Biological control of whiteflies and scale insects

Biological control of whiteflies.

There are several varieties of whitefly. The cabbage whitefly, as its name suggests, attacks cabbages planted in a vegetable garden. This pest is not picky about what it eats. If it cannot find cabbage, it is equally fond of tomatoes, squash, cucumbers, aubergines, beans, strawberries, and many others. It is often found in vegetable gardens, but because the pest is so small, it is not easy to detect. Organic methods using physical processes such as black soap solutions are generally not very effective in controlling this insect. In addition, black soap can trap useful beneficials such as the small black ladybird that preys on whitefly larvae. As the females can lay up to 600 eggs, whitefly multiplication is very fast.

The whitefly Trialeurodes vaporariorum, also known as the ‘greenhouse whitefly’, and the whitefly Bemisia tabaci are known to be damaging to tomatoes, as they are formidable vectors of viruses such as tomato chlorosis virus (ToCV) and tomato yellow leaf curl virus (TYLCV). Some whitefly populations have become resistant to one or more insecticides. Fortunately, there are useful predators or parasitoids that have been known for several decades, capable of regulating their proliferation such as Eretmocerus eremicus and Encarsia formosa. These small Hymenoptera, which are systematically used in greenhouse cultivation, parasitise whiteflies. Both hymenoptera are available by clicking here.

Ladybirds also feed on whiteflies when they do not find aphids (see the article on ecosystem services; Albizia, an often overlooked friend of the gardener)

Biological control of mealy bugs

In the vegetable garden, the mealy bug is a sucking insect that can cause serious damage. It feeds on sap, which weakens the plants and makes them more susceptible to disease. Mealybugs thrive mainly in warm, humid regions and on houseplants.

A mealy bug infestation is not always easy to diagnose, as the insect is relatively inconspicuous due to its size and means of protection. The scale insects protect themselves with a brown carapace attached to the stems and the underside of the leaves or their veins. Scales can also be found surrounded by a somewhat pinkish white cluster.

All scale insects produce water-resistant waxy secretions. These various protective coatings make them resistant to insecticides. The parasitized plant is covered with a sticky honeydew that later turns into black smoke. In the vegetable garden, if left unchecked, an attack of mealy bugs can jeopardise the harvest. You can get rid of them with a solution containing white oil (paraffin oil) to be sprayed especially in winter to avoid harming the useful auxiliaries that are present especially in summer.

Mealybugs also have their own predators; ladybirds and certain small parasitoid wasps (eptomastidea abnormis – Microterys falvus). You can also use black soap diluted at a ratio of one teaspoon in 1 litre of water with one teaspoon of alcohol. This effective solution penetrates the protective cuticle of these sucking insects. If this does not work, there are commercially available oily preparations combined with a pesticide.