Integrated Pest Management Manual
This module is intended to serve as a source of basic information needed to implement an Integrated Pest Management program for aphids. Any pest management plan or activity must be formulated within the framework of the management zones where it will be implemented. Full consideration must be given to threatened and endangered species, natural and cultural resources, human health and safety, and the legal mandates of the individual parks. Recommendations in this module must be evaluated and applied in relation to these broader considerations.
The term aphid is applied to a large number of species of small, soft-bodied insects of the superfamily Aphidoidea, order Homoptera. The majority of aphid problems likely to be encountered within the National Park Service are caused by species in three families: the Aphididae (true aphids or plantlice), the Adelgidae (pine and spruce adelgids), and the Phylloxeridae (phylloxerans). These families comprise thousands of species and include some of the most important plant pests in the world.
Aphids vary greatly in their patterns of reproduction, use of hosts, and types of damage that they cause. In a single season some species may produce sexual and asexual forms. In other species sexual forms are unknown. Some species remain on the same host throughout the entire year and others may have obligatory alternation between two different hosts. Because of the large number of species and the variation which occurs within species, it is recommended that aphid identification problems be referred to your local or state Cooperative Extension Service agent. Color photographs and descriptions of important species can be found in Johnson and Lyon (1988). A detailed treatise on the biology, natural enemies, and control of aphids was edited by Minks and Harrewijn (1988).
IDENTIFICATION AND BIOLOGY OF APHIDS
Other insects and abiotic disorders can produce plant injury similar in appearance to aphid injury. Use the information presented in Table 1 to distinguish between these.
Table 1. Distinguishing between aphids and aphid-like symptoms.
|Problem||Symptoms||Time of Appearance||What to look for|
|Aphids||New leaves are distorted||After flushes of new growth||Small (1/8”) green, yellow, black, white, or orange insects
are seen on new growth. White cast skins are seen. Honeydew and
sooty mold are present
Ref: Johnson and Lyon
|2, 4-D herbicide injury||All growth is distorted, twisted, including petioles and leaves||Often seen after windy or very hot weather. Appeears very suddenly (24 hours). Often only affects windward side of plant.||No cast skins may be seen.
|Leafhoppers||New growth is stunted or leaf margins or interveinal areas are chlorotic or necrotic.||Injury develops quickly (1 week).||Cast skins may be seen.
Ref: Johnson and Lyon
|Eriophyid mites||New growth, or current season's growth or buds are distorted, stunted and necrotic.||Can be seen in spring or summer.||Very small, cigar-shaped mites with only four legs seen with a
Ref: Johnson and Lyon
Aphids are small (usually less than 1/4" long), soft-bodied, pear-shaped insects. They may be pale yellow, green, red, blue, gray, or black, and may have spots or stripes. Winged forms have two membranous pairs of wings, with the front pair larger than the hind pair. Immature aphids closely resemble adults, but may differ in color and do not have wings. Most members of the family Aphididae possess a pair of elongate tubular structures, called cornicles, on the back of the fifth or sixth abdominal segment; in some species the cornicles are very small or absent. The antennae have six segments. Some species are covered by white, waxy fibers secreted from glands on the body, giving them a thick covering of fuzzy white wax; these are known as woolly aphids. Aphids are also characterized by the production of a sugary excretion called honeydew that may be produced in large quantities, often resulting in the growth of sooty mold on plant surfaces. This is unsightly and can reduce the photosynthetic capability of the plant.
The adelgids have been called pine and spruce aphids in the older literature but are not true aphids. They lack cornicles and the antennae have three to five segments. All winged forms have five-segmented antennae, sexual forms have four-segmented antennae, and wingless parthenogenetic females have three-segmented antennae. Many species produce waxy threads that cover the body. Many species produce galls on spruce and some species, such as the balsam wooly adelgid, Adelges piceae (Ratzberg) and hemlock woolly adelgid, A. tsugae Annand, are capable of killing trees.
The Phylloxeridae, or phylloxerans, are close relatives of adelgids and aphids. Like adelgids they also lack cornicles and in all forms the antennae are three-segmented. These insects do not produce waxy threads, but some species are covered with a waxy powder. Like some species of aphids and adelgids, many species of phylloxerins produce galls and some use multiple hosts.
Aphids, adelgids, and phylloxerans are distributed worldwide. Some species have restricted distributions that correspond to the range of their host plants. Hamamelistes agrifoliae Ferris is found only in California where it attacks coast live oak. Some species are more widespread; the woolly alder aphid, Prociphilus tesselatus Fitch, occurs in the east from Canada to Florida and west to the Mississippi River, and alternates between alder and silver maple in this range. A few species, including some of the most significant pest species, are cosmopolitan. The green peach aphid, Myzus persicae (Sulzer), is distributed worldwide.
Many pest species have been introduced from abroad. The balsam woolly adelgid, a native of Europe, was first discovered in North America in Maine in 1908 and quickly spread throughout the Appalachians from the maritime provinces of Canada to Georgia and North Carolina. In 1928 it was discovered in the Pacific Northwest and now extends along the Coast and Cascade mountains from British Columbia to California. The hemlock woolly adelgid, Adelges tsugae Annand, is probably a native of the orient. It was first reported in this country in 1927 and now occurs in California, the Pacific Northwest, and from the maritime provinces to the mid-Atlantic states in the east.
Aphids consume only plant material and are most often found on their host plants. Different species show a preference for different hosts, and their habitat is in large measure determined by the distribution of their hosts. Species habitats are further defined by their location on the host plant. Many species are foliage feeders, others prefer tender shoots and stems, some are found on the woody parts of trees and shrubs, and a few are found on the roots. Some species utilize different parts of the host at different times of the year, their choice being dependent on the season, stage of growth, or the species of host plant. Each host plant has its own chemistry, so aphids may select one individual plant in a stand and feed on it exclusively.
Wind may dislodge aphids from their host plant by causing the leaves to rub together, knocking off the insects. Wind also is considered the most important factor in the dispersal of aphids from one plant to the next, as in the case of the balsam woolly adelgid, and in the migration of winged forms from one area to another. Because of their small size and weak flight muscles aphids, adelgids, and phylloxerans are not strong fliers and have little control over the direction of their flight when the wind speed is more than a few miles per hour.
Aphids often have very complicated life cycles that involve alternation of host plants, sexual and asexual generations, and winged migrant and wingless nonmigratory generations. In temperate regions most species overwinter as eggs, and the eggs hatch in the spring into females that give birth to live young. Several generations may be produced asexually, resulting in very rapid growth of populations. In some species, such as the apple aphid, Aphis pomi De Geer, the aphids may remain on a single host throughout the year. In other species, winged females migrate to new plants and produce more young asexually. These hosts may be quite different from the primary host. For example, the rosy apple aphid, Dysaphis plantaginae (Passerini), moves from a woody primary host (Malus) to a herbaceous secondary host (Plantago ). Late in the season, a sexual generation consisting of winged males and females is produced. These return to primary hosts where they mate and the females lay eggs that overwinter. Details of the life cycle and patterns of host utilization vary considerably among aphid species.
Adelgids also can have complex life cycles that include sexual and asexual generations and alternation of hosts. Many species of adelgids utilize spruce as a primary host and other conifers such as pines, larch, firs, and Douglas fir as secondary hosts. For example, the Cooley spruce gall adelgid causes pineapple-like galls on spruce and needle distortion on Douglas fir. Another unusual life cycle is that of the balsam woolly adelgid. Populations in North America consist entirely of females. They do not give birth to live young, but instead lay eggs. There are two to four generations per year, depending on the locality and elevation of the population, with fewer generations produced in the northern parts and higher elevations within the range.
Phylloxerans produce galls on several species of deciduous plants. In most species of phylloxerans the entire life cycle is completed on a single host. However, in at least two species, Phylloxera texana Stoetzel and P. castanea Pergande, Carya serves as the primary host and oak or hickory are secondary hosts (Stoetzel 1985).
The level of precipitation also affects the vigor of the host plant, which affects the aphids feeding on it. Some aphid species do better when a plant is well-watered and fertilized, while others do better if the plant suffers from stress.
The development of winged individuals in a population seems to be triggered by the degree of crowding on the host plant, but the way in which this works depends on the species (Hille Ris Lambers 1966). Temperature, daylength, and host plant food quality all combine to play a role in aphid dispersal.
Ants have an important role in the development and success of many aphid species (Way 1963). The presence of aphid-tending ants may inhibit the production of winged forms. The ants collect honeydew excreted by the aphids and stimulate the aphids to produce large quantities of honeydew by stroking them with their antennae. Ants protect aphids from parasites and predators, and even transport them to suitable host plants and safe places to hibernate. Some species of aphids live in the nests of their benefactors and are dependent on their ant protectors for their survival. Monitors should look for ants as a clue to the presence of aphids.
Certain horticultural practices may affect the abundance of aphids and adelgids. It has long been known that plants and plant parts rich in nitrogen are exploited by aphids (Minks and Harrewijn 1987). Recently, McClure (1991) demonstrated that populations of hemlock woolly adelgid increased dramatically when hemlocks were fertilized. Resource managers should be aware that the over- fertilization of plants may facilitate population increases of some associated pests such as aphids and adelgids.
Feeding by aphids, adelgids, and phylloxerans withdraws sap from the host plant and can interfere with the physiology of the plant by altering the balance of plant growth hormones. Aphids feeding on leaves can cause yellowing, spotting, and premature death, and can reduce the ability of the leaf to photosynthesize by reducing the amount of fluid in the leaf and reducing the surface area as a result of curling. Aphids are believed to inject toxins into plant tissues as they feed. These toxins may produce local and systemic effects in plants that include reductions in growth and alterations in the normal patterns of nutrient distribution in the plant (Minter and Harrewijjn 1987). Twigs may develop swelling or gouting.
The production of large amounts of honeydew by aphids may foul plant tissues and structures, walkways, or vehicles beneath heavily infested trees. The honeydew may attract stinging insects, ants, and other insects and may create a nuisance. Furthermore, honeydew on plant tissues may facilitate the growth of sooty mold which itself is unsightly and may reduce the photosynthetic capacity of plants.
Many species of aphids, adelgids, and phylloxerans produce galls on their host plant. Galls are distinctive growths on leaves, shoots, stems, or roots, that are a response of the plant to certain stimuli provided by the aphids. Galls such as those produced by adelgids on spruce may interfere with normal patterns of plant growth by killing terminals. Leaves heavily galled by phylloxerans may be dropped prematurely. Galls may also reduce the aesthetic quality of plants. In some cases it is possible to identify the species of aphid appearance of the gall. Useful references include Felt (1940), Johnson and Lyon (1988), and Russo (1979).
Aphids and adelgids can cause distortions of other plant parts. Symptoms caused by balsam woolly adelgid on fir begin with curling and dieback of the current year's growth, swelling of buds and gouting of shoots, and thinning of the crown. In trees with a heavy infestation on the bole, the wood becomes reddish and coarse, a condition known as "rotholz" or redwood (Knight and Heikkenen 1980). Susceptibility varies with the species; subalpine fir dies within a few years, sometimes before terminal swelling occurs; Grand fir may survive fifteen years; Noble fir, Shasta red fir, and white fir may show gouting but usually are not killed. In Great Smoky Mountains National Park, balsam woolly adelgids kill Fraser fir in two to six years (Allen-Reid 1984).
In addition to direct injury caused by their feeding, aphids are serious plant pests because of their role as vectors of plant diseases. Hundreds of plant viruses are transmitted by aphids. The green peach aphid, Myzus persicae (Sulzer), is the single most important vector. It is known to transmit over 100 virus diseases to plants in about 30 different families (Ossiannilsson 1966; van Emden et. al. 1969). The ability of aphids to transmit plant diseases is related to their piercing-sucking feeding habit, rapid growth, and life histories that involve host alternation and migration.
MONITORING AND THRESHOLDS FOR APHIDS
Population Monitoring Techniques for Aphids
At least two methods have been used for monitoring aphids, adelgids, and phylloxerans. The first and most commonly used technique involves visual observation of the plant. On small plants, examine the entire plant; on larger plants and trees, examine representative leaves, twigs, stems, or other portions of the plant. The part of the plant to be examined also will be determined by the biology of the pest. For example, in the Great Smoky Mountains National Park, balsam woolly adelgid infestations are greatest at about 15' above the ground rather than at breast height, the standard position for sampling on trees. Therefore, monitoring populations of this pest is done at this greater height (C. Eagar, pers. comm.). Data can be recorded on monitoring forms such as the one shown in Table 2.
A system of classifying relative levels of infestation was presented by Heathcote (1972). He used the following population density index to classify infestations.
None (O) - no aphids seen.
Very light (V) - one to a few aphids per plant and only a few scattered young plants infested, or one to a few aphids per leaf, shoot, or other section of larger plant or tree and only a few colonies per large plant with the colonies on the young tender leaves or buds.
Light (L) - 5-25 aphids per plant and many plants infested, or with many colonies on larger plants or trees, and the colonies not confined to young shoots.
Medium (M) - 25-100 aphids per plant and most plants infested, or with large numbers of aphids on larger plants or trees and not in recognizable colonies, but diffuse and infesting many leaves, stems, etc.
Heavy (H) - more than 100 aphids per plant with virtually all plants infested, or with stems, leaves, buds, etc., solidly covered with aphids.
Where direct observation of aphids is difficult, such as in tall trees, monitoring may be done indirectly by quantifying production of honeydew. Dreistadt and Dahlsten (1988) used water- sensitive spray droplet cards to collect honeydew beneath tall tuliptrees infested with aphids. The honeydew droplet counts correlated well with the abundance of aphids in the canopy of the trees.
When monitoring through visual observation, also survey for the presence and effectiveness of aphid pathogens, parasites, and predators. Aphids killed by fungi, bacteria, or other pathogens usually remain on the plant and can be recognized by their immobility and peculiar coloration. Parasitized aphids usually are darker than unparasitized aphids, at least near the completion of the parasite's life cycle. Aphids that have been killed by parasitic wasps are mummified; that is, they are discolored and papery in texture. If the adult wasp has emerged there is a round hole in the mummy where the wasp exited. Color photographs of aphid mummies are given in Yepsen (1984). Look for predators among the aphids on the plant, as well as flying or perching nearby.
Decision-Making and Thresholds for Aphids
Data on the number of aphids, adelgids, and phylloxerans may be combined with other information, including the injury caused by the pest, the value of the plants being managed, and the cost of control activities, to create economic or aesthetic injury levels and thresholds (Raupp et al. 1988, 1989). Because of the economic importance of aphids, thresholds and action levels have been established for several aphid-crop systems. However, there has been relatively little work concerning aphids, adelgids, and phylloxerans infesting ornamental plants. Notable exceptions include the following studies. In 1978 Olkowski et al. published a decision-making guideline for ornamental spruces being attacked by the blue spruce aphid, Elatobium abietinum Walker. They determined that about 34 aphids per quadrant of a tree collected with a beating cloth were sufficient to cause defoliation. They used this level as an aesthetic injury level to justify intervention. In 1988 Dreistadt and Dahlsten presented a methodology for determining a threshold for managing the tuliptree aphid, Illinoia liriodendri (Monell), based on honeydew counts and public complaints about the honeydew. This methodology could be used as a model for establishing decision-making rules for other aphid problems in the National Park Service. Finally, Nielsen (1989)
published action thresholds for aphids found on the leaves of hardwood trees. He suggested that two aphids per leaf in the spring and four aphids per leaf in the summer justify intervention.
In setting thresholds and action levels in the National Park Service, the particular needs of each park must be considered. Unless threatened or endangered plants are being attacked, control is not generally recommended in natural areas. Under normal circumstances natural mortality factors will keep aphid populations in check in these areas. However, this may not be the case for certain exotic species such as balsam and hemlock woolly adelgids. These species appear to lack natural enemies in this country that are capable of establishing control.
Thresholds will be quite low and may approach zero for plants that are valuable due to their size, age, beauty, rarity, or historic significance. Low thresholds will also be established for plants that are extremely vulnerable to a pest capable of causing death, such as Canadian hemlocks under attack by the hemlock woolly adelgid. Specimen plants and small groups of plantings are good candidates for the establishment of thresholds and other decision- making guidelines.
A further complication arises if aphid-borne diseases threaten plants. If this is the case, the threshold level will be much lower than for aphid damage alone. Accurate identification of both the aphid vector and the disease is essential to be positive that the suspected vector and the disease are causally related. Consult diagnostic experts at your Cooperative Extension Service or commercial laboratory for aid in identification of aphids and plant diseases that may be transmitted by them.
NON-CHEMICAL CONTROL OF APHIDS
1. Avoid planting species and cultivars susceptible to aphids, adelgids, and phylloxerans.
2. Begin monitoring plants early in the growing season and record your observations using a standardized system such as the one presented in the decision-making section. Record the presence of aphid parasitoids and predators.
3. If decision-making guidelines have not been established for the resource under management, use the methodology outlined in the decision-making section to establish thresholds or action levels. Intervention may also be necessary if sooty mold or honeydew are problems.
4. If aphid populations exceeded thresholds in the previous season, consider using a dormant oil to kill overwintering life stages.
5. Use mechanical and cultural controls where feasible. Use aluminum foil or white plastic mulches in newly planted areas if possible. The reflection of light from these materials will confuse aphids and prevent them from landing on plants. Crush aphids with fingers if infestations are not too extensive. Control aphid-tending ants by preventing them from reaching plants, using sticky substances around tree trunks, or bone meal or crushed charcoal barriers in and around gardens. Destroy ant colonies if necessary. Eliminate alternate host plants in the vicinity of more desirable plants.
6. Encourage natural predators and parasitoids. Release lacewings, ladybird beetles, syrphid flies, predaceous midges, and aphid mummies. Plant nectar-producing flowering plants that attract adults of these insects. Provide suitable habitat that will encourage predators to remain in the vicinity.
7. Consider release of exotic parasitoids and predators in cooperation with federal and state experts.
8. Spot treat with insecticidal soap, oil, or other approved insecticides when necessary. Spot treatment will have less impact on biological control agents than widespread spraying.
Aphids, adelgids, and phylloxerans have many natural enemies, including diseases, parasites, and predators. The ecology of aphid predators has been reviewed by Hodek (1966), and the impact of the natural enemies of aphids has been reviewed by Hagen and van den Bosch (1968). Reviews dealing with specific groups of pathogens, parasites, and predators of aphids include Madelin (1966), Hodek (1967), Schneider (1969), Stary (1970), Ferron (1978), Hall (1981), Wilding (1981), Viggiani (1984), and Minks and Harrewijn (1988).
Diseases of aphids include several species of fungi that are capable of drastically reducing aphid populations under appropriate conditions. Excessive moisture in cool weather favors the development of entomogenous fungi. Outbreaks of fungal pathogens are more likely to occur in cool, moist seasons than in warm dry seasons. Aphids also are susceptible to infection by bacteria, viruses, protozoa, and nematodes, but none of these is known to cause high mortality in natural populations. Pathogens for the control of aphids have been used successfully in greenhouses but only with limited success in field situations. A fungal pathogen, Verticillium leucanii, is available commercially in Europe, but not yet in the United States.
Aphids are parasitized by many insects, the most important belonging to the hymenopteran families Aphidiidae and Aphelinidae. The family Aphidiidae contains over 300 species, all of which are parasites of aphids. In the family Aphelinidae, only species in the genera Aphelinus, Mesidia, and Mesidiopsis parasitize aphids, but certain of these have proven successful in biological control programs. Two other hymenopteran families, the Encyrtidae and the Mymaridae, also include aphid parasites. Similarly, many insects feed on aphids, including beetles, flies, lacewings, earwigs, and predaceous bugs. Ladybird beetles (Coccinellidae) and green lacewings (Chrysopidae) feed on aphids as larvae and adults. Hover flies (Syrphidae) and predaceous gall midges (Cecidomyiidae) eat aphids only as larvae. All are thought to play important roles in reducing aphid populations (Hagen and van den Bosch 1968, Minks and Harrewijn 1988). Few vertebrates have been reported feeding on aphids, but Smith (1966) reported that in Great Britain birds may have a significant impact on aphid populations under some circumstances.
Many parasites have been successfully introduced for control of aphids (Clausen 1978, Olkowski et. al. 1976, Minter and Harrewijn 1988). Parasites often are specific for one or a few aphid species, requiring accurate identification of the aphid species in order to match the correct parasite species to the problem. Consult with federal and state extension officials and your regional National Park Service Integrated Pest Management coordinator before considering implementation of a parasite release program. Predators also have been used successfully against some species of aphids. Specific predators have been imported from overseas to help combat a variety of aphids (Mitchell et al. 1970). Native predators, such as ladybird beetles, lacewings, predaceous midges, and syrphid flies have also given good results. Ladybird beetles, lacewings, midges, and several species of parasitoids can be obtained from commercial supply houses. A list of suppliers is available through the Biological Control Services Program, 3288 Meadowview Road, Sacramento, CA, 95832.
Natural predators and parasites may be augmented by various techniques. A sugar or sugar and protein food supplement may be sprayed on plants to attract green lacewings and ladybird beetles (Hagen et. al. 1970; Schiefelbein and Chiang 1966). Commercial preparations of such supplements are available. Larvae of predators and aphid mummies may be collected in one area and released in the control area. The adults of predators such as syrphid flies and parasitic wasps may be encouraged to stay in an area by planting nectar producing flowering plants to provide food for the adults. Carroll and Hoyt (1984) report good control of apple aphids in orchards by using earwigs reared on dog food and released at five or six per tree. "Earwig retreats" made of cardboard and paper towels were placed in the trees and straw was scattered on the ground under the trees to encourage the earwigs to stay. Aphid densities declined dramatically in augmented trees.
Beneficial organisms may also be preserved by using pesticides with short residual activities such as soap and oil and avoiding treatments of large numbers of plants in favor of spot treating only individual plants that require intervention.
Use of plant cultivars and species that are less susceptible to these aphids should be encouraged. Sadof and Raupp (1991) reported that aphid populations increased more on variegated euonymus cultivars than on green cultivars. Cranshaw (1989) found that green individuals of Colorado blue spruce were more likely to be infested by the Cooley spruce gall adelgid compared to blue individuals. Avoid planting Douglas fir near spruces infested with Cooley spruce gall adelgid as Douglas fir is the alternate host for this species.
A simple approach to aphid control is to crush them between your fingers. This will work on garden plants and other ornamentals with light infestations. Another simple approach is to knock the aphids off the plants with a stream of water from a hose or sprayer, although the efficacy of this technique is unknown.
Cultural practices may also reduce aphid problems. Aluminum foil and white plastic mulches can inhibit the migration of winged aphids into newly planted areas (Wyman et. al. 1979, Yepsen 1984). These work best with young, small plants up to 1' tall. The highly reflective surface of the mulch causes migrating aphids to become disoriented, reducing the number of migrants that land and become established on the plants. Controlling alternate hosts of the pest species can also successfully control aphids (Knipling 1979). For example, to control the green peach aphid in gardens and orchards, Yepsen (1984) recommends clearing plants such as plantain, bindweed, and lamb's quarters from nearby land.
Ants play an important role in the success of aphids. Therefore, control of ant populations can cause a significant reduction in aphid populations. If ants are observed on aphid- infested trees, apply a commercial adhesive designed for such purposes in a band around the lower trunk of the tree. Caution should be exercised in applying these materials directly to the bark of trees; they can cause long-lasting discoloration or may injure thin-barked trees. In situations where individual treatment of plants is impractical, a barrier of bone meal or crushed charcoal may keep ants away. Destroy colonies of aphid-tending ants, if necessary; keep in mind that ants often are beneficial insects and eliminating them may not be the best strategy.
CHEMICAL CONTROL OF APHIDS
Dormant oils are applied during the dormant season of the plant, either in fall or spring before bud break, to kill eggs or other overwintering life stages on oil-tolerant plants. Recent improvements in the formulation of oil products have facilitated the use of these materials on a wide variety of plants during the growing season. Used in the nondormant seasons, summer oil or horticultural spray oil, has proven very effective in reducing population of adelgids on ornamental trees (McClure 1987, Baxendale and Johnson 1990). The efficacy of oils in controlling aphids has been equivocal. For some species such as the crapemyrtle aphid, Tinocallis kahawaluokalani (Kirkaldy), oil provided good control (Booth et al. 1990); for other species such as the birch aphid, Euceraphis betulae (Koch), oil provided little or no control (Nielsen 1990).
Insecticidal soaps are also recommended for aphid control. Like oils they have been effectively used to control adelgids (McClure 1987). However, their efficacy against aphids varies (Booth et al. 1990, Nielsen 1990).
Several conventional pesticides control aphids. Contact your regional Integrated Pest Management coordinator to determine which, if any, pesticide is best suited for your management program.
APHID INTEGRATED PEST MANAGEMENT DEMONSTRATION PROGRAMS
In an integrated control program in several cities in California, many aphid management techniques were combined to provide superior levels of aphid control and a dramatic reduction in pesticide use (Olkowski 1973, Olkowski et al. 1976, Flint and van den Bosch 1981). The first step was to institute a monitoring program to accurately assess the aphid problem on trees lining city streets. Pest species were identified. Aesthetic injury levels were established and management techniques were applied only if the thresholds were exceeded. Parasites of exotic aphid species were located and imported for release. Several species of imported parasites became established and have contributed to the management program. Heavily infested trees were pruned to remove the highly susceptible inner canopy. Where aphid-tending ants interfered with predators and parasites, bands of a commercial sticky substance were applied around the bases of trees. In Berkeley, where the program began, pesticide usage went from hundreds of pounds per year to zero, and the aphid problem became negligible.
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