WSU Tree Fruit Research & Extension Center

Little Cherry Disease

Wednesday, October 18, 2017


Apple mealybug

Apple mealybug nest
 Apple mealybug nest (E. Beers)
 Apple mealybug
 Apple mealybug adult (E. Beers)

The apple mealybug is the historical vector of Little Cherry Virus 2 (LChV2), spreading it from one tree to another.  It has one generation per year, and overwinters as a second instar nymph in a cocoon under bark scales or in cracks in the bark. Feeding is done by inserting the proboscis into plant tissues (bark or leaves) and sucking plant sap. They emerge from overwintering sites very early in the spring, feed on twigs and under loose bark, mature to the adult stage (male and female) and mate. Egg laying begins in early May in central Washington.

The mealybugs appear to be quite indiscriminate about their egg-laying sites; many of the nests are on twigs, especially in the crotches, some are in pruning scars on heavy wood, and some are on the leaves (underside is common, but they can also be found on the upper surface). In heavy infestations, nests can be found on twine used for tree training or on dead leaves. Just before laying her eggs, the female becomes sedentary, and develops a loose, fluffy covering of waxy filaments. Eggs are laid, and the ovisac is constructed as she moves along. The female can be found at one end of the ovisac, where she eventually dies. Initially the female in the nest will retain the sage green color, but becomes progressively more yellow as she begins to dessicate. Eggs begin to hatch in early June, although crawlers may not emerge from the nests immediately. Gradually, they disperse to nearby tissue (leaves, especially near the midribs; twigs; leaf axils, and fruit) and begin feeding. Nymphs grow slowly over the course of the summer, and partially grown nymphs seek overwintering quarters in the fall.

For more information about apple mealybug, see: http://jenny.tfrec.wsu.edu/opm/displayspecies.php?pn=135


Grape mealybug

As of studies completed in 2013, grape mealybug is also capable of spreading LChV2 from infected to healthy trees. Grape mealybug overwinters as eggs or crawlers within the loose cottony egg sac under bark scales on scaffold limbs, in other sheltered places on trees, or in duff at the bases of trees. Crawlers start emerging from egg sacs at the beginning of bud swell and begin feeding on the bases of buds. When buds open they go directly to new shoots and leaves. Because some overwintering sites are exposed to sun while others are shaded, crawler movement occurs over a long time, ending at about petal fall. First generation nymphs mature during late June and July in the Northwest. Adult males appear first, mate with last instar nymphs or adult females and die. Receptive females release a pheromone to attract males. Mated females migrate to sheltered areas, lay eggs and die in the egg sac. A partial second generation matures in late August and September.

For more information about grape mealybug, see: http://jenny.tfrec.wsu.edu/opm/displayspecies.php?pn=130.

Grape mealybug overwintering Grape mealybug on sweet cherry
 Grape mealybug overwintering (E. Beers)  Grape mealybug on sweet cherry (E. Beers)
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