Pest 2 - Greenhouse Whitefly (Order - Hemiptera)

Found not only in greenhouse settings, but outdoors as well, in protected situations with humid atmosphere in summer and autumn. This infestation was in a sheltered, semi-shaded corner of the house on an ornamental fuchsia. 

Appearance : 

Adults are small, white sap suckers about 1mm long, with 2 pairs of wings. They are typically located on the underside of leaves and if plant is disturbed will quickly flutter out and then return to its original site. Males live for 1 month, females a few more months. Nymphs are no bigger than 0.75mm.

Damage : 

Adults and nymphs are sap suckers. Upper surface of leaves develop a sandy mottle. Honeydew is secreted leading to formation of black sooty mould. Leaves, fruit and stems may be coated with this black deposit. Whiteflies are often found feeding without causing any obvious damage and often require no control measures, especially ornamentals. 

Host :

Greenhouse white fly has the largest host range of the whiteflies including tomatoes and beans.

There are other separate species that infest rhododendrons, citrus.

Lifecycle : 

Each female lays several hundred eggs. Eggs are embedded in the leaf tissue in an upright fashion, like very tiny pegs. Nymph crawls about for 3 days on underside of leaf, sap-sucking. Oval shaped immature forms look like scale. Egg - adult cycle can take 5-8 weeks. 

Controls :

cultural - reducing humid environment, or use of companion planting eg. basil or nasturtium. 

biological  -  parasitizing wasp Encarsia formosa - lays an egg inside the body of nymph. These parasitized nymphs are black. 

physical - Trapping. Painted yellow-orange boards covered with a sticky material like clear grease. Whitefly are attracted to this colour. Boards have to be cleaned regularly. 

Chemical - in severe infestations can use pyrethrin spray or dimethoate to undersurface of leaves, but may need to be repeated. 

In this example, the plant was looking reasonably healthy despite infestation and there was no indication to do anything but continue to observe regularly. 

Reference :  Kerruish, A-164