Mealybug

Mealybugs are common insect pests that tend to live together in clusters in protected parts of plants, such as leaf axils, leaf sheaths, between twining stems and under loose bark. They 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.

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 and redcurrant.

Most mealybugs thrive in warm conditions, which is why they are not usually a problem on outdoor plants. Mealybugs are active all year round on houseplants and in greenhouses.

You may see the following symptoms:

• Infestations are 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
• Heavy infestations 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

Non chemical 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 for a month or so 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.

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.) are also available for use against this pest.

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 chemical control. They are available by mail order from suppliers of biological controls.

Chemical 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.

• The systemic insecticide, acetamiprid (e.g. Bug Clear Ultra) can be used on ornamental plants and some listed edibles such as greenhouse-grown tomatoes, peppers, cucumbers and aubergines, but not other edible plants, provided manufactures instructions on application and harvest intervals are followed
• Contact action insecticides deltamethrin (e.g. Bayer Sprayday Greenfly Killer and Bayer Provado Ultimate Fruit & Vegetable Bug Killer), lambda-cyhalothrin (e.g. Westland Resolva Bug Killer) and cypermethrin (e.g. Py Bug Killer) can be used on ornamental plants and some listed fruits and vegetables, provided manufactures instructions on application and harvest interval 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
• Other organic treatments for use during the growing season include fatty acids (e.g. Bayer bug free or Doff Greenfly and Blackfly Killer), plant oils or extracts (e.g. Vitax Organic Pest and Disease Control or  Bug Clear for Fruit & Veg Gun!) or natural pyrethrins (e.g. Defenders Bug Killer). These organic pesticides have a contact action and short persistence and so may require more frequent use. They can be used on all edible plants provided manufactures instructions are followed.

Plants in flower should not be sprayed due to the danger to pollinating insects.

Download

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

Biological control suppliers (Adobe Acrobat pdf)

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).

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.

Aphids
Glasshouse leafhopper
Glasshouse red spider mite
Glasshouse whitefly
Golden root mealybug
Scale insects
Woolly aphid