Sir: Many wind-pollinated plants can trigger asthma, as you report ("The disease that lurks in the dust", 13 October). However, three trees can be removed from the list. Elder and horse chestnut are insect-pollinated. They have obviously been confused with the wind-pollinated alder and sweet chestnut. Willows also are insect-pollinated. Their stiff, showy catkins attract a variety of insects, especially early bees. There have been suggestions that some birds might also pollinate willows, but almost certainly any bird showing an interest will be after the pollinating insects or, even better, the caterpillars that feed on the catkins.