Mealybug Damage To Yews

Yews are a popular shrub in ornamental home gardens. Their deeply green foliage contrasts beautifully in the gardens spring, summer, fall and winter. Their slow growth habit and ability to take a hard pruning has made them a favorite for hedges and topiary for thousands of years. One of the insect pests that is commonly quite damaging to yews are mealybugs. There are many varieties of mealybugs but there are only 2 that feed on yew (Taxus). Yew mealybug (Dysmicoccus wistariae), and grape mealybug (Pseudococcus maritimus (Ehrhorn)). Yew mealybugs also feed on rhododendron, maple, linden, dogwood and other woody plants. Grape mealybugs feed on grape vines and most deciduous fruit crops, as well as members of the rose family and other herbaceous ornamental trees and shrubs. Typically, in north America, yew mealybugs occur in the east while grape mealybugs occur in the west.

Signs and Symptoms

Look for shiny, sticky branches and needles, caused from the honeydew secretions from the mealybugs. These sticky areas tend to attract wasps and ants who like to feed on the honeydew. Black sooty mold also commonly grows on this sticky honeydew. The mealy bugs themselves appear white and are usually found on inner branches often in branch forks. Also look for dead females the on bottom and top of leaves. On affected branches there will be yellowing of leaves, and leaf loss. Branch and twig dieback may be observed along with stunted or distorted growth.

Yew Mealybug (Dysmicoccus wistariae)

Meet the Pest

Adult females are oval in shape with obvious body segments and waxy filaments extending from their sides. They are wingless and measure up to .5cm wide by 1 cm long (3/16″ wide by 3/8″ long). The body is reddish brown in color and covered with white mealy wax secretions.

Adult Males are tiny, white, 2 winged flies who are incapable of feeding.

Nymph females will pass through 3 instar stages in all, while the males will pass through 5. The first instar nymphs are light yellow to brown with smooth bodies and functional legs. They are the able to crawl around in search of a good feeding site. The second instar stage looks closer in appearance to the adult females, only smaller. They are less mobile and begin covering themselves in white mealy like secretions for protection. The 3rd instar stage for the female is immobile and looks like the adult female.

Eggs are laid in masses in white cottony sacs typically at branch crotches and cracks and crevices in the bark.

Life Cycle

Yew mealybugs overwinter as immatures (crawlers). The crawlers begin feeding again in early spring (7-91 growing degree days (Adams N.E. (n.d.))). The first instar nymphs have functional legs and are more mobile. They molt into nymphs that resemble adult females. The males and females from this point on develop differently. “Female nymphs increase in size with each molt, reaching adulthood in about a month. In contrast, male mealybugs go through two or so molts and then enter a pupal state in a flimsy cocoon before hatching as a tiny, fly-like, two-winged adult, incapable of feeding” (Missouri Botanical Garden (n.d.)). The males and females’ mate then the male dies. The females lay about 500 eggs in a cottony mass which hatch in 1 to 2 weeks and the cycle begins again. (Second generation occurs between 246-618 growing degree days (Adams N.E. (n.d.)). Two generations a year are common. The immatures feed until the onset of winter, then they overwinter under bark scales or beneath the waxy covering of a dead female.

Yew mealybugs (Dysmicoccus wistariae) on a yew shrub showing their characteristic oval shape, obvious body segments, waxy filaments extending from their sides and white mealy like coating.
Yew mealybug (Dysmicoccus wistariae) on a yew shrub. The shiny, sticky branch and needles is caused from the honeydew secreted by the mealy bugs.
Yew mealybug (Dysmicoccus wistariae)
Yew mealybug (Dysmicoccus wistariae)
Yew mealybug (Dysmicoccus wistariae) a yew shrub, attached at the base of a branch. The waxy filaments extending from the sides are clearly visible here.

Grape Mealybug (Pseudococcus maritimus (Ehrhorn)

Looks very similar to Yew Mealybugs but they are smaller, measuring about 5mm long (Burts E.C. and Dunley J. (1993)). The eggs are salmon coloured, and like the yew mealybugs, are laid in sacs covered in white cotton waxy secretions.

The first instar nymphs (crawlers) are pink and covered in a light coating of waxy granules, giving it the appearance of being coated with flour. The crawler molts into a stationary nymph, which looks very similar to the adult female.

The adult female is wingless, pink to purple in colour and covered in an even thicker layer of white mealy wax secretions. Waxy filaments extend from the sides of its body. The adult male is much smaller than the female, light brown and has clear wings held flat over its back.

Life Cycle

Grape mealybugs overwinter as eggs or crawlers (if the eggs hatch during warm fall weather). The overwintering crawlers begin feeding in early spring while overwintering eggs hatch in a week or two later. First generation nymphs mature during late June and July in the west. In the east the insect is not a problem. Adult males’ mate with adult females and die. The females lay their eggs in white cottony covered sacs in sheltered areas and die in the egg sac. Eggs hatch by mid-July and the second-generation of immatures begin feeding. This generation matures by late summer or early fall. The adults’ mate, the male dies and the female lays her eggs in white cottony covered sacs that overwinter.

Management and Control of Mealybugs

Integrated Pest Management Strategies

  1.  Inspect any new plants thoroughly for mealybugs before installing them. 
  2. Monitor your yews on a regular basis. Seek out the white cottony egg masses and remove them before they hatch in early spring.
  3. Prune out badly infected branches.
  4. Ease up on nitrogen fertilizer that will produce a lot of tender new growth and stimulate more egg laying.
  5. Attract and conserve natural enemies such as lacewings, syrphid flies, ladybugs, and several small parasitic wasps that lay their eggs in or on developing mealybugs. Avoid unnecessary insecticide use to minimize damage to these beneficial insects.
  6. Remove mealybugs by hand if the infestation a small, using a cue tip or cotton swab dipped in rubbing alcohol.
  7. Control ants: Ants are known to transport mealybugs from one plant to another.

Sprays:

  • Dormant oil: spray a thorough coating during dormancy.
  • horticultural oil: sprayed at crawler stage (young nymphs).
  • Insecticidal soap: sprayed at crawler stage.
  • Trounce : an organic insecticide. Active ingredients are potassium salts of fatty acids 20%, pyrethrins 0. 2% (not available in Ontario and New Brunswick)
  • BotaniGard ES: a biological insecticide containing Beauveria bassiana, an entomopathogenic fungus (available for use in Ontario).
  • Chemical insecticides  Acephate (Orthene), bifenthrin, and pyrethrins are effective against mealybugs. Follow label directions carefully (Missouri Botanical Garden (n.d.).
  • Malathion a broad spectrum insecticide (not available for home use in Ontario).

Note: always follow label directions and check with your province or state to see what is legal to use in your area.

Summary:

Mealybugs while easily identifiable are difficult to spot within the dense growth of yews. They are most commonly found on branch forks. You will likely notice their damage before you notice them. Sticky leaves, branches and surfaces below the shrub that tend to develop black sooty mold is one sure sign of their presence, along with yellowing leaves, stunted growth and thinning and dying twigs and branches. They can be difficult to eradicate once established so regular scouting is important.

Photo Credits: all photos taken by the author.

Updated on Dec.12, 2022

References:

Adams N.E. (n.d.), (Extension Educator, Agricultural Resources) University of New Hampshire Cooperative Extension, ‘Using Growing Degree Days For Insect Management’, Retrieved on Jan. 23 from: http://ccetompkins.org/resources/using-growing-degree-days-for-insect-management

Bartlett Lab Staff (n.d.); RESEARCH LABORATORY TECHNICAL REPORT ‘Mealybugs on Taxus’; Retrieved on Jan. 15, 2021, from: https://www.bartlett.com/resources/mealybugs-on-taxus.pdf

Burts E.C. and Dunley J. (1993), Washington State University, ‘Grape Mealybug’ Retieved on Jan. 21, 2021 from: http://treefruit.wsu.edu/crop-protection/opm/grape-mealybug/

Missouri Botanical Garden (n.d.); ‘Mealybugs – outdoors’; Retieved on Jan 15,2021 from: https://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/pests-and-problems/insects/mealybugs/mealybugs-outdoors.aspx

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