Page 7 - Volume 3, Issue 2, July 2010

Mosquito oviposition aggregation pheromone

37

pheromone material produced from the

seed oil of the summer cypress,

Kochia sco-

paria

(Chenopodiaceae). Laboratory bio-

assays with 3 and 5 μg of each pheromone

(plant-derived and synthetic) revealed that

these two compounds were equally attrac-

tive. The bioactivity of the plant-derived

pheromone was comparable to that of the

synthetic oviposition pheromone (1:1:1:1

mixture of the four stereoisomers).

Apart from the reaction to the phero-

mone,

Culex

mosquitoes may also have an

intrinsic neurophysiological response to

specific indoles, which could explain why

it oviposits in polluted water such as ska-

tole water (3-methyl indole) (Pickett and

Woodcock, 1996). According to Millar

et al.

(1994), responses to the pheromone com-

bined with several doses of 3-methyl indole

were shown to be additive rather than syn-

ergistic. The same additive pattern was also

observed in the presence of skatole water

at two concentrations with plant-derived

pheromone (Olagbemiro

et al.

, 2004).

Three larvicides, an insect growth reg-

ulator (pyriproxyfen), an organophosphate

(temephos) and a microbial (

Bacillus thu-

ringiensis

subsp.

israelensis,

Bti) were test-

ed separately or combined with synthetic

pheromone, as agents that can keep water

free of mosquito larvae (Kioulos

et al.

, 2007-

2008). Preliminary results of oviposition bio-

assays revealed that, with the exception of

temephos, all the other tested larvicidals,

when used separately, repel gravid females

of laying eggs for the first two days. How-

ever, when synthetic pheromone was com-

bined with the three larvicidals, only teme-

phos and the microbial agent (Bti) followed

the same attractant pattern as the synthetic

pheromone alone.

For the first time a slow release systemhas

been developed that incorporates 6-acetoxy-

5-hexadecanolide (mixture of the four stere-

oisomers). Laboratory bioassays studying

attractancy over time, and showed a dose-

dependent response. The microencapsulat-

ed pheromone was found to be sufficient-

ly attractive to gravid female mosquitoes for

almost 40 days (Figure 5) (Michaelakis

et al

.,

2007). The addition of temephos did not af-

fect the activity of the pheromone while the

bioassays showed 100% mortality. When an

aged infusion was combined with aged pher-

omone (microencapsulated) there was a syn-

ergistic effect only for the first day, while all

the other days the oviposition pattern was

similar to that of the microencapsulated

pheromone, with a minor exception on the

15th day (Michaelakis et al., 2009).

Recently, Leal

et al.

(2008) employed im-

munohistochemistry to prove the stereo-

specific interaction between the receptor

of the

Culex

mosquitoes and the phero-

mone and that the receptor accepts only the

(–)-(5

R

,6

S

)-enantiomer. The studies revealed

that an odorant-binding protein from

Cx.

p. quinquefasciatus

, CquiOBP1, is expressed

in trichoid sensilla on the antennae. Cqui-

OBP1 exists in two forms, monomeric and

dimeric; the mosquito oviposition phero-

mone is bound in the monomeric form in a

pH-dependent manner. While all previous

reports measured the effect of pheromone

(and also of other attractants) on egg laying,

in this report authors evaluated the phero-

mone for attraction of gravid females (i.e.,

numbers captured). The results suggested

that pheromone might be an arrestant rath-

er than an attractant because it could in-

crease egg laying but not necessarily grav-

id female capture.

Figure 5.

Oviposition behaviour of gravid females

Cx. p.

bi-

otype

molestus

to aged microencapsulated pheromone (slow

release system for the synthetic mixture of the four diastere-

omers of 6-acetoxy-5-hexadecanolide). The broken lines rep-

resent the upper and lower values of the control mean±SEM

(50.45 ± 2.6%,

n

= 10) [for more details see Michaelakis

et

al.

(2007)].