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Identification & Distribution:

Microlophium carnosum is a large spindle-shaped aphid with an adult body length of 3.1-4.3 mm. Apterae (see first picture below) are various shades of green, pink or reddish purple. The antennae are curved and much longer than the full body length. The antennal tubercles are smooth, with the inner faces divergent. Microlophium carnosum siphunculi are long and tapering with flared apices, 2.3 to 3.1 times the length of the cauda.

Common nettle aphid alates (see second picture above) have dark marginal sclerites but only faint spino-pleural markings.

The clarified slide mounts below are of adult viviparous female Microlophium carnosum : wingless, and winged.

Micrographs of clarified mounts  by permission of Roger Blackman, copyright AWP  all rights reserved.

There is a sexual stage in the life cycle and there is no host alternation. Common nettle aphids live on stems and leaves of common nettle (Urtica dioica). Microlophium carnosum is generally common and often abundant throughout Europe and Asia east to Mongolia, Africa & North America. It is not ant-attended.

 

Biology & Ecology:

Life cycle

In autumn, during the sexual stage in the common nettle aphid's life cycle, eggs are laid near the base of nettle stems. The overwintering eggs hatch in March or April. However, in mild winters a number of parthenogenetic individuals at various stages of development survive over winter, which gives the population a head-start in spring. The picture below shows an overwintering young Microlophium carnosum nymph on nettle in January 2012 - the middle of winter.

As the weather warms in spring, the first generations reach maturity and start to reproduce. The Microlophium carnosum population size then increases rapidly during April and May to give dense colonies by June.

Common nettle aphid populations reach their peaks in June, and there is then a rapid decline in numbers as a result of the combined effects of intraspecific competition and a deterioration in the food quality of the host plant. Perrin (1976)  showed that the size of the Microlophium carnosum population in any particular nettle patch was determined by the food quality of the nettles. Thus each nettle patch had a particular 'carrying capacity' for aphids. Perrin also recorded a biennial fluctuation between relatively large and small aphid populations. It was unclear whether this was because of a natural or aphid-induced cycle in host plant quality, or alternatively, the persistent effects of intraspecific competition over several generations. Levels of predation and parasitism also peak in June.

Colour polymorphism

There are two colour forms - pink and green (see pictures below) with various intermediates. The colour is most likely determined genetically. Araya (1996)  looked at color forms of another aphid species - Sitobion avenae - and found that the developmental rates of the two colour forms were different.

 

The picture below shows a dark green adult giving birth to a pink nymph.

Pink and green forms mix together on the plant as shown below:

Competition/coexistence

Microlophium carnosum is one of two aphid species that only feed on stinging nettle - the other is Aphis urticata.  In some areas both species of nettle aphid may be common, but in other areas Aphis urticata is much less common. This has led to some hypothesizing that the local rarity of Aphis urticata might be explained by competitive exclusion by Microlophium carnosum, mediated indirectly through natural enemies.

Although it seems to be unusual to find mixed colonies of both aphid species on the same plant - it is not unknown. We have found them on occasion to be sharing the same plant (see picture below - Microlophium carnosum is pale green, Aphis urticata is dark green).

Aphis urticata was the commoner of the two species, with Microlophium carnosum scattered amongst the Aphis urticata. Perhaps significantly, this mixed colony was the only time we have found Aphis urticata to be unattended by ants.

Kean & Müller (2004)  used experimental aphid colonies on potted nettles to test for effects of Microlophium carnosum on Aphis urticata in the field. Despite the presence of numerous predators, the colony dynamics of the rarer aphid were not different on potted nettles when a colony of the common aphid occurred on the same plant or on a plant nearby or when the common aphid was altogether absent. It was concluded that there was no evidence for competition being a major factor causing the local rarity of Aphis urticata. A more likely explanation for the scarcity of Aphis urticata in some sites is the lack of suitable ants to attend the colonies (Müller & Godfray, 1997 ).

One can perhaps sum up the situation thus - whilst Microlophium carnosum cannot live with ants (presumably because of predation by the ants), Aphis urticata prefers not to live without them (because of a mutualistic interaction involving honeydew and protection from predators).

Natural enemies

Several parasitoids attack the common nettle aphid. Until recently the dominant parasitoid was recorded as 'Aphidius ervi' a common polyphagous parasitoid which also attacks some cereal and pea aphids (Stary, 1983 ). Hence nettle was considered to provide a valuable reservoir of this beneficial parasitoid (Perrin, 1975 ) with up to 10% of Microlophium carnosum populations affected in June. The first picture below shows the mummy of an aphid parasitized by this species, and the second shows (what was thought to be) Aphidius ervi reared from Microlophium carnosum.

 

It was subsequently shown that Aphidius ervi reared on pea aphids will not attack Microlophium carnosum, whilst 'Aphidius ervi' reared on Microlophium carnosum lays very few eggs in pea aphids (Cameron et al., 1984 ). The Aphidius ervi-like parasitoid on Microlophium carnosum has now been reclassified as a different species, namely Aphidius microlophii (Pennacchio et al., 1994 ). Since other primary parasites on Microlophium carnosum are quite rare, it now appears that Microlophium carnosum does not provide reservoir of useful parasitoids for crop pests.

Not all aphid mummies yield the primary parasitoids when they are reared through. A range of hyperparasitoids attack the primary parasitoid, two of which are shown below. The first two pictures show a Dendrocerus species (male, then female) and the third shows an Alloxysta species.

   

As well as parasitoids, common nettle aphids also have predators. Syrphid larvae are common predators of nettle aphids, especially the syrphid Eupeodes luniger (see images below of larva on nettle, and of adult in flight).

 

The orange eggs on this nymph (see first picture below) will hatch to give predatory cecidomyiid larvae. The second picture below shows another common predator - an anthocorid bug. Coccinellidae (see below) are also abundant predators of nettle aphids.

 

The aphid below is covered with the waxy secretion produced from its siphunculi, in response to predator activity. In some species this secretion is known to contain an alarm pheromone which alerts other aphids of the same species.

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There are also a number of pathogens that attack nettle aphids. Barta et al. (2003)  surveyed populations of the common nettle aphid for entomophthorean infections in Slovakia. Five species were detected, but only three were found during each year of the three year survey: Erynia neoaphidis, Neozygites fresenii and Neozygites microlophii.

The species above is the commonest in colonies we have examined, and is most probably Erynia neoaphidis  

Beneficial effects

Does nettle provide a reservoir for natural enemies of crop pests?

It has long been postulated that the large aggregations of natural enemies of common nettle aphid move to nearby plants and hence reduce populations of pest aphid species. Such indirect interaction between aphid species is sometimes termed 'apparent competition'. Much research has focused on predation by coccinellid species such as the two spot ladybird (Adalia bipunctata) (see first picture below) and the 14-spot ladybird (Propylea quattuordecimpunctata) (see second picture below).

 

For example Alhmedi et al. (2007)  carried out a field 'experiment' in Belgium to compare the aphid and aphidophagous populations in nettle margin strips with those in nearby fields of wheat, green pea or rape. (This is correctly termed an observational study - not an experiment.) There were more natural enemies in nettle strips than in field crops, and predatory anthocorids, mirids and green lacewings were only observed on nettle. Coccinellidae were found in both nettle strips and in crop fields, although species composition differed. The invasive harlequin ladybird (Harmonia axyridis) (shown below, feeding upon a common nettle aphid) was dominant in nettle whilst Coccinella septempunctata was dominant in field crops.

Laboratory experiments showed that harlequin ladybirds preferred to consume and oviposit amongst common nettle aphids (Alhmedi et al., 2008 ) so it was concluded that harlequin ladybirds are unlikely to be a suitable candidate for biological control of pest aphids. Using data from a further year of study, Alhmedi et al. (2009)  confirmed this, but also found that predators of crop pests such as the 7-spot ladybird and syrphid larvae were present on stinging nettle earlier in the season, albeit in low numbers, and hence could (potentially) supplement numbers in field crops later on.

Of course movement may occur in the other direction, with predators present in the crop environment moving to the non-crop areas. Thus Möller & Godfray (1997)  placed populations of the common nettle aphid on potted nettles adjacent to grass plots infested by bird-cherry aphids (Rhopalosiphum padi ). Populations of the common nettle aphid declined much more rapidly when many bird-cherry aphids were nearby. This was because Coccinellid predators that had fed on bird-cherry aphids shifted to the nettles, where they laid their eggs, and where they and their offspring fed on the common nettle aphids. A similar experiment by Rott et al. (1998)  demonstrated the same effect with pea aphids (Acyrthosiphon pisum  ) and common nettle aphids.

The non-crop habitat also serves as a source for aphid pathogens. The image below shows a colony of nettle aphids severely affected by an entomophagous fungus, probably Erynia neoaphidis.

Van Veen et al. (2008)  has shown that the common nettle aphid may act as a major source of fungal spores attacking aphid species on crops. They predicted that, under meteorological conditions favouring fungal pathogens, the presence of nettle aphids will contribute to the control of other species feeding on nearby host plants.

 

Other aphids on Urtica dioica (common nettle):

Blackman & Eastop list about 18 species of aphid  as feeding on Urtica dioica worldwide, and provide formal identification keys. Of those aphid species, Baker (2015)  lists 10 as occurring in Britain: Aphis fabae,  Aphis urticata,  Macrosiphum euphorbiae,  Microlophium carnosum, Myzus ascalonicus,  Myzus ornatus,  Myzus persicae,  Neomyzus circumflexus,  Rhopalosiphoninus latysiphon and Rhopalosiphoninus staphyleae.

Acknowledgements

Our particular thanks to Roger Blackman for images of his clarified slide mounts.

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).

Useful weblinks 

References

  •  Abe, J. et al. (2011). Descriptions of two new endoparasitic cecidomyiids (Diptera: Cecidomyiidae) from Japan. Applied Entomology and Zoology 46(1), 15-25. Abstract 

  •  Alhmedi, A. et al. (2007). Aphidophagous guilds on nettle (Urtica dioica) strips close to fields of green pea, rape and wheat Insect Science 14, 419-424. Abstract 

  •  Alhmedi, A. et al. (2008). Role of prey-host plant associations on Harmonia axyridis and Episyrphus balteatus reproduction and predatory efficiency. Entomologia Experimentalis et Applicata 128, 49-56. Abstract  

  •  Alhmedi, A. et al. (2009). Effect of stinging nettle habitats on aphidophagous predators and parasitoids in wheat and green pea fields with special attention to the invader Harmonia axyridis Pallas (Coleoptera: Coccinellidae). Entomological Science 12, 349-358. Abstract 

  •  Araya, J.E. et al. (1996). Development and reproduction of two color forms of English grain aphid (Homoptera: Aphididae). Environmental entomology 25(2), 366-369. Abstract 

  •  Barta, M. & Cagan, L. (2003). Entomophthoralean fungi associated with the common nettle aphid (Microlophium carnosum Buckton) and the potential role of nettle patches as reservoirs for the pathogens in landscape. Journal of Pest Science 76, 6-13. Abstract 

  •  Blackman, R.L. & Eastop, V.F. (2006). Aphids on the world's herbaceous plants and shrubs. Vols 1 and 2. John Wiley & Sons.

  •  Cameron, P.J. et al. (1984). Reservoirs for Aphidius ervi Haliday (Hymenoptera: Aphidiidae), a polyphagous parasitoid of cereal aphids (Hemiptera: Aphididae). Bulletin of Entomological Research 74, 647-656. Abstract 

  •  Kean, J. M. and Müller, C. B. (2004). Can competition explain local rarity of a nettle aphid? Ecological Entomology 29, 706-710. Abstract 

  •  Müller, C.B. & Godfray, H.C.J. (1997). Apparent competition between two aphid species. Journal of Animal Ecology 66(1), 57-64. Abstract 

  •  Pennacchio, F. et al. (1994) Host recognition and acceptance behaviour in two aphid parasitoid species: Aphidius ervi and Aphidius microlophii (Hymenoptera: Braconidae) Bulletin of Entomological Research 84, 57-64. Abstract 

  •  Perrin, R.M. (1975). The role of the perennial stinging nettle, Urtica dioica, as a reservoir of beneficial natural enemies. Annals of Applied Biology 81(3), 289-297. Abstract 

  •  Perrin, R.M. (1976). The population dynamics of the stinging nettle aphid, Microlophium carnosum (Bukt.). Ecological Entomology, 1, 31-40. Abstract 

  •  Rott, A.S. et al. (1998). Indirect population interaction between two aphid species. Ecology Letters 1(2), 99-103. Abstract 

  •  Stary, P. (1983). The perennial stinging nettle (Urtica dioica ) as a reservoir of aphid parasitoids (Hymenoptera, Aphidiidae). Acta Entomologica Bohemoslovaca 80(2), 81-86.

  •  Van Veen, F.J.F. et al. (2008). Food web structure of three guilds of natural enemies: predators, parasitoids and pathogens of aphids. Journal of Animal Ecology 77, 191-200. Abstract   Full text 

 

Identification requests

Alan Outen, 13 May 2014, Aphids on Urtica dioica

One of the aphids that I found in the village yesterday (Clifton, Beds, UK) was abundant on nettles just down the footpath from me (though I found it nowhere else in the village. Remarkably masses of nettles by the River Ivel were devoid of any aphids!). This group seem very variable but do not seem to me fit well to either of the species that you give for nettles and I note that you say you have not found either Aulacorthum circumflexum (crescent-marked lily aphid) or Myzus ornatus (ornate aphid), on this host. This anyway does not look to me like one of these either! I am sure it is very common and the problem may perhaps well be because they are just immature but I would appreciate any comments, though I am aware that images alone are often not enough. I do also have specimens.

Bob, Influentialpoints:

  • The aphids on nettles are all Microlophium carnosum of various stages & colour forms + mummified specimens (parasitized by braconids) + specimens attacked and killed by a fungal pathogen:

Images copyright Alan Outen,  all rights reserved.

  • Your photo (above) shows III and IV instar nymphs:

 

  • Both of these photos show a nymph, + ones killed by Entomophthora, + right-hand-top an empty parasitized mummy.

    The parasitized mummies can be kept alive in tubes and reared out - identification is difficult but not impossible.

  • This shows an aphid killed by a fungal pathogen, probably an Entomophthora. These fungi can also be identified, although I haven't really got into that yet. Despite having lots of piccies of them, I haven't put a piccie of one killed by Entomophthora yet. Will do so soon.

As a mycologist I have often found Entomophthora, mostly on Diptera but on whatever host always keying out to E. muscae! I have also found, identified and photographed many other hyphomycete parasites on various insects but was not aware that Entomophthora attacked aphids. I will see if I can get anything fertile out to try and identify species as well as seeing if I can rear through any braconids.