Monoterpene Pheromones

A large number of insect pheromone compounds are either simple terpenes or are derived from them. The few examples of monoterpenes given in Figure 6.4 illustrates the variety of structures that can be made from geranyl pyrophosphate or linalyl pyrophosphate without much further reaction. Both the Coleoptera and the Hymenoptera make frequent use of terpenes as secretory substances. Bark beetles have particularly employed monoterpenes in their aggregation pheromones.

The males of the cotton boll weevil (Anthonomus granĂ¡is, Coleoptera, Plate 6), a serious pest of cotton, produce a sex pheromone to attract females. The pheromone, known as grandlure, consists of a mixture of at least four monoterpenes (Figure 6.7). The males make these compounds from geraniol and nerol (the cw-isomer of geraniol), present in the cotton buds on which they feed.

Figure 6.7 The aggregation pheromone of male Anthonomis grandis made from geraniol and nerol. The four compounds A, B, C and D together make up grandlure

It is often difficult to know whether terpene compounds found in insects are made de novo by the insect or whether the insect has altered some terpenes it finds in its food or environment. The boll weevil is known to alter plant terpenes from cotton. Ips beetles which bore under the bark of pine trees produce an aggregation pheromone consisting of ipsdienol or ipsenol (Figure 6.8) which were thought to be conversion products of myrcene from the pine tree. However studies with radiolabelled acetate and mevalonate showed that at least two species, Ips paraconfusus and Ips pini were able to synthesize their pheromones 'from scratch'. On the other hand, both males and females of Ips paraconfusus convert a-pinene from the tree to cw-verbenol, which adds to the attraction of the male-produced pheromone.

os-verbenol

Figure 6.8 The structure of myrcene and the aggregation pheromones of two species q/Ips beetles that are now found not to be made from plant-derived myrcene, but Ips paraconfusus males and females convert a-pinene to cis-verbenol

The southern pine beetle Dendroctonus frontalis is another bark beetle with an aggregation pheromone for which the origin is difficult to see. Taken stepwise, it can be seen to derive from geraniol (Figure 6.9). The mountain pine beetle Dendroctonus ponderosae and other species use exo-brevicomin (Figure 6.10) with a quite different origin in the acetogenin (Z)-6-nonen-2-one. By labelling the ketone with lsO, and a very detailed argument, it was shown that the epoxide was not opened first but that the carbonyl oxygen atttacked the epoxide, much as shown in Figure 6.10. Both enantiomers of the epoxide were formed but males favoured making brevicomin from the (6S,7i?)-keto-epoxide, while the females preferred the (6JR,7S)-keto-epoxide as intermediate. The full myrcene ^(5)-ipsdienol (fl)-ipsdienolj In pine trees Y

Ips paraconfusus myrcene ^(5)-ipsdienol (fl)-ipsdienolj In pine trees Y

Ips paraconfusus

(S)-ipsenol Ips pini

(S)-ipsenol Ips pini a-piner

Figure 6.9 The formation offrontalin from geraniol through loss of two carbon atoms from the hydroxyl end, oxidation of a double bond to an epoxide and then ring opening and ketal formation

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