I. Introduction: The rosy apple aphid (RAA) has been a major pest of apple trees in North America since the end of the 19th century. It is the most serious of the five aphid species attacking apple, causing leaf, fruit and systemic root damage. In severe outbreaks, up to fifty percent of the fruit have been injured.

II. Hosts: The primary (overwintering) host of the RAA is apple. During early summer, winged females move to the secondary (summer) host, narrowleaf plantain. This plant is an introduced weed pest.

III. Description: The egg, which is deposited on the bark of spurs and shoots in the fall, is oval and about 2/100 inch (0.4 mm) long. When first laid it is a bright yellow, but it gradually changes to greenish-yellow and finally becomes a shiny jet black. The time required for these color changes varies under normal outdoor conditions from about 9 days to more than 2 weeks. The immature aphids that hatch from the eggs in spring are all viviparous (giving live birth), wingless females and when mature are called "stem mothers". The body of the immature aphid changes color as it matures, from a dark green immediately after hatching to a more purplish or rosy tinge when full grown (Plate 38). The immature aphids possess long cornicles at the base of the abdomen and also have long antennae which extend almost half the length of the body. The antennae of young apple grain aphid stem mothers reach only about to the end of the thorax. The winged adults are black and also possess long cornicles and antennae.

The summer generations on plantain look distinctly different from the spring aphids on apple. They are a pale yellowish color, and occur singly or in low numbers, rather than in dense colonies. Plantain leaves are not curled by aphid feeding. Males are scarce and are only found in the fall. Oviparae, also present only in the fall, have a prominent central tubercle on the front of the head, separating this species from green aphids and apple grain aphid. There are six antennal segments, further separating rosy apple aphid from apple grain aphid.

IV. Biology: The eggs usually hatch when buds are at the silver tip stage in spring. The eggs do not hatch all at once but continue over a period of about 2 weeks. Egg hatch is complete by the half-inch green stage. The young, as soon as they hatch, seek out the opening buds of apple, seeming 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. One nymph feeding for 24 hours is sufficient to cause the leaf to be curled when it unfolds. The first stem mothers usually reach maturity when apple trees are coming into pink. They settle down and content themselves with feeding and producing young at a rapid rate. The production of young usually begins 2 or 3 days after the last molt and continues without interruption for over a month. A single female produces an average of about 185 young. Normally, the period of reproduction extends from about pink to June 20 or later. Usually the maximum period of productive activity is around the last week of May and the first week of June. This is usually the period when young fruits are beginning to set and to start active growth. Populations are usually greatest in the inner and upper parts of the canopy. There are approximately three generations produced on apple with the second generation occurring 2 to 3 weeks after petal fall and the third generation appearing by mid- to late June. With each succeeding generation of aphids, a larger percentage of alate (winged forms) are produced. By early to mid July all aphids have developed into alates and have dispersed from apple to summer hosts. About six generations occur on narrowleaf plantain (WSSA photo) during the summer before winged females fly back to apple in the fall. These females produce wingless oviparae, which mate with returning winged males from the summer hosts. The mated females deposit an average of 4-6 overwintering eggs, usually in late October and November.

V. Injury: RAA remove plant juices from the leaves, causing severe curling (Plate 39) and abscission, and twisting of growing shoots (Plate 40). They secrete large quantities of honeydew which provides a substrate for a black sooty fungus which can affect fruit finish. However, the most serious effect results from the translocation of saliva from the leaves to the fruit. This causes the apples to remain small and deformed and renders them unmarketable. Systemic effect of the toxic saliva include reduced growth of roots and other woody tissue. Research at VPI&SU has shown that this can have an important impact on young trees as they develop a mature bearing structure.

VI. Monitoring: Examine twigs in the dormant period for overwintering eggs, to aid in selecting timing of delayed dormant sprays. Research at the Penn State Fruit Research Laboratory in 1980 demonstrated that an application of a contact insecticide at the green tip to half-inch green stage provided optimum control of the RAA. This application timing was selected initially because no method exists for distinguishing the overwintering eggs of RAA from those of the other four species of aphid overwintering on apple as eggs, or for predicting aphid populations based on egg numbers. If this application is not made or the orchardist wishes to assess the effectiveness of this application, RAA can be sampled by selecting five to ten trees (preferably from the cultivars `Rome Beauty', `Yorking', `Golden Delicious' or `Stayman') in an orchard at the early pink to pink stage of apple development. The sampling should be done early enough to allow the application of an aphicide to the trees if the threshold is exceeded. Make a 3-minute examination of each tree and count the number of fruit spurs showing curled leaves with live aphids. Average the number of fruit spurs across all trees to calculate the number of spurs showing evidence of rosy apple aphid infestation.

In 1981, researchers at the Penn State Fruit Research Laboratory established a relationship between the density of the RAA and fruit damage. They found that the number of infested fruit spurs per tree in a 3-minute search at the early pink to pink stage of apple development was directly related to the number of damaged apples per tree at harvest. This relationship assumes that the aphids attack the blossom clusters at random and the aphid-infested cluster yields one injured apple. From this relationship, they proposed that if the grower finds an average of one infested cluster per tree at the pink stage then a chemical treatment is needed. If any live colonies of aphids are found at petal fall, an additional application of insecticide should be applied.

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