Large Yellow Underwing
According to Claudia Copley (Victoria Naturalist 63, 14 (2006)) this abundant European species was first reported in North America in Halifax in 1979 and is now known through most of Canada and much of the United States. I first noticed the caterpillars on Vancouver Island in October 2004, when I could go out almost anywhere at night with a flashlight and find as many as I wanted within minutes. They were usually feeding on grasses, but will also feed on many herbaceous plants - especially those (such as Primula) growing in your garden.
Its fecundity is enormous. The eggs are laid in neat rows on a blade of grass or other herbaceous plant, and, because of the regularity of their placement, it is not hard to make an estimate of the numbers in a single batch. Look at the eggs shown here for two minutes, and try and estimate how many there are. The exact answer is the cube root of 360944128. How did you do? I have seen one batch with more than twice this number on a single blade of grass.The hindwings, of course, are yellow (with a black border), but it quite deliberately does not exhibit these when it is at rest - so I have yet to obtain a worthwhile photograph showing these.