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Dark Green Nettle AphidIdentification & Distribution Biology & Ecology Beneficial Effects
Identification & Distribution:
Aphis urticata is a small to medium size aphid. Early generations of the dark green nettle aphid are dark bluish-green with no wax covering and a body length of 1.7-2.2 mm. Later generations of apterae are yellow and much smaller with a body length of 0.9-1.4 mm. The abdominal dorsum is either unsclerotized or rarely with rather narrow dusky bands across tergites 7-8. The pale siphunculi taper towards their tips which are usually slightly dusky. The tongue shaped cauda is also pale. Alates have more sclerotization tham apterae with bands across tergites 7-8 and some marginal sclerites. The siphunculi of alates are uniformly dusky and cylindical. The body length of the alate is 1.5-1.8 mm.
Early generations of the dark green nettle aphid form dense colonies on stems and leaves of Common Nettle (Urtica dioica). The dwarf summer form of Aphis urtica can be found scattered on the undersides of the leaves. The dark green nettle aphid is generally common and sometimes abundant throughout Europe and into Asia.
The first picture above left shows a blue-green adult aptera of Aphis urticata in early summer. Note the pale tongue-shaped cauda and the pale siphunculi which taper towards dusky tips. The picture above right shows an alate with dark sclerotized bands on the dorsum along with an alatiform nymph.
The picture above shows the unusual dwarf aptera form which is present on the leaves in summer. It is yellowish rather than blue-green. The dwarf form requires lower levels of nutrition which enables aphids to get through mid-summer when available levels of nitrogen in the phloem are very low. Another strategy for getting through this period is to have a period of reproductive diapause in summer as with the common sycamore aphid Drepanosiphum platanoidis.
Biology & Ecology:
The overwintering eggs of Aphis urticata laid on nettle stems hatch in early May to give the fundatrices. These reproduce parthenogentically to produce relatively small but very dense colonies. The first picture below shows a mature colony in late May.
Aphis urticata is one of two aphid species that only feed on stinging nettle - the other is Microlophium carnosum. In some areas both species of nettle aphid may be very common, but in other areas Aphis urticata is much less common. This has led to some hypothesizing that the local rarity of A. urticata might be explained by competitive exclusion by the Microlophium carnosum, mediated indirectly through natural enemies.
Kean & Möller (2004) used experimental aphid colonies on potted nettles to test for effects of M. carnosum on A. urticata in the field. Despite the presence of numerous predators, the colony dynamics of the rare 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 A. 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 ). We have always found successful Aphis urticata colonies to be ant-attended, whether by red ants (picture above right), black ants (picture below left) or southern wood ants (picture below right).
Lang & Menzel (2011) used Aphis urticata as one example of an ant-loving (myrmecophilous) aphid in their study to test their hypothesis that ants would discriminate between ant-loving and non-ant-loving aphids using cuticular hydrocarbons, rather than honeydew quality or quantity. This was on the basis that ant-loving aphids only produced honeydew of higher quality or quantity after tending commenced, so any signal to ants has to be independent of honeydew quality.
They looked at the responses of ants to live aphids, dead aphids and dummies covered with aphid cuticular hydrocarbons. Lasius niger invariably attacked individuals covered with non-ant-loving aphid hydrocarbons, but tended those covered with ant-loving aphid hydrocarbons. Further analysis suggested that myrmecophilous aphid species had a common cuticular hydrocarbon signal, which L. niger used to classify aphids into potential partners and potential prey.
Colonies of Aphis urticata often suffer high rates of parasitism as indicated by the picture below which shows a cluster of parasitized mummies.
The parasitoid adults are evidently not discouraged by the attending ants, similar to the situation with Lysiphlebus confusus parasitizing Aphis farinosa. Lysiphlebus fabarum is the key parasitoid of A. urticata on stinging nettles, whilst Aphidius ervi (now Aphidius microlophii) is the key-parasitoid of of the other nettle aphid Microlophium carnosum (Stary, 1983 ).
We have not found any predators present on colonies tended by southern wood ants which seem to be remarkably effective at protecting their honeydew providers. Predators are sometimes present on Aphis urticata colonies attended by black ants (Lasius) in particular larvae and adults of the seven-spot ladybird Coccinella septempunctata. The larva is shown in the first picture below and the adult in the second picture below.
The seven-spot ladybird has a rather pronounced proclivity for cannibalism (Khan et al., 2003 ) which the one in the picture demonstrated by attempting to eat a larva of its own species (see image below). This larva did eventally manage to escape, but it looked somewhat worse for its experience.
There remains the issue of whether nettle provides a reservoir for aphid natural enemies which move to nearby crops when aphid numbers on nettle decline in the summer. We consider this on the page on the common nettle aphid Microlophium carnosum. The commonest predator of the common nettle aphid is the coccinellid Harmonia axyridis, but that species tends not to be found on crops (Alhmedi et al., 2007 ). Similarly the main parasitoid of Microlophium evansi - Aphidius microlophii - will not attack crop aphids.
However, the coccinellid Coccinella septempunctata that we have found attacking Aphis urticata (at least early in the year) certainly does attack crop aphids, as does the parasitoid of Aphis urticata - namely Lysiphlebus fabarum. Hence it seems that the Aphis urticae offers a much better prospect of providing useful natural enemies from nettle strips to nearby crops than does Microlophium carnosum. Having said that, we still need some experimental work to demonstrate that predators really will move to crops, and more importantly will actually reduce pest populations.