Mealy Bugs

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Mealybug

Pseudococcus Long Tailed Mealybug:Pseudococcus longispinus

Brief
Mealybugs are small insects covered with a white mealy coating; some have white hairs attached to their bodies. The bugs feed by sucking on the plant juices. Mealybugs excrete a sticky substance called honey dew which ants like to feed on. The honeydew also provides a perfect medium for sooty mould growth. Mild temperatures and high humidity are perfect conditions for mealybugs to breed as eggs hatch every 2-3 weeks. Prolonged hot weather reduces numbers. Heavy infestations can occur on citrus trees, daphne, and other ornamental plants. Orchids and ferns, especially in shadehouse, can also become infested. Mealybugs can also attack bulbs in storage and the roots of some plants such as polyanthus, liliums and callas. Mealybugs are insects in the family Pseudococcidae, unarmored scale insects found in moist, warm climates. They are considered pests as they feed on plant juices of greenhouse plants, house plants and subtropical trees and also acts as a vector for several plant diseases.

Description
Mealybugs are sexually dimorphic, meaning that the sexes have distinct morphological differences.

Females are nymphal, exhibit reduced morphology, and are wingless, though unlike many female scale insects, they often retain legs and can move. The females do not change completely and are likely to be neotenic (exhibiting nymphal characteristics).

Males are winged and do change completely during their lives. Since mealybugs (as well as all other Hemiptera) are hemimetabolous insects, they do not undergo complete metamorphosis in the true sense of the word, i.e. there are no clear larval, pupal and adult stages, and the wings do not develop internally. However, male mealybugs do exhibit a radical change during their life cycle, changing from wingless, ovoid nymphs to "wasp-like" flying adults.

Mealybug females feed on plant sap, normally in roots or other crevices. They attach themselves to the plant and secrete a powdery wax layer (therefore the name mealybug) used for protection while they suck the plant juices. The males on the other hand, are short-lived as they do not feed at all as adults and only live to fertilize the females.

Male citrus mealy bugs fly to the females and resemble fluffy gnats.

Some species of mealybug lay their eggs in the same waxy layer used for protection in quantities of 50–100; other species are born directly from the female.

The most serious pests are mealybugs that feed on citrus; other species damage sugarcane, grapes, pineapple (Jahn et al. 2003), coffee trees,cassava, ferns, cacti, gardenias and orchids. Mealybugs only tend to be serious pests in the presence of ants because the ants protect them from predators and parasites. Mealybugs also infest some species of carnivorous plant such as Sarracenia (pitcher plants), in such cases it is difficult to eradicate them without repeated applications of insecticide such as diazinon. Small infestations may not inflict significant damage. In larger amounts though, they can induce leaf drop.

Biology
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.

Life Cycle
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.

Control

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.

Chemicals: There are a number of insecticides that are registered for control of mealybugs. Please call us for further information on 1300 882 787 or email us.

Symptoms
New growth appears distorted with a whitish substance in the leaf axils. Close inspection will reveal the the bugs.


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