METEOROLOGICALLY, the seasons changed on Tuesday. We moved from summer into autumn. Astronomically, though, we’ve still got three weeks of summer to go, and phenologically, the picture is rather confused – particularly if you look out of our sitting room window where it has looked like autumn for weeks.

There are three ways of working out when autumn begins. The Met Office likes things orderly, so in their calculations, each of the four seasons has three months which means autumn begins promptly on September 1.

Astronomers study the skies and note that at the summer and winter solstices (June 21 and December 21), the sun reaches its most northerly and southerly points. These are the longest and shortest days, when summer and winter begin. Autumn and spring, therefore, must begin at the midpoints between these dates – the equinoxes, when night and day are roughly equal. By this calculation, autumn starts about September 21.

The third method is phenological – the study of phenomenon. This means looking out of your window and seeing when things start happening: the tinting of the leaves on the trees, the ripening of the blackberries/brambles (please delete as appropriate) on the bushes.

This method usually places the start of autumn in mid-September, although down our street the blackberries/brambles have already been ripe for weeks and the horse chestnut has looked brown and autumnal for a month or more.

For a second year, our neighbouring trees appear to have been infested by the horse chestnut leaf miner moth. This tiny brown and silver moth, just 5mm long, lays its eggs on the leaves. They hatch into larvae, or caterpillars, which burrow into the leaves, sucking out the sap, turning the withering leaves a dismal brown.

In each of the afflicted trees outside our window there may be several million caterpillars sucking away.

Despite this, the trees’ overall health seems not to be affected, although its conkers may be smaller – last year we had a very bad conker crop.

The horse chestnut leaf miner moth is a classic migrant. It was first spotted in Macedonia, in northern Greece, in 1985. It then popped up in Austria in 1989, and by 2002, it had smuggled its way into Britain as it was noticed in Wimbledon. Last year – when we first noticed it in our trees on the banks of the Tees – it made its debut in Scotland.

It is believed that its spread has been so rapid because, like a migrant, it manages to stowaway in vehicles – it has been nicknamed the "hitch-hiker moth".

It has no natural predators to keep it in check, although blue tits, wasps and dragonflies seem to learn that the caterpillars can be tasty.

The summer sun fades and the year grows old, but our trees’ prematurely brown leaves won’t fall into a golden gown for us to kick our way through although, because of the actions of the leaf miner moth, outside our windows it has for some time looked like forever autumn – whenever it may officially start.