The resistance problem could be compounded if excessive spray programmes and zero whitefly levels are demanded by plant health authorities if plant exports are restricted due to whitefly infestations. Only the tobacco whitefly (Bemisia tabaci) is a Notifiable Pest, whereas glasshouse whitefly (Trialeuroides vaporariorum) is not a quarantine pest. They are easily distinguished by the pupae: glasshouse whitefly pupae are white and have a raised straight edge wall with a circle of waxy filaments on the edge, whereas tobacco whitefly pupae are flatter and are a yellowish colour with less waxy filaments. An intensive IPM programme which includes natural enemies will be just as effective as an intensive pesticide programme and will not leave a residue of pesticide resistant whitefly.
Most of the mature whitefly scales will be on the lower side of the canopy of the plant, so effective under-leaf cover with pesticides is important. Take advice from spray technology experts on whether improvements could be made in spray cover by changing nozzles, pressure and droplet size. More water volume does not always mean better spray cover.
A single female whitefly lays from 100 to 600 eggs depending on the suitability of the host plant. These will hatch and develop into mobile ‘crawlers’ which eventually settle to insert a feeding tube into the leaf. It completes its lifecycle in this final position, by drawing the sugary sap out of the plant. Much of this sap floods out of the ‘other end’ and drips onto the leaf surface below. This sugary solution is an ideal food source for sooty moulds which blacken the surface of the leaf below – reducing yield and quality of flowers. Sooty mould can be washed off with insecticidal soap sprays.
Whitefly adults prefer certain plants such as mint, aubergine, Calendula and tobacco, to many other flower crops. This preference can be used to draw whitefly away from the flower crop, or slow down their movement within or between crops. If biological control agents such as Encarsia formosa are introduced to these trap-crops, then the levels of whitefly parasitoids can build up on the farm. Trap crops must be regularly checked and whitefly found there should be treated with pesticides, bio-pesticides or natural enemies, to prevent these areas becoming infection hotspots for the crops nearby. Calendula is a useful trap for whitefly eggs, as the adults lay many eggs on this plant but few develop into scales – it acts like a whitefly egg sink.
Whiteflies are cold-blooded and need to accumulate a lot of heat from the sun before they are able to fly. By removing old infested crops during the early evening or early morning, when air temperatures are still low, migration from old crops can be almost completely stopped because they are unable to fly at low temperatures. Contact insecticides applied at these times may also be more effective if the adult whitefly do not fly away.
Physical controls such as insecticidal soaps (Savona) can be sprayed under-leaf to destroy the cuticle of the scale and kill the pest by dehydration. Take care to test for phyto-toxicity and avoid spraying in the heat of the day to prevent scorch. Potato starch dextrin (Majistick) has also been used to kill whitefly scales by osmosis, drawing out water from the body of the whitefly to dilute the more concentrated starch solution. Majistick can be integrated with the use of Encarsia because parasitised pupae are not affected.
Entomopathogenic fungi such as Lecanicillium lecanii, Paecilomyces fumosoroseus and Beauveria bassiana are biopesticides for whitefly. Fungal spores land on the whitefly and are able to germinate, penetrating the whitefly and growing in its body fluids. These fungi generally need a relative humidity of 75% for at least 10 hours after application in order to germinate. Careful timing of the fungicide programme in relation to the biopesticide applications needs to be taken care of because these are themselves fungi and could be killed by some fungicides.
The predatory mite Amblyseius swirskii is a new biocontrol for whitefly control, but according to Wageningen University researchers, it should be used in conjunction with Encarsia because A. swiskii prefers to prey on the whitefly eggs and crawlers. Encarsia is still needed at six Encarsia per meter square per week, to combat the second, third and forth whitefly larvae stages. Encarsia is more susceptible to insecticides and fungicides – so a well designed spray programme for other pests is essential. Wageningen recommend that if swirskii is used in roses, that the slow release bags are more effective than the loose swirskii products because it is more difficult for swirskii to build up in the crop (if there are only low whitefly populations) if the loose product is used. The bags are more expensive and at least 4,000 slow release bags are needed per hectare, replaced every 6 weeks to provide effective control. The best reproduction of swirskii occurs if both thrips and whitely are present at the same time. It will not control high infestations of whitefly. There is minimal survival of swirskii and no population growth at temperatures below 15 deg C and minimal growth at 18 deg C. Other predators such as Macrolophus, will eat all stages of whitefly but they belong to the capsid family and care needs to be taken if capsid cause damage in the crop.
If the parasitic wasps, such as Encarsia formosa and Eretmocerus are being used, more care needs to be used in the deployment of yellow sticky traps, as they are also attracted to yellow sticky traps. Most bio-control companies offer these parasitoids and their websites can be referred to for detailed programmes. (www.syngenta-bioline.co.uk and www.biobest.be). High introduction rates of Encarsia adults at an introduction ratio of 1 adult Encarsia to 20 whitefly scales on the crop, will force the Encarsia to ‘sting out’ the scales, making them drop completely off the crop. This might be a preferred way of using this parasitoid, if the customer does not want to see black scales on the leaf. Since Encarsia is one of the cheapest bio-control agents for whitefly, this may still be the most economic way of controlling whitefly.