Mealybug: Identification and Treatment

What are the symptoms of a scale insect infestation?

Small and discreet, scale insects are often difficult to spot.

Yet a few telltale signs betray their presence for those who know what to look for: clusters of whitish secretions, small brownish spots, sticky droplets on the leaves. The foliage turns yellow prematurely or becomes covered with a black, sooty substance. Young shoots become deformed. Growth and flowering are reduced.

A black sooty mould deposit almost completely covers a eucalyptus leaf. Meanwhile, ants feast on the honeydew secreted by scale insects (Wikipedia)

Bear in mind that a scale insect attack rarely kills a plant. The damage is primarily aesthetic, though vegetable crops may suffer. Regular monitoring helps prevent infestations and stay one step ahead.

Mealybug

In our temperate regions, mealybugs are more of a problem in greenhouses and indoors.

Outdoors, it’s in tropical and subtropical zones that they cause damage in vegetable gardens, orchards and ornamental gardens. Mealybugs can be observed on date palms, citrus trees, succulents…

The most common species are: Planococcus citri, Pseudococcus viburni and Pseudococcus longispinus.

The males resemble small wasps and are not harmful to plants – they are rarely observed. The females are white, ovoid in shape. Wax filaments of varying lengths can be seen depending on the species. The larvae are yellow in colour.

Their life cycle depends on temperature: the warmer it is, the faster their development and consequently the greater the proliferation. Fertilised females lay several hundred eggs in a white cottony mass before dying. The larvae hatch and feed on plant sap. They will go through 3 larval stages before becoming adults.

Planocuccus citri (ephytia.inra.fr) – Pseudococcus-viburni (ephytia.inra.fr) – Pseudococcus longispinus (bugguide.net)

The scale insect

Scale insects attack many fruit and ornamental trees such as apple, pear, olive, citrus trees, walnut, oleander… as well as some houseplants like succulents and cacti.

Most species do not produce honeydew and the females are not attached to their shells, whose shape and colour vary from one species to another. There are no males in these scale insects.

Eggs are laid in autumn and overwinter under the shell. The larvae appear in spring and migrate onto branches before settling permanently. They develop into females which will then lay eggs in turn.

These include Diaspis boiduvalii, Chrysomphalus aonidum, Lepidosaphes ulmi and Saissetia oleae, the black scale of the olive tree.

They are also responsible for crusty deposits on branches and fruits. Splitting of the bark as well as drying out of affected parts can also be observed.

Diapsis boiduvalii (masdevallia68.blogspot.fr)- Chrysomphalus aonidum (entnemdept.ufl.edu) – Lepidosaphes ulmi (entomofaune.qc.ca) – Saissetia oleae (naturalist.fr)

The scale insect

Scale insects thrive both under shelter and outdoors. In the garden, you may encounter Pulvinaria regalis or Pulvinaria hydrangeae. These species produce large amounts of honeydew. The female’s body is attached to a protective shell. These are the pulvinaria scales most commonly found on trees, forming large whitish cottony masses that protect them and their eggs.

Fertilised females lay eggs in June before dying. The eggs hatch in July, and the mobile larvae colonise nearby plants. In autumn, the larvae migrate to the bark to overwinter. However, they continue to feed. After winter, the larvae metamorphose into adults, and the females return to infest plants, starting a new cycle.

Scale insects mainly develop on trees and bushes: hydrangea, magnolia, lime, dogwood, elm, maple…

Pulvinaria hydrangeae – Pulvinaria regalis (picssr.com)

Natural treatment against scale insects

How to Get Rid of Scale Insects?

Mealybugs are the easiest to eradicate, unlike hard scale insects which are very difficult to combat. You can significantly reduce their numbers, but be aware there are no miracle solutions.

Rather than using an insecticidal product that harms not only scale insects but also beneficial auxiliary insects living nearby, we recommend natural solutions.

Preventive Measures:

• Monitor carefully and regularly to intervene quickly.
• Improve growing conditions (a stressed plant is more vulnerable) and note that good air circulation reduces the risk of infestation.
• Take your plants outside as often as possible (avoid frost and direct sunlight). Scale insects dislike well-ventilated, cooler environments compared to indoors and especially dislike rain. Additionally, they will be exposed to predators.
• Always check the health of plants before purchasing them.

In case of infestation, you have several options:

• • Remember that scale insects are sensitive to weather conditions: take your plants outside.
  • Remove mealybugs and pulvinaria with a cotton bud or cloth soaked in 90° alcohol or soapy water. Rinse afterwards with clean water. Scrape off the waxy shells hiding the insects and eggs using a blunt knife that won’t damage the plant. Slide it between the shell and the plant. Collect the shells in a container and burn them. Use the soaked cloth to remove any survivors.
  • Try a scale insect solution. For this, dilute in 1 litre of water:

– 1 teaspoon of liquid black soap
– 1 teaspoon of methylated spirits
– 1 teaspoon of vegetable oil, such as rapeseed oil.

Mix and spray twice at 30-minute intervals, then spray every 8 days until the scale insects disappear.

Rigour and repetition help eliminate many scale insects or even eradicate them completely. The oil suffocates the eggs, larvae, and adults. The black soap improves spreading and adhesion of the oil while having a caustic effect. Additionally, it is known for its insecticidal properties and acts by disrupting the permeability of the insects’ cuticle. Finally, the alcohol irritates them.

Place plants in the shade or at least out of direct sunlight to prevent the treatment from damaging them.

In this recipe, alcohol is sometimes replaced with white vinegar, but it appears to be less effective.

Note that delicate-leaved plants like ferns or those with fuzzy foliage cannot tolerate this treatment. Similarly, do not spray hydrangeas in full growth, as the treatment will scorch the leaves. Treat them when the leaves have fallen or in spring before they reappear.

You can also:

• Use a jet of water on less fragile plants to dislodge mobile scale insects on the branches. You can also brush them off trunks.
• During winter, prune infested branches (under 2 cm in diameter) and burn them. This significantly reduces the population the following spring.
• White oil (or paraffin oil) is effective against mealybugs. However, it has no effect on hard or armoured scale insects, as nothing penetrates their shells!
• Black soap can be used on larvae, which have no protection. Repeat this treatment for several consecutive weeks to reduce the population of hard scale insects.

A Balanced Garden: The Best Prevention

The best prevention against scale insects is to encourage the presence of beneficial insects in your garden. Ladybirds, certain wasps, lacewings, bugs, hoverflies, not to mention tits, are natural predators of scale insects. And the sedentary lifestyle of female scale insects makes them particularly vulnerable to predators.

To help them:

• Preserve wilder corners of the garden to allow space for host plants of many beneficial insects, such as nettles,
• Grow melliferous flowers like phacelia,
• Don’t hesitate to install shelters, nesting boxes, insect hotels,
• Promote biodiversity and avoid monocultures,
• Avoid using any pesticidal products.