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Aphids and their natural enemies
Aphis fabae fabae
This blog describes the aphids we found during a short survey we did during the Sussex Biological Recorders weekend visit (May 24-26 2014) to the "re-wilding" Knepp Castle Estate project in their Southern Block (and to the Castle) near the village of Dial Post - off the A24 north of Worthing.
Knepp Castle Estate used to be a traditional arable and dairy farm. However, for the past 12 years the land has been managed to boost biodiversity and allow less intensive meat production. Boundaries between fields have been removed, and the 1400 hectares are now grazed by free roaming herds of Longhorn cattle, Exmoor ponies, Tamworth pigs and Fallow deer. This has resulted in an interesting mixture of long established woodland patches, grazing land and arable land, with various areas going over to shrubs. Of concern parts of the southern block are now densely covered in 'weeds' that are not palatable to livestock, in particular fleabane (Pulicaria dysenterica).
Aphids provide an excellent indicator of biodiversity, partly because there are a huge number of species present in Britain - over 600 at the last count. Many aphid species have a very restricted host range, and to some extent aphid species richness will mirror plant species richness. Some individual plant species such as oak host a large number of aphid species. The presence of some of those species will often depend on how old the woodland is, and whether other organisms are present - in particular the ant species that tend aphids for their honeydew.
To maximise the information gained on associations with other organisms aphid samples were obtained hand-searching. We found a total of 24 aphid species. Below we briefly describe the woodland species, followed by the shrub and herb fauna.
English oak (Quercus robur) has over 30 aphid species. A special effort was made to check very old oak trees at Knepp in the hope of finding some of the real rarities of the aphid world such as Stomaphis quercus. Sadly it was probably too early in the year to stand much chance of finding that species, but we did find two of the commoner ones.
Sweet chestnut aphids are found on both English oak (Quercus robur) and sweet chestnut (Castanea sativa). All adults are winged. Winged adults are yellow and distinctively marked with a dark median strip on the head and thorax, and paired black spinal and marginal patches on the dorsal abdomen.
We only found this species once at Knepp on Quercus on 25/5/14, but it is probably widespread.
This species was common at Knepp on several Quercus trees on 24/5/14.
None of the ant-attended oak aphids was found, possibly reflecting a shortage of suitable ant species such as Lasius fuliginosus.
The wingless aphids are pale yellowish green to pale yellow to almost white, although in late summer to autumn they may have patches of darker pigment. The siphunculi are smooth and short. They are mainly found on the undersides of leaves of downy birch (Betula pubescens), but they also occur on silver birch (Betula pendula) and occasionally on grey alder (Alnus incana).
A few colonies were found at Knepp on the underside of Betula pubescens leaves on 25/5/14.
Myzocallis carpini feeds on the undersides of leaves of hornbeam. Winged adults are pale yellow to yellowish white, with no dorsal abdominal markings. The antennae are ringed with black, and the forewing has a black spot at the base of the pterostigma.
The species is generally rather infrequent, but can become abundant when hornbeam is used for hedging.
A few colonies were found at Knepp on Carpinus betulus leaves on 25/5/14.
The aphids develop within galls (shown below) on elm. The galls are stalked, approximately bean-shaped, smooth and shiny, and coloured reddish-green and/or yellow.
The aphid that creates the gall is light green with the head, thorax, antennae and legs dark and no wax glands. The offspring have a shiny black head, thorax, antennae and legs, greyish black abdominal segments and wax glands.
A few galls were recorded at Knepp on 23 & 24/5/14 on regrowing elm trees along field boundaries.
The elm-currant aphid host alternates from the primary host elm (Ulmus spp.) to the secondary host currant (Ribes). The aphids that hatch from the overwintering eggs develops in yellowish or whitish green galls on elm. These galls are formed by downward curling, twisting and blistering of one edge of a leaf. Along with their wingless offspring they are dark green and wax-covered. Their offspring are brownish or dull green and develop to adult winged aphids which are dark green to bluish grey with dark cross bands on the abdomen.
Unlike Tetraneura ulmi, the gall is not sealed and predators can enter. The image below shows a larva of Syrphus ribesii inside the gall.
This was recorded once at Knepp on elm in woodland beside campsite on 25/5/14.
This species is found almost exclusively on common alder (Alnus glutinosa). Both the winged and wingless adults are yellowish white to yellowish green.
This was recorded once at Knepp on alder on 24/5/14.
The pale sallow leaf aphid lives mostly on sallows (broad-leaved Salix spp.) especially great sallow (Salix caprea) and grey sallow (Salix cinerea). The aphids are white to yellowish-white. There are no distinct dark dorsal abdominal markings. These aphids are not attended by ants.
A few were found at Knepp scattered on the undersides of sallow leaves on 24/5/14
The common periphyllus aphid feeds on wide range of Acer species. They are dirty-dark-green to dark-brown aphids with a clear pattern of dark abdominal sclerites. Winged forms have dark dorsal abdominal cross-bands and marginal sclerites, which are darker than the light-brown pterostigma of the wing.
This species was common and widespread at Knepp from 23-25/5/14 on field maple (Acer campestre).
The large hazel aphid spends its entire life cycle on hazel and is not attended by ants. It feeds on the fast growing shoots and is generally rather uncommon. Aphids are yellowish-green often mottled with red spots. Their siphunculi are long, thin and tapering.
A small colony was found at Knepp on the young shoots of hazel (Corylus avellanae) on 25/5/14.
Hazel aphids live on the undersides of hazel leaves, and may become abundant when its host is used for hedging. The winged adult viviparae (known as alates) are pale yellow to yellowish white, 1.3-2.2 mm long, and lack dorsal abdominal markings. Their antennae are ringed with black, and the forewing has a black spot at the base of the pterostigma.
One nymph was found on a hazel leaf at Knepp on 25/5/14.
The large pine aphid feeds on Scots Pine (Pinus sylvestris). Adults are shiny orange-brown early in the year and grey or dark brown later on. The body is finely spotted with black and dusted with wax. The siphuncular cones are small to medium sized and reddish-brown or dark brown. The large pine aphid is unusually large for an aphid at up to 5 mm in length. They are usually attended by ants.
Several colonies of the large pine aphid were found on Scots Pine at Knepp on 25/5/14.
Some pine trees had the adelgid Pineus pini on them (not photographed).
The black cherry aphid alternates between cherry (Prunus cerasus, Prunus avium) as the primary host and bedstraws (Galium), eyebrights (Euphrasia) and speedwell (Veronica as secondary hosts. The wingless adults on cherry are shiny, very dark brown to black with a sclerotized dorsum. Their siphunculi are cylindrical and black with the distal part slightly curved outward.
Colonies were found at Knepp around the camp site and elsewhere on 23/5/14 on cherry. They were attended by ants (Lasius sp.).
The leaf-curling plum aphid alternates between various plum (Prunus) species (especially domestic plum and blackthorn) and a wide range of Asteraceae such as asters, chrysanthemums, yarrow and groundsel. On the primary host it lives in a gall of curled leaves. These aphids are variable in colour ranging from yellow to green to brown. Their siphunculi are pale, tapered and short.
This was widespread at Knepp on blackthorn (Prunus spinosa) from 23-25/5/14.
The dock aphid feeds only on dock (Rumex species). It is probably fairly common in Britain, but many older localities records are unreliable because the species has often been confused with Aphis fabae. Unlike that species, the immatures of Aphis rumicis do not have the white pleural wax spots typical of Aphis fabae. Also unlike Aphis fabae, Aphis rumicis rolls and crumples the leaves of its host before later in the year moving up stems and into the inflorescences. Aphis rumicis is usually attended by ants.
The dock aphid was widespread at Knepp on dock (Rumex) from 23-25/5/14 in pasture.
There are two stinging nettle aphids and both feed exclusively on stinging nettle.
These are large spindle-shaped aphids, the adults of which may be winged or wingless. The antennae are much longer than the body length and the siphunculi are long, tapering with a large flange. They are not ant attended.
The common nettle aphid was common and widespread at Knepp on Urtica (nettle) from 23-25/5/14 in pasture.
These are medium sized aphids which are usually attended by ants. Early generations are dark bluish-green with yellowish cauda and siphunculi. Later generations are yellow and much smaller. Two of the aphids in the second image, below, belong to the smaller yellow form.
The dark green nettle aphid was found in one site at Knepp on 25/5/14. They were attended by ants (Lasius sp.).
The primary hosts of the plum-thistle aphid are various Prunus species, mainly cherry, plum and apricot. Aphids migrate to various wild and cultivated daisies especially thistle (Carduus and Cirsium spp.) and ragwort (Senecio). The wingless adults are brownish-yellow, pale green or brown, with a large black spot situated dorsally on the abdomen. The siphunculi are black, thick and cylindrical. Immatures often have reddish patches on a greenish background. Brachycaudus cardui are usually ant attended.
This was widespread at Knepp on Senecio from 23-25/5/14 in pasture.
The interaction between ragwort, aphids, ants and grazers is a complex and dynamic balance. Ragwort produces toxins to repel herbivores - including grazers such as cattle. However plants infested with aphids - and most particularly with their attendant ants - also deter herbivores. In addition Brachycaudus cardui prefer ragwort with comparatively low amounts of toxin. Producing toxins, or supporting aphids, are metabolic costs to the ragwort plants. The end-result of which is, in the presence of grazers, aphids, and ants, relatively non-toxic ragwort strains tend to be at an advantage.
The rose aphid may alternate from the primary host rose to the secondary hosts, teasels and valerians - or it may remain on rose all year round. The wingless or winged adults are green (see the first image, below) or red (the second and third images, below). The siphunculi are black and bent outwards.
This was widespread at Knepp on Rosa caninum from 23-25/5/14 in scrub.
The melon aphid varies in size and colour depending on conditions. In cool favourable conditions they are medium sized and blackish green or green mottled with dark green. In hotter conditions they are a very pale whitish yellow. It has dark siphunculi and a dusky cauda.
Note: we originally misidentified this aphid as Aphis chloris, a dark green aphid found on Hypericum perforatum which feeds at the base of the plant.
Several colonies were found at Knepp on Hypericum calycinum on 23/5/14. They were attended by ants (Lasius sp.).
The black bean aphid host alternates between spindle (Euonymus europaeus) and several secondary hosts including broad beans (Vicia faba), poppies (Papaver spp.) and Chenopodium spp.
A colony was found at Knepp on the camp site galling the leaf of Chenopodium album. This colony is somewhat aberrant in that the leaf is folded round as a gall (shown in the first image, above) and the immatures do not have white pleural wax spots (shown in the second image, above).
The hawthorn-mint aphid alternates from hawthorn and apple (Rosaceae) to mint (Mentha). The wingless adults are yellow-green to apple-green, sometimes mottled with darker green markings. Their antennae are curved and somewhat longer than the length of the body.
Aphids were found at Knepp on mint in the kitchen tent. The mint was apparently picked locally in the garden.
The mealy cabbage aphid spends its entire life cycle on cabbage (Brassica sp.). The adults are green and wax-powdered. Their siphunculi are thick and very short.
Several colonies were found at Knepp on Brassica in the garden. They were parasitized (see the second image, above) by the specialized Braconid wasp, Diaretiella rapae.
Due to the number of old hedgerows, there were a good diversity of hedgerow shrubs and trees - and a corresponding diversity of aphids feeding upon those species. This is important because modern farming practices (such as herbicide use and hard mechanical trimming) tend to impoverish hedgerows.
Given the tree species present, which included oak, birch and elm, we would have expected more aphid species than we found. This may be, in-part, because this survey was performed in May. Many species such as Lachnus roboris on oak build up slowly in numbers through the year, and the very low numbers present in May are unlikely to be detected by hand searching. But a more important factor constraining aphid biodiversity is probably the lack of mutualist ant species, especially wood ants (Formica spp.). Much of the area was too wet for wood ants, but we would expect to find jet black ants (Lasius fuliginosus). We did not find any aphid species being attended by this ant, although these ants have been found on the Knepp Estate (nesting in an old tree) by the Knepp Castle Estate baseline ecological survey.
We did find a few nest mounds of Myrmica ants (see below), but we could not find any aphids being tended by them in the vicinity.
The number of aphid species on herbs was very low - partly because in May host-alternating aphids would still have been on their primary (woody) hosts - but more importantly because of the comparatively low herb diversity. This is presumably due to prolonged (conventional) cattle grazing - and associated farming practices such as fertilizers and herbicides. One way to remedy this would be to exclude cattle from some areas, or to graze the areas briefly but intensively. An additional factor is the excessive number of rabbits, possibly due to predator control to promote game-bird populations.
No aphids were found on fleabane (Pulicaria) although a number of aphid species do feed on it. In particular Ovatus inulae, alternates from hawthorn and apple (Rosaceae) to fleabane. Whether this aphid has any control potential for fleabane is unknown.
We wish to thank everyone at Knepp Wildland Safaris for assistance during this survey, and inviting the Sussex biological recorders to their Safari Camp opening weekend. We also thank Penny Green, BRC Manager, Sussex Biological Records Centre for helping arrange this recording event.
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).