Dutch elm disease is caused by the fungal pathogen Ophiostoma novo-ulmi which is transmitted by the native elm bark beetle, Hylurgopinus rufipes. We have found that four semiochemicals (the monoterpene (-)-beta-pinene and the sesquiterpenes (-)-alpha-cubebene, (+)-spiroaxa-5,7-diene and (+)-delta-cadinene) from diseased American elms, Ulmus americana, synergistically attract H. rufipes, and that sesquiterpene emission is upregulated in elm trees inoculated with O. novo-ulmi. The fungus thus manipulates host trees to enhance their apparency to foraging beetles, a strategy that increases the probability of transportation of the pathogen to new hosts.

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DiscussionMr J. V. Spddhg said that he wished to speak on the question of maintenance.When the catchment boards were set up in 1930, in general, river and drainage systems were in a very neglected state, largely because the riparian interests concerned could not meet the very high cost of maintenance, which tended to exceed the value of any direct benefits.After 1930 the catchment boards were formed with funds, sometimes inadequate, which, however, were wider and more general in their incidence, so they were able to try to catch up with the arrears of maintenance. There had been a tendency, once having caught up with maintenance, to forget that maintenance must be continued if rivers were to maintain their efficiency.Engineers naturally preferred spectacular schemes so, being human, they tended to concentrate on new and important works, leaving maintenance