The apple aphid, Aphis pomi, which is usually referred to as the green apple aphid, is the most common aphid species found in most Ohio apple orchards in most years. Two other aphids found in apple orchards will also be described in this fact sheet because they look quite similar to green apple aphid; these are spirea aphid, which is economically important, and apple grain aphid, which is not economically important. Green apple aphid develops on pear, quince, hawthorn, and crabapple as well as on apple. Other aphid species that cause serious but more sporadic problems in Ohio orchards are rosy apple aphid (see Extension FactSheet, HYG-2207-94) and woolly apple aphid (see Extension FactSheet, HYG-2208-94).
|Shoot infested with green apple aphid|
from Quaintance and Baker 1917, USDA Farmers Bulletin 804
Feeding by green apple aphid on succulent tissue of bearing and nonbearing trees can lead to leaf curling, stunting, and distorted tip growth. During heavy infestations, aphid feeding may stimulate growth of lateral branches which affects tree shape, and aphids may also feed on immature fruit and cause russetting. Heavy infestations can cause serious injury to young trees and nursery stock.
In addition to damage caused by feeding, aphids are associated with the development of black-sooty mold. As they feed on sugary sap from the tree's phloem tissue, aphids excrete excess sap in a form known as honeydew. Honeydew on leaves and fruit provides a medium for growth of black sooty mold. Sooty mold on leaves can affect photosynthesis and may reduce fruit yield, while sooty mold on fruit can lower fruit quality and marketability.
Green apple aphid is usually found feeding on the leaf undersurface, close to major veins, on young terminal leaves in early-summer. Further infestations can develop on water sprouts and excessive shoot growth caused by heavy fertilization or summer pruning. Small populations of green apple aphid can be tolerated and are often suppressed by naturally occurring predators. When predators are too few and conditions are favorable for aphid reproduction, aphid populations can quickly grow to high densities. Heavy populations can be found covering the terminal shoots on the outside portion of the tree.
Like other aphids, green apple aphid is a small, soft-bodied insect with piercing-sucking mouthparts and two cornicles (or "tailpipes") projecting from the back of the abdomen. Green apple aphid in the nymph stage has a dark-green body and short cornicles. Wingless adults are bright green to yellow-green with black legs, black cornicles, and black antennal tips. Winged adults have a black head, a black thorax, and a yellowish-green abdomen. Nymphs are about 1 mm long; adults are about 2 mm long. Green apple aphid has shorter cornicles and shorter antennae than rosy apple aphid.
|Green apple aphid, adult female (left) that gives birth|
to live young; young nymph (top center); egg-laying female
(bottom center); adult winged female (right) (from USDA)
Green apple aphid overwinters in the egg stage. The eggs are shiny-black and look identical to those of rosy apple aphid and apple grain aphid. Overwintering eggs of green apple aphid are found at the base of buds and leaf scars on terminal shoots and water sprouts. Most eggs can be found 15-20 cm (6 to 8 inches) from the tips of twigs; they are seldom found on scaffold limbs or tree trunks.
In the spring, eggs begin to hatch as buds are unfolding. Newly hatched nymphs are all females. After feeding for about two weeks and molting several times, the nymphs mature into wingless adults that are able to reproduce without being fertilized by males. They give birth to live young, thus skipping the egg stage and allowing for the rapid build-up of large populations. Each female produces 50 to 100 offspring that reach maturity in about seven to ten days.
Depending upon weather conditions, one generation is completed in two to three weeks. There are many generations per year. Adult aphids in a colony are generally wingless until crowded conditions induce the formation of winged individuals that can disperse to new hosts. Green apple aphid usually stays on apple throughout the summer. In late-summer, it goes through the sexual phase of its life cycle; males are produced as well as females, which mate and produce offspring that then lay overwintering eggs.
Aphid populations are affected by weather, the quality of their host plant, and natural enemies. Aphid reproduction slows down when temperatures are high or when the quality of the foliage declines. Heavy rains can wash aphid populations off of leaves. A cool, wet spring favors aphid development because these conditions are unfavorable for the aphid's natural enemies.
The most commonly observed predator of green apple aphid is an orange maggot of the gall midge species Aphidoletes aphidimyza. The mosquito-like adult midge seeks aphid colonies as sites for laying eggs that hatch into predatory maggots. Maggots are 0.3 mm long when first hatched and 2.5 mm when fully grown. They inject a poison into the aphid prey, then suck out the aphid's body fluids. Once fully grown, maggots drop to the ground and pupate in a cocoon in the top layer of soil.
Other natural enemies of apple aphids include predators such as hover fly larvae, lacewing larvae, lady beetle larvae, lady beetle adults, and earwigs. Small parasitic wasps also attack aphids; they lay their eggs in aphids by stinging with their ovipositor (egg-laying organ). The wasp egg hatches within the aphid and the young wasp larva consumes the aphid. Parasitized aphids turn brown or black. In time, the wasp larvae emerge as adults from the aphids, leaving behind empty aphid skins. These skins, called "aphid mummies," can be found attached to leaves.
Scouting for green apple aphid should begin at petal-fall and continue until the terminals harden off. Examine 3 to 10 growing shoots from each of 5 to 10 trees. In Ontario, insecticide treatment is suggested if 20 percent of the terminals have 100 or more aphids per terminal, or a solid line of aphids along the mid-rib of the endmost three leaves. In Pennsylvania, insecticide treatment is suggested if an average of at least 4.2 leaves per shoot are infested by green apple aphid.
Practices that minimize the growth of succulent leaves will aid in suppressing this pest. Fewer green apple aphids are found in trees that are summer pruned to remove water sprouts and that had minimal fertilizer treatment.
In commercial orchards, endosulfan (Thiodan), dimethoate (Cygon), chlorpyrifos (Lorsban), or methomyl (Lannate) can be used to control green apple aphid. In home plantings, diazinon or insecticidal soap can be used for aphid control. Thorough coverage of leaves is needed for soap or diazinon to be effective.
Spirea aphid, Aphis spiraecola, is green in color and is nearly identical to the green apple aphid and to the citrus aphid, Aphis citricola, which is an important pest of citrus in California and Florida. These species can be separated only by microscopic examination of the abdomen. Spirea aphid is found on spirea, apple, pear, quince, and haw. Spirea aphid has a life cycle similar to green apple aphid; overwintering eggs are laid on spirea or apple. Spirea aphid should be managed in the same way as green apple aphid.
Apple grain aphid, Rhopalosiphum fitchii, is noticeable on apple from green tip until bloom. Clusters of apple grain aphid are often found on swelling buds. Unlike the green apple aphid that remains on apple throughout the year, apple grain aphid leaves the apple tree after one or two generations, and migrates to grains and grasses. Apple grain aphid is of little importance as an apple pest because it does not cause serious injury to the foliage or fruit. It is yellowish-green marked with dark crossbars down the middle of its back. It has pale-yellow legs with darkened tips, and short stubby cornicles. Nymphs are dark green when young, but lighter green as they grow. Host plants include apple, pear, plum, crabapple, and grains and grasses.
|Shoot infested with green apple aphid
(from Matheson 1919, Cornell Ag. Ext. Stn. Memoir 24)
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