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The pheromone blend of M. vitrata
Okeyo-Owuor & Agwaro (1982) were
the first to trap male M. vitrata moths in water traps baited with virgin
females in Kenya, thus suggesting the production of a sex pheromone by female
M. vitrata. Later, Adati & Tatsuki (1999) reported (E,E)-10,12-hexadecadienal
(EE10,12-16:Ald) to be an electroantennagram-(EAG) active component in gland
extracts from female M. vitrata abdominal tips. Synthetic EE10,12-16:Ald
was shown to be attractive to male moths, in laboratory bioassays. However, high
(99%) isomeric purity was found to be critical to this attraction. The
corresponding alcohol, (E,E)-10,12-hexadecadienol (EE10,12-16:OH), was
also noted as being present at 3-4% of the aldehyde. No field testing of the
compounds was carried out.
Our own work (Downham et al., 2003) in the laboratory commenced in the mid-1990s.
All laboratory and the subsequent field experiments
employed blends in which the isomeric purity of the EE10,12-16:Ald major
component was >99% unless otherwise stated. In analyses of ovipositor washings
and entrained volatiles from virgin female M. vitrata moths by
gas-chromatography (GC) linked to EAG recording from the male moth, the two most
consistent male responses were to the major component, EE‑10,12-16:Ald, and
EE10,12-16:OH at 2-5% of the major component, confirming the report of Adati &
Tatsuki (1999). Another response indicated the presence of a monounsaturated,
16-carbon aldehyde, which we could not positively identify because of the small
quantity of material present - estimated to be much less than 2% of the major
component. Several EAG, wind-tunnel and field bioassays of a range of
hexadecenal compounds then suggested that this third blend component was most
probably (E)-10-hexadecenal (E10-16:Ald). None of the synthetic blends
were as attractive as natural gland extracts.
Field trapping experiments commenced in 1998 in cowpea fields at the International
Institute of Tropical Agriculture (IITA) station outside Cotonou, in the
Republic of Benin (West Africa). The first trapping experiment clearly indicated
that all three components, EE10,12-16:Ald, EE10,12-16:OH and E10-16:Ald, were
essential for optimal attraction of male M. vitrata moths when tested in
100 : 5 : 5 ratio respectively (see Figure). Neither two-component blend tested
attracted more males than the single component alone, but the presence of both
minor components together produced a four‑fold increase in catches. This blend
also attracted far more moths than virgin females.
Catches of M. vitrata with four synthetic
blends of EE10,12-16:Ald, EE10,12-16:OH and E10-16:Ald, respectively, averaged
across two loadings (0.1 and 0.01 mg) and two dispenser types (polyethylene vial
and rubber septa) and with female-baited and un-baited control traps. Error bars
= standard errors of the means. Catches with the 100: 5: 5 blend were
significantly greater than all other treatments for both males and females.
[Data from: Downham et al. (2003)]
Surprisingly, a subsequent experiment indicated that amounts of the two minor
components, when varied together over the range 0.1 – 50% of the major
component, made no significant difference to catch levels. However, a third
experiment was more consistent with the first in showing that blends with
E10-16:Ald present at 0.5% or less were no more attractive than the single- or
two-component blends, while if E10-16:Ald was present at 5%, mean catches were
more than doubled. This was consistent with laboratory analytical results which
indicated that the amounts of the hexadecenal compound in female ovipositor
washings were less than 2% of the major pheromone component.
Several aspects of the results, not least the surprising phenomenon of attraction of female M. vitrata to
synthetic lures, showed that pheromonal attraction in M. vitrata was not
yet fully understood. These are considered further under under ‘Problems with
the pheromone blend’ [link to page]. However, from an immediate practical
standpoint, the development of an effective synthetic blend for M. vitrata
in Benin was considered complete. It consisted of EE10,12-16:Ald (100µg),
EE10,12-16:OH (5µg) and E10-16:Ald (5µg).
This was the first time a synthetic pheromone blend had been identified that was more attractive to male
M. vitrata than virgin female moths under field conditions.
Problems with the pheromone blend
Despite progress with the pheromone blend identification three findings have shown that pheromonal
attraction in M. vitrata was not yet fully understood. These are the
consistently weaker responses of males to synthetic pheromones in laboratory
bioassays, inconsistencies in the effect of minor blend components on trap
catches and the attraction of female moths to synthetic lures in traps.
Female attraction to the synthetic blend
The phenomenon of attraction of female M. vitrata to synthetic lures in traps is very striking and
unusual. To our knowledge, there is only one other moth species for which
captures of females in traps baited with the synthetic version of the
female-produced pheromone have been reported, i.e. the noctuid,
Trichoplusia ni (Mitchell et al., 1972; Birch, 1977). In this case
it was subsequently shown that males produce a pheromone (Heath et al.,
1992) which attracts females under field conditions (Landolt, 1995). The
probable explanation for the observations of Mitchell et al. (1972) and Birch
(1977) was thus attraction of females to previously trapped males.
In one M.Sc. study at NRI the possibility of female attraction to a male-produced pheromone was investigated
but not supported by wind-tunnel results (Mondhe, 2001). Our observation that
moth captures in individual traps in the field were frequently exclusively
female also argues against this explanation. Mondhe (2001) also found that
neither EE10,12-16:Ald, EE10,12-16:OH nor E10-16:Ald elicited EAG responses from
female M. vitrata moths, so direct attraction of females to the pheromone
lures seems unlikely.
An important further point to explain is the variation in the proportion of female captures, which in all
trapping experiments has ranged from 11% to 50% of all captures. It may be that
the behaviour of M. vitrata in the field is affected by factors such as
host plant volatiles not yet investigated. Regardless of explanation, catches
of females may actually improve the predictive power of traps, since catches of
both sexes should more accurately reflect population events.
Further components?
In addition to the investigations of pheromone blend in Benin, different blends and lure loadings
were compared in trapping experiments* in Sri Lanka, India, Kenya, Taiwan and
northern Nigeria. Although in most cases some M. vitrata adults were
trapped, catches were generally too low or inconsistent to be useful.
Later re-investigations of the
pheromone blend of M. vitrata have since been carried out through the
Ph.D study of Mr M.N. Hassan, which will be completed in the first half of
2006. Laboratory experimentation suggested the possible existence of a small
quantity of (E)-10-hexadecen-1-ol (E10-16:OH) within the blend. In late
2004 trapping experiments were conducted by Mr Hassan, in association with
ICRISAT staff, in pigeonpea fields in southern India to investigate the new
compound. These have showed significantly improved catches with blends with
including E10-16:OH, but the results await publication.
Unfortunately, the same compound had little effect on catches in experiments we ran at five locations in
Benin, Ghana, Burkina Faso and northern Nigeria, although intriguingly we did
demonstrate high catches in Burkina Faso with lures containing the major blend
component, EE‑10,12-16:Ald, alone.
*We thank the following for assistance with these experiments:
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Ms L.D. Galanihe then of the Food Crops Research and Development Institute, Sri Lanka
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Dr G.V. Ranga Rao of the International Centre for Research in the Semi-Arid Tropics, Patancheru, India
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Dr T. Adati then of the International Centre of Insect Physiology and Ecology, Nairobi, Kenya
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Dr N.S. Talekar of the Asian Vegetable Research and Development Center, Taiwan
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