Tag Archives: super-cooling

Midwinter Madness – The Snow flea

Between 1982 and 1992 I worked as a research and advisory entomologist for the UK Forestry Commission based at their Northern Research Station just outside Edinburgh. For the first five years of my time there I worked almost exclusively on the pine beauty moth, Panolis flammea. The pine beauty moth is

snowflea 1

a native insect that became a pest of a non-native tree, Pinus contorta, then a tree that was widely planted over northern Britain. The majority of planting in Scotland was in the north and this meant that my study sites were in Sutherland and Caithness and Aberdeenshire. My main experimental forest was west of Aberdeen in the Spey valley (very handy for the whisky trail) in the Elchies block of Criagellalchie Forest.

snow flea 2

My experimental forest with nearby distillery marked 😉

In Mid-January 1984, I headed north to do some maintenance on my head capsule collecting funnel traps.

snowflea 3

In those days, snow was a perennial hazard, even in the south of Scotland and as I progressed northwards the drifts at the side of the road became increasingly higher. When I reached the forest gates, it was obvious that I was not going to be able to drive to my site. The sun was shining, the sky was blue and the snow glistened. A perfect day for a walk, albeit one of 10 km. Luckily, the weather had been sunny for the last couple of days so the snow was mostly hard enough to walk on. Only in a few places did I break the surface and find that I was standing on about a metre depth of snow. Two hours later as I was approaching my field site, squinting against the sun bouncing off the white untouched snow, I saw black spots moving on the surface. My immediate thought was that I was suffering the first stages of snow-blindness, but as I got nearer I saw that the black dots were actually insects. At first sight I thought I was hallucinating, was this some strange bizarre form of life perhaps an aphid-fly hybridization experiment gone wrong? On closer examination I realised that I was looking at wingless Mecopterans.

snow flea 4

Male snow flea, Boreus hyemalis http://mecoptera.free.fr/Boreus-hyemalis.html


snow flea 5

Female Boreus hyemalis, note the sting-like ovipositor. http://www.wbrc.org.uk/WORCRECD/32/Bingham–John–Snow_Flea_Boreus_hyemalis.html

Although I was familiar with Scorpion-flies, I had never seen these critters before.

snow flea 6

The aptly named Scorpion fly Panorpa communis : https://commons.wikimedia.org/wiki/File:Scorpion_Fly._Panorpa_communis._Mecoptera_(7837166610).jpg

I collected a few to send off for identification and confirmation and carried on into the depths of the forest to check on my funnels. On returning to civilization a day or so later I sent my specimens off to the Natural History Museum and shortly after was informed that I had they were the snow flea, Boreus hyemalis and that I had extended the recorded range of this particular species, albeit only by a few miles.

snow flea 7

My record – it lasted 10 years as the furthest north before M.S.C. Elliott recorded it in February 1994 in Easter Fearn in the north-west Highlands.

Boreus my record

Distribution of Boreus hyemalis in 1994; my record, then the furthest North.


snow flea 9

Current recorded distribution of Boreus hyemalis – obviously widespread – just lacking people willing to go and look for it in the winter 🙂

So what is a snow flea. It is of course, not a flea, being a Mecopteran or Scorpion fly, albeit non-winged.  In Britain there are three species with wings (in the genus Panorpa), the larvae and adults both being predatory on other insects. The adult snow flea is about 5mm long, and lives among moss on which it feeds as both a larva and adult (Withycombe, 1922, 1926). Interestingly, the BugLife site states that they are predatory in both the larval and adult stage. I am not sure where they got this information as they do not cite a reference and all the published literature I have seen indicates that they are moss feeders (Withycombe, 1922, 1926; Fraser, 1943; Hågvar, 2010). Indeed, Wthycombe (1922) conducted a series of experiments on the larvae and conclusively demonstrated that they were unable to complete their development unless fed on moss, although the adults will apparently also feed on dead insects.

These are true winter-active insects, adults emerging in October and November when they mate and lay their eggs the eggs at the base of moss plants), Polytrichium commune being the preferred host (Fraser, 1943). The eggs start to hatch in November and the larvae forage within the moss clumps, pupating towards the end of the summer, emerging as adults after 6-8 weeks.  The adults, which are wingless, thus come out in the coldest months of the year, usually between October and April.  They are most easily seen when walking or jumping on the snow surface. Considering that the adults are winter-active they have a surprisingly high super-cooling point (-6.5oC) (Sömme & Östbye, 1969), especially when compared with the cereal aphid, Sitobion avenae, which has a super-cooling point of -24oC but rarely survives English winters (Knight & Bale, 1986). The BugLife site wonders “how they (snow fleas) manage to jump up to 5 cm without muscular hind legs” but Burrows (2011) found that their jumping prowess is by virtue of large depressor muscles within the thorax which enables them to jump distances of up to 10 cm with a take-off velocity of 1 m s-1, indicating a force of about 16 times their body weight.  So aptly named in this respect too.

The Snow flea is not found (or at least has not been recorded) in the mild south-west of Britain, seeming to prefer areas with a harsher winter. Climate warming may thus pose a threat for this intriguing and little studied insect. Perhaps it is time for us all to venture out in mid-winter and start scanning the surface of snow drifts in heathland areas for these elusive creatures before it is too late.



Burrows, M (2011) Jumping mechanism and performance of snow fleas (Mecoptera, Boreidae). Journal of Experimental Biology, 214, 2362-2374.

Fraser, F.C. (1943) Ecological and biological notes on Boreus hyemalis (L.) (Mecopt., Boreidae). Journal of the Society for British Entomology, 2, 125-129

Knight, J. D. & Bale, J. S. (1986). Cold hardiness and overwintering of the grain aphid Sitobion avenae. Ecological Entomology 11, 189-197.

Sömme, L. & Östbye, O. (1969) Cold-hardiness in some winter active isnects. Norsk Entomologisk Tidsskrift, 16, 45-48

Withycombe, C. L. (1922). On the life history of Boreus hyemalis L. Transactions of the Entomological Society of London, 1921, 312-318.

Withycombe, C. L. (1926). Additional remarks upon Boreus hyemalis L. Entomologist’s Monthly Magazine, 62, 81-83.


Useful link

For more images and observations see http://www.wbrc.org.uk/WORCRECD/32/Bingham–John–Snow_Flea_Boreus_hyemalis.html



Filed under EntoNotes

A Winter’s Tale – aphid overwintering

Aphids that live in temperate or boreal regions have to be able to survive overwinter. Aphids, depending on species, are able to pass winter in two ways. If they are holocyclic i.e. possess an egg-laying stage, they usually overwinter as eggs. Aphid eggs are extremely cold-hardy; they have been reported to have super-cooling points of about -42oC (Somme ). If laid on a woody host, eggs are usually laid in the bud axils as in the case of the apple aphid, Aphis pomi, the black bean aphid Aphis fabae and the bird cherry aphid, Rhopaloishum padi.

aphid eggs

In some instances, such as the sycamore aphid, Drepanosiphum platanoidis, eggs are laid directly on the tree bark or in crevices in the bark or even in lichen growing on the bark.  See if you can spot the eggs in the picture below.


If however, the aphid in question lives on an herbaceous host, the eggs may be laid directly on the ground, on or amongst fallen leaves or at the base of grass tussock.

The other strategy adopted by those aphids that are anholocyclic, such as the green spruce aphid, Elatobium abietinum, is to pass the winter as an active stage, either as an adult or immature nymph. Those holocyclic aphids that have anholcyclic strains are also able to adopt this strategy. Despite their soft bodies and fragile appearance, aphids have quite low super-cooling points values such as -26oC having been reported (Griffiths & Wratten, 1979).

A potential advantage of using an active overwintering stage and not an egg, is that if they survive the winter, they are able to start reproducing sooner, particularly if they are a host –alternating aphid, where the aphids hatching from eggs, have to spend time developing and reproducing on the primary woody host before being able to migrate to the secondary hosts. This also applies, to a lesser extent, to those holoyclic aphids living on herbaceous plants, although the temporal advantage is not as great. One would assume that given the relative cold-hardiness attributes of aphid eggs and adults that in a country such as the UK where winter temperatures below -10oC are both infrequent and short lasting, winter survival of aphids would be extremely high if not guaranteed. This is not the case. For example, eggs mortality of the bird cherry aphids is typically around 70-80% as shown in my first ever publication (no fancy graphics packages in those days, just Letraset , Indian ink, stencils and tracing paper). Actually people had measured aphid egg mortality much earlier than this (Gillette, 1908) but I was the first person to monitor mortality throughout the winter and show that it occurred at a steady rate irrespective of the severity of the winter.

 Egg survival

It is actually a function of the length of the winter that determines how many eggs survive, the longer the winter the greater the mortality.

Egg mortality

This level of mortality is typical for all aphid species for which I have data (Leather, 1993). Some of this mortality can be attributed to predation, but most of it is intrinsic (Leather, 1981), possibly due to cryo-injury.

Similarly, those aphids that overwinter as adults or nymphs, despite their ability to super-cool to temperatures below -20oC, experience even greater levels of mortality as shown elegantly by Jon Knight and Jeff Bale in 1986 studying overwinter survival of the grain aphid Sitobion avenae near Leeds.

Knight & Bale

In fact one wonders how any aphids at all survive winter this way, but they certainly do if the winters are mild enough, as in the case of Myzus persicae and Sitobion avenae in southern England and E. abietinum throughout most of its range (Day et al., 2010). An interesting anomaly is Iceland where hot springs abound and the bird cherry aphid is able to survive anholocyclically on grasses growing around the springs whereas in other countries with similar winter temperatures it would only be able to survive as the egg stage.

Despite the importance of winter to aphid population dynamics we still know very little about their winter ecology, our knowledge being confined to a handful of economically important species. Despite the discomfort of field work in the winter this is an area which would be very rewarding to anyone in need of an interesting and good research project.  Finger-less mittens are, however, definitely recommended 😉

Useful references

Bale, J. S. (1996). Insect cold hardiness: a matter of life and death. European Journal of Entomology 93, 369-382. http://www.eje.cz/pdfs/eje/1996/03/09.pdf

Day, K. R., Ayres, M. P., Harrington, R. & Kidd, N. A. C. (2010). Interannual dynamics of aerial and arboreal spruce aphid populations. Population Ecology 52, 317-327. http://link.springer.com/article/10.1007/s10144-009-0190-0#page-1

Gillette, C. P. & Taylor, E. P. (1908). A few orchard plant lice. Colorado Agricultural Experimental Station Bulletin, 113, 1-47.

Griffths, E. &Wratten, S. D. (1979). Intra-and inter-specific differences in cereal aphid low temperature tolerance. Entomologia experimentalis et applicata 26, 161-167. http://onlinelibrary.wiley.com/doi/10.1111/j.1570-7458.1979.tb02912.x/abstract

Knight, J. D. & Bale, J. S. (1986). Cold hardiness and overwintering of the grain aphid Sitobion avenae. Ecological Entomology 11, 189-197.

Leather, S. R. (1980). Egg survival in the bird cherry-oat aphid, Rhopalosiphum padi. Entomologia experimentalis et applicata 27, 96-97. http://onlinelibrary.wiley.com/doi/10.1111/j.1570-7458.1980.tb02951.x/abstract

Leather, S. R. (1981). Factors affecting egg survival in the bird cherry-oat aphid, Rhopalosiphum padi. Ent omologia experimentalis et applicata 30, 197-199. http://onlinelibrary.wiley.com/doi/10.1111/j.1570-7458.1981.tb03097.x/abstract

Leather, S. R. (1993). Overwintering in six arable aphid pests: a review with particular relevance to pest management. Journal of Applied Entomology 116, 217-233. http://onlinelibrary.wiley.com/doi/10.1111/j.1439-0418.1993.tb01192.x/abstract;jsessionid=9FC2ED8174E96317F192CF42A19092FE.f03t03?deniedAccessCustomisedMessage=&userIsAuthenticated=false

Strathdee, A. T., Howling, G. G. & Bale, J. S. (1995). Cold hardiness of overwintering aphid eggs. Journal of Insect Physiology 41, 653-657. http://www.sciencedirect.com/science/article/pii/002219109500029T


Filed under Aphidology, Aphids