Byers, J.A. and Levi-Zada, A. 2011. Individual variation of (S)-4-methyl-3-heptanone in heads of braconid wasp,
Leiophron uniformis, and Pogonomyrmex ants indicates costs of semiochemical production.
Chemoecology 21:35-44.
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Abstract--
(S)-4-Methyl-3-heptanone is an alarm pheromone
released from the mandibular glands in heads of
harvester ants (Pogonomyrmex spp.). We used gas chromatography-mass
spectrometry (GC-MS) to study the
variation in amounts of this ketone among individual ants
of a colony. P. barbatus contained about 2,000 ng per
head, while only about half of this amount was found in
heads of P. rugosus and P. californicus. Individuals of
P. barbatus from three different nests contained rather
uniform amounts of the alarm pheromone within each
colony (16-30% coefficient of variation CV; normal distributions
skewed left), but one nest under food stress had a
significantly lower mean amount. In contrast, both sexes of
a small braconid wasp Leiophron uniformis, a parasitoid of
Lygus plant bugs, contained up to 10 ng of the same volatile
enantiomer in their heads; and groups of either sex of
the wasp exhibited normal distributions of quantities (64%
CV, skewed right). The differences in the distributions
between parasitoids and ants suggest that related members
within a social ant colony may attempt to maintain a uniform
level of ketone compared to independent variation in
unrelated, solitary wasp individuals. When the wasp’s leg
was grasped with forceps, it tried to escape and bite the
forceps as it ejected (S)-4-methyl-3-heptanone (detected by
solid phase microextraction, SPME, and GC-MS). Since
adult wasps are nonsocial and feed only on nectar, their
sharp piercing mandibles in combination with this escape/biting
behavior indicate the ketone is used for defense
rather than for an alarm function as in harvester ants. Costs
of producing the semiochemical in wasp L. uniformis and
ants P. barbatus and P. californicus are suggested since
populations exhibited a significant linear increase in the
amount of (S)-4-methyl-3-heptanone with an increase in
body weight of individuals.
Chemical Ecology