Page 8 - Volume 3, Issue 2, July 2010

Mihou & Michaelakis

38

1.3. Bioassays with pheromone under

semi-field and field conditions

In western Kenya oviposition phero-

mone was tested for the first time in the

field. Five milligrams of the active (–)-(5

R

,6

S

)

enantiomer was exploited in a tablet for-

mulation and used in established breeding

sites. Compared to the control, 82% more

females oviposited around the pheromone

source. The activity of the pheromone con-

tinued for four days after application (Otie-

no

et al.

, 1988). The combination of the

pheromone with an insect growth regula-

tor showed both an acceptable oviposition

activity and sufficient larvicidal effect. Dur-

ing that field test, there was an attempt to

attract

Culex

mosquitoes in a non-breeding

site. Despite using a very high dose of pher-

omone, only one egg raft was found and fur-

ther analysis showed a higher pH than that

in the breeding sides (established). This fact

suggests that the lack of oviposition in non-

established breeding sites involves factors

in the attraction mechanisms other than the

pheromone.

The same phenomenon was observed

in Tanzania when synthetic oviposition phe-

romone was used to non-breeding sites

(Mboera

et al.

, 1999). When water from es-

tablished breeding sites was treated with

the pheromone, it received more egg rafts

than the untreated water with nine days re-

sidual activity.

The mixture of the pheromone with

soakage pit water or grass infusions indicat-

ed a synergistic effect. As mentioned before,

Culex

mosquitoes

prefer breeding in water

with a high organic content (polluted water)

rather than clean water. This explains why fe-

males prefer water that consists of complex

mixtures of compounds that vary over time,

including products of bacterial degradation

such as soakage pit water or grass infusions.

Apart from laboratory studies, plant-de-

rived pheromone was also employed in field

studies (Olagbemiro

et al.

, 2004). Both lab-

oratory and field bioassays revealed equal

attractiveness level for plant-derived and

synthetic pheromones. Of the two phero-

mones, only the plant-derived one was used

in field tests in a mixture with skatole.

Under semi-field conditions, oviposi-

tion was induced by an amount of pherom-

one equal to a single egg raft (0.3 μg). This

was the first report that compared a syn-

thetic pheromone with the natural product

(Braks

et al.

, 2007). The oviposition jars were

arranged in two squares (small and large,

“near” and “distant” respectively). In the

“near” bioassays, a single egg raft with hay

infusion had the highest oviposition activi-

ty, while by increasing the egg rafts from 1

to 10 (or by adding 3.0 μg synthetic pherom-

one) synergistic effects were observed be-

tween the oviposition pheromone and the

hay infusion at both distances. Authors sug-

gested three possible explanations for the

number of females ovipositing and the rela-

tion to the distance: a) females may fly along

the edge of the cage searching for a suitable

oviposition site by making a distinction be-

tween hay infusion based on the oviposition

pheromone, b) females fly along the edges

of the cage and after encountering the phe-

romone at close range land and/or oviposit

and c) females visit and land at several ovi-

position sites before the final decision.

2. Synthetic approaches

According to Pickett and Woodcock (1996)

“The characterization of the oviposition

pheromone for mosquitoes in the genus

Culex

as a novel chiral lactone ester provid-

ed the impetus for a number of sophisticat-

ed asymmetric syntheses and economical

large-scale routes to racemic products”.

This section focuses on the most suc-

cessful asymmetric synthetic approaches

published in literature. The synthetic strat-

egies are grouped depending on the meth-

od used to induce chirality to the mole-

cule. Carbohydrates and aminoacids were

used as starting materials. Kinetic resolution

(Sharpless asymmetric epoxidation or oxi-

dation protocol) of racemic starting mate-

rials and Sharpless asymmetric epoxidation

or dihydroxylation of olefins have also been

utilized for the enantioselective synthesis of