II. Hosts: The AA feeds primarily on apple and pear, but will also attack both hawthorn and quince. The SA is known to feed on apple and spirea. It is the main aphid pest of citrus worldwide, having made a shift to several tropical crops in the 1950's. It feeds on a variety of vegetable crops as well. Due to the apparent shift in species composition from the AA to the SA, this aphid now probably uses the same hosts as the AA. For years it has been assumed that SA used spirea as its primary host, on which overwintering eggs are placed, and shifted to other hosts in the late spring. In 1983, it was shown to use citrus as a primary host for the first time, in Japan. It has recently been shown in Virginia that SA uses apple as a primary host, as well.
III. Description: The eggs of both species are small, shiny and black. They cannot be differentiated from the two other species of aphids inhabiting the tree at the same time, rosy apple aphid (RAA) and apple grain aphid (AGA). It is difficult to distinguish young stem mothers of these species; they have not yet developed their characteristic color patterns. The cornicles of the young nymphs are smaller than those of the RAA but larger than those of the AGA especially when the nymphs are very young, when the cornicles of AGA are barely swollen protuberances. RAA also has longer antennae than the others; antennae of AGA are the shortest. The young GA nymphs which develop into stem mothers in the spring are wingless females and are bright green in color, although not as dark as the AGA. As the colonies develop throughout the summer, the aphids can vary considerably in color from a yellow-green to a light green, with black cornicles (Plate 37). The alate adults (with wings) of SA and AA can be distinguished by the following characteristic: for the AA, the veins in the forewing are distinctly pigmented, especially the cubitus and media (the two veins crossing from the thick front vein across the central part of the wing). Other characters to separate the two species, while usable for wingless viviparae, are impossible to use in the field. Oviparae (egg-laying females, produced in the fall) of SA have greatly swollen tibiae on the hind legs. The oviparae of both species have six antennal segments (unlike AGA); the front of the head is gently rounded, lacking the central tubercle of RAA. Males (also occurring in the fall) of SA are winged, unlike those of AA.
IV. Biology: The eggs are laid mostly
bark or on the buds in the fall by wingless females after they
the males. Warm periods during the winter, as well as cold rains
cause some natural mortality of eggs. Hatch occurs in the spring
the silver tip and half-inch green stages, especially around
The young nymphs develop into stem mothers which are wingless,
females, bright green in color. Stem mothers require 12-20 days
maturity. Adults often appear around bloom. These give birth to
of green viviparous (producing live young) aphids, ranging from
per female. About three-quarters of this generation develop into
females; the rest remain wingless. The winged forms spread
other parts of the tree or other trees and orchards. About
the second generation and some of the later generations may
and disperse. Wingless aphids produce more offspring than
the RAA and the AGA, GA can live on the apple tree all year
species breed continuously during the summer. There are seven to
depending on whether first or last young of each generation are
In August and during the autumn months, they are found almost
on water sprouts or terminals of young trees that are still
it is at such locations that the male and female sexual forms
at about the fourteenth generation. Males mate throughout their
period, but many oviparae fail to become fertilized because of
scarcity of males (males are less numerous than oviparae and do
as long). Oviparae may deposit eggs from early October until
Spirea aphid is more tolerant of certain
insecticides (esfenvalerate, methomyl and imidacloprid) than
aphid. On the other hand, spirea aphid is somewhat more
susceptible than apple aphid to azinphosmethyl.
V. Injury: Recent research at VPI&SU has shown that SA and AA have generally similar effects on photosynthesis and tree growth. Both nymphs and adults suck sap from apple trees. They prefer to feed on the succulent, young tissue found at the ends of terminal shoots and water sprouts. They can curl the foliage if high populations develop and stunt tree growth, especially on young trees. Indirect injury results when aphids secrete large amounts of honeydew on which grows a black fungus that smuts both the fruit and leaves and causes considerable discoloration, especially of early apples. Honeydew is often prevented from accumulating by rains.
VI. Monitoring: During the dormant period, examine twigs for overwintering eggs, to aid in selecting timing of delayed dormant sprays.
Neither SA and AA is usually an early season problem (i.e., before bloom), therefore, monitoring usually begins in late May and June when populations generally begin to increase. At this time select the major cultivar in the block and sample 10 actively growing shoots (not water sprouts) on each of five trees. On each shoot, determine the number of leaves that have wingless aphids. Calculate the average number of leaves per shoot infested with aphids across all trees. Aphid populations should be scouted for and managed until the terminal shoots harden off. This will occur later in the season for young trees than mature trees.
If an average of >4 leaves per shoot are
with one or more wingless aphids, an application of an
There are a number of natural enemies of aphids which have
for biological control of aphid in mid-Atlantic apple orchards.
a number of
larvae, and coccinellid
adults and larvae (see section on natural enemies of aphids). If
than 20% of GA colonies have predators, there is a good chance
control. Growers should recognize and monitor these species
on management practices.