Biology, images, analysis, design...
|"It has long been an axiom of mine that the little things are infinitely the most important" |
Common nettle aphidOn this page: Identification & Distribution Biology & Ecology Life cycle Colour polymorphism Competition / coexistence Natural enemies Beneficial Effects Other aphids on the same host
Identification & Distribution:
Microlophium carnosum is a large spindle-shaped aphid. 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 (cf. Aphis urticata which has pale tapering siphunculi, usually slightly dusky at the tips, which are 0.90-1.78 times the length of the cauda). Body length of the adult aptera is 3.1-4.3 mm.
The clarified slide mounts below are of adult viviparous female Microlophium carnosum : wingless, and winged.
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:
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.
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).
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.
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.
Other aphids on the same host
Microlophium carnosum has been recorded from 6 Urtica species (Urtica cannabina, Urtica dioica, Urtica fissa, Urtica massaica, Urtica parviflora, Urtica urens).
Blackman & Eastop list about 18 species of aphids as feeding on common stinging nettle (Urtica dioica) and 11 on small nettle (Urtica urens) worldwide, and provides formal identification keys for aphids on Urtica (Show World list).
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.