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Active wood ants & cold-hardy green spruce aphids
in mid-winter 2014On this page: Wood ants active in January! Cold-hardy green spruce aphids
Even if you're into wildlife, and especially insects, you might think that looking for aphids in the middle of winter is just a wee bit daft. After all, aphids are small soft-bodied insects found on vegetables and flowers in mid-summer.
Searching for aphids in winter is hard or pointless: their adults have died off, the overwintering eggs are tiny, and their ants are hibernating. But whilst those simple assumptions may be cosy, there are exceptions, especially when the weather is unusual...
Wood ants tending aphids in January - is this a first?
Aside from their scarcity in winter, the lack of easily noticeable ant attendants makes aphid-spotting hard work. Wood ants, for instance, are usually inert below-ground by mid-November - where they remain until March, then re-emerge and sunbathe en-masse on their denuded nest-mounds.
On 11th January 2004 we ventured out on an unusually mild morning to look for aphids close to Selwyn Wood in East Sussex. The area is well known for its southern wood ant (Formica rufa) populations, which favour the presence of giant conifer aphids (Cinara species) on the trees. But as per-normal in mid-winter all their nests looked completely inactive - not an ant in sight.
Nevertheless since not all aphids are tended by ants we pressed on, then stopped to examine some young Scots Pine trees. Almost immediately, we spotted wood ants on each of the growing tips - one climbed to the top of its tip to give a rather subdued threat display.
And yes, they were very definitely (if not entirely devotedly) tending aphids in the depths of winter!!
Photographing this remarkable scene presented some difficulty since, at the slightest disturbance, the attending ants dropped from the plant - and only slowly returned thereafter. (Their tardiness was quite understandable given the cold wind and the already-vanished sunshine.)
The aphids were large pine aphids (Cinara pinea), one of the biggest aphid species in Britain, reaching up to 5.2 mm in length. This is a fairly common species, being found in pine woodland throughout Britain, and over much of the world wherever Scots pine is grown.
Colonies of this aphid are generally small, and they are not considered harmful to established trees. In parts of Europe they are regarded as beneficial as the honeydew provides a useful source of forage for honeybees - which in turn produce very fine honey.
Well, most insects do not actually freeze when the temperatures drop below zero. This is partly because they have electrolytes in their haemolymph which lower the freezing point and partly because they are able to supercool below the freezing point. Most aphids including the large pine aphid (and possibly wood ants) are described as chill susceptible where they have extensive supercooling and can survive at zero to five degrees Celcius, but will die within hours at temperatures below minus-five degrees Celcius.
Cinara aphids normally overwinter as eggs which are much more cold hardy than the nymphs or adults. Finding adult and nymphal Cinara aphids in January is exceptional, and is a direct result of the unusually mild weather. It is similarly surprising to find wood ants active in January. Wood ants usually enter a state of facultative hibernation in winter, and are protected from the worst of the weather by their nest. Clearly however, if the weather is unusually mild they will go out foraging - even if we very rarely see them!
Green spruce aphids - a winter specialist
Not all aphids pass winter as eggs - some continue to feed and grow despite the cold.
So, on this mid-January foray, we subjected some young spruce trees to long and careful examination and were eventually rewarded by finding some, well-camouflaged, Elatobium aphids, specifically the green spruce aphid (Elatobium abietinum). This aphid species spends all year on the needles of Spruce, especially Sitka Spruce (Picea sitchensis) and Norway Spruce (Picea abies) and, much less commonly, Firs (Abies spp.).
Unlike Cinara pinea the green spruce aphid is a cold weather specialist par excellence.
Newly-born unfed Elatobium abietinum are able to withstand temperatures below minus-19 degrees Celcius, although their high supercooling ability decreases markedly when they begin to feed.
The green spruce aphid has an unusual life cycle in that populations often continue to feed and reproduce through the winter. In spring and early summer winged-forms are produced in response to the changing nutritional status of their host. These winged forms migrate to other spruce trees where their nymphs aestivate for the summer before resuming development in autumn.
Despite it being a much smaller aphid than Cinara pinea, Elatobium abietinum is a more serious forestry pest. Colonies on spruce cause discoloration (see picture below) and loss of old needles, sometimes resulting in serious defoliation. Affected trees fail to grow to their full potential.