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Holly AphidOn this page: Identification & Distribution Biology & Ecology Life cycle Colour Ant attendance Natural enemies Other aphids on the same host Damage & Control
Identification & Distribution:
Aphis ilicis apterae are dark olive-brown, reddish-brown or greyish-brown. The sclerotic dark banding of the dorsum is variable - it is confined to the latter abdominal tergites in small apterae, but there are broken bands on some anterior tergites in larger apterae. Aphis ilicis siphunculi are black, rather short and slightly tapered distally. The siphunculi are 1.15-1.67 times longer than the dark, blunt, finger-shaped cauda. Aphis ilicis is a member of the Aphis fabae group, so immatures may have white pulverulence (but see biology & ecology section below). Adult apterae have a body length of 1.7-2.9 mm.
The alate (see second picture above) has variable banding on the abdominal dorsum, but when present it is more regular than in the aptera. The picture below shows a dorsal view of an apterous adult in alcohol.
The holly aphid lives in dense colonies on young shoots and undersides of young leaves of holly (Ilex aquifolium). Attacked leaves curl towards their undersides. Later in the year, when the leaves have matured, aphids can be found colonizing the berry petioles. Sexual forms with winged males are recorded from July onwards. It is usually attended by ants. Aphis ilicis is widely distributed in western and northern Europe eastward to Turkey.
Biology & Ecology:
The eggs laid the previous year on holly twigs hatch out in April, and the nymphs move to the youngest available shoots and leaves. The first picture below shows a group of immature holly aphids (mostly fourth instar) feeding on a young holly leaf.
Feeding induces curling of the leaves towards their undersides.
This produces leaf rolls where the aphid colonies develop.
So how do holly aphids get through late summer when the holly leaves are too hard to feed on?
Stroyan (1984) correctly postulated that later generations of the cycle develop on the petioles of the flowers and immature berries. Such colonies are shown below in July and August, after most of the aphids had left the leaves.
The main colour variation is from reddish brown (the normal colour for immatures) to greyish brown or black (see pictures below).
In colonies that are not attended by ants, the aphids can look rather different. For a start they may have a white pulverulence on all but newly moulted aphids (see picture below). Note this is contrary to the account of Stroyan (1984) who states they have no general pulverulence. He presumably only encountered ant-attended colonies.
Secondly, if there is no ant attendance, it is more likely that the immatures, especially fourth instar, will have white pleural markings, such as those shown in the picture below.
The protection offered by the curved holly leaves is usually supplemented by ants. We have found this invariably to be the case in locations where wood ants are present. In the pictures below, southern wood ants (Formica rufa) are guarding the aphids assiduously (the attacker is the camera lens).
The wood ants continue to attend and defend the aphids when they move to the berry petioles.
The aphids appear completely unconcerned by the wood ants, and in the picture below the aphid has its front leg touching the ant.
We have generally found very few predators and parasitoids attacking Aphis ilicis, which probably reflects the frequent attendance and protection by Formica rufa. The exception was a population of Aphis ilicis at Burton Pond (in West Sussex) where there were no wood ants. There were a number predatory species present including larvae of the harlequin ladybird (Harmonia axyridis, see first picture below), and an adult of the pine ladybird (Exochomus pustulatus, see second picture below).
We have only once found evidence of parasitoid attack, again where there were apparently no wood ants - see image below taken in Devon, UK in May. These aremummies of Aphis ilicis parasitized by a braconid parasitoid, probably Aphidius. No live holly aphids were present, but the leaves showed signs of a past infestation, presumably the previous summer.
Other aphids on same host:
Blackman & Eastop list 7 species of aphid as feeding on holly (Ilex aquifolium) worldwide, and provide formal identification keys (Show World list). Baker (2015) lists all 7 as occurring in Britain (Show British list).
It is rare to find any species other than Aphis ilicis on holly, but we have encountered the rose aphid, Macrosiphum rosae, colonizing the young holly shoots.
Damage and control
Aphis ilicis only causes damage to holly to the extent that the curled leaves might be considered 'unsightly'. Yet we note that some ornamental bushes have been bred specifically to produce such contorted foliage - for example a variety of hazel (Corylus avellana var contorta).
Given the rather extraordinary shapes produced, one might wish that gardeners would appreciate this aphid as part of the natural environment. Application of insecticide after the curling has been produced seems rather pointless.