Aphids in fruit trees
Description and life history traits
The six most common aphid species infesting fruit trees in the western United States are the green peach aphid (Myzus persicae), mealy plum aphid (Hyalopterus pruni), black cherry aphid (Myzus cerasi), apple aphid (Aphis pomi), rosy apple aphid (Dysaphis plantaginea), and woolly apple aphid (Eriosoma lanigerum). The green peach aphid primarily feeds on peach and nectarine but can also attack a variety of vegetable crops in summer. The black cherry aphid can infest cherry trees grown in nurseries. The apple aphid and rosy apple aphid tend to feed on tender shoots of apple trees, which causes curling of the terminal shoot that provides a protective shelter for the aphids. Woolly apple aphids feed in the tree canopy on new growth and on roots below the ground, resulting in the formation of galls. All aphids have winged and wingless forms, with the latter typically being more abundant and more commonly observed.
All aphids are parthenogenic and can give birth to live young through asexual reproduction. This natural history trait allows aphid populations to increase very rapidly under favorable conditions, especially when natural enemies are sparse or absent. Multiple aphid generations are produced each year.
- Several species of aphids can be serious pests of fruit trees in Colorado. They are the green peach aphid, mealy plum aphid, black cherry aphid, apple aphid, rosy apple aphid, and woolly apple aphid. In addition to attacking fruit trees, many of these species can also feed on vegetable crops and shade trees.
- The specific feeding injury depends on the species of aphid. Feeding of the green peach aphid, black cherry aphid, apple aphid, and rosy apple aphid results in curling of leaves. Feeding of the woolly apple aphid produces galls on tree roots and twigs. All aphids excrete honeydew, which can lead to sooty mold growth on tree structures.
- Aphids are parthenogenetic and give live birth rather than lay eggs, which can lead to rapid population growth. Healthy, mature trees can tolerate aphid infestations, especially when the natural enemies complex is conserved by limiting the use of broad-spectrum insecticides. Aphid feeding on young trees can lead to stunted growth, deformation, and death of the tree.
Green peach aphids on the underside of a wild plum leaf. Aphids can feed on a wide variety of plants including shade trees and fruit trees. They are prone to outbreaks under favorable environmental conditions. Image credit: Eugene E. Nelson, Bugwood.org
Multiple life stages of apple aphid. Note the dark green appearance. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Apple aphids on a leaf. Note the yellow bodies and two dark cornicles at the end of the abdomen. These aphids can also be dark green. Image credit: Kansas Department of Agriculture , Bugwood.org
Black cherry aphid on sour cherry leaf. Note the dark color of adults. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Colony of mealy plum aphids. Note the white waxy covering. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Apple leaf infested with rosy apple aphid. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Woolly apple aphid. Note the white, cotton-like waxy substance that serves as a protective layer for the aphids against their natural enemies. Image credit: University of Georgia Plant Pathology, University of Georgia, Bugwood.org
Overwintering eggs of the apple aphid. Note the dark, shiny appearance. Image credit: Jim Baker, North Carolina State University, Bugwood.org
Plant with leaf curling and discoloration due to apple aphid infestation. Image credit: John A. Weidhass, Virginia Polytechnic Institute and State University, Bugwood.org
Example of sooty mold growing on leaves. Sooty mold can grow on surfaces containing honeydew, which is exuded by aphids and some other sucking insects. Image credit: Joseph OBrien, USDA Forest Service, Bugwood.org
Galls forming on red delicious twig due to feeding of the woolly apple aphid. Image credit: University of Georgia Plant Pathology , University of Georgia, Bugwood.org
Deformed fruit caused by feeding of the woolly apple aphid. Image credit: University of Georgia Plant Pathology, University of Georgia, Bugwood.org
Description and life history traits continued…
Green peach aphid
These aphids are usually pale green but can also be pink, with darker pigmentation observed during cooler weather. They have oval or pear-shaped bodies that are about 1.6 mm (1/16 inch) long and cornicles (i.e., tubes) that are swollen and club shaped. Winged individuals have a yellow green abdomen with a dark spot on the back. Nymphs are smaller than adults and have three dark lines marking the abdomen.
The green peach aphid overwinters at the base of buds as eggs that hatch when buds begin to open. Newly emerged nymphs begin feeding immediately. When population densities are high, these aphids produce winged forms that can disperse to weeds and grasses that serve as alternate hosts. They migrate back to fruit trees to lay eggs before the winter.
Mealy plum aphid
Wingless adults are pale green with three dark lines running down their back and bodies covered with a white mealy wax for protection. They are about 1.6 mm (1/16 inch) long, and winged forms have a dark thorax and bands on the abdomen. The life history of the mealy plum aphid is the same as the green peach aphid described above.
Black cherry aphid
At 3.2 mm (1/8 inch) long, the black cherry aphid is relatively large and has a shiny metallic black body with a round abdomen. The eggs, when present, are also shiny and black. The color of nymphs varies from shades of dark brown to black or amber. These aphids also have long dark cornicles.
This species overwinters as eggs on twigs and hatches near budbreak. Newly emerged nymphs feed on buds and the undersides of leaves. Winged forms are produced after several generations and migrate to alternate hosts which includes weeds, ornamental plants, vegetables, and plants in the mustard family. More generations are produced on the alternate hosts, and winged forms migrate back to fruit trees to mate and lay eggs for overwintering.
Apple aphids attack apple and pear trees. The oval shaped nymphs are yellow green or dark green and measure about 1.6 mm (1/16 inch). They also have black cornicles at the rear of the abdomen. Wingless adults of the apple aphid have oval-shaped, bright green bodies that measure 3.2 mm (1/8 inch) with black legs and black cornicles. The winged adults are similar in size to the wingless form, but have a black head and thorax with wings, yellow to green abdomen, and a narrow body. Eggs are oval shaped, long, black, shiny, and 0.5 mm (1/50 inch) in length.
Apple aphids overwinter as eggs laid in late summer and fall on smooth twigs and water sprouts of host trees. Nymphs develop through several instars, and all adults arising from overwintered eggs are females that reproduce asexually. When winged forms are produced, the apple aphid can colonize new plants. The last generation produced each growing season includes males and egg-laying females that mate and lay eggs that overwinter until the following season.
Rosy apple aphid
The rosy apple aphid also attacks apple trees and has a rose-colored body, which easily distinguishes this species from the apple aphid. Nymphs are 1.6 mm (1/16 inch) and have rosy or purple bodies covered in a white chalky substance with long rosy cornicles. At 3.2 mm (1/8 inch) long, adults are larger than nymphs but have similar appearance. The winged adults are also 3.2 mm (1/8 inch) long but are dark green to black and have elongated bodies with wings.
Rosy apple aphid populations tend to increase sooner than green apple aphids. This species overwinters as eggs on smooth twigs and water sprouts in bark crevices. After emerging from the egg, nymphs develop through several instars. All overwintered eggs give rise to females that reproduce asexually. Winged forms are produced later in the growing season and migrate from apple to plantain where they reproduce asexually. Winged females are eventually produced on plantain and migrate back to apple where egg-laying females are produced. Adults mate in early fall, and mated females lay overwintering eggs on apple.
Woolly apple aphid
Woolly apple aphids are different in appearance than the other two apple aphid species and only cause problems in apple orchards. The adults are about 1.6 mm (1/16 inch) long with red or purple bodies covered in a white, cotton-like waxy substance. At 1.3 mm (1/20 inch) long, the nymphs are smaller than adults but have similar coloration and a protective waxy covering.
Wooly apple aphid overwinters as nymphs within galls on limbs and roots of the host tree. Nymphs develop through several instars and are most active during the first instar when they migrate to a suitable feeding site and establish a new colony. Egg production of the woolly apple aphid is uncommon in western North America and is associated with the American elm, which is an alternate host of this species.
The exact type of feeding injury depends on the species of aphid. However, all aphids excrete honeydew which can accumulate on plant structures and result in the growth of sooty mold. This mold can interfere with leaf photosynthesis and lower fruit quality. Young trees are typically most susceptible to aphid infestations.
Feeding of the green peach aphid and mealy plum aphid leads to curling and discoloration of leaves. Leaves may drop as a result, and fruits can be deformed. Feeding of the black cherry aphid also causes leaves to curl and reduces the growth of terminal buds.
Feeding of apple aphid and rosy apple aphid causes curling and discoloration of leaves and less commonly shoots, which can reduce photosynthesis and root growth. In heavy infestations, apple aphids will also be found on fruits. Feeding on immature apples leads to deformation and stunted growth, while feeding on mature apples can cause russeting. Saliva produced by the rosy apple aphid can also cause stunting or distortion of fruits, especially when feeding occurs early in the season.
Feeding of the woolly apple aphid produces galls on roots that gradually increase in size as feeding continues. The galls interfere with root function and can stunt, weaken, or even kill young trees. Twig galls and yellowing of leaves are sometimes observed in heavy infestations. Furthermore, ruptured galls on stems serve as infection sites for Cryptosporiopsis perennans, which is a fungus that causes perennial canker.
Monitoring for aphids should be done shortly before budbreak to inform decisions related to management since aphids are more easily controlled when immature and exposed. The undersides of leaves can be inspected for new colonies, or limbs can be shaken over a tray to dislodge the insects.
Many natural enemy species attack aphids. Predators include lacewing larvae, lady beetle adults and larvae, syrphid fly larvae, predaceous midge larvae, and other predatory insects such as pirate bugs and damsel bugs. A small parasitic wasp, Aphelinus mali, is present in some regions and can attack woolly apple aphids and provide adequate control in orchards with low to moderate aphid populations. When assessing aphid densities in an orchard, it is recommended to also scout for the presence of predators. If present, the orchard can be re-sampled a week later to assess whether natural enemies are providing some control. When possible, it is advised that natural enemies be allowed to reduce aphid densities. Often, complete control is possible by natural enemies in established orchards since healthy, mature trees can tolerate some aphid feeding. The use of broad-spectrum insecticides should be avoided since they are not species-specific and may kill natural enemies, which can lead to an increase in the aphid population.
Aphids can be washed from plants by hand-wiping or with a strong stream of water. Removing alternate hosts is also recommended and is most practical in areas that have few alternate hosts since winged forms are capable of migrating.
Aphid populations are more likely to build when there is new growth in an orchard. Water sprout and lush shoot growth should be removed with proper pruning and fertilization practices. Fruit tree varieties that produce lush growth may be more susceptible to heavy aphid infestations. For the woolly apple aphid, planting varieties with resistant roots is recommended when replanting or when establishing new orchards. Proper plant care is strongly encouraged since established trees can tolerate aphid infestations without any significant loss in vigor or productivity.
When aphid populations are high the previous season and overwintered eggs are present on young limbs and twigs, chemical management may be necessary. For aphids other than the woolly apple aphid, applying dormant oil with an insecticide after bud swell will eliminate most of the eggs and newly emerged nymphs. After the application, monitoring is recommended to inform decisions related to further chemical control. If aphid populations are still high later in the season after the initial application, additional controls may be needed. It is important to note that aphid populations can develop resistance to repeated applications of an insecticide. In addition, many insecticides that are effective against aphids are also toxic toward natural enemies. Therefore, insecticide use should be limited when possible.
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