Greenhouse whiteflyTrialeurodes vaporarium (Westwood)
Dominant in the Southern states
Silverleaf whiteflyBemsia argentifolii (Bellows and Perring)
Queensland, Northern NSW and WA. Not known to occur in the southern states.
Description and life cycle
Whitefly feed, mate and lay numerous eggs on the underside of leaves. The adult whitefly are small white insects, 1.5mm long. The crawlers that emerge do not move far and after 2-4 weeks of feeding turn into pupae.
Greenhouse whitefly are normally identified at the pupal stage. They are scale like, oval shaped and only 1.5-2mm long. Under magnification they have a flat top and box like sides with waxy filaments emerging from the top edge. The adult whitefly that emerges from these pupa cases are small white insects, 1.5mm long with four powdery white wings. They disperse mainly onto the underside of young leaves where they feed, mate and lay eggs. The eggs are bullet shaped and laid vertically onto the leaf in a semi circle shape. When first laid the eggs are creamy white, but they turn purplish as they mature and are difficult to see without a 10X hand lens. The crawlers that emerge from the egg are the only motile (moving) juvenile stage, but they do not move far from where they were laid, usually staying on the same leaf. Within a few days the crawler settles down and begins actively feeding, soon looking more like a scale than a bug. As it progresses through two scale-like larval instars, its outer covering hardens giving it extra protection. The pupa then forms under this scale-like covering and the adult emerges 12 - 23 days later, from a T shaped slit made in this hardened covering.
Silverleaf whitefly pupa are smaller than Greenhouse whitefly pupa, their profile is flatter and dome shaped and they do not have the waxy filaments around the top edge. The eggs are still bullet shaped and laid vertically, but they scattered on the leaf and do not turn as dark as greenhouse whitefly. Silverleaf whitefly has a similar life cycle to that of greenhouse whitefly, but it prefers a hotter climate and is a pest in northern Australia. Silverleaf whitefly adults hold their wings tent-like over their body with the body visible down the centre, while greenhouse whitefly hold their wings flatter over the body with no part of the body showing.
Crops attacked and problems caused
Greenhouse Whitefly are serious pests of most greenhouse vegetables and many ornamentals.
Whitefly are sap-sucking insects in both the adult and immature stages. The scale like immature nymphs are the most damaging. Their feeding can cause yellowing and mottling on leaves. Honeydew excreted by the feeding insect onto the plant foliage can cause sooty mould to grow, which detracts from the plant and harvested fruits' appearance. Heavy infestations will reduce the overall plant vigour and cause stunted growth, defoliation and poor yields.
Reducing the threat of invasion and attack
Greenhouse and silverleaf whitefly have a wide host range of about 250 plant species, mostly in the families Cruciferae, Leguminosae, Malvaceae, and Solanaceae. Capsicum, cucumber, eggfruit are members of these families, but so are many broad leaf weeds including mallow, sow thistle and verbena. Controlling these weeds well ahead of planting out a new crop is very important. Use a fallow period, if possible, when no crop is grown to clear whitefly populations. Use seedlings or cuttings that have been grown away from whitefly infested areas and are free of whitefly i.e. start with a clean crop.
DO NOT LEAVE OLD CROPS, ESPECIALLY IF THEY ARE ALREADY INFESTED WITH WHITEFLY.
There are some further simple things that can be done to avoid high WFT numbers:
- Use seedlings that have been grown away from infested areas, i.e. start with a clean crop.
- Avoid introducing any infested plant material into the crop
- Avoid moving whitefly around the crop on staff moving from infested to clean areas.
- Use a fallow period, if possible, when no crop is grown to clear pest populations
- Use fine mesh/netting if growing in a Greenhouse (400microns or less). Cover all doors (double doors are even better) and vents if the crop is likely to be invaded by whitefly from outside.
- Rolls of yellow sticky tape may be useful in some greenhouse designs if placed near entry points and hot spots.
Monitor pest populations for early control. Use yellow cards to alert you of new infestations (only adults with wings are caught on traps) and scout plants by turning leaves to work out were they are. Control is simpler and less expensive when plants are young and spray coverage is not an issue.
MAKE INSPECTIONS ROUTINE AND KEEP GOOD RECORDS!
Monitor for early detection and control of whitefly. Control of whitefly is simpler and less expensive when plants are young and spray coverage is not an issue. Whitefly adults and eggs are usually found on the under side of young upper leaves, while the larval and pupa stages are found on lower older leaves. Use yellow cards to alert you of new infestations and scout plants by turning leaves to work out were they are.
- Get into the habit of walking right through your crops in a set pattern (a M or Z)
- Check about (about 1%) of your plants very carefully
- Check underside of leaves for feeding larvae, adults and eggs
- Keep good records of pest levels and treatments used
Monitoring with sticky traps (only adults with wings are caught on traps):
For insecticide-based control programs economic thresholds need to be worked out from monitoring and spray records using sticky trap and plant leaf checks. Decide on a threshold level for whitefly in your crop, above which you must spray and below which you can safely withhold spraying.
- Plan the layout of traps to identify hot spots and estimate overall pest levels
- Place traps just above the plant tops
- Do a weekly count of pests on each trap and look for signs of pest activity. Mark affected plants with tape and check nearby plants to determine the size of hot spots.
- Check pest numbers on plants 1-2 days after spraying to check results
- If able, count the proportion of adult to larvae (larvae but no adults = spray worked, but high breeding levels still in the crop; adults only = new flight; both adults & larvae = pests not killed by spray applications indicates resistance/coverage issues)
- Record trap and plant results
- If pest numbers are above the threshold you must spray ASAP to prevent loss of control
- If you can spot isolated hot spots early you may only need to spray a small area!
- Plan to introduce biological control agents as soon as thrips are found
- Chemical control of this pest has been difficult due a number of factors: Resistance to insecticides is fairly common
- Nearby weed and crop host plants readily reinfest new crops
A resistance management and prevention strategy needs to be in place to reduce the chance of whitefly becoming resistant. Although pupae are not susceptible as with WFT, the precise chemical strategy is a bit different because some new whitefly chemistry acts very differently, takes longer to kill and has minimal impact on adults.
The five distinct life cycle stages (adult, eggs, crawler, larvae (scale) and pupa) differ in their tolerances to insecticides but all stages can be on a single plant at the same time. It is very important to find out what stage of the whitefly lifecycle is susceptible to each chemical being used. Some of the "soft" moulting inhibiting chemicals will only kill larval/nymph stages and not effect the adults! The adult and crawler stages are the most susceptible to contact insecticides but the egg, scale and pupa vary in their resistance to these chemicals. A single spray of any chemical will only kill the susceptible stages present at the time of treatment or during the time the chemical remains active. All other stages will survive and continue their life cycle. Thus clusters of 2-3 applications are usually required during the cropping period.
Whitefly feed on the underside of leaves and it is important to remember that it is difficult to obtain thorough coverage with sprays to these parts of the plant and this often leads to repeated failures to control this pest. Spray crop after pruning and training plants to maximise chemical application by improving penetration into the crop. If monitoring indicates the need to spray frequently, then insecticide resistance, inappropriate spray application or inadequate farm hygiene should be suspected and expert advice sought.
Relevant beneficial insects
There is a good biological control option. Encarsia formosa a small parasitic wasp has been extensively used as a biological control for greenhouse whitefly especially in protected environments such as greenhouses. The adult wasp lays its eggs in the 3rd or 4th larval stage of the whitefly but the adults also feed on the young scale like larvae. The parasitised larvae turn black as they mature and a small wasp emerges leaving a small round emergence hole. An average daily temperature of 23oC (15oC or higher at night) is required for good whitefly control by Encarsia.
If using biological control by introducing Encarsia formosa for Greenhouse whitefly do it early and encourage parasite activity by only using soft sprays and only pruning leaves after parasites have emerged. The exotic parasitoid, Eretmocerus hayati, has been released for Silverleaf whitefy by CSIRO and Growcom staff on Queensland grower farms in the summer of 04/05. The parasitoid has established readily at most locations. Toxic broad spectrum sprays should be avoided to encourage this parasites' activity and spread into other growing areas. For more information: www.growcom.com.au/media/March05/March_23_05.html
Whitefly are also preyed upon by lacewing larvae, and other general predators. As with WFT can also boost the numbers of wild lacewings and other beneficial insects in your crop naturally by holding back on broad spectrum insecticides, providing safe plant species as habitat near the crop and maintaining higher levels of organic soil carbon.
The only other natural enemy is a fungal pathogen Veticillium lecanii but more research is needed on formulations which will improve the effectiveness of V. lecanii in controlling whitefly. It is important to prolong the period of its effectiveness. At present, the ‘conidial solution’ is capable of infecting target insect pests only for a short time after it is applied to crops.
Commercial suppliers of bio-control agents in Australia can be found listed at the Goodbugs website (external link). The suppliers on this page will help you develop an IPM program suitable for your crop and situation. Many also provide IPM monitoring services. Fungal pathogens are supplied by companies dealing in microbial products. Nutri-Tech Solutions - Microbial Products
Download the White Fly factsheet (PDF)
Information on other greenhouse pests:
• Western Flower Thrips
• Two Spotted Mites
• Broad mites