Brimstone Moth has a complex ecology as sometimes one brood is produced each year but sometimes three broods are produced over a two-year period, with the result that adults can be seen on the wing at any time from April to October.

Finding this moth as an adult in spring depends on how long it takes to get to adult after hibernating. This is based on what stage it hibernated as, either a half grown caterpillar or a pupae in a cocoon.

In England, Scotland and Wales the Brimstone moth is found between April and October. In Mainland Scotland and on the Orkney Islands, there is considered to be only one brood found in June to July, but in Aberdeenshire and Baniffshire in 2003, a second brood was found from August to September.

In Ireland this moth is found from April to October with peak records being in June.

In Northern Ireland this moth is found from around the 2nd of May to the 3rd November, with peaks in the records in early June then again in late July, possibly meaning two separate broods but the first brood can overlap into the second broods flight period.

This table has information on the flight period of the Brimstone Moth. The flight period represents an area as a whole based on overall records of flight times. This may vary in areas within the locations mentioned.

The shaded cells in orange, indicate what months the moth is found as an adult. Half coloured cells indicate the first or second half of a month when the moth flies.

N Ireland is Northern Ireland and S Ireland is South of Ireland and W Ireland is West of Ireland (Any part of Ireland not included in Northern Ireland).