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Giant willow bark aphids attended by jet black ants

Aphid wildlife in Britain in mid-spring 2014

Pterocomma aphids  Californian Maple Aphids  Bird cherry - oats aphid  Cypress aphid and Spruce shoot aphid 

  <April 2014>  

Spring arrived in April (officially or otherwise) and insects numbers started taking off. We were especially interested in finding the the aphid fundatrices - these are the female aphids that develop from the overwintering eggs. In some species the fundatrix is much larger than the normal viviparous female, and has quite a different appearance...
 

Pterocomma aphids on willow - Giant aphids warn off predators

Woods Mill is the headquarters of the Sussex Wildlife Trust and is also where the Sussex Biodiversity Records Centre is based. We visited there in April to discuss improving the identification and recording of aphid species in Sussex and the rest of the UK. As a result we are now responsible for verifying aphid records in iRecord. 

Following our meeting we had a good look round the nature reserve at Woods Mill, and were rapidly rewarded with some colonies of giant bark aphids, Pterocomma spp.,  being attended by jet black ants (Lasius fuliginosus). One species apparently being attended was the giant black willow bark aphid (Pterocomma salicis) shown below - a black aphid up to 4.5 mm long marked with white and with striking bright orange siphunculi.

Black, white and orange are a clear example of warning (aposematic) colouration designed to dissuade vertebrate predators from attacking them. Most aphids that are attended (and presumably to some extent protected) by ants do not have aposematic colouration since the ants provide sufficient disincentive to potential predators. So why does Pterocomma salicis go to the energetic expense of producing suitable honeydew for ants.

It seems the answer is they don't!! Close examination of the colony below revealed that in nearly every case (including the colony in the photo below) the ants were primarily tending another species Pterocomma pilosum, often first instar nymphs 'buried' in amongst large numbers of Pterocomma salicis. Moreover when there were no Pterocomma pilosum present, there were sometimes no ants in attendance. In these cases, wasps were often feeding on the honeydew, and syrphid larvae were found attacking the colony.

The picture below shows an ant tending Pterocomma pilosum - in this case a winged female depositing young nymphs around her - whilst ignoring Pterocomma salicis. Pterocomma pilosum is cryptically coloured, with a green-brown body and yellowish siphunculi.

Our own data suggest that when Pterocomma salicis is on its own, it is usually not ant attended, relying instead on its aposematic colouration. In such a situation, wasps often remove the honeydew. Pterocomma pilosum, on the other hand, is cryptically coloured and is nearly always ant attended.

But the two aphid species tend to associate in mixed colonies. In such mixed species colonies, the ants will tend and protect both species, but focus more on Pterocomma pilosum.

 

... and now for something completely different

Let's take brief diversion from aphids to look at colour in butterflies: in this case an orange tip butterfly (Anthocaris cardamines).

The orange tips has a good year this spring in the Sussex Weald. This one was taking nectar at a Cuckooflower (Cardamine pratensis) which is the larval foodplant of the orange tip.

 

The upper side is white with bright orange tips making the insect very conspicuous when in flight. When it settles and closes its wings only the mottled green and white underside is visible which is cryptic in a habitat of green leaves and white flowers. This contrast is described as flash colouration and is thought to confuse predators - when in flight it flashes orange, but then it disappears from sight when it settles.

 

Californian maple aphids - an invasive species on Japanese maples

The Californian maple aphid, Periphyllus californiensis, is dark olive-green to brown with dark dorsal cross bands. It is not indigenous to Europe (nor to California), but is from East Asia. It has proved highly invasive and has spread to Europe, North America, Australia and New Zealand on planted Asian ornamental maples such as Acer palmatum.

We have never found it ourselves in UK until April 2014, although it is common in some parts of the UK including South Wales (Baker pers.comm). Then on April 21st we found several large female aphids on a neighbour's Japanese Maple tree. Given the host plant had only recently come into leaf, it seems likely that these aphids were fundatrices.

The reason why we had not noticed the species before was probably because it produces its barely visible aestivating nymphs as early as April, in response to low soluble nitrogen levels in already expanded leaves. These nymphs then do not resume development until autumn.

 

The aestivating nymphs ( pictured above) are fascinating. They are quite different in appearance from usual nymphs, being very flattened with foliate marginal hairs. They suffer very high mortality over summer, but some remain to grow and reproduce in autumn.

 

The bird cherry-oat aphid - fundatrices in rolled leaf galls

The bird cherry (Prunus padus) with its long drooping white flowers (below left) is always a welcome harbinger of spring. As well as the flowers, the rolled leaf galls of the bird cherry-oat aphid, Rhopalosiphum padi, (below right) are also characteristic of spring. This aphid has bird cherry as its primary host, before moving to grasses in summer.

 

Each leaf gall we found contained a fundatrix or stem mother. In this species the fundatrix is much larger than the normal viviparous female and is light green with a reddish spot at the base of each siphunculus.

Unlike the fundatrix, the progeny are covered in greyish wax which serves to protect them from predators. They mature to winged forms before moving to cereals and grasses.

 

The conifer aphids Cinara pilicornis and Cinara cupressi - the good, the bad and the ugly?

A visit to to the Bedgebury Pinetum in Kent usually turns up some interesting aphids, such as the newcomer to Britain, Essigella californica found for the last two years on Montezuma Pine (Pinus montezumae). In April we did not find any Essigella, but we did find the spruce shoot aphid (Cinara pilicornis). This is a supposedly common species, but one that we have only occasionally encountered.

It comes in two colour forms, a plain orange brown (pictured below) or a greyish green and has small brown siphuncular cones. The whole aphid is densely clothed with numerous fine hairs and a dense mealy secretion, the latter not very apparent on the specimen below.

The young nymphs (pictured below) feed on the youngest available shoots. By the second instar the mealy covering is becoming more apparent. The species is usually not ant attended, but the nymphs are very cryptic and can be difficult to spot amongst the young needles.

 

It is invariably found living on the youngest available branch shoots and provides an important source of honeydew for bee populations. Hence bee keepers view this aphid very favourably, although it can damage young trees.

On our Bedgebury visit we also found large colonies of cypress aphid, Cinara cupressi on an exotic cypress Xanthocyparis nootkotensis. This aphid strikes terrror into the heart of any gardener because it has a remarkable ability to kill the tree it is feeding on.

Some colonies are attended by ants which consume most of the honeydew. But unatttended colonies quickly accumulate honedydew deposits, which then leads to the growth of ugly and damaging black sooty moulds shown in the first picture below.

 

The combined effects of direct feeding damage and the black sooty mould can kill part or all of the tree, as shown in the second picture above. The damage tends to be especially severe on clipped hedges.

So why 'hunt' for aphids?? 

Acknowledgements

We have made provisional identifications from high resolution photos of living specimens, along with host plant identity. In the great majority of cases, identifications have been confirmed by microscopic examination of preserved specimens. We have used the keys and species accounts of Blackman & Eastop (1994)  and Blackman & Eastop (2006)  supplemented with Blackman (1974) , Stroyan (1977) , Stroyan (1984) , Blackman & Eastop (1984) , Heie (1980-1995) , Dixon & Thieme (2007)  and Blackman (2010) . We fully acknowledge these authors as the source for the (summarized) taxonomic information we have presented. Any errors in identification or information are ours alone, and we would be very grateful for any corrections. For assistance on the terms used for aphid morphology we suggest the figure  provided by Blackman & Eastop (2006).

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