Whitefly - A pesky pest

Whitefly seem to have the ability to find their way into most cropping situations - and large populations of these insects get up most grower's noses in more ways than one. Whitefly will make themselves known fairly early on in the infestation process - making early control possible. Small (1.5mm long) white moth like insects which flutter around the plants when disturbed and rest on the undersides of leaves are whitefly, of which there are a few different species. The whitefly is not actually a fly, but belongs to the order Hemiptera, which also includes other pests such as aphids, spittlebugs, scale and the green vegetable bug.

Plant Damage

High populations of whitefly in a crop have the potential to be extremely damaging. Plants will be seriously weakened by heavy infestations, as large volumes of sap are removed in a short period of time, particularly by the developing nymphs. A heavily infested crop will be stunted, grow poorly, the foliage may turn yellow and drop prematurely. Yields and quality will be seriously affected. What causes more concern is the production of `honeydew' a sticky substance which is excreted in large quantities by the pests as they feed. A single whitefly nymph (the juvenile stage) has the ability to excrete 20 drops of honeydew per hour and a single leaf can easily contain over 2000 nymphs. This sticky honeydew provides an ideal medium for the growth of `sooty mould' a black fungus which uses this sugar for growth and development Black sooty mould over large surfaces of the plant not only looks unsightly, it interferes with photosynthesis and transpiration, thus stressing the plant further. To make matters worse, both the greenhouse whitefly and the sweet potato whitefly are two species which are known to transmit over 25 different virus diseases.

Whitefly Life cycle and identification

Most growers are already familiar with the adult whitefly - these pests actually have yellow bodies which are covered in a white waxy powder. Whitefly develop from eggs which are laid on the undersides of leaves. These eggs are small and oval shaped and the female will often cut slits in the leaf surface or deposit eggs into stomatal openings to provide support. The time taken for eggs to hatch depends on temperature, the higher the temperature the faster the life cycle and population growth. After hatching from the egg, the first instar, also called a nymph or crawler, moves a short distance away, locates a vein and settles down to feed. The second and third larval stages remain fixed once in position, feeding all the time - these are often referred to as `scale-like' since they are flat, transparent ovals. These stages are visible on the undersides of leaves. The fourth instar lasts for a few days in which the larva pupates into an adult. Adults only live for 10 - 40 days, but females begin laying eggs (400 over 6-8 weeks) only a few hours after emergence. Given that the feeding nymphs can remove large quantities of sap and produce volumes of honeydew, it is not just the adult insects growers need to control - the whole life cycle needs to be broken, otherwise newly hatching larvae will rapidly reinfest a crop.

Control options

As with many other insect pests, prevention is the best form of management, but this is easier said than done. Whitefly eggs are small and almost impossible to detect - so many infestations begin with infected plants or seedlings being brought into the cropping area. Adults can also hitch a ride on plants, crop residues, equipment or even people, as well as flying in through vents from outside. Insect mesh over all vents and quarantining all new plants away from the final cropping area will help prevent introducing the pest. After removing a crop which was heavily infested, the growing area needs to be thoroughly cleaned, and all plant debris removed. Usually a host-free period of at least two weeks will starve whitefly - so any alternative plants, such as weeds outside the growing area which can act as hosts, must also be destroyed.

If whitefly appear to be present the next option is to monitor populations and try to trap those few adults before they lay eggs and begin a population explosion. Whitefly adults are attracted to yellow - for this reason growers use sticky yellow traps to monitor flying adults. Sticky traps should be placed just inside the plant canopy or around the edges of plantings - there are many brands of commercially available traps for whitefly. For small areas, sticky traps can sometimes give control of whitefly populations, but for larger areas they are usually only a detection device. An acceptable threshold for whiteflies is 0.5 per card per day when the crop is young and 2 per card per day as the crop develops.

Insecticides

Chemical control of whitefly with insecticides was once simple and highly effective, however this is no longer the case. Whiteflies began showing resistance to synthetic insecticides early on and by the 1980's were becoming difficult to control. Apart from the development of insect resistance to chemical pesticides - which can be delayed by alternating the types of chemical used - the eradication of whitefly populations has other difficulties The egg and non feeding pupa stages are not as susceptible to insecticides as are the adults and nymphs. Also eradication of a population usually requires a number of applications of insecticide at close intervals to ensure the life cycle is stopped. There are however a few limited chemical insecticides which will give initial control of some whitefly populations, however whitefly has such a prolific reproduction rate, they can develop resistance faster than we can develop new chemicals to control them. For this reason, control of whitefly is now focused on using products based on biological control, predators, insect growth regulators, botantical and biorational products and physical control with soap/oil sprays

Biological control - predators

Whitefly have several natural enemies including predators, parasitoids and pathogens, many of which have been made available for control of this pest in greenhouses and growing areas. The most commonly used predator of whitely is the small parasitic wasp Encarsia formosa which was one of the first biological control agents developed. Encarsia wasps kill whitefly nymphs by laying an egg inside the nymph, parasitizing it and causing it to turn black in the case of the greenhouse whitefly and yellow/brown in the case of the silverleaf whitefly pupae. Encarsia wasps are introduced into the growing area in the black scale stage, glued to cards which are hung in various positions in the crop. The wasps start to emerge within a few days under warm conditions and begin to work immediately. Encarsia are best introduced when whitefly numbers are low, as heavy infestations and the presence of large amounts of honey dew will reduce the parasite's ability to locate the whitefly scale and parasitise them. The mobility of the Encarsia adults is largely determined by temperature and at 15 - 18 C they don't travel far on the leaf surface in search of prey, once temperatures rise above 21 C the wasps are much more active. Plants with hairy leaves such as cucumbers inhibit the parasite as the hairs make the wasp's movement across the leaf difficult and also because they deposit honeydew on the wasp's body. Encarsia wasps covered with sticky honeydew will spread a great deal of their time cleaning themselves rather than searching out prey. These predators will often need to be re introduced at regular intervals during the growing season to keep the levels of parasites high enough to give adequate control.

Silverleaf and sweet potato whitefly are not that well controlled by Encarsia formosa and two other wasp parasites - Encarsia luteola and Eretmocerus californicus are available for control of these. Another natural predator - a predatory beetle, Delphastus pusillus, can also be used for biological whitefly control. This beetle works best at high population levels of whitefly as adults can feed on up to 150 whitefly eggs per day. Delphastus is best used in conjunction with Encarsia to get a good reduction in pest numbers. These beetles however do no perform well at low temperatures and need conditions of 21 C and 70% humidity or above to survive.

Bio-insecticides

Along with parasites and predators there are a number of fungal diseases which can provide an effective control mechanism against whitefly. Beauvaria bassiania is a common soil borne fungus that occurs world-wide. Two strains of this have been made commercially available under the names `Naturalis O' and `Botanigard'. These are effective against eggs, immature and adult whiteflies, athough good coverage is required to ensure the fungal spores make contact with the insect to begin the infection process. Another fungal pathogen of whitefly, Paecilomyces fumosoroseus (trade name PFR-97) is also commercially available, it controls not only whitefly but a number of other insect pests. Verticillium lecanii is yet another fungus which can be used to control whitefly. The spores of this fungus develop inside the whitefly, killing it then erupting though the insect's outer layer to produce more spores for further infection. Most of these fungal based control agents require high humidity levels and warm temperatures to be successful

Insect growth regulators

Insect growth regulators control pests by disrupting their development and these are a less toxic option than chemical insecticides. Growth regulators may work in a number of ways: they can mimic juvenile hormones so that the insect's moulting is disrupted, they can interfere with the production of chitin which composes the shell of many insects or they can prevent insects from entering the adult stage of development, thus ending the life cycle. There are many insect growth regulator products including: Adept, Dimlin, Citation, Precision, Distance, EnstarII, Preclude, Pyrigro and others. Some of the most interesting growth regular type products are based on a botanical insecticide from the Neem tree: Neem based products containing the active ingredient Azadirachtin have been widely proven to be effective against many insect pests if used on a regular basis. Neem based formulations (such as Neemazal, Neemix and Azatin to name a few) are widely available and safe to use.

OTHER CONTROL METHODS

Smothering agents

There are a number of oil/soap based sprays for control of whitefly (such as Sunspray Ultra-fine spray oil and I-Pede insecticidal soap)- these rely on smothering the insects, so good coverage is essential for control. One mechanical method of controlling whitefly populations which has been used quite successfully on both a large and small scale is deleafing. This involves removing the older leaves from the crop which contain the immature stages, before the adults have hatched. This is particularly effective in vigorously growing crops where the foliage is removed from the growing area before more adults are produced to continue the life cycle. This method however relies on timing and being able to identify the different stages of the whitefly life cycle.

There is recent evidence to suggest that the composition of the atmosphere in the growing area can give some degree of whitefly control. Either reducing oxygen or increasing carbon dioxide can result in whitefly death even after an exposure of just a few hours. It has been reported that whitefly populations are generally lower in some greenhouses where carbon dioxide is used on a daily basis. The likely reason given for this is that plants grown in atmosphere with high CO2 levels tend to have higher concentrations of carbohydrates in the plant tissue relative to nitrogen, resulting in a nitrogen dilute diet which slows the growth and reproduction of the whitefly pests.

Summary

The battle against whitefly is expected to be a long-term one, but nowadays technology is developing some of the best weapons against this pest ever seen. Apart from trying to prevent infestations, control options now focus on a more integrated approach with predators, botanical extracts, bio-rational products and smothering agents. Luckily these types of products are considerably safer to use than the old chemical insecticides which were often just as toxic to the grower as to the pest. As with many other insect pests, control of whitefly in your particular growing situation will probably be a matter of trying a number of approaches and seeing what works best for your crop.

Sources:

Tripp, Kim and Peet. 1993. New Use for CO2: slowing whiteflies. American Vegetable Grower. November 1993 pp43-44.

Schmutter, and Ascher, 1986. Natural pesticides from the Neem tree (Azadirachta indica A. Juss) and other tropical plants. Proceedings of the1 third annual international Neem conference, Nairobi, Kenya 10 - 15 July 1986. Published by Deutsche Gesellschaft fur Technische Zusammenarbeit (GTZ) GmbH, Eschborn.

Succop, 1997. The parasitism of Greenhouse Whitefly, Trialeurodes vaporariorum by the parasitic wasp Encarsia formosa: A biological control method in greenhouses. Insect Behaviour Review Article 1997.