While large numbers of any type of aphid can stunt new growth and cause sooty mold to develop on fruit and leaves, the rosy apple aphid injects a toxin with its saliva that causes the leaf to curl and the fruit to be distorted. Of the aphid species that can be found on apple trees, rosy apple aphid causes the most severe damage and is the most difficult of the three to control.
The aphid overwinters on apple trees as eggs laid on twigs, bud axils, or in bark crevices. The black eggs are 1/2 mm long and football shaped. The overwintering eggs give rise to only female aphids which give birth to live young. Shortly after silver tip the eggs hatch. The nymphs' color changes from dark green to slight purplish or dull pink with variable amounts of greyish-white wax bloom. The aphids continue to reproduce on apple until summer, then winged forms are produced which migrate to other hosts such as dock and narrow-leaved plantain to spend the summer. Recent evidence, however, shows that the biology of this pest has changed and populations in orchards may no longer need to go to the alternate host plantain but can breed continuously on apple. In the late fall, winged forms migrate back to apples and lay eggs in bark crevices and on twigs.
A cool, wet spring favors aphid development because it provides conditions unfavorable for parasites and predators of aphids.
These aphids cause a decrease in tree vigor because of foliage loss and damage to the fruit through dwarfing, misshaping, and staining. The rosy apple aphid injects a toxin with its saliva that causes the leaf to curl and the fruit to be distorted. A single stem mother located on the underside of a leaf near the midrib will cause the leaf to fold almost as tightly as the outer wrappings of a cigar. The presence of only a few stem mothers can cause severe curling of all leaves surrounding an opening flower bud; within such curls, ideal protection is afforded to the rapidly developing aphids.
‘Cortland', 'Ida Red', and 'Golden Delicious' are the varieties most frequently showing fruit injury. Fruit adjacent to rosy apple aphid colonies is stunted, puckered at the calyx end, and ridged like a pumpkin.
Monitoring for rosy apple aphid is possible after egg hatch begins since the eggs of apple grain aphid and apple aphid are identical to rosy apple aphid eggs. Starting at early pink, select 5 to 10 trees per block. Sensitive varieties such as ‘Cortland', 'Ida Red', and 'Golden Delicious' should be selected if present. For 3 minutes, on each tree, count the number of fruit spurs showing curled leaves. The presence of more than one aphid-infested cluster per tree justifies an insecticide treatment to prevent fruit injury. Samples should be taken from the upper parts of the canopy on the inside of the tree where rosy apple aphid colonies are most common.
The green apple aphid, apple-grain aphid, and rosy apple aphid overwinter as eggs on twigs and bark crevices of apple trees. A delayed dormant oil application between green-tip and half-inch green controls newly hatched aphids.
Before leaf curling, an organophosphate insecticide or a 1 to 2 % application of insecticidal soap or summer horticultural oil can provide effective control of these aphids. Thorough coverage is essential. After petal fall, because the curled leaves protect the aphids, then the best control will be achieved with a systemic insecticide. Some insecticide options include Admire Pro and Movento (active ingredient: spirotetramat*, at a rate of 6 to 9 fl. Oz).
*Spirotetramat is an insecticide derived from tetramic acid, a systemic material, for the control of sucking insects in their juvenile, immature stages, including aphids, scale insects, and whitefly. It produces growth inhibition of younger insects, reduces the ability of insects to reproduce, resulting in mortality. Spirotetramat is harmless to slightly harmful to beneficials such as hoverfly larvae, spiders, predatory bugs, wasp parasites, ladybird beetles and lacewings.