Possible Involvement of Microorganisms in Pheromone Synthesis

Larvae of the seedcom maggot, Hylemya platura, damage certain crop species by feeding on the cotyledons and plumules. It has been assumed that the stimulus for oviposition was provided by the germinating seeds. However, Eckenrode et al. 431) found that significantly fewer eggs 

In the case of certain bark beetles a number of their identified pheromones are derivatives of a-pinene 136, 373, 435), and it is probably correct to assume that these derivatives arise by enzymatic action on the a-pinene of the host plant. There are two possibilities for the origin of the enzymes concerned. Firstly, they could be secreted by the gut tissue of the insect and effect the transformation of a-pinene in this region. The induction of mixed-function oxidases in insect gut tissue by host plant monoterpenes has been mentioned. Secondly, they could be produced by microorganisms present either within the gut or in the host plant tissue itself or in the frass after excretion. It is the second possibility that concerns us now. 

From these results we must conclude that certain gut-associated pheromone components may be produced by the enzyme systems of the insect itself and/or by microorganisms. If this is the case, then the resulting blend occurring in the frass will be the result of the dynamic balance between those substances that are most readily produced by both microbes and insect and those that are most readily metabolized by both microbes and insect. Furthermore, the types of microbes present in the gut and the nature of the enzymes secreted by the gut tissue will be affected by the diet of the insect. Therefore, the occurrence of frass-associated pheromones should probably be considered the result of a delicate equilibrium involving a number of biological species. 

An increase in the verbenone concentration released from a successfully colonized tree has been proposed as an important factor in inhibiting further attacks on the tree by both D. frontalis and D. brevicomis (443). It is therefore possible that SJB-133, which is important in the beetle s nutritional regime (440), could also play a significant role in regulating response to the plant host. More recently c rfo-brevicomin has been implicated in the shifting of attack from one tree to another (444). 

Inquiries into the possible production of behavioral chemicals of D. frontalis by microorganisms associated with it have been carried further. As mentioned above, two fungi and two yeasts are associated with the mycangium of the female (440, 441). The production of various volatile substances, other than ethanol, by actively fermenting yeasts is well established (445). Brand et al. (446) grew three yeasts obtained from D. frontalis, namely H. holstii, P. pinus, and P. bovis, on Sabouraud s dextrose broth, and identified isoamyl alcohol, 2-phenylethanol, isoamyl acetate and 2-phenylethyl acetate as the main volatile substances (other 

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