CHEMICAL ECOLOGY OF THE PALM WEEVILS (COLEOPTERA: CURCULIONIDAE):AGGREGATION
PHEROMONE AND HOST PLANT ATTRACTION
CHEMICAL ECOLOGY OF THE PALM WEEVILS (COLEOPTERA: CURCULIONIDAE):AGGREGATION
PHEROMONE AND HOST PLANT ATTRACTION
AHMED ALJABER
Colorado State University
Fort Collins, Colorado 80523
ABSTRACT: Weevils of the genus Rhynchophorus (Coleoptera,
Curculionidae) are considered important pests of the palm family
Palmae. Rhynchophorus spp. are widely distributed throughout southern
Asia and South America. Male of Rhynchophorus palamrum, Rhynchophorus
phoenicis, Rhynchophorus vulneratus, and Rhynchophorus ferrugineus,
produce several volatile pheromone related compounds. They are
classified as aggregation pheromones since they were produced only by
males and will attract both sexes. These compounds were identified
using volatile collections and gas chromatography . Standard analytical
methods were used to identify chemical structures and major and the
minor active compounds. (5E) - 2 - methyl - 5 - heptan - 4 - ol
Rhynchophoral, was the major compound emitted by Rhynchophorus palmarum,
whereas , 2 , 3 - epoxy - 6 - methyl - 4 - heptanol was identified as
a minor compound. 4 - methyl - 5 - nonanol was identified as the major
compound in both Rhynchophorus vulneratus and Rhynchophorus ferrugineus.
Despite recent advances of the knowledge of chemical ecology of
Rhynchophorus spp, the function of all semiochemicals being released by
the weevils are not fully understood. Additionally the interaction of
the semiochemicals emanating from palm stems and those being released by
the weevils are unknown.
In this paper I will discuss the aggregation pheromones of the four
Rhynchophorus spp. that are mentioned above, the major and the minor
compounds that elicit the behavioral responses the host plant volatile
compounds, and the laboratory and field assays of the aggregation
pheromones.
Introduction:
The Rhynchophorus spp. are members of the family Curculionidae the
largest natural family of animals . The sub family Rhynchophorinae
contains several genera that are closely associated with Palmae,
Zingiberaceae, Musaceae, Amaryllidaceae and Gramineae, all
monocotyledons (Kalshoven, L.G.E. 1981). There are more than 40,000
species of weevlis in the world and many more to be discovered and
named. Practically all feed on plants, most of the larvae burrowing
infesting nuts, twigs, and the like (Borror et al. 1989). The adults are
usually readily recognized by the shape of the head. The larvae are
whitish, usually C-shaped, more, or less cylindrical, and usually
legless. Many species occurring in the cultivated areas are harmless or
not noticeably noxious; other are important pests.
The chemical communication importance in insect systems is an obvious
fact. These olfactory signals as sex, trail, aggregation pheromones play
a vital role in the reproductive success of most insects. Chemical
signals in their widest sense are a universal attribute of life; in some
form, they exist within cells and between all organisms (Harborne 1989).
Insects are sensitive to chemical aspects of their environment,
particularly with regard to host and mate location. Palm weevils
individuals can determine the location of other sex or the host plant
from long distance. This ability relies upon the detection of secondary
plant compounds as well as the aggregation pheromone that emitted by
males. The olfaction plays a major extensive role in the chemical
ecology of palm weevils.
Rhynchophorus spp. are major pests of coconut, oil palm crops, and
sugarcane. (Restrop et al 1982 ; Wattanapongsiri.,1966). They adults
cause indirect damage to palm trees by vectoring the nematode
Bursaphelenchus cocophilus which responsible for red ring disease
(Griffity, 1987). Direct damage is also caused by the larvae that
develop in the stems, bud, rachis of leaves, and inflorescence of
cultivated, ornamental, or wild palm trees (Lepesne et al.,1947 ;
Wattanapongsiri, 1966). The study of palm weevils chemical ecology
involving semiochemicals, particularly males pheromones and volatile
plants compounds, has been greatly enhanced by the development of
chemical analysis instruments. The high-resolution capillary column gas
chromatography provides a powerful tool for locating the active
components in behavioral active samples. Analysis by other highly
sensitive techniques such as combined GC - mass spectrometry ( GC - MS)
enables compounds to be identified, synthesized, and made available for
use in further behavioral and ecology Palm weevils control.
Host plant volatile compounds effects:
Much is known about the chemical interaction between insects and plants.
For instance, insect hormones occur in plants in relatively large
quantities and with a variety of chemical structures. Their function is
still speculative but it is clearly possible that they are deliberately
produced by plants to interfere with insect metamorphosis and, hence,
reproduction (Harborne 1989). Rhynchophorus spp. attack or are attracted
to several species of plants: such as date palm, oil palm, sago palm,
coconut palm, palemera palm, banana, pineapple, sugarcane, Arenga,
Coelcoccus, Corypha, Elaeis, Livistona, Matroxylon, Nypa, Oncosperma,
Aloes, and screw-pine (Najeeb 1988, Jaffe, et al. 1993). The coconut,
pineapple, banana, and sugarcane have been used as a trap crops for
most Rhynchophorus spp.(Fig. 1).
Plant tissues (coconut and pineapple) were analyzed using CH2CL2
extracts and the heatspace method (Jaffe, et al, 1993) GC-MS of
volatiles from infested and healthy coco plants revealed the presence of
water, CO2, ethyl-acetate, pentane, hexanal, isopentanol, and ethanol.
Fruits of pineapple produced ethanol and ethyl-acetate (Jaffe, K. 1993).
The most four attractive plant tissue compounds are presented in Table
(1).
Those analysis results correspondes with the field observations that
showed the weevil used odor cues to orient to its host plant (Moura, et
al., 1989; Hernandez et al., 1992). This fact is known to the farmers of
the host plants, so the traditional method of controlling palm weevils
has been the use of a poisoned traps baited with damage tissue of
coconut, pineapple, or banana fruits (Chavero, 1976; Dean and Velis,
1976; Morin et al., 1986; Vera and Orellana, 1988).
Among the volatiles identified, ethanol was shown to be attractive (
Table 2); in addition, isoamyl-acetate reported to be attractive to the
weevil (Hagley, 1965), and known to be present in pineapple and banana
(Falth and Forrey, 1970; Macku and Jennings, 1987), and was active in
the olfactometer. Pure ethanol or isoamyl-acetate were as attractive or
even more attractive than natural plant tissue (i.e., pineapple vs.
isoamyl-acetate, (Table 2). The attractiveness of the plant tissue
compounds increased by two ways. The first, is the decomposition of the
plant material.
The temporal analysis of these compounds in healthy and infested
coconut tissue (Fig.2) which showed that the proportion of ethanol vs.
ethyl-acetate changed during decomposition of the material, reaching a
proportion of 7:3 after three days of decomposition and remained
relatively constant up to day 9. This timing coincides with the moment
when plant tissue showed maximal attraction in the laboratory and in the
field (Cerda et al., 1993; Hernandez et al., 1992).
The second, by mixing compounds with each other (total volumes of 3 ml)
as it indecated in (Table 3) (Jaffe, K. et al 1993). Jaffe, et al (1993)
observed that ethanol and ethyl- acetate in proportion of 70:30, as
found in coco plants, was the most attractive two compound mixture for
the weevils.
Successive enrichment of the ethanol-ethyl-acetate mixture with traces
of pentane, isoamyl-acetate, hexanal, and isopentanol increased the
attractiveness of the mixtures, although differences between different
complex mixtures of these substances were not always statistically
significant. A mixture of 68% ethanol, 27% ethyl-acetate, and 5% pentane
was statistically as attractive as natural plant tissue. Isoamyl-acetate
in pure form was the most attractive compound tested. None of these
compounds or mixtures attracted a single weevil in the field. However,
they are not effectiveness, since Rhynchophorol 2(E)-6-methyl-2-hapten-
4-ol, attracts weevils in the olfactometer and in the field only if
plant tissue are present.
Moreover, Rhynchophorol will become not effective if the male does not
detect the odor of the ethyl-acetate or a mixture of plant tissue odor
as Jaffe et al.(1993) stated in their study (Table 4 )
Field-trapping date (Rochat et al., 1991) supported the hypothesis that
American Palm Weevil males produce Rhynchophoral only when they feed on
the host plant. The biological significance of the male APW pheromone
would thus be a signal to aggregate on a suitable food resource, as
occurs in many other beetles (Walgenbach and Burkholder, 1986; Borden,
1985; Renwick, 1988).
American Palm Weevil:
In tropical America, R. palmarum is a maor pest of coconut and oil palm,
through direct attack and as a vector of the red ring nematode
Bursaphelenchus cocophilus baujard (A.C. Oehlschlager, et. al 1993, K.
Gerber & Davis, 1990). Rochat et al. 1991 showed that only APW males
produce a volatile aggregation pheromone (Fig.3). This pheromone
provided a very strong response from both males and females of R.
palmarum. EAG activity of male-produced pheromone for extracts
containing male-produced + sugarcane confirmed Rochat et al.(1991)
studies (Fig.4). Further investigation by Dider et al. (1991) clarified
that APW males produced two sex-specific compounds. One was a minor
compound, not always detected. The major male- prodced volatile was
ideneified as (2E)-6- methyl -2- hepten -4- ol.
The identification of (2E)-6- methyl -2- hepten -4- ol as the major
pheromone compound was confirmed by all GC-EAD recordings from both male
and female APWs, which showed high EAG responses to R1 and no EAGs were
recorded in response to the sugarcane volatiles or R2 (Fig.5) Dider et
al. (1991). Dider et al. (1991) proposed that Rhynchophorol as trivial
name for the new pheromone. Rhynchophorol is a new eight-carbon aliphtic
and oxygenated pheromone. Such pheromones have been found in three
coleopteran families : Cerambycidae (2,3-octanediol and 2-hydroxy-3-
octanone; Sakai et al. 1984), Curculionidae (sitophilure, 5-hydroxy-4-
methyl- 3-heptanone : Phillips et al., 1985 ; 5-methyl-4,6-heptanedione
; Blight et al., 1984) and Scolytidae (ipsinol, 2-methyl-6-methylene-7-
octen-4-ol, and ipsdienol,2-methyl-6-methylene-2,7- octadien-4-ol ;
Silverstein et al., 1966 ; sulcatol, heptanol ; Pearce et al., 1975.
African Palm Weevil :
The African Palm Weevil R. phonicis causes damage particularly to young
oil palms. Boring by the larvae into the crown or root bulb of a young
palm causes yellowing of the leaves, while boring close to the growing
point is lethal to the palm Gries et al., (1993). In 1993, 3- methyl-4-
octanol was identified by Rochal et al.(1991) as the male produced
aggregation pheromone of the African Palm Weevil R. phonicis.
Asian Palm Weevils:
R. ferrugineus and R. vulneratus are sympatric species of palm weevils.
They are important pests of coconut, oil palm, sago and aren palms in
India and Southeast Asia (Sadadathulla 1991). Adults are attracted to
wounded palms where eggs are laid, the larvae tunnel into the terminal
bud or trunk of the tree, leading directly to its death (Sadakathulla
1991). Analyses of volatile extracts vacuumed from 20 male and 20 female
R. ferrugineus and 25 male and 25 female R. vulneratus feeding on
sugarcane with coupled gas chromatographic- electroantennographic
detection (GC-EAD), revealed in both species the presence of two male-
specific compounds that elicited strong electrical potentials by male
and female antennae (Fig. 6).
Hallett et al. 1993 have identified 4-methyl-5-nonanol as an aggregation
pheromone of both R. ferrugineus and R. vulneratus (Fig.7). They found
also that 4-methyl-5-nonanone was antennally active in both weevils, but
it induced observable behavioral responses only in R. ferrugineus
(Fig.8). In the base of this results 4-methyl-5-nonanone can be classed
as a pheromone only for this species.
Conclusion:
The identification of the palm weevils aggregation pheromone and
synthesization was an important step towards improving traditional
methods used to control this major pests (Table 5). The understanding of
doses amount of the pheromone and the presence of plant tissues or their
active chemical compounds are a key factors of the pheromone application
success. However, there are a lot of things that should be concerned
about to understand and utilize these pheromones. For example, what is
the function of the minor pheromones that some Rhynchophorus species
release, why do these aggregation pheromones do not work in the absence
of the host plant volatile compounds.
Finally, all these mysteries around us, the new sciencific secrets that
are discovered every day and the resultant organized system nature leads
us dierctly to God. He created us and we should thank him in a way that
he wants us to do ...!
Table 5: The aggregation pheromones in Rhynchophorus species.
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