First identified in Australia from cotton crops in the Emerald and Burdekin regions during the 2009-10 season, the solenopsis mealybug (Phenacoccus solenopsis) is widely distributed throughout eastern Australia at low densities, and outbreaks in cotton and horticulture occur when local conditions are suitable. Possible triggers may include host availability (allowing population build up), climatic conditions, use of broad-spectrum insecticides, water stress, high levels of nitrogen fertiliser use, or limited natural enemy activity.
On this page:
- Description and lifecycle
- Host plants
- Dispersal and survival
- Natural enemies
- Further information
Description and lifecycle
Solenopsis mealybug form dense, white, waxy colonies on stems, shoots and leaves. Female adults are wingless with a 3-4 mm long oval-shaped body covered by a waxy coating (giving them a ‘mealy’ or ‘cottony’ appearance). Dark bare areas on the thorax and abdomen appear as dark longitudinal lines. Adult males are about 1 mm long, with a grey body and a single pair of transparent wings. Two long filaments of white wax project from the end of the abdomen. The adult male has no feeding mouthparts and causes no damage.
Australia has a large number of native mealybug species including Paracoccus, Ferrisia and Pseudococcus spp.
Mature females lay between 150-600 eggs in waxy sacs. After 3-9 days the eggs hatch into nymphs called ‘crawlers’ that are relatively bare and pearly white to yellow, without a thick coating of wax. Crawlers are very mobile and move rapidly around the plant. Later instars are similar in appearance to adult females.
The female crawler has four larval instars before moulting to adult (with no pupal stage). The total life span of the female is around 30 days. Male crawlers undergo three larval instars before spinning a cottony cocoon in which it spends about a week in a pupal stage. A male adult lives for only 3-5 days. Mealybugs can have 12-15 generations per year.
Cotton (Gossypium hirsutum) and a wide range of other cultivated plants (e.g. tomatoes, eggplant, chilli, melons, potatoes, mungbean) and broadleaf weeds are hosts. In Pakistan it has been recorded on 154 plant species including field crops, vegetables, ornamentals, weeds and trees. In Australia, solenopsis mealybug has been recorded from a range of common weed species such as pigweed, sow thistle, bladder ketmia, native rosella, vines (cow, bell and potato), crownbeard and volunteer cotton.
Adults and nymphs pierce and suck the sap from both soft and hard plant tissue – this can occur at all stages of cotton crop development. Damage is often patchy, and may be worse in areas where the crop is under stress (e.g. poorly drained areas). Heavy infestations that start early and persist can lead to plant death.
Symptoms of cotton plants infested during the vegetative phase include:
- distorted and bushy shoots
- crinkled and/or twisted and bunchy leaves
- stunted plants that die in severe cases.
Symptoms of late season infestations:
- reduced flower and boll development
- fewer, smaller and deformed bolls
- reduced plant vigour
- early crop senescence
- honeydew contamination of lint.
Little is known about the potential impact of this pest on other broad-acre crops under field conditions. Preliminary trial work conducted in 2011 under glasshouse conditions has shown that solenopsis mealybug can have a significant impact on the growth of seedling and reproductive sunflowers but little impact on pulse crops (peanut, soybean and mungbean).
Produced during feeding, honeydew forms a sticky deposit on the leaves, stem and lint. Honeydew promotes the growth of sooty mould fungi which can inhibit photosynthesis and plant growth. Ants feed on the honeydew and spread the mealybugs. Ants also clean the colonies and protect the mealybugs from predatory insects (such as ladybird beetles).
Dispersal and survival
The female mealybug cannot fly, but is capable of dispersive movement around a plant, and perhaps from plant to plant. Mealybugs are generally dispersed between plants and fields as first/second instar crawlers. While this type of movement is localised, mealybugs can also be transported long distances by wind and rain or in water in irrigation channels. Long-distance movement through the transport of infested plants and farm machinery is also possible.
Mealybugs can overwinter (an inactive state) to survive cold conditions, both on the host plant and in the soil. In warm climates, mealybugs reproduce all year round.
Their broad host range provides the potential to survive for significant periods on a series of crop and non-crop hosts. Research has shown that the species can also live for relatively long periods without food or water, for example on farm machinery.
Numerous biological control agents are active in Australia. Predators include ladybird beetles, lacewings, red and blue beetles, smudge bugs, earwigs and native cockroaches. One of the most successful predators is the mealybug ladybird (Cryptolaemus montrouzieri ), whose larvae look like an oversized mealybug.
At low densities, mealybugs can be present anywhere on the plant, although they prefer sheltered positions (undersides of leaves or in bracts) in the upper crop canopy.
Crop stress, such as waterlogging, may make cotton more susceptible to mealybug, so it is important to include stressed areas such as tail drains when checking. Investigate patches of stunted or dead plants. As solenopsis mealybug has a very wide host range, also monitor surrounding vegetation including gardens.
If hotspots (areas of dead plants) are found, mark the spot to allow regular checking of population dynamics (to see if beneficial populations are having an impact), and minimise movement of staff and machinery through the area.
Mealybugs tend to colonise protected parts of the plant (e.g. undersides of leaves or in bracts), making chemical contact difficult. A focus on minimising mealybug populations in weeds and volunteer hosts will limit the size of infestations, and the overall impact of this pest in crops.
Natural enemies have proven to be very effective at reducing high mealybug populations, and minimising the build up of populations in crops. Farm hygiene is very important to minimise spread of this pest.
A basic mealybug management strategy includes:
- Minimise on-farm sources of mealybug survival and build up – remove weeds in and around the field. The early removal of affected plants may reduce mealybug numbers in the rest of the crop. Do not place removed plants near water channels.
- Include mealybug in pest monitoring schedules. Visit areas that are under stress where populations may develop first. Monitor abundance of adults, nymphs and natural enemies over time to provide a picture of population changes.
- Practice good farm hygiene and clean all equipment that has been in affected fields. Avoid physical contact with infested plants as mealybugs easily adhere to clothing and implements. Use industry Come-Clean-Go-Clean protocols to minimise the spread of mealybug.
- Avoid early season use of broad spectrum insecticides – these kill natural enemies that may otherwise control mealybug. Select the softest option when controlling other pests to conserve mealybug predators.
- Consider innundative releases of beneficials such as cryptolaemus and/or lacewings in hotspots.