days, the percentage
mortality of larvae fed with
Nostoc commune
,
Phormidium corium
,
P
.
tenue
and
Nannochloropsis
oceanica
was found to be 44%, 48%, 48% and 100%,
respectively (Table 1).
The development of larvae fed with phytoplankton
cells was delayed compared to the control (larvae fed
with normal food) with respect to the first pupation
period. But larvae fed with
P
.
tenue
did not show
significant difference with control. The days taken for
the 50% of adult emergence was comparatively high
than the control, whereas growth and adult emergence
was faster in larvae fed with diatom
Chaetoceros
calcitrans
as that of control. The adults emerging from
the treatments were further analyzed for their body
size by measuring the wing length. The wing length of
the adults emerged from the larvae fed with
phytoplankton cells were shorter than control and
those of adults emerged from
Chroococcus turgidus
,
Oscillatoria geminata
,
Chaetoceros calcitrans
and
Skeletonema costatum
treatment were similar in size
to that of control. From the statistical analysis, very
few significant treatment effects on development
times and mosquito size in the individual trials were
observed. The larvicidal property of the isolates was
determined by calculating the Lethal Time (LT
50
and
LT
90
). More than 50% of mortality was seen only in
the larvae fed with
N. oceanica.
The LT
50
and LT
90
were found to be 4.98 and 6.14 days. The percentage
of undigested
N. oceanica
cells was found to be
91.23% (Table 2).
4 Discussions
Phytoplankton are consumed by mosquito larvae
