What is mealybug?

Glasshouse mealybugs are common insects that tend to live together in clusters in inaccessible parts of plants, such as leaf axils, leaf sheaths, between twining stems and under loose bark. There are also mealybug species found on plant roots. Mealybugs suck sap from plants and then excrete the excess sugars as a substance called honeydew. This lands on the leaves and stems were it is often colonised by sooty moulds, giving the surfaces a blackened appearance.

• Glasshouse mealybugs are found mainly on greenhouse plants and houseplants, especially cacti and succulents, African violets, bougainvillea, citrus plants, fuchsia, grape vines, hoya, orchids (especially Phalaenopsis), oleander, passion flower, peach and tomato
• Some other mealybug species can attack outdoor plants, such as ceanothus, laburnum, New Zealand flax (Phormium mealybug) and redcurrant.
• Glasshouse mealybugs thrive in warm conditions, which is why they are not usually a problem on outdoor plants. Glasshouse mealybugs are active all year round on houseplants and in greenhouses
• Some species of mealybug feed on plant roots, most of these are Rhizoecus species and are also confined to glasshouse and house plants. One species the golden root mealybug, will survive on roots out of doors

Symptoms

You may see the following symptoms:

• Mealybug is usually first noticed as a fluffy white wax produced in the leaf axils or other sheltered places on the plant. The insects or their orange-pink eggs can be found underneath this substance
• Large numbers of mealybugs may result in an accumulation of honeydew. This makes plants sticky and encourages the growth of sooty moulds, giving leaf and stem surfaces a blackened appearance
• Severe infestations will reduce plant vigour and stunt growth. Heavy infestations may cause premature leaf fall
• Root mealybugs (Rhizoecus species) are also covered in a white waxy substance and found on plant roots. The golden root mealybug is yellow in colour

Control

Non-pesticide control

• Female mealybugs do not fly or crawl far, so infestations are usually brought in on an infested plant. Inspect new plants carefully before putting them in a greenhouse or conservatory and, where possible, keep them in quarantine (isolated from other plants) for at least a month before adding new acquisitions to an existing collection
• Dead leaves and prunings should be removed from the greenhouse as these may have mealybugs or eggs on them
• It can be simpler to dispose of heavily infested plants rather than try to eliminate mealybugs
• Plant can tolerate some levels of mealybug, although popuations can quickly build if left unchecked

Biological control

A ladybird, Cryptolaemus montrouzieri, can be released into greenhouses to control mealybugs. Note that the ladybird's larvae look like large mealybugs! Both the adult ladybirds and their larvae are able to find and eat mealybugs and their eggs in confined spaces on the plants. Parasitic wasps (Leptomastix spp., Leptomastidea spp. and Anagrus spp.) are also sometimes available for use against these insects. The parasitic wasps can give control of mealybug populations where population levels are fairly low.

The ladybird and parasitic wasps need relatively high temperatures and so are only likely to be successful during May to September. They are susceptible to most insecticides and should therefore be used as an alternative, rather than in addition to pesticide control. They are available by mail order from suppliers of biological controls.

Pesticide control

Due partly to the waxy covering mealybugs are difficult to control with insecticides, affected plants should be sprayed thoroughly. In many cases it may be impossible to eliminate mealybugs.

• Organic sprays, such as natural pyrethrum (e.g. Bug Clear Gun for Fruit & Veg, Ecofective Bug Killer), fatty acids (e.g. Solabiol Bug Free, Doff Greenfly & Blackfly Killer) or plant oils (e.g. Vitax Organic Pest & Disease Control, Bug Clear for Fruit and Veg) can give some control. These pesticides have a very short persistence and so may require reapplication to keep mealybugs in check. Plant oil and fatty acid products are less likely to affect larger insects such as ladybird adults. These products can be used on most edibles provided label instructions are followed
• Organic Winter Wash (contain plant oils e.g. Growing Success Winter Wash) can be used on peach and grape vines in December while they are fully dormant. Scrape loose bark off grape vines before treatment in order to expose hidden mealybugs. Spread newspaper under the vine to collect the bark scrapings for disposal
• More persistent insecticides include the synthetic pyrethroids lambda-cyhalothrin (e.g. Westland Resolva Pest Killer), deltamethrin (e.g. Provanto Ultimate Fruit & Vegetable Bug Killer) and cypermethrin (e.g. Py Bug Killer). These can be used on ornamental plants and some listed fruits and vegetables, provided manufactures instructions on application and harvest interval are followed
• The systemic neonicotinoid insecticide acetamiprid (e.g. Bug Clear Ultra) is also available. This product can be used on ornamental plants and some listed edibles such as greenhouse-grown tomatoes, peppers, cucumbers and aubergines, but not other edible plants
• Follow label instructions when using pesticides. On edible plants make sure the food plant is listed on the label and follow instructions on maximum number applications, spray interval and harvest interval
• Plants in flower should not be sprayed due to the danger to pollinating insects
• Inclusion of a pesticide product does not indicate a recommendation or endorsement by the RHS. It is a list of products currently available to the home gardener

Download

Pesticides for gardeners (Adobe Acrobat pdf document outlining pesticides available to gardeners)

Biological control suppliers (Adobe Acrobat pdf)

Biology

Several species of mealybug occur in greenhouses or on houseplants. These include Pseudococcus calceolariae (Glasshouse mealybug), P. longispinus (Long tailed mealybug) and Planococcus citri (citrus mealybug) and Rhizoecus species (root mealybugs).

The adult females have flattened oval-shaped soft bodies up to 4mm in length; they are sometimes pink in colour but appear whitish due to the white, waxy powder that covers their bodies. Waxy filaments project from the edges of their bodies. Some species are all female; others have small winged males, but the latter are infrequently seen.

Female mealybugs lay eggs under a white, waxy coating. Mealybug nymphs resemble the adult insects and can complete their development in about a month in mid-summer. Breeding continues throughout the year in greenhouses, but takes place at a slower rate in winter.