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Mealybugs - fact sheet
Mealybugs belong to the scale insect group, of which they make up about a third of known species. They have a worldwide distribution, occurring in all except the polar regions, and are one of the most economically important groups of insects known to humans because they attack many cultivated food and ornamental crops. Mealybugs are so named because many of the known species are covered in a whitish ‘mealy’ wax, which helps retard the loss of water from their soft bodies. They generally prefer warm, humid, sheltered sites away from adverse environmental conditions and natural enemies. Different species of mealybugs prefer different feeding sites - some species feed in and under bark, while others feed on fruits, flowers or seed heads.
Mealybugs can build up in huge numbers in a very short time and cause considerable damage. They feed by inserting their straw-like mouthparts, known as ‘stylets’, into plant tissue. Feeding damage can be either by direct removal of plant fluids and nutrients, and/or by the excretion of toxic salivary compounds into plant tissue.
Honeydew - the waste product of the mealybug feeding process - is a perfect growth medium for sooty mold fungi. These molds damage plants by covering leaves and reducing light available for photosynthesis. Australia has a number of native mealybugs (and other scale insects) which are now worldwide pests, the worst being the Long-tailed Mealybug (Pseudococcus longispinus) and the Citrophilus Mealybug (Pseudococcus calceolariae). One of the more interesting mealybugs occurring in Australia is the Golden Mealybug (Nipaecoccus aurilanatus), which thrives on the Norfolk Island Pine (Araucaria heterophylla) and other araucarias. This mealybug has a dark purple body with a vivid yellow longitudinal stripe of mealy wax on its dorsal (upper) surface. The insects may be seen nestling under the leaf scales of the Norfolk Island Pine, and affected trees are immediately recognisable by their black coating of sooty mold.
Various species of ants ‘farm’ mealybugs in much the same way that humans farm cows. In return for honeydew, the mealybugs are given shelter in the form of ‘barns’ constructed by their ant ‘farmers’ from pieces of dead plant material, soil, etc. ‘Barns’ can be found on protected flat surfaces, or in the axils of plant leaves. Most species of ants that ‘farm’ mealybugs also aggressively defend their ‘herds’ from predators and parasites. Ants ‘milk’ the mealybugs - when they stroke a mealybug’s abdomen it responds by exuding a drop of honeydew.
Most mealybugs (especially pest species) have numerous, often overlapping, generations per year. Like all insects, their development is dependant on temperature: there is a threshold temperature for each particular species of mealybug, below which development either ceases totally (dormancy) or is slowed to a greater or lesser degree (quiescence). Just as there is a minimum threshold temperature, there is also a maximum threshold temperature, beyond which development is slowed or ceases all together.
If temperatures remain elevated for prolonged periods, insect mortality increases rapidly with a consequent crash in population size. Mild to warm conditions are therefore favourable for insect development, and mealybugs are no exception to this rule. Temperatures of about 25°C and a high relative humidity are optimum for mealybugs in Australia, and their populations reach peaks in spring and autumn.
Eggs can be laid singly or in clusters, and female Long-tailed Mealybugs have been recorded as laying as many as 200 eggs in a lifetime. Egg clusters are usually embedded in a cocoon of waxy filaments, with the structure varying between species, from tightly packed to loose.
On hatching, the juveniles, known as ‘crawlers’, move away from the cocoon and search for suitable feeding sites in sheltered areas. The juveniles progress through five moults before reaching adulthood. In the case of the males, the last juvenile instar pupates in a silk cocoon, and emerges as a winged adult. Adult males do not feed, having no mouthparts - their sole purpose is to mate with females and pass on their genes to their many offspring.
Because mealybugs have high reproductive capacities and multiple generations in a year, they have the potential to become resistant to pesticides very quickly. The use of stronger and stronger pesticides breeds more and more resistant mealybugs, until the stage is reached where efficient and practical chemical control of the pest is no longer possible. Fortunately mealybugs can be controlled using ‘soft’ methods including biological agents and low-toxicity pesticides, most of which are readily available to the horticultural industry and the home gardener.
Biological agents: Good control of mealybugs can be achieved by releasing parasitic wasps such as Leptomastix dactylopii and Anagyrus fusciventris into the infested area. The wasps lay their eggs into young mealybugs: on hatching, the wasp larvae feed on the internal fluids of the mealybugs. The mealybugs are usually killed when the wasp larvae pupate. The wasps emerge from the dead bodies of the mealybug hosts as winged adults and fly off in search of mates and mealybugs (if female), then the cycle is repeated.
Predatory ladybird beetles such as Cryptolaemus montrouzieri also attack many species of mealybugs, including the Long-tailed Mealybug, although they don’t match the effectiveness of the parasitic wasps in terms of control. Cryptolaemus is most effective when both it and mealybugs are present in large numbers - it therefore needs to be released en masse, e.g. in lots of at least 1000 or more. The actual population of mealybugs also needs to be quite high so that the beetles can locate them quickly without becoming discouraged and flying off in search of ‘greener pastures’. Parasitic wasps tend to outperform Cryptolaemus beetles in most situations because they are more active and persistent hunters, and because the wasps are usually species-specific. It is harder, however, to use the wasps as control agents because many of the species are currently very difficult to mass produce.
Predatory insects (and mites) are available from a number of companies in Australia. For your nearest supplier, please contact the Australasian Biological Control Association.
Chemicals: There are a number of insecticides registered for control of mealybugs. Please see your local nursery for information.