greenfly n : greenish aphid; pest on garden and crop plants

English

Noun

greenfly (plural greenflies)

1. A green aphid that is a common pest of garden plants, houseplants, and crops.

"Aphid" is also the NATO reporting name for the Soviet/Russian Molniya R-60 air-to-air missile.

Aphids, also known as plant lice, are small plant-feeding insects, members of the superfamily Aphidoidea. About 4,000 species of aphids are known, presently classified in 10 families, though historically there were many fewer, with most species included in the family Aphididae. Around 250 species are serious pests for agriculture and forestry as well as an annoyance for gardeners. They vary in size from 1-10 mm long.

Important natural enemies include the predatory lady beetles (Coleoptera: Coccinellidae), hoverfly larvae (Diptera: Syrphidae), and lacewings (Neuroptera: Chrysopidae), and entomopathogenic fungi like Lecanicillium lecanii and the Entomophthorales.

Aphids are distributed world-wide, but are most common in temperate zones. It is possible for aphids to migrate great distances (mainly through passive dispersal riding on winds) depending on the weather patterns; for example, the lettuce aphid spread from New Zealand to Tasmania. They have also been spread by human transportation of infested plant materials.

Taxonomy

Aphids are in the superfamily Aphidoidea in the homopterous division of the order Hemiptera. Recent classification within the Hemiptera has reduced the old taxon "Homoptera" to two suborders: Sternorrhyncha (aphids, whiteflies, scales, psyllids...) and Auchenorrhyncha (cicadas, leafhoppers, treehoppers, planthoppers...) with the suborder Heteroptera containing a large group of insects known as the "true bugs". More recent reclassifications have resulted in a substantial rearrangement of the constituent families with the Aphidoidea, with some old families reduced to subfamily rank (e.g., Eriosomatidae), and numerous old subfamilies elevated to family rank.

Anatomy

Aphids contain sucking mouthparts called stylets. They have soft bodies; long, thin legs; two-jointed, two-clawed tarsi; and usually a pair of cornicles, abdominal tubes through which droplets of defensive fluid are exuded. Aphids have two compound eyes and two ocular tubercles made up of three lenses, each of which is located behind and above the compound eyes. When host plant quality becomes poor or is crowded, female aphids will produce winged offspring that can disperse to other food sources.

Diet

Many, but far from all, aphids are monophagous (i.e. feeding only on 1 species of plant). Others, like Myzus persicae feed on hundreds of plant species across many families.

Similarly to related families, aphids passively feed on sap of phloem vessels in plants. Once a phloem vessel is punctured, the sap which is stored under high pressure is forced into the food canal. As they feed, aphids often transmit plant viruses to their food plants. These viruses can sometimes kill the plants.

Some species of ants "farm" aphids, protecting them on the plant they eat, and eating the honeydew that the aphids release from their anus; this is a mutualistic relationship. Aphid honeydew is rich in carbohydrates, of which the aphids ingest an excess, being phloem-feeders. Many aphids are host to endosymbiont bacteria, Buchnera, which live in specialized cells called bacteriocytes inside the aphid. These bacteria synthesize some essential amino acids that are absent in the phloem that the aphids eat.

Reproduction

Aphids are known for having unusual reproductive adaptations in some species. Many aphids undergo cyclical parthenogenesis. In the spring and summer, only females are present in the population. Reproduction is typically parthenogenetic and viviparous. Females undergo a modified meiosis that results in eggs that are genetically identical to their mother (parthenogenetic). The embryos develop within the mothers ovarioles, and give live birth to 1st instar nymphs (viviparous). Aphids typically live from 20-40 days and thus undergo multiple parthenogenetic, viviparous generations each summer. In the fall, a change in photoperiod and temperature cause females to parthenogenetically produce sexual females and males. The males are genetically identical to their mothers except they have lost one sex chromosome. Sexual females and males mate and females lay eggs that will develop outside of the mother. Thus in the fall aphids undergo sexual, oviparous reproduction. The aphids will overwinter as eggs and hatch out as females in the following spring.

Aphids have been known to have what is called telescoping generations. The parthenogenetic, viviparous female aphid will have a daughter within her, who is already parthenogenetically producing her own daughter at the same time. This leads to the situation where the diet of a female aphid can have inter-generational effects on the body size and birth rate of aphids. In other words, what the aphid eats can directly change the size and fertility of the aphid's daughters and granddaughters (Nevo and Coll 2001, Jahn et al. 2005).

The following is the life cycle of the rose aphid (Aphis rosae), and may be regarded as typical of the family, though exceptions occur in other species: Eggs produced in the autumn by fertilized females remain on the plant through the winter and hatching in the spring give rise to female individuals which may be winged or wingless. From these, females are born parthenogenetically: that is to say, without the intervention of males, and by a process that has been compared to internal budding, large numbers of young resembling their parents in every respect except size are produced, which themselves reproduce their kind in the same way. This process continues throughout the summer, generation after generation being produced until the number of descendants from a single individual of the spring-hatched brood may amount to many thousands. In the autumn winged males appear; union between the sexes takes place and the females lay the fertilized eggs which are destined to carry the species through the cold months of winter. If, however, the food-plant is grown in a glasshouse or greenhouse where protection against cold is afforded, the aphids may go on reproducing agamogenetically (asexually) without cessation for many years.

The young may be born by the oviparous or viviparous methods and either gamogenetically or agamogenetically, and may develop into winged forms or remain wingless, and males only appear in any number at the close of the season. Although the factors which determine these phenomena are not clearly understood, it is believed that the appearance of the males is connected with the increasing cold of autumn and the growing scarcity of food, and that the birth of winged females is similarly associated with decrease in the quantity or vitiation of the quality of the nourishment imbibed. Sometimes the winged females migrate from the plant they were born on to start fresh colonies on others often of quite a different kind. Thus the apple aphid (Aphis mali), after producing many generations of apterous females on its typical food-plant, gives rise to winged forms which fly away and settle upon grass or corn-stalks.

Some species of cabbage aphids (like Brevicoryne brassicae) reproduce rapidly during the summer. They are all females, and can produce up to 41 generations of offspring. If none of these died, a female would have more than one and a half billion billion billion offspring (1.5 x 1027) by the end of the season.

Evolution

Aphids probably first appeared around 280 million years ago, in the early Permian period. They probably fed on plants like Cordaitales or Cycadophyta. The oldest known aphid fossil is one of the species Triassoaphis cubitus from the Triassic. There were relatively few species of aphids at that time, and the number of species only considerably increased since the appearance of angiosperms 160 millions of years ago. This is due to the fact that angiosperms provide an occasion for aphids to become specialized. Organs like the cornicles did not appear until the Cretaceous.

Gallery

References

• G. B. Buckton, British Aphides (Ray Soc. 1876-1883)
• Nevo, E., and M. Coll. 2001. Effect of nitrogen fertilization on Aphis gossypii (Homoptera: Aphididae): variation in size, color, and reproduction. J. Econ. Entomol. 94: 27-32.
• Jahn, GC, LP Almazan, and J Pacia. 2005. Effect of nitrogen fertilizer on the intrinsic rate of increase of the rusty plum aphid, Hysteroneura setariae (Thomas) (Homoptera: Aphididae) on rice (Oryza sativa L.). Environmental Entomology 34 (4): 938-943.http://docserver.esa.catchword.org/deliver/cw/pdf/esa/freepdfs/0046225x/v34n4s26.pdf

See also

• Specific case of Adelgidae - adelgids, affecting Douglas Fir and Cooley Spruce
• Aeroplankton
• Pineapple gall