Approximately 20 species of soft scale insects are known to attack grapevines in various regions of the world. Five species are now known to infest grapes in B.C., but only two are commonly found: European fruit lecanium scale (EFLS) (Parthenolecanium corni) and cottony vine scale (CVS), also known as cottony grape scale (Pulvinaria vitis).
Both EFLS and CVS have been upgraded from minor to major pests due to their ability to transmit certain grapevine leafroll diseases, including grapevine leafroll-associated virus 3 (GLRaV-3), which is the most widespread and economically damaging leafroll disease in B.C.
EFLS and CVS are also a concern in the production of table grapes where the large quantities of honeydew that they produce supports the growth of sooty mould fungus that can leave fruit unmarketable.
Soft scales have become more abundant recently.
Female EFLS, also known as brown apricot scale, when fully grown, have a chestnut brown, smooth and slightly shiny protective shell that is 5-8 mm long and slightly longer than wide. Certain races in some regions reportedly produce males, but most females reproduce without mating.
The small nymphs or crawlers that hatch from the small oval, pearly white eggs are yellow to orange in colour.
Mature female CVS are similar in size and colour to EFLS, but their bodies are slightly more elongate. The masses of cottony material containing orangey-red eggs that extrude from underneath the scale tilts the females upward.
In other regions, male CVS are produced in late summer, do not have mouthparts, live for only a few hours to one week and mate with immature females. In North America, it is presumed that the males are not present and that the females reproduce without mating, resulting in female offspring only.
Mature female scales of both species become brittle after the eggs are laid and can easily be detached from the one- to three-year-old wood with a fingernail.
EFLS overwinter on older wood where they can find shelter, often under the bark. In spring, when the sap begins to flow, nymphs go through a growth spurt and moult in April to become adult females.
In May, females produce 2,000-3,000 eggs under the body cavity, which hatch into crawlers in June and July. The crawlers disperse to the undersides of leaves where they settle and feed until early fall when they moult into second instars for the winter.
Occasionally, there is a partial second generation with females producing fewer eggs while settled on the current year’s shoots, clusters and leaf petioles.
CVS overwinter as young adult females primarily on new canes. In spring, the adult female will increase in size, become darker in colour, more convex in shape and form a cottony-like egg mass. Adult females produce, on average, 3,500 eggs but have been known to produce up to 5,000.
Crawlers disperse in June and July to the undersides of leaves where they settle and feed until early fall. Mortality is as high as 90% during the crawler phase, primarily due to environmental conditions and failure to successfully settle on the host. Lacking a protective waxy covering, the crawler stage is susceptible to environmental conditions and is the stage most susceptible to insecticides.
CVS increase in size over the summer, with the final moult in fall giving rise to the overwintering young adult female that is no larger than 3 mm in size. In autumn, the scale migrates from leaves to woody tissues and develops a hardened protective shell for the winter.
Both species of soft scale feed on the phloem of grapes and other woody plants, including fruit trees and ornamentals. Direct feeding damage from the removal of plant fluids is rarely a cause for concern, but the additional stress caused to weak vines might be sufficient to hasten death.
The greatest causes for concern are the ability of EFLS and CVS to transmit GLRaV-3 in vineyards and the vast amounts of honeydew they produce that fouls the fruit and supports the growth of sooty mould fungus, potentially rendering table grapes unmarketable.
Look for partly grown CVS during pruning and for mature females with cottony egg masses in late spring.
Watch for honeydew on leaves and fruit during late spring and summer. Ants feed on the honeydew and will protect the scale from predators. They can often be seen in large numbers hurriedly running up and down posts or vine trunks to and from infested canes.
To detect when small crawlers are active, remove the loose bark in an area near the infestation and encircle the vine with clear tape applied sticky side out or use double-sided adhesive. Replace the tape weekly and check for the orange-coloured crawlers. A good magnifying glass or dissecting microscope is useful for this purpose.
There are no established thresholds for direct damage from soft scale infestations, and small to moderate numbers of scale on a healthy vine are unlikely to result in direct damage due to the removal of fluids and nutrients. Treatment of table grapes is required when the honeydew they excrete fouls the fruit.
It is important to control infestations of soft scale if vines or neighbouring vines are known to be infected with GLRaV-3 or other leafroll viruses.
For more information about chemical control, see the Pesticides page.
In European vineyards, scale populations are often controlled by parasitic wasps and predators. Very little was known about the natural enemies of soft scale, specifically CVS, in the Okanagan prior to a study initiated over the 2019 field season. The study of soft scale parasitism and updated information (Brauner) was conducted as part of the Canadian Grapevine Certification Network grapevine virus project.
Of 545 adult female CVS, with and without egg masses, collected in late May to early June, just over 80% were parasitized. A total of 990 parasitoids of six different species were reared from 42 adult females.
On average, 23 parasitoids emerged per adult scale, while 68 were collected from one individual. The most abundant parasitic wasp species was Coccophagus scutellaris (94%).
Of the scales producing egg masses, 93% had silver fly larvae (Leucopis species) consuming a very large portion of the eggs. Of nymphs collected from leaves throughout the summer, approximately 67% were parasitized by 13 different parasitoid species.
In addition to parasitoids and silver flies, soft scale are also preyed upon by lady beetles and other generalist predators. English sparrows and several species of warblers that feed on fully grown soft scale have been known to curtail infestations in some regions.
Healthy vines of moderate vigour are less susceptible to scale infestations and are better able to withstand damage.
Routine pruning removes many scales, and a small infestation might be pruned out in spring or early summer. Removal of basal leaves in late June from the fruiting zone for the control of leafhoppers will also help reduce scale populations.
