Page 42 - Volume 2, Issue 1, January 2009

Effect of an IGR on

Culex pipiens

43

of dilutions (day 0). Every 2 days the jars

were weighted and tap water was added up

to initial volume to supplement water loss

due to evaporation.

For the typical larval mortality bioassay,

twenty larvae of 3

rd

and early 4

th

instars were

placed in a glass beaker with 100 ml of stock

solution of each insecticide. Five replicates

were made per concentration and a con-

trol treatment with tap water was included

in each bioassay. Beakers with larvae were

placed at 25±2ºC, 80±2% relative humidity

and a photoperiod of 14:10 (L:D) h.

For the bioassays with the egg rafts, 100

ml of each stock solution were added in a

250 ml glass beaker and one newly laid egg

raft (less than 20 h old) was transferred by

means of a wooden stick on the water sur-

face (70±5 eggs per egg raft). In addition,

1 ml of baby fish food solution (TetraMin®,

Baby Fish Food) was added to each beaker

every 2-days to provide larvae with food.

Oviposition bioassays

Two-choice oviposition experiments

were set in sieve covered wooden framed

cages (33x60x33 cm). Two to three days old

male and female adult mosquitoes were re-

moved daily from the maintenance cages

(not containing oviposition beakers) and in-

troduced into the bioassay cages. The bioas-

say cages were kept under the above-men-

tioned rearing conditions. Two glass beakers

(10 cm diameter x 5 cm depth), one contain-

ing 100 ml distilled water and the other 100

ml distilled water plus the larvicidal, were

placed into the cages in approximately 40

cm distance between each other as more

centrally as possible in order to provide ovi-

position sites. Each oviposition bioassay

lasted six days.

Data recording and analysis

Larval mortality was assessed by count-

ing the number of dead larvae every 24 h.

In the cases where 100% larval mortality

did not occur, pupal mortality was assessed

too. Percentage mortality was calculated for

each treatment and replicate by dividing the

number of dead and moribund larvae to the

total (dead and alive). Dead larvae were con-

sidered those that could not be induced to

move when they were gently touched with

a glass pipette in the siphon or the cervical

region. Moribund larvae were those who

were incapable of rising to the surface, with-

in a reasonable period of time, or those not

showing the characteristic diving reaction

when the water was disturbed; they could

also show discolorations or unnatural posi-

tions (22).

Efficacy of each insecticide was assessed

as the mortality noted at each treatment

compared to the mortality of the controls.

In addition, the percentage of larvae that

pupated was estimated for the evaluation of

pyriproxyfen effect.

For the oviposition bioassays the num-

ber of egg rafts was counted and removed

every 24 h after the introduction of the ovi-

position beakers into the bioassay cages.

The number of egg rafts in the treated bea-

ker was converted to percentages of the to-

tal number of egg rafts in both beakers for

each cage. These results refer to three ex-

periments for each case.

Results and Discussion

Pyriproxyfen and temephos were bioas-

sayed against egg rafts of

Cx. pipiens

bio-

type

molestus

at a concentration of 0.2 gr/l

and 150 μl/l respectively. The bioassay so-

lutions were stored from 1 to 6 days under

constant conditions before use (post treat-

ment days).

None of the tests were discarded be-

cause control mortality was lower than 20%

in all cases.

In the bioassays with the larvae of 3

rd

and early 4

th

instars all the larvae were dead

within a 24 h period (mortality 100%), at the

doses tested. These results for both insec-

ticides are quite expected as a general rule

the recommended by the producer applica-

tion rate usually gives the maximum effec-

tiveness against susceptible strains of the

target organisms.

The efficacy of both insecticides when