Several aphid species are present in apple orchards. The green apple aphid, the Oat apple aphid or Apple-grass aphid and the Rosy apple aphid. In the private gardens mostly, rosy apple aphids have some importance. This aphid leads to heavy hormonal changes in the tips of the shoots with infestations. The leaves become curled and the shout becomes shorted. Later the leaves become yellow and die. This aphid is mostly affecting young shoots. The Rosy apple aphid can produce severe damage already in a small abundance. Whereas the green apple aphid and the apple grass aphid can occur in higher abundance without leading to economic damage. The apple grass aphid has to be rated to be of non-economic importance at all.

Rosy apple aphid

This is an old European species introduced into the United States about 1870. It became a major pest of apples near the end of the 19th century. To thrive, this species must have an abundance of its summer host plant, the narrow-leaved plantain. A remarkable parallelism exists between the introduction and spread of rosy apple aphid and the narrow leaved plantain. This aphid occurs throughout all fruit-growing areas worldwide.

Life stages
Egg: The egg is oval and slightly flattened on the side next to the bark. The length varies from 0.49 to 0.56 mm. When first laid, it is bright yellow and covered with a glutinous substance that hardens with age. The color gradually changes to greenish yellow and finally to a shiny jet black. The time required for this change in color varies under normal outdoor conditions from about nine days to more than two weeks.
Nymph: The individuals that hatch from the eggs are all viviparous wingless females. There are five instars. The last is the mature stem mother, which shortly after the fourth instar, begins to produce living young parthenogenetically. She produces an average of five to six young per day. The nymphs of the second generation, all of which are females, reach maturity in two to three weeks. The great majority of the nymphs begin to reproduce on apple, though a few may develop wings and migrate to the plantain.
The third generation is produced in June and early July. The majority of this generation develop wings and migrate to the narrow-leaved plantain. In some seasons, wingless females of the third generation produce a fourth generation on apple. In recent years, it has been observed in some areas that damaging populations of rosy apple aphid have persisted in orchards until midsummer or later. It is not known whether this change is due to the selection process, changes of habit or lack of need for an alternative host.
Adult: The adult varies considerably in color markings. The general color is rosy brown, with a pinkish cast due to a powdery covering. Some of the older adults are purple, while the younger adults are decidedly reddish pink.

Host range
Apple is the preferred host, but the aphid also feeds on pear and hawthorn. Cortland, Golden Delicious, Rhode Island Greening and Ida Red are all particularly susceptible apple varieties. The aphid is found in all fruit-growing areas of the United States and Canada.
Injury or Damage: Rosy apple aphid feeding often causes apple leaves to curl, starting at petal fall. These leaves may later turn bright red. Feeding on the leaves around fruit clusters often results in the bunching, stunting and malformation of the developing fruit. These abnormalities become worse as fruit develops and can eventually render the fruit unsalable.
Large aphid populations may produce large amounts of honeydew as waste from the sap on which they feed. Honeydew excreted onto fruit will serve as a growing ground for sooty mold fungus, which will affect the finish of the apple. Toxins in the aphids’ saliva also serve as a “stop drop,” preventing the fruits’ abscission (natural separation from the tree) at normal harvest.
Life History: The aphid passes the winter in the egg stage. Hatching occurs early in the spring, about a week to 10 days later than the apple grain aphid and at about the same time as the apple aphid. The eggs hatch when the buds start opening in the spring, over a period of two weeks. As soon as they hatch, the young seek out the opening buds of the apple; they seem to prefer the fruit buds. They feed on the outside of the leaf bud and fruit bud clusters until the leaves begin to unfold. Then they work their way down inside the clusters and begin sucking the sap from the stems and newly formed fruits.
Their feeding causes the leaves to curl, protecting the aphids from sprays and some enemies. The severe curling of the foliage caused by this species is probably the most characteristic feature of its work. A single stem mother located on the underside of a leaf near the midrib will cause the leaf to fold tightly. It takes only a few stem mothers to cause a severe curling of all leaves surrounding an opening flower bud, providing ideal protection to the rapidly developing aphids. The stem mothers reach maturity when apple trees are coming into bloom.
The mature stem mothers are very inactive. They settle down and feed and produce young at a rapid rate. When distributed, they quickly remove their beaks from plant tissues and seek out another spot in which to continue. The stem mothers mature about two weeks after hatching. The length of time depends largely on weather conditions. The production of young usually begins two or three days after the last molt and continues without interruption for more than a month.
The total production by a single female averages about 185. Normally, the period of reproduction extends from about early May to June. Usually, the maximum period of reproductive activity is around the last week of May and the first week of June, when the young fruits are beginning to set and start active growth. Rosy apple aphid is rarely found attacking the young and rapidly growing shoots. It restricts itself to the foliage, the flower stalks and the young fruits.
One of the characteristic features of this species is the congregation of the young about the mother. Each individual stem mother or group of mothers will have massed about it hundreds of young. The infested leaves may soon be covered-in some cases, by more than one layer of aphids. This habit of congregating soon kills the infested leaves and causes the forced migration of the aphids. The young move actively and hurriedly, seemingly anxious to locate a suitable feeding ground. They are frequently found during this period congregated on the forming fruits or attacking the new, succulent unfolding foliage.
The second generation required four to 40 days to reach maturity and produce young. The majority of the second generation is wingless females. The average total production of each individual in the second generation about 119. The habits and activities of the third generation do not differ from those of the second. The aphids congregate in immense numbers on the undersides of the foliage, causing severe curling. They also attack the setting and developing fruits, producing characteristic injuries.
The majority of this third generation acquires wings and migrates to narrow-leaved plantain, the summer host plant. A fourth generation may be produced. All of these produce wings and migrate to the narrow-leaved plantain. After the last molt, the winged adults are very tender and inactive. They remain secreted in the curled leaves for two or three days before venturing on their migratory flight. Just before flying, they become very active and nervous, running about or moving their wings up and down in anticipation of their flight.

Green apple aphid

The green apple aphid, Aphis pomi (DeGeer), is widely distributed in all apple growing areas. In the northern hemisphere it first appears in apple orchards in late May to early June. The insect sucks sap from the leaves on water sprouts and succulent terminal growth. Green apple aphids are usually found close to the major veins on the under surface of the leaf. The green apple aphid also attacks pear, hawthorn, quince, crab apple and spiraea.

Description of green apple aphid
The eggs of the green apple aphid are oval and shiny black. The nymphs and adults are dark or light green. The green apple aphid has a green head, uniformly green or yellowish green abdomen, black legs and long black cornicles – the “tail pipes” (at the rear end of the insect). Adult females are dark green to black and winged. During the summer these females, or alates, give birth to live young parthenogenically.

Biology of green apple aphid
The green apple aphid overwinters as an egg on suckers, at the base of buds and on leaf scars of terminal shoots. The eggs usually hatch when the buds burst and the first leaves are unfolding. The nymphs begin to feed immediately on the developing leaves. They are initially present on the terminal shoot and later move to older cluster leaves. Adult female aphids produce wingless females in two weeks, each female being capable of producing 50-100 live offspring. Young aphids develop in seven to ten days. Green apple aphid populations usually build on apple slowly in early spring (bloom, petal, fall), and more rapidly as average daily temperatures increase. Winged aphids, or alates, and wingless aphids are produced during the summer. Green apple aphids are most numerous during July and early August. There are more nymphs that alate adults in the initial stages of an infestation. The percentage of alates increases as the population grows. Green apple aphid colonies can double in a week if conditions are favorable. If the aphids do not have succulent new growth and they have to feed on older leaves, then the number of young produced can drop by up to 50%. If daily temperatures are 30-32°C and higher, females do not reproduce well. When temperatures are high for several days, aphids will die. Several generations of the green apple aphid develop during the summer, and winged forms disperse throughout the orchard. Sometimes the green apple aphid and the rosy apple aphid, Dysaphis plantaginea Pars., have colonies on the same leaf. The rosy apple aphid is pink or purple. In the fall, wingless males and females develop and mate, and the females lay eggs for the winter. The greatest numbers of eggs are 15-20cm from the tips of twigs. Eggs are seldom on the large scaffold limbs or on the trunks of apple trees.

Damage
The green apple aphid sucks sap from the leaf. Heavy infestations reduce vigor and growth of shoots. This is of particular concern in nurseries and young, non-bearing orchards. The insect can reduce bud size and internode length, and cause leaf curling. Aphid feeding may also stimulate lateral branch growth that can affect tree shape. Leaf curling and weakened terminals are susceptible to winter injury. Honeydew produced by the aphids may drip onto fruit allowing sooty fungi to grow. The fungus blemishes the fruit and lowers the market value. When infestations are heavy, the green apple aphid may feed on immature apples and cause russeting.

Propagation risk model
Aphid propagation is mainly influenced by temperature. As more hours are close to the optimum temperature (20-32°C) and there is a food source available as higher will be the propagation rate. Whereas eggs and young instars are susceptible against dry air. Low relative humidity’s can damage aphid populations too (below 30% r.h.). Beside of insecticides, aphids are controlled by predators and parasitic fungi. The parasitic fungi will be depending on moisture. Most of this species like Beauvaria bassiana are favored by free moisture.

We increase risk when the optimal temperature is between 20 and 32°C and relative humidity between 30 and 95% r. h. and decrease the risk when it is too wet (leaf wetness, rain, or relative humidity above 95%), too cold (temperatures lower than 20°C) or too hot (temperatures above 32°C), wet nights (leaf wetness in night hours).