Identification & Description:
Aphids are soft-bodied insects that use their piercing sucking mouthparts to feed on plant sap. They usually occur in colonies on the undersides of tender terminal growth. Heavily-infested leaves can wilt or turn yellow because of excessive sap removal. While the plant may look bad, aphid feeding generally will not seriously harm healthy, established trees and shrubs.

However, some plants are very sensitive to feeding by certain aphid species. Saliva injected into plants by these aphids may cause leaves to pucker or to become severely distorted, even if only a few aphids are present. Also, aphid feeding on flower buds and fruit can cause malformed flowers or fruit.

There are about 4 000 species of aphids in the world of which about 250 are serious pests. Scientists believe that the number of species remained relatively small until the angiosperms (flowering plants) became more common, then as the aphids adapted to these new and rapidly speciating plants so their species numbers increased also. Twenty-five percent of all plant species are infested with aphids, and though it is believed that the speciation of aphids has followed that of plants not all groups of plants are equally parasitised. The Compositae, the 3rd most specious plant family, supports the most aphid species (605 species), but the Orchidacea, the 2nd most specious plant family, only support 9 species of aphids while the Rosaceae which is only the 22nd most specious plant family supports the 3rd highest number of aphid species (293 species). The plant family which supports the 2nd highest number of aphids is Coniferae (363 species) but these are non-flowering plants.

Aphids have a world-wide distribution but there are far more species in temperate zones, we have more than 500 species in the UK while there are about 1350 species in North America. Most aphids are monophagous, though some feed on more than one species of plant, species like Myzus persicae which feed on a number of different plant species are very rare.

Aphids are part of the Superfamily Aphidoidae and are described as small (1-10 mm) soft bodied insects with or, more commonly without, wings. They possess a proboscis which originates between and behind the forelegs, their antennae has two thick basal segments and a flagellum composed of up to 4 segments, the last of which is divided into a proximal part and a thinner distal part called a ‘process terminalis’. They possess two compound eyes and two ocular tubercles made up of three lenses each which are situated behind and above the compound eyes. They have two tarsal segments, the wings when present have only one prominent longitudinal vein. The fifth abdominal segment bears a pair of upward and backwardly pointing tubes on the dorsal surface called siphunculi (cornicles) and a cauda is usually present below and between them on the last abdominal segment.

Aphids produce large amounts of a sugary liquid waste called “honeydew”. The honeydew that drops from these insects can spot the windows and finish of cars parked under infested trees. A fungus called sooty mold can grow on honeydew deposits that accumulate on leaves and branches, turning them black. The appearance of sooty mold on plants may be the first time that an aphid infestation is noticed. The drops can attract other insects such as ants, that will feed on the sticky deposits. sooty mold
Univ. of Kentucky Entomology

Some aphids are very important vectors of plant viruses. However, it is seldom possible to control these diseases by attempting to kill the aphid vectors with an insecticide. Aphids carrying viruses on their mouthparts may have to probe for only a few seconds or minutes before the plant is infected. Resistant varieties or sequential plantings may be helpful in reducing problems with some viruses that attack annual plants.

Life Cycles and Habits
Aphids are often parthenogenetic for part or all of their lives and both viviparous and oviparous at different times of year. Parthenogenetic (asexual) reproduction generally results in viviparity (live birth) and all aphids are both parthenogenetic and viviparous during Spring and Summer. Some aphids which are parthenogentic during the Summer produce sexual offspring (males and females) in Autumn this is termed ‘cyclical parthenogenesis’ or a ‘holocyclical’ life cycle. The sexual males and females mate in the Autumn. Sexual females, like asexual ones have 2 sex chromosomes i.e. XX. Males have only one sex chromosome i.e. OX. In theory this means males could produce sperm with either no sex chromosomes i.e. a O, or one sex chromosome i.e. an X. However, in reality sperm with an O sex chromosome degenerate very rapidly and never contribute to an embryo. This means that all offspring of a sexual mating must have XX as their sex chromosomes, because females always contribute an X chromosome, and therefore all aphids resulting from sexual matings are female. Eggs are laid during the Autumn as the overwintering stage in many temperate forms, and as explained above, give rise to females whether they are the result of sexual mating or not. In other species a special overwintering form develops in the Autumn called a ‘hiemalis’, while in some species the adults are the overwintering stage. Ova within a viviparously reproducing female start to develop immediately after ovulation, this occurs long before birth (even human females are born with all the ova they will ever need throughout their life, though they remain undeveloped for many years.) This means that an embryo can exist inside another larger and more mature embryo. In fact a newly born Summer aphid can contain within herself not only the developing embryos of her daughters but also those of her grand-daughters which are developing within her daughters. Parthenogenesis combined with this ‘telescoping of generations’ give aphids an exceedingly rapid turn-over of generations meaning they can build up immense populations very quickly.

Though aphids belong to the group of insects with simple life cycles, many species have very complex life histories. They can be conveniently grouped into three types: 1) those with single hosts and asexual reproduction, 2) those with single hosts and asexual alternating with sexual reproduction, and 3) those with alternating hosts and alternating asexual and sexual reproduction.

Aphids with simple life histories live on one plant species or a group of related plants. The females give live birth (ovoviviparous) to tiny female nymphs that start sucking sap immediately. After shedding their skin several times, the nymphs reach adulthood and soon produce new aphids. A complete cycle may take only 10-14 days. As the colonies grow, winged forms may be produced that seek out additional host plants. Some of these species may lay eggs, asexually, in the fall, which overwinter.

The next group again lives on one plant or group of plants, but alternates from the spring asexual form of reproduction to a fall sexual cycle. Generally, these aphids overwinter in the egg stage. The egg hatches in the spring into a female called the “stem mother.” This female gives live birth to female aphids. Asexual reproduction continues with wingless or winged forms being produced. In late summer or fall, the asexual reproducing females produce sexuals, males and females. These sexual forms mate and lay eggs for overwintering.

The most complicated aphid life cycles include not only alternation from asexual to sexual reproduction but also a switch of host plants. The spring and summer asexual forms live on one host but fly to an alternate plant for the production of the sexual forms. The sexual forms, after mating, return to the spring host plant to lay eggs.

The important things to remember about aphids is that they are tremendous reproducers, and with their ability to fly, tend to constantly reinfest plants.

Taxonomy
Aphids are divided up into 10 families all of which have been extant for 50 to 70 million years. Two families, the Aphididae and the Drepanosiphidae make up 70% of the species.

Superfamily Aphidoidea
Pemphigidae
Anoeciidae
Hormaphididae
Mindaridae
Thelaxidae
Drepanosiphidae
Phloeomyzidae
Greenideidae
Aphididae
Lachnidae

Feeding Ecology
Aphids feed from the phloem of plants which they tap into with the stylets of their proboscis. They gain access to the phloem vessels from 3 main parts of the plant, stems, leaves, and roots. Their stylets, which are contained within the proboscis when the aphid is not feeding, are very thin and could suffer damage while being pushed into the plant or bend in an unwanted direction. Therefore aphids secrete a special liquid from the tips of their stylets which starts to harden as soon as it leaves the stylets forming a hard protective sheath around the stylets as they are slowly pushed into the plant in search of the phloem tubes. When the stylets reach a phloem tube the aphid injects saliva into the plant cell. It is suspected, but not known for sure, that this saliva helps prevent the plant cell from sealing the puncture (i.e. the aphids mouthparts) with special proteins. These special proteins are the plants normal defence mechanism. They are deposited on the wall of the cell around a puncture as a result of the drop in redox potential that occurs along the cell wall following puncture damage. Aphids insert their stylets slowly and it takes quite a bit of time to tap into a phloem tube, it can be anywhere from 25 minutes to 24 hours from starting to insert the stylets to actually getting something to eat.

Some aphids always feed by inserting their stylets through the stomata of the plant leaves they are feeding on,i.e. Ceratovacuna lanigera on Miscanthus spp. and Schizolachnus orientalis on pine needles. Other than this aphids penetrate to the phloem either intracellularly (pushing the stylets between cells), intercellularly (pushing the stylets through cells) or by a bit of both. Plant phloem saps are rich in sugars and poor in amino-acids or nitrogen. This results in aphids excreting large amounts of sugary liquid, called honeydew, because the amount of sap they have to drink in order to get enough nitrogen in their diet means they have far more sugar and liquid than they need. This honeydew can often be seen on the lower leaves of infested trees which it falls on giving them a sticky coating. This is then fed on by other insects like the Common wasp Vespula vulgaris and the Brown Hairstreak Butterfly Thecla betulae and a yeast-like fungus which makes the leaves look all black as if they were covered in soot.

Nearly all aphids contain endosymbionts in special groups of cells called bacteriocytes. The exceptions are some members of the Cerataphidini which have yeast cells in their haemocoel instead. In most aphids the main endosymbiont is Buchnera aphidicola a relative of Escherichia coli which is common in our guts. Buchnera aphidicola is believed to complement the aphids diet by synthesising vitamins, sterols and certain amino acids. Whatever their role they are important for the aphids growth and reproductive potential as they decrease as the aphid gets older and are absent from most soldiers and males. The relationship between Buchnera and the aphids it lives with is obligatory on both sides, i.e. the bacteria can’t live outside of the aphids and the aphids can’t reproduce successfully without it. Buchnera is transmitted maternally via the ovary i.e. new aphids get the symbiont from their mums when they are just an egg.

Most aphids are autoecious (living on one or a few species of closely related plants). About 10% are heteroecious spending Autumn, winter and spring on one plant species (its primary host) and Summer on a different unrelated plant (its secondary host). For example the Rosy Apple Aphid Dysaphis plantaginae has Apple Malus sp. as its primary host and Plantain Plantago lanceolata as its secondary or summer host. Some heteroecious aphids such as Myzus persicae the Peach or Potatoe Aphid and Aphis fabae the Black Bean Aphid have a wide range of secondary hosts, but this is relatively rare. Most heteroecious aphids have just one primary and one secondary host.

Though aphids are very successful at feeding on plants they do not always have things all their own way, and small spines, hairs and sticky latex secretions are among the more successful methods that plants have evolved to protect themselves against these parasites