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Adult greenhouse whitefly is a small white to yellowish winged insect about 1.5 mm long. There are two pairs of wings which are covered with a powdery wax and, when at rest, are held over the body. Males and females appear similar.
There are other species of whitefly, and some small moths that look like whitefly, so it is important not to confuse the species. Seek expert advice if you are unsure about identification.
Greenhouse whitefly eggs are elliptical and are pale yellow when first laid, but turn black after several days. They are often found on the lower leaf surface.
The first nymph stage or instar, called a ‘crawler’, is about 0.3mm long, flat, oval and almost transparent with very short legs. The second, third and fourth instar nymphs which are often called the ‘ scale’ stages, are also oval and almost transparent. The final instar nymph is about one mm long.
The nymph body is surrounded by a short, transparent fringe of hairs which in the early instars lies against the host plant. Longer hairs are found on top of the body. In the fourth instar the fringe of hairs is raised up from the surface of the plant.
The nymph pupates in the four instar’s exoskeleton, and is now called a puparium.
Sometimes scales or puparia turn black, indicating it has been parasitised by Encarsia formosa.
Distribution and host range
Greenhouse whitefly is the most common whitefly species found in New Zealand, and may be found all year round in greenhouse crops and outdoors in the warmer parts of the country.
It is found on a wide range of solanaceous plants such as tomatoes; cucurbits, such as cucumbers and pumpkins; legumes, such as beans and Compositae, e.g., the milky thistle. A strain of greenhouse whitefly has adapted to tamarillo, while others have adapted to feeding and breeding on weeds such as dock.
Signs and symptoms
Greenhouse whitefly adults and nymphs feed by sucking plant juices and heavy infestations can weaken the host plant. Feeding may also cause physiological disorders in the plant.
Greenhouse whitefly are capable of transmitting plant viruses, but this is not regarded a problem in New Zealand.
The signs and symptoms of greenhouse whitefly include:
Life history and habits
Following emergence, female greenhouse whitefly will begin to mate and lay eggs after about 2-3 day. Eggs are laid on the lower surfaces of younger leaves, often in small circles around the point of feeding.
Females lay about 50-150 or more eggs during their life time, depending on temperature and the host plant on which they are feeding.
Eggs hatch into the first nymph stage called the ‘crawler’ which may move a short distance, and then settle to begin feeding.
When the crawler moults, it turns into a legless scale-like nymph. There are four nymph stages or instars.
The full grown fourth instar stops feeding and develops into a puparium (pupa in the nymphal skin). Inside the puparium the nymph transforms into an adult.
On tomato, the average length of life cycle ranges from 18-64 days at 28 °C and 12 °C, respectively.
Host plants also affect the reproduction and survival of greenhouse whitefly. Egg plant is a highly favourable host (high reproduction and survival) whereas capsicum is a poor host (low reproduction and survival). Cucumber and tomato are intermediate in suitability.
Greenhouse whitefly has no overwintering stage and all stages may be found throughout the year. Adults are highly mobile and readily take to wing when disturbed by handing or brushing past plants. Adults are readily attracted to yellow sticky traps.
High infestations of greenhouse whitefly can have a severe impact on plant growth and yield. If left uncontrolled, enormous populations can develop on greenhouse crops over a production season.
Lesser infestations can produce sufficient honeydew which, under humid conditions, supports the growth of black sooty mould fungus. Black sooty mould does not cause plant disease, but is a contaminant which can be difficult to remove.
Black sooty mould fungus spoils the quality of fruit, and may reduce the photosynthesis capacity of leaves by reducing light penetration into leaf cells.
Early detection is the key to success for control of greenhouse whitefly.
Yellow sticky traps can be used to monitor the presence and activity of greenhouse whitefly adults, and provide an early warning indicator of their presence. Traps should be placed near to the growing tips of plants, and moved up as plants grow.
Plant inspection should be used to verify the presence of greenhouse whitefly populations in a crop.
Threshold levels for whitefly infestation have been suggested for many crops, however, there is no universally accepted threshold level for greenhouse whitefly. It is recommended that there be less than 1 whitefly adult per 20 plants when introducing new plants to a greenhouse, and control action is recommended when adults exceed 2 per plant.
A number of practices are recommended to prevent or minimise the establishment of greenhouse whitefly on greenhouse crops. These include:
On outdoor crops:
A variety of natural enemies (predators, parasites and diseases) has been researched for the management of greenhouse whitefly. The parasite (parasitoid), Encarsia formosa is the most commonly used biological control agent for this pest.
Encarsia formosa is available from BioForce Ltd, who sells the product Enforce™.
Encarsia formosa is most effective when it is used in an Integrated Pest Management programme, when the use of harmful pesticides is avoided and effective crop management practices are adopted.
A range of insecticides are claimed to be effective against greenhouse whitefly, although few have specific registration claims for use on greenhouse crops.
The nymphs and adults of greenhouse whitefly are more susceptible to insecticides than eggs and puparia. Insecticide applications should therefore be targeted at nymphs and adults.
Resistance to insecticides by greenhouse whitefly is known to occur in New Zealand and elsewhere. Care must be taken to not apply more than the recommended number of applications of insecticide from any one chemical in a year, e.g., no more than 2 applications of buprofezin per year.
Insecticide applications should be made as cluster of sprays, rather than at regular intervals throughout the crop cycle, e.g., 2-4 sprays, 2-4 weeks apart depending on the chemical used. A different chemical group should be used at each cluster of sprays.
Apply sprays to crops in a manner to ensure good coverage of the undersides of leaves. Crops recently de-leafed allow better spray penetration. Spot spray ‘hot spot’ areas to minimise the spread and build of greenhouse whitefly in a crop.
Care should be taken to consider the effect of insecticides on beneficial insects or mites that may be being used in integrated pest management programmes. Consult with BioForce Ltd before applying any insecticide when beneficial insects or mites are being used.
If you are uncertain about the identity of any pest in your crop, or need advice on the management of pests contact BioForce Ltd, firstname.lastname@example.org .