Biology, images, analysis, design...
|"It has long been an axiom of mine that the little things are infinitely the most important" |
Identification & Distribution:Aphis ilicis apterae have a body length of 1.7-2.9 mm, and 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 cauda is dark, blunt, finger-shaped and somewhat shorter than the siphunculi. Aphis ilicis is a member of the Aphis fabae group, so immatures often have white pleural wax markings, but without any general pulverulence. Sexual forms with winged males are recorded from July onwards.
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. Aphis ilicis apparently does not host alternate, although it is unclear how colonies survive through the summer under natural conditions as mature leaves are not colonised. The holly aphid aphid is widely distributed in western and northern Europe eastward to Turkey.
These two pictures are of adult apterae of Aphis ilicis on holly, and show the range of colouration from reddish brown to a dark greyish-brown. Note the black and slightly tapered siphunculi and the black, blunt, finger-shaped cauda, both characteristic of the black Aphis fabae group aphids. The nymphs do not appear to have the typical white pleural wax markings of this species group.
Biology & Ecology:
The pictures below shows holly aphid colonies within rolled young holly leaves which protect the aphids.
This protection is usually supplemented by ants. We have found this invariably to be the case at one of the sites in East Sussex (Ashdown Forest) where we find it regularly. In the picture below, southern wood ants (Formica rufa) are guarding the aphids assidously (the attacker is the camera lens).
We have yet to find any parasitoids or predators attacking Aphis ilicis when attended by wood ants. There are, however, several pictures on the web (see for example Studyjunkie ) showing holly aphid colonies attended by Lasius ants being attacked by coccinellid predators - the seven-spot ladbird (Coccinella septempunctata) or the harlequin ladybird (Harmonia axyridis).
We have found evidence of parasitoid attack where there were apparently no wood ants - see image below taken in Devon, UK in May. These are mummies 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.
So we are left with the issue of how holly aphids get through late summer when the holly leaves are too hard to feed on. It seems that Stroyan (1984) was quite correct when he suggested that later generations of the cycle may 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.
Damage and controlThe only damage this aphid is likely to cause is to make ornamental holly trees look 'unsightly' becauser of the curled leaves. One can only 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 extaordinary shapes produced, one can only hope a gardener would appreciate this aphid as part of the natural environment. Application of insecticide after the curling has been produced would serve no useful purpose.