Soft Scales (Coccidae) on Grapevines in Australia

Note Number: AG1369
Published: October 2008
 

This information note discusses the biology of soft scales that infest grapevines in Australia. The information is intended as the basis for grapegrowers and vineyard managers to develop sustainable control strategies for the pests.

Introduction

Grapevine scale, Parthenolecanium persicae, is a well known and widely distributed pest of grapevines in Australia. A recent study of soft scales on grapevines in Australia (Rakimov 2008, in preparation) has shown that five other soft scales are present on grapevines. The most common of these are frosted scale, Parthenolecanium near pruinosum, and soft brown scale, Coccus hesperidum, which were often in mixed infestations with grapevine scale. Further research is needed to resolve the taxonomy of frosted scale, as specimens from Australia were not identical to type specimens of Parthenolecanium pruinosum (Ben-Dov, personal communication). For the purpose of this document, the common name of “frosted scale” has been used for Parthenolecanium near pruinosum in Australia.

Nigra scale, Parasaissetia nigra, and a Saissetia sp. (probably black scale, Saissetia oleae) were found in a few vineyards and backyard grapevines, but are not considered as significant grape pests.

Long soft scale, Coccus longulus, was found on table grapes at Carnarvon and winegrapes at Margaret River, WA, and in the Hunter Valley of NSW. It may become a minor grape pest as table grapes become more common in warmer areas of Australia.

Identification of scales

It is difficult to distinguish the different species of soft scales on grapevines, particularly the juvenile stages. However, mature females on grapevines in spring have characteristic features which help to visually distinguish the different species (Table 1). In late winter and spring, grapevine scales grow to mature females and produce up to 5000 eggs. At this stage, grapevine scales can be distinguished by the shape and colour of the adult female (Figs. 2-5), and the colour of the eggs (Table 1).

Table 1. Characters that can help to visually distinguish mature soft scales found on grapevines in Australia.

Scale species	Mature females	Eggs and/or crawlers
Grapevine scale	Oval shape, about 7mm long, dark brown scale.	Yellow eggs laid in mass beneath the body of the adult scale, crawlers yellow.
Frosted scale	Oval shape, 6-8mm long, strongly convex. Scale dark brown, sculptured and covered with white powdery wax	White eggs, laid in mass beneath the scale body. Crawlers white.
Soft brown scale	Oval shape, 3-4 mm long, yellow-brown mottled with brown spots.	Crawlers are light brown, born alive.
Black scale	Oval shape, 3-5 mm long, convex, scale black to dark brown. Ridges on top surface of scale forming a “H” shape, most obvious in early stage adults.	Pink eggs, laid in mass beneath the scale body. Crawlers light brown.
Nigra scale	Elongate oval in shape, 3-4 mm long. Smooth, shiny dark brown to black scale.	Eggs laid in mass beneath the scale, crawlers light brown.
Long soft scale	Elongate oval shape, 4-6 mm long, yellow to grayish brown, with visible eye spots.	Crawlers born alive.

Life cycle of grapevine scale and frosted scale.

The life cycle of grapevine scale and frosted scale is very similar. Both scales have predominantly female populations, although Rakimov (2008) has recorded the presence of male grapevine scale in Australian vineyards. There is generally only one generation of grapevine scale and frosted scale per year.

In autumn and early winter, juvenile grapevine scales and frosted scales move from leaves to the shoots and wood of the grapevines. The juvenile scales are most obvious on the underside of canes and spurs, but are also found under the bark of the grapevines (Fig 1).

The adult females die after laying eggs, which remain under the scale cover of the dead female until the crawlers hatch. They colonise the leaves and shoots of grapevines after budburst, settling on the newly emerged leaves and shoots.
The young scales are mostly on the underside of the basal leaves of the shoots, but are small, translucent, and difficult to see without a microscope or hand lens. One obvious clue to the presence of soft scales on grapevines is the activity of ants, which attend the juvenile and maturing scales to harvest honeydew excreted by the scales.

The scales remain on the leaves and green shoots, growing slowly during the summer months, until moving from the leaves and shoots in autumn.

Effects of scale on grapevines

In high numbers, grapevine scales can reduce the vigour of grapevines, but more commonly the main consequence is the production of honeydew by soft scales, which attracts ants and is a substrate for the development of sooty moulds. The honeydew and sooty moulds reduce the quality of grapes for fresh markets, and where the grapes are processed for dried fruit or wine. The ant activity on the grapevine disrupts biological control, not only of the grapevine scale, but also of other honeydew producing pest insects such as mealybugs, which can lead to increased and sustained pest populations. Ant suppression (e.g. by vineyard soil management or treatment of ant nests) should help sustain biological control and thus reduce populations of honeydew producing insects.

Biological control of soft scales on grapevines

The grub of native ladybird beetle, Rhyzobius submetallicus, the caterpillar of the native moth Mateomera dubia, and the larvae of green lacewings are useful predators of the eggs and crawlers of grapevine scale. They are probably most effective in reducing high populations of soft scales on grapevines.

There are several minute wasps that parasitise the different soft scales. The most common and effective parasitic wasp attacking grapevine scale was Metaphycus maculipennis, a solitary parasite of juvenile scale and a gregarious parasite of the mature grapevine scales. Up to 62 wasps of M. maculipennis were recorded emerging from a single grapevine scale (Rakimov 2008). Parasitism of the mature scales by M. maculipennis reduces the number of eggs produced by the female scale. The parasitised scales are easily recognised by the multiple emergence holes in the scales (Figures 6, 7).

Other parasitic wasps reared from soft scales on grapevines are Metaphycus helvolus, Coccophagus lycmnia,and C. semicircularis. Metaphycus helvolus is a parasite of soft brown scale, nigra scale, and black scale, while C. lycmnia and C. semicircularis parasitise soft brown scale and black scale (Malipatil et al 2000). Coccophagus ceroplastae was recorded as a parasite of long soft scale.

Management of scale infestations in vineyards

The best time for growers to assess scale populations in their vineyards is during winter, after leaf fall.

The immature scales can be seen with the naked eye or a hand lens, and are most abundant on the underside of canes or spurs. Where scales are found on a vine, a good idea of the extent of the infestation can be found by stripping loose bark away to expose scales sheltered beneath the bark. Both grapevine scale (Fig. 2) and frosted scale (Fig. 4) will appear as dark, oval shaped scales, but it is difficult to distinguish between the juvenile stages of these species.

The scale infestations across the vineyard are typically patchy, with some vines having high scale populations, and others lightly infested. Marking of the infested vines (e.g. with flagging tape) is useful where later spot spraying of the heavily infested vines is planned.

Spraying infested grapevines with oil is an effective way of controlling the scales. As pruning operations remove some of the scale infested wood, it is more economical and efficient to spray after pruning, but care must be taken to complete spraying while the vines are fully dormant, as winter oil sprays can have phytotoxic effects if sprayed too close to budburst. The spray machines should be set up to achieve good coverage of the underside of canes or spurs. Spot spraying of the vines with dense infestations only will help preserve predators and parasites of the scales on the unsprayed parts of the vineyard.

Growers should also inspect scale infestations in early spring, to determine the species of scale present (Table 1), and in the case of grapevine scale, the extent of parasitism of mature scales as evidenced by the exit holes in the parasitised scales (Figs 6.7). This information is useful for planning integrated pest management practices in the vineyard.

During the growing season, scales are on the stems and underside of leaves. The scale populations on leaves are likely to be reduced by predators such as green lacewings and by high temperatures, and in some cases by broad spectrum pesticides applied for control of other vineyard pests. The presence of ants attending the scales is a useful indicator of scales and other honeydew producing insects. The suppression of ant populations may assist in reducing populations of Figure 7. Female Metaphycus maculipennis emerging from a mature grapevine scale.

honeydew producing insects. In most cases it is doubtful that specific control measures are warranted for scales during the growing season on the basis of economic damage, and the effectiveness of sprays will depend on the ability to achieve spray coverage on the underside of leaves. Insecticidal options for scale control in vineyards are discussed in Bernard et. al. (2007).

References

Bernard et. al. (2007). Guidelines for environmentally sustainable winegrape production in Australia: IPM adoption self-assessment guide for growers. Australian Grapegrower and Winemaker, March 2007.

Malipatil, M.B., K.L. Dunn, and D. Smith (2000). An illustrated guide to the parasitic wasps associated with citrus scale insects and mealybugs in Australia. Department of Natural Resources and Environment, Institute for Horticultural Development, Private Mail Bag 15, South Eastern Mail Centre, Victoria 3176.

Rakimov A. (2008, in preparation). “Soft scales on grapevines in Australia: aspects of their distribution, ecology and biological control”. PhD Thesis, University of Melbourne.

Further information

Contact DPI Mildura, Division of Biosciences Research, P.O. Box 905, Mildura Vic 3502.

Acknowledgements

The information presented here is a result of a research project carried out by Mr. Adrian Rakimov, a PhD candidate based at DPI Mildura and supervised by Prof. Ary Hoffmann (University of Melbourne), Dr. Mallik Malipatil and Dr. Greg Buchanan (DPI Victoria). The project was jointly funded by DPI Victoria, the Grape and Wine Research and Development Corporation, and the University of Melbourne.

The photographs in this publication were taken by Mr. Adrian Rakimov. This publication was compiled by Dr. Greg Buchanan, DPI Mildura.

This Agnote was developed by Greg Buchan, Biosciences Research. October 2008.