Pea Aphid

In the autumn, female pea aphids lay fertilized eggs overwinter that hatch the following spring. The nymphs that hatch from these eggs are all females, which undergo four moults before reaching sexual maturity. They will then begin to reproduce by viviparous parthenogenesis, like most aphids. Each adult female gives birth to four to 12 female nymphs per day, around a hundred in her lifetime. These develop into mature females in about seven to ten days. The life span of an adult is about 30 days. Population densities are at their highest in early summer, then decrease through predation and parasitism. In autumn, the lengthening of the night triggers the production of a single generation of sexual individuals (males and oviparous females) by the same parthenogenetic parent females. Inseminated sexual females will lay overwintering eggs, from which new parthenogenetic females will emerge in early spring. When the colony begins to become overcrowded, some winged females are produced. These disperse to infest other plants, where they continue to reproduce asexually. When temperatures become colder and day lengths shorter, sexual winged females and males appear. These mate, the females lay diapausing eggs and the life cycle starts again. Pea aphids can complete their whole reproductive cycle without shifting host plant. Inbreeding is avoided by the recognition of close kin. …

Like other insects of the order Hemiptera, A.pisum utilizes an endosymbiotic bacterium to overcome the nutritional deficiencies of phloem sap. A. pisum feeds on phloem sap of host plants including Medicago sativa (alfalfa), Pisum sativa (pea), Trifolium pretense (red clover), and Vicia faba (broad bean). The phloem saps of these plants are nutritionally rich in carbohydrates but poor in terms of nitrogen. The ratio of essential amino acids to nonessential amino acids in these phloem saps ranges from 1:4-1:20. This ratio of essential to nonessential amino acids is severely disproportional compared to the 1:1 ratio present in animal tissues and necessary for survival. Animals, including A. pisum, can produce nonessential amino acids de novo but cannot synthesize nine essential amino acids that must be obtained through their diets: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. In addition to these nine essential amino acids, A. pisum is unable to synthesize arginine due to missing urea cycle genes. The endosymbiotic relationship with Buchnera allows A. pisum to overcome this lack of essential amino acids in the phloem sap When provided with nonessential amino acids, Buchnera converts nonessential amino acids into essential amino acids to be returned to A. pisum. …