International Journal of Marine Science 2015, Vol.5, No.57: 1-5
3
Table 1 Effect of phytoplankton isolates on larval survival and development.
Phytoplankton
% mortality after
3 days
% mortality after
7 days
Days to first
pupation
Days to 50%
emergence
Wing length
(mm)
Chroococcus turgidus
-
-
C<S
C<S
C=S
Lyngbya confervoides
-
-
C<S
C<S
C>S
Nostoc commune
-
44
C<S
-
C>S
Oscillatoria fremyii
-
-
C<S
C<S
C>S
O. geminata
-
-
C<S
C<S
C=S
O. sancta
-
-
C<S
C<S
C>S
Phormidium corium
-
48
C<S
C<S
C>S
P. tenue
-
48
C=S
C<S
C>S
Spirulia major
-
-
C<S
C<S
C>S
Chaetoceros calcitrans
-
-
C<S
C=S
C=S
Skeletonema costatum
-
-
C<S
C<S
C=S
Nannochloropsis oceanica
32
100
-
-
-
*C Control; S: Phytoplankton suspension.
Table 2 Lethal time values for 50 % (LT
50
) and 90 % (LT
90
) mortality of the mosquito larvae fed with
Nannochloropsis oceanica.
LT
50
(Days)
LT
90
(Days)
χ
2
[df = 5]
p
Intercept
Slope
% of undigested cells
4.98
(3.65 - 6.31)
6.14
(4.56 - 7.18)
51.206
0.001
a
-3.105
7.337
91.23
a
Since the significance level is less than 0.150, a heterogeneity factor is used in the calculation of confidence limits.
(Marten, 2007). But some species of phytoplankton
such as
Oscillatoria agardhii
,
Microcystis aeruginosa
,
Anabaena solitaria
,
A. circinalis
,
Akashiwo sanguine
and
Chlorella ellipsoidea
have lethal effects in the
development and survival of mosquito larvae (Kiviranta
and Abdel-Hameed, 1994; Saario et al., 1994; Harada
et al., 2000). The effects of phytoplankton could be
toxic to aquatic stages of mosquitoes, local reduction
or elimination of mosquito populations by their
indigestibility or modification of the reproductive
cycles (Tuno et al., 2006; Rey et al., 2009).
In the present study, the planktonic green alga,
Nannochloropsis oceanica
was found to be the most
effective larvicide against the test mosquito larvae,
with most dying with their guts full of microalgae
cells. The larvae showed no growth and died within
few days during the second or third instar stage of
development. The larvae which reached the fourth
instar stage were usually in an emaciated condition.
The 100% mortality of larvae fed with
N. oceanica
cells was observed. Ahmad et al. (2004) reported the
similar observation when
Aedes aegypti
larvae treated
with
Chlorella vulgaris
culture. They also observed
the shrunken appearance of treated larvae and concluded
that microalgae may induce some morphological
abnormalities in the mosquito larvae. In the remaining
culture suspension, no significant larvicidal effect was
seen. Rashed and El-Ayouty (1992) investigated that
Chlorella vulgaris
has some mosquito regulating
effects and it is not a sufficient food source for larval
development when tested against
Culex pipiens
.
The digestibility of microalgae cells by
Culex
quinquefasciatus
showed that
Nannochloropsis oceanica
cells were found to be resistant to digestion (92.23%).
This suggested that death of the larvae at second/third
instar stage of development might be due to the
indigestibility of microalgae cells. The poor digestibility
of
N. oceanica
may be caused by the resistance of the
thick and rigid cell wall to disruption by digestive
processes (Becker, 2007; Marshall et al., 2010).
Ahmad et al. (2004) reported that digestibility of the
microalgae in larval food is determined by the
resistant properties of their outer wall and duration of
exposure in the gut. Many researchers found that cell
wall of Chlorophytes consist of sporopollenin, a
carotenoid polymer impervious to all mosquito larval
digestive enzymes (Atkinson et al., 1972; Ahmad et al.,
2004).
The larvae fed with phytoplankton isolates such as,
Chroococcus turgidus
,
Lyngbya confervoides
,
Nostoc
