Spruce Carpet (Thera britannica or Thera variata)
Up till 1925 we knew of only one Spruce Carpet (Thera variata). But then it was discovered that there are actually two seprate species. Thera britannica was born. The common English name has never been changed to make a distinction between the two, though. Both species are real look-a-likes and both are very variable indeed. In many ways they often are also identical to the Grey Pine Carpet. The latter however often has a brownish glow all over the wings. It is important to look at the entire wing and not just at the central band, for this is brownish in many Thera species at times. In our garden the Grey Pine Carpet often is brownish all over, while the Spruce Carpet usually is entirely greyish. Below we do not separate the two Spruce Carpet species. They need to be killed and dissected to be absolutely sure and that's something we don't do for our site. The basic colour of the Spruce Carpet usually is grey, but may be reddish brown as well. Apparently the reddish brown animals are the rule in northern and western Britain and, judging from our pictures, in eastern Holland. Rather typical of the Thera species is the band in the middle of the wing. It is made up of oval shaped markings. Usually the markings have black borders and a centre that's slightly darker than the other parts of the wing. However in very brown specimens the ovals may be absent or hardly visible. The wingspan is typically that of many Geometers: 30 to 36 mm.
The Spruce Carpet is clearly double-brooded. The caterpillars of the first generations are seen in June and July. Those of the second generation appear in autumn. The larvae are green with a bright thick yellow line on the sides. The head and legs are also green. The young caterpillar overwinters. By the end of May they pupate in a hole or just a ditch in the ground. They spin a cocoon there and inside the pupa develops. The caterpillars live on some spruce and fir species. The needles are eaten top down, however the last bit isn't touched. When they sit in their typical Geometer position the caterpillars are very hard to find between the needles. They will reach a length of some 28 mm.
Both species of Spruce Carpet fly in two distinct generations, with near to zero animals on the wing in between. The first generation flies in May and June, the second in September and October. In the north of Scotland however there's just one generation in June and July. You might be able to catch an adult by shaking a tree, but best is to let them come to light. They are easily attracted to light, sometimes even in great numbers. A modern energy saving bulb will do. These are very good for Geometers, but are not very good attracting Owlets. The Spruce Carpet is a widespread species, but common only in certain areas. In Scotland and Ireland a very local species only. In most other parts of Europe only common in spruce plantations, elsewhere a widespread, but local or even very local species.