Solanum mealybug Phenacoccus solani (C) Whitney Cranshaw, Colorado State University, Bugwood.org
Mealybugs - a problem for home gardeners
By Howard Drury
Introduction
As a grower of several genera of plants susceptible to mealy bug, I have decided to look into the problem and try to find answers to help other growers affected by this pest.
It would appear there are at least four species of native mealybugs attacking our plants, plus several that have arrived in the UK in recent years despite tight biosecurity measures. Combined, they pose significant problems to both amateur growers and those working in commercial horticulture, as few effective methods of control seem successful.
What are The Problems With Mealybug?
They are small (3-5 mm) long sap sucking insects which are able to cover themselves and their eggs and young in a usually white mealy waxy fluffy coating that not only protects the mealybug from predators, it also makes it much harder to control with chemicals. They thrive in warm, humid conditions. Furthermore, they can also act as a vector transmitting viral diseases. Mealybugs are a worldwide problem.
They can suck sap from leaves, stems fruit and even roots in some cases, weakening plants and leaving them prone to infection and stunted growth. As part of their life cycle they exude a clear sticky substance which drops onto foliage below and quickly becomes infected with black moulds, hence the term covered with a sooty mould. This in turn makes it difficult for plants to photosynthesize, leading to poor health.
They are often confused with woolly aphid, which is more commonly seen out of doors.
Life Cycles
These vary slightly according to species. Most continue to bred throughout the year, the higher the temperature, the more numbers increase in frequency. Female mealybugs generally appear as nymphs, and are wingless, they have legs to make them mobile and can reach 3-5 mm in length. Males are much smaller (1 mm) more gnat like, and in the later stages of metamorphosis have wings and are able to fly. Adult females lay up to 500 eggs which hatch in 6 -14 days depending on temperature and these are termed crawlers. They crawl to find their own space to live and ultimately breed. The females go through several stages of moulting and getting bigger, reaching adulthood in around a month. Males only go through two moults and then pupate before hatching as a tiny fly. Adult males have no functioning mouth parts so only survive a few days, during which time they fertilize females, although in most species females can reproduce with being fertilized. There can be eight or more generations per year under warm conditions.
Glasshouse Mealybugs
There are four UK mealybugs commonly found in UK greenhouses all the year round on infected plants, such as Citrus, Fuchsia, Orchids, Passiflora and food crops such as Tomatoes and Peaches. They are sometimes seen on outdoor plants during the summer months, but cannot survive out of doors in cold winters. They include Planococcus citri, Pseudococcus longispinus, P. calceolariae and others Some species of glasshouse mealy bug are often missed as they feed on plant roots, the main culprit are usually Rhizoecus species
Symptoms
Visual presence of white soft bodied insects on host plants the female adults have a wool like covering and they often live together in colonies away from predators and can build up numbers quickly during warm weather. Stunted plants. Yellowing of foliage. Presence of honeydew and sooty moulds. In certain cases, premature leaf fall (Citrus)
Controls
Non-Pesticidal Controls
Avoid soft lush plant growth
Regularly check for honeydew and sooty moulds
Watch for ant activity as they often indicate mealy bug presence
Install yellow sticky traps just above susceptible plants to attract the males
Biological Controls
(These are all temperature dependant, and the predator not escaping in search of other food.)
An Australian ladybird, Cryptolaemus montrouzieri, can be released into greenhouses to control mealybugs. This species is ineffective against mealybug on cacti where they cannot again access to the pests, and to a certain degree also true on some succulents with close foliage.
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.
Pesticidal Controls
These are difficult as the contact type sprays are largely ineffective due to the presence of their mealy coverings. Organic sprays, such as natural pyrethrum (e.g. Bug Clear Gun for Fruit & Veg, Neudorff Bug Free Bug and Larvae Killer), fatty acids (e.g. Solabiol Bug Free, Doff Greenfly & Blackfly Killer) or plant oils (e.g. Vitax Plant Guard Pest & Disease Control, Bug Clear for Fruit and Veg) can give some control of glasshouse mealybugs. These pesticides have a very short persistence and so may require reapplication to keep mealybug numbers in check. Plant oil and fatty acid products are less likely to affect larger insects such as ladybird adults.
Plant invigorators combine nutrients to stimulate plant growth with surfactants or fatty acids that have a physical mode of action against aphids (e.g. Ecofective Bug Control, RHS Bug and Mildew Control, SB Plant Invigorator and Westland Resolva Natural Power Bug & Mildew). These are not considered organic.
More persistent contact-action insecticides include the synthetic pyrethroids lambda-cyhalothrin (e.g. Westland Resolva Bug Killer), deltamethrin (e.g. Provanto Ultimate Fruit & Vegetable Bug Killer, Provanto Sprayday Greenfly Killer) and cypermethrin (e.g. Py Bug Killer)
The systemic neonicotinoid insecticide acetamiprid (e.g. Bug Clear Ultra) is also still available
(Grateful to the RHS for their pesticide guide)
Other Species of Mealybug
The New Zealand flax mealy bug (Trionymus diminutus) iswidley found on phormiums and cordylines in the UK. Other non-native mealy bugs have found on imported plant material include the Bougainvillea mealy bug (Phenacoccus peruvianus), the Coffee mealy bug (Planococcus lilacinus) Notifiable pests include the lantana mealy bug (Phenacoccus parvus) and root mealy bug (Rhizoecus hibisci). It is important that imported stock is carefully Checked. Some root mealy bugs are not notifiable, i.e. Rhizoecus falsifer.
The golden root mealybug, Chryseococcus arecae was first recorded in Britain in 2012 and is thought to be a native of New Zealand. It has established in the UK and now can be found on the roots of many outdoor plants throughout the year. It has been detected on plants of Meconopsis and Primula, where it causes yellowing of the foliage as the mealybug eat plant roots. Infected plants lose vigour. Typical of mealybugs these3- 5 mm long insects, it covers itself in a pale yellow waxy covering, making it different from all other species of mealybug. Unlike most other mealybug this species seems able to survive out of doors in the UK all the year round, making it a potentially serious pest. However, it does not seem to kill host plants
Suppliers of biological controls include
The information given in this Fact Sheet is provided in good faith. It is however of necessity general information and advice on the topic. Howard Drury will not be under any liability in respect of the provision of such advice and information, and you are strongly advised to seek independent advice on any particular gardening problems or queries you may have, preferably from experts who can (when appropriate) inspect the problem before providing advice.
© 2022. This material has been produced by Howard Drury and must not be reproduced in part or full without the written consent of Howard Drury, Kings Heath, Birmingham, B13 0SJ.
JHD/03/04/2022
