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Woolly Plant Louse

Native to Asia, the hemlock woolly adelgid, or HWA, is an invasive, aphid-like insect that attacks North American hemlocks. HWA are very small (1.5 mm) and often hard to see, but they can be easily identified by the white woolly masses they form on the underside of branches at the base of the needles.

woolly plant louse


Hemlocks are ecologically important due to the unique environmental conditions they create under their dense canopies. These cooler, darker and sheltered environments are critical to the survival of a variety of species that rely on them for food, protection, and ideal growing conditions. Moose, black bears, salamanders, and migrating birds, as well as unique lichen and plant communities, are all closely associated with the hemlock ecosystem.

Well-suited for growing on steep slopes where not many other species can grow, hemlocks stabilize shallow soils and provide erosion control. In addition, they are often found along streams, where their shade helps moderate water temperatures, maintaining a suitable environment for cold-water species such as trout. Removal of hemlocks from NYS ecosystems can dramatically change ecosystem processes and may result in the loss of unique plants and wildlife.

The woolly apple aphid differs from other apple aphids in appearance, life cycle, and the type of damage inflicted. A colony appears as a cottony mass generally clustered in wounds and pruning scars on the trunk and branches of the tree. The aphids themselves are purplish in color surrounded by white, cottony, thread-like secretions. Woolly apple aphid is a sucking insect pest that weakens the tree by feeding on limbs and roots. It gets its name from the woolly appearance of its colonies. Long strands of white wax are produced that help to protect the colony of purple aphids from predators and pesticide sprays.

As the number of aphids on the above ground portion of the tree increase, many work their way down to the roots and trunk below ground surface. It is the feeding on the roots that produces the greatest damage. Mature trees usually suffer little damage from the root infestations, but the root infestations are very damaging to young trees. Control of these aphids is very difficult when they attack the roots. Yellowish foliage is a sign that woolly apple aphid may be infesting roots. The root systems of nursery stock can be damaged, and severe root infestations can stunt or kill young trees. Infested trees often have short fibrous roots, which predisposes them to being easily uprooted. Swollen galls also form on roots; galls increase in size from year to year and are sites where fungi can attack. Aphid feeding on the root systems also disrupts the nutrient balance of root tissue, which can affect growth of other parts of the tree. Trees can have above-ground infestations of woolly apple aphid but no root infestations. Rootstocks vary in susceptibility to woolly apple aphid and susceptible rootstocks will form galls around the infestation sites. Use M111 or M106 if woolly apple aphid is a serious problem. Rootstocks appearing more susceptible to woolly apple aphid infestation include B9, M9, M26 and the P series.

During the summer, repeated woolly apple aphids generations of wingless individuals are produced. In the fall, winged individuals are produced which fly to search for elms on which to lay overwintering eggs, while some wingless forms may remain on both above and below ground parts of the apple tree throughout the winter.

It is relatively easy to find where the colonies have formed. When monitoring for woolly apple aphid, examine four pruning scars on each of 5 scaffold limbs per tree. Carefully examine woolly apple aphid colonies to determine if live aphids are present. Predators, such as lady beetles, hover fly larvae, and lacewing larvae can completely destroy the colony, but the waxy residue will remain. When examining colonies, blow hard on the branch to remove the waxy filaments to reveal live aphids. Treatments for woolly apple aphid are recommended when 10% of the pruning scars are infested with live colonies.

There are few insecticides specifically labeled for control of woolly apple aphid. Diazinon, Closer, Beleaf, Movento, and Admire Pro are recommended for control of above-ground infestations. There are no insecticides to control root infestations on bearing apple trees.

Mouthparts are important sensory and feeding structures in insects and differences in mouthpart structure reflect differences among lineages in feeding strategy and behavior. The woolly apple aphid (WAA), Eriosoma lanigerum (Hausmann), is an important pest of apple orchards worldwide, causing direct damage through feeding by the highly specialized piercing-sucking mouthparts. To obtain a better understanding of feeding, the morphology of mouthparts of the WAA was examined using scanning electron microscopy (SEM). The mouthparts of E. lanigerum are similar to those of previously studied aphid species in most aspects and composed of a cone-shaped labrum, a tube-like, four-segmented labium with a deep groove on the anterior side, and a stylet fascicle consisting of two mandibular and two maxillary stylets. The sculpturing on the lateral margin of the distal extremity of the maxillary stylets and a dentate protuberance at the very sharp tip are newly observed features that distinguish E. lanigerum from other aphids and Auchenorrhyncha. Also, there is a common duct in E. lanigerum as based on SEM. Two types of sensilla trichodea and three types of sensilla basiconica occur at different locations on the labium; the labial tip has eight pairs of small sensilla basiconica. The morphology of the mouthparts and the distribution of sensilla located on the labium in E. lanigerum are discussed with respect to their possible taxonomic and functional significance.

The original primary (or overwintering) host of the woolly apple aphid is American elm. In areas where this species of elm occurs, elm is the overwintering host, and apple is one of several summer (or alternate) hosts. Winged females dispersing to elm give live birth to wingless males and females. These in turn mate and the females lay eggs which overwinter on elm. However, woolly apple aphid has adapted to live and reproduce asexually on apple year-round in most fruit growing areas of the world (where the American elm does not occur), including the western United States. There is evidence that sexual reproduction occurs on apple in New Zealand, but the importance of this has not been established. Other alternate hosts include hawthorn, mountain ash, and cotoneaster.

Shortly after birth, the nymph is salmon colored and lacks the woolly coating. This stage is known as the crawler. The waxy filaments begin to form after the aphid settles to feed. There are four nymphal instars, averaging 0.64, 0.67, 1.2, and 1.3 mm in length. Eyes are dark brown to black (no oceli). The cornicles are circular, and only slightly elevated from the surface of the abdomen.

Winged adults (alates) are normally the form that would migrate back to the overwintering host (elm) in the fall. Winged forms have been noted in colonies of woolly apple aphid in the fall in Washington orchards, although their fate is uncertain. There is a persistent speculation that the winged forms may form part of the dispersal mechanism to other apple trees, but the meager evidence on this subject indicates that egg production on apple is rare, and the eggs fail to hatch.

Galls, or swollen enlargements, form on the plant where aphid colonies feed on twigs or roots. These are not very noticeable after one year of feeding but increase in size as feeding continues in an area. Subterranean aphid colonies cause the most damage. Roots of infested trees have large, abnormal swellings. Continued feeding can kill roots and cause reduced growth or even death of young trees.

Honeydew produced by the woolly apple aphid can drip onto the fruit resulting in sooty mold and downgrading of fruit because of blackened or russeted areas. High populations of woolly apple aphid can create sticky and unpleasant working conditions for harvest crews. Woolly apple aphid can also infest the stem and calyx end of the apples; the presence of live (or even dead) aphids in packed apples is a potential quarantine issue. In some instances, especially varieties with an open calyx, aphids can also infest the apple core.

No specific monitoring procedures or treatment thresholds have been developed for woolly apple aphid. Generally, monitoring should begin in midsummer or perhaps earlier if the winter was mild. If many colonies are in the fruiting zone of the tree, treatment will probably be needed.

The parasitoid Aphelinus mali is generally given the most credit for biological control of woolly apple aphid. Although A. mali play a role, research in Washington has shown that a complex of generalist predators including lady beetles, syrphid fly larvae, green lacewings, Deraeocoris brevis, and European earwig is also important. Whereas A. mali leaves behind evidence of parasitism in the form of mummies, the important role of predators can be overlooked because they consume all or part of the colony, leaving no trace of their activities. The more sedentary predators (syrphid larvae) are the easiest to find in aphid colonies; however, more mobile predators also have an impact, and nocturnal predators such as the European earwig may go unnoticed. The latter is considered an important predator of woolly apple aphid in Washington and worldwide. (For more information on specific natural enemies, see Predators or Parasitoids on the Beneficials page).

The pest status of woolly apple aphid in the Pacific Northwest has varied over time. During the era predominated by organophosphate use, it was not considered a serious pest, or at least one that was easily controlled. The transition of pest management programs away from organophosphates has been associated with an increase in the incidence and severity of woolly apple aphid outbreaks. While the causes for this can only be speculative, reducing the use of specific organophosphates in the delayed-dormant and mid-late summer (2nd generation codling moth) are plausible explanations. Organophosphates have been replaced by other groups of pesticides (including IGRs, neonicotinoids, and other novel modes of action), which have little or no toxicity to woolly apple aphid, but may be equally toxic to its natural enemies. In addition, milder winters may improve overwintering survival, and contribute to earlier or higher populations. 041b061a72

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