White flies and parasitoids

Unless you are a fan of horror films like Aliens, you probably have little experience or knowledge of parasitoids. Researcher Steve Hazell introduces these little known but fascinating creatures, and makes a suggestion about how white fly might be controlled.

Steve Hazell © C. Jameson

Parasitoids are neither predators, which kill their prey immediately after attacking them, nor parasites, which live on or in their prey but rarely kill them. Parasitoids are insects - most are wasps or flies - that lay their eggs in, on or near their prey. The young parasitoid then develops inside - or occasionally on - its host. To begin with, the host may not suffer, but the young parasitoid grows, and eventually kills and consumes the host almost entirely. Despite this apparently sinister lifestyle, parasitoid wasps are often beneficial to man. Many parasitoids are common in gardens and help to protect our garden plants from herbivorous insects.

Herbivorous insects such as aphids attack many important plants but rarely do much damage because of the actions of their natural enemies, including parasitoids. Aphid parasitoids lay their eggs in young aphids, and the eggs develop when the aphid has almost reached adulthood. The young parasitoid grows rapidly after killing the aphid and attaching its body to a leaf with silk threads. After consuming all of the insides of the aphid, the parasitoid then spins a silk cocoon within the aphid and pupates. Only the swollen skin of the aphid is left.

The best place to observe parasitoids in action is on plants - such as peppers - that are infested with aphids. Careful observation, with or without a magnifying lens, will reveal the pale brown, papery swollen skins of the dead aphids. Look out for the neat holes cut into the top of these ‘mummies’ by the adult parasitoid as it leaves. To see the adult wasps, carefully remove mummies without exit holes and store them in a sealed container. After a week or two the adult parasitoids will emerge to mate and find new aphids to attack.

White fly, a major pest problem to Seychelles plants © S. Hazell

Fortunately for Seychelles, the most recent insect pest to arrive here, the spiralling whitefly also has several parasitoid natural enemies. Work is underway by Nature Seychelles and the Department of Plant Protection to establish whether the parasitoid which kills spiralling whitefly, which is known as Encarsia haitiensis, is present in Seychelles.

The best solution to the spiralling whitefly problem may be to introduce Encarsia haitiensis. Other countries, including India and Benin, have managed to control spiralling whitefly in this way. This course of action would be completely safe. Like many parasitoids, Encarsia haitiensis is host specific – this means that they attack spiralling whitefly, and no other insect species. The history of pest control by introducing non-native predators is a litany of disasters, as the introduced predators often decimate non-target, native species. But in this case, Encarsia haitiensis appears to be a safe option.

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