Ant mutualism Some species of
ants farm aphids, protecting them on the plants where they are feeding, and consuming the honeydew the aphids release from the
terminations of their
alimentary canals. This is a
mutualistic relationship, with these dairying ants milking the aphids by stroking them with their
antennae. Although mutualistic, the feeding behaviour of aphids is altered by ant attendance. Aphids attended by ants tend to increase the production of honeydew in smaller drops with a greater concentration of amino acids. Some farming ant species gather and store the aphid eggs in their nests over the winter. In the spring, the ants carry the newly hatched aphids back to the plants. Some species of dairying ants (such as the European
yellow meadow ant,
Lasius flavus) manage large herds of aphids that feed on roots of plants in the ant colony. Queens leaving to start a new colony take an aphid egg to found a new herd of underground aphids in the new colony. These farming ants protect the aphids by fighting off aphid predators. Another
ant-mimicking gall aphid,
Paracletus cimiciformis (Eriosomatinae), has evolved a complex double strategy involving two morphs of the same clone and
Tetramorium ants. Aphids of the round morph cause the ants to farm them, as with many other aphids. The flat morph aphids are
aggressive mimics with a "
wolf in sheep's clothing" strategy: they have hydrocarbons in their cuticle that mimic those of the ants, and the ants carry them into the brood chamber of the ants' nest and raise them like ant larvae. Once there, the flat morph aphids behave like predators, drinking the body fluids of ant larvae. File:Ant guards its Aphids.jpg|An ant guards its aphids File:20220726 Aphis farinosa 04.jpg|Ants tending aphids File:ant feeding on honeydew.JPG|Ant extracting honeydew from an aphid File:Aphids and ants.webm|thumbtime=35|
Ants tending aphids and collecting
honeydew secreted. A
wrinkled solder beetle flies in and eats an aphid before being chased away by the ants.
Bacterial endosymbiosis Endosymbiosis with
micro-organisms is common in insects, with more than 10% of insect species relying on intracellular bacteria for their development and survival. Aphids harbour a vertically transmitted (from parent to its offspring) obligate
symbiosis with
Buchnera aphidicola, the primary symbiont, inside specialized cells, the
bacteriocytes. Five of the bacteria genes have been transferred to the aphid nucleus. The original association is estimated to have occurred in a common ancestor and enabled aphids to exploit a new
ecological niche, feeding on phloem-sap of vascular plants.
B. aphidicola provides its host with essential amino acids, which are present in low concentrations in plant sap. The metabolites from endosymbionts are also excreted in honeydew. The stable intracellular conditions, as well as the bottleneck effect experienced during the transmission of a few bacteria from the mother to each nymph, increase the probability of transmission of mutations and gene deletions. As a result, the size of the
B. aphidicola genome is greatly reduced, compared to its putative ancestor. Despite the apparent loss of
transcription factors in the reduced genome,
gene expression is highly regulated, as shown by the ten-fold variation in expression levels between different genes under normal conditions.
Buchnera aphidicola gene transcription, although not well understood, is thought to be regulated by a small number of global transcriptional regulators and/or through nutrient supplies from the aphid host. Some aphid colonies also harbour secondary or facultative (optional extra) bacterial symbionts. These are vertically transmitted, and sometimes also horizontally (from one lineage to another and possibly from one species to another). So far, the role of only some of the secondary symbionts has been described;
Regiella insecticola plays a role in defining the host-plant range,
Hamiltonella defensa provides resistance to parasitoids but only when it is in turn infected by the
bacteriophage APSE, and
Serratia symbiotica prevents the deleterious effects of heat.
Predators and her eggs near a
milkweed aphid colony. Aphids are eaten by many bird and insect predators. In a study on a farm in
North Carolina, six species of
passerine bird consumed nearly a million aphids per day between them, the top predators being the
American goldfinch, with aphids forming 83% of its diet, and the
vesper sparrow. Insects that attack aphids include the adults and larvae of predatory ladybirds,
hoverfly larvae, parasitic
wasps,
aphid midge larvae, "aphid lions" (the larvae of
green lacewings), and arachnids such as
spiders. Among ladybirds,
Myzia oblongoguttata is a dietary specialist which only feeds on conifer aphids, whereas
Adalia bipunctata and
Coccinella septempunctata are generalists, feeding on large numbers of species. The eggs are laid in batches, each female laying several hundred. Female hoverflies lay several thousand eggs. The adults feed on pollen and nectar but the larvae feed voraciously on aphids;
Eupeodes corollae adjusts the number of eggs laid to the size of the aphid colony. Aphids are often infected by
bacteria,
viruses, and fungi. They are affected by the weather, such as
precipitation, and
wind. Fungi that attack aphids include
Neozygites fresenii,
Entomophthora,
Beauveria bassiana,
Metarhizium anisopliae, and entomopathogenic fungi such as
Lecanicillium lecanii. Aphids brush against the microscopic spores. These stick to the aphid, germinate, and penetrate the aphid's skin. The fungus grows in the aphid's
hemolymph. After about three days, the aphid dies and the fungus releases more spores into the air. Infected aphids are covered with a woolly mass that progressively grows thicker until the aphid is obscured. Often, the visible fungus is not the one that killed the aphid, but a secondary infection. Excessive heat kills the symbiotic bacteria that some aphids depend on, which makes the aphids infertile. Rain prevents winged aphids from dispersing, and knocks aphids off plants and thus kills them from the impact or by starvation, but cannot be relied on for aphid control.
Anti-predator defences on
narrow-leaf milkweed is attacked by a
hoverfly larvae. It thrashs and release
pheromones and sticky wax from its
cornicles. Nearby aphids flee. Video played at 4X speed. Most aphids have little protection from predators. Some species interact with plant tissues forming a
gall, an abnormal swelling of plant tissue. Aphids can live inside the gall, which provides protection from predators and the elements. A number of galling aphid species are known to produce specialised "soldier" forms, sterile nymphs with defensive features which defend the gall from invasion. For example, Alexander's horned aphids are a type of soldier aphid that has a hard
exoskeleton and
pincer-like mouthparts. Although aphids cannot fly for most of their life cycle, they can escape predators and accidental ingestion by herbivores by dropping off the plant onto the ground. Others species use the soil as a permanent protection, feeding on the vascular systems of roots and remaining underground all their lives. They are often attended by ants, for the honeydew they produce and are carried from plant to plant by the ants through their tunnels. Peptides produced by aphids,
Thaumatins, are thought to provide them with resistance to some fungi. It was common at one time to suggest that the cornicles were the source of the honeydew, and this was even included in the
Shorter Oxford English Dictionary and the 2008 edition of the
World Book Encyclopedia. In fact, honeydew secretions are produced from the anus of the aphid, while cornicles mostly produce defensive chemicals such as waxes. There also is evidence of cornicle wax
attracting aphid predators in some cases. Some clones of
Aphis craccivora are sufficiently toxic to the invasive and dominant predatory ladybird
Harmonia axyridis to suppress it locally, favouring other ladybird species; the toxicity is in this case narrowly specific to the dominant predator species.
Parasitoids pushes the sticky wax drop from its
cornicle against an attacking
parasitic wasp, extruding another drop. Two scenes at one-tenth speed. A different aphid has captured a wasp. Aphids are abundant and widespread, and serve as hosts to a large number of
parasitoids, many of them being very small (c. long)
parasitoid wasps. One species,
Aphis ruborum, for example, is host to at least 12 species of parasitoid wasps. Parasitoids have been investigated intensively as biological control agents, and many are used commercially for this purpose.
S. berthaultii and other wild potato species have a further anti-aphid defence in the form of glandular hairs which, when broken by aphids, discharge a sticky liquid that can immobilise some 30% of the aphids infesting a plant. Plants exhibiting aphid damage can have a variety of symptoms, such as decreased growth rates, mottled leaves, yellowing, stunted growth, curled leaves, browning, wilting, low yields, and death. The removal of sap creates a lack of vigor in the plant, and aphid saliva is toxic to plants. Aphids frequently transmit
plant viruses to their hosts, such as to
potatoes,
cereals,
sugarbeets, and
citrus plants. The green peach aphid,
Myzus persicae, is a vector for more than 110 plant viruses. Cotton aphids (
Aphis gossypii) often infect
sugarcane,
papaya and
peanuts with viruses. The coating of plants with honeydew can contribute to the spread of fungi which can damage plants. Honeydew produced by aphids has been observed to reduce the effectiveness of fungicides as well. A hypothesis that insect feeding may improve plant fitness was floated in the mid-1970s by Owen and Wiegert. It was felt that the excess honeydew would nourish soil micro-organisms, including nitrogen fixers. In a nitrogen-poor environment, this could provide an advantage to an infested plant over an uninfested plant. However, this does not appear to be supported by observational evidence. ==Sociality==