| ||Whiteflies love poinsettias and often growers may end up with a serious infestation at a later stage when it is difficult to use insecticides because of possible bract burn. There can be a variety of sources for the whiteflies on a poinsettia crop. Most commonly the insects are brought to the greenhouse with the plant material (new cuttings) or move from the other crops grown in the greenhouse (i.e. tomatoes) to poinsettias. They can enter the greenhouse from outside through vents, doors and other openings. Monitoring the crop will greatly help in an early assessment of insect population. A good rule of thumb is: when you see one whitefly in your greenhouse there are already more than one hundred bugs hidden in your plants. There are two major types of whiteflies attacking greenhouse crops: 1) Greenhouse whitefly (Trialeurodes vaporariorum) and 2) Sweet potato whitefly (Bemisia tabaci), the second being very specific to poinsettias in Alberta. Both insects have a relatively short life cycle and it may only take two weeks to have a serious outbreak.
Greenhouse whitefly adults are tiny, white, moth-like insects. Females lay about 200 eggs on the underside of leaves over the life-time period of 30-40 days. Nymphs hatch in 10-14 days. There are four nymphal stages and a final 'pupal' stage. The life cycle is about 3-4 weeks depending on temperature. At 26°C, egg to adult takes 24 days.
Sweet potato whitefly has a short life cycle of 18 to 28 days at 20°C. Adult longevity is about 10 to 22 days. Females lay about 300 eggs. The whitefly also serves as a vector for many plant viruses, transmitting about 20 viruses which cause over 40 crop diseases.
Screening of vents and other greenhouse openings is one option for keeping insects out. Anti-virus insects screens prevent small insects such as whiteflies and aphids from entering the greenhouse. The chemical application is still the first option used among poinsettia growers. Growers, as well as general public, have very low tolerance for whitefly nymphs, adults, and honeydew in poinsettia crops. The use of regular insecticide applications ensures that whitefly populations do not develop to levels that are able to reduce plant vigor, foliage production or bract quality. There is a number of insecticides registered for whiteflies on poinsettias including Ambush 50EC, DDVP smoke fumigator and/or fog solution, Thiodan 50% WP or Endosulfan 50W, Sulfotep smoke fumigator and/or fog solution, Pounce 384 and Enstar 65 EC (Enstar should not be applied after September 15). Enstar (kinoprene) is a growth regulator that affects only whiteflies and aphids, it works very well in conjunction with establishment of Encarsia formosa. Remember that conventional insecticides are harmful to bio-control agents. Under the name of Impower 60 WP (or Merit 60 WP), imidacloprid was registered for use on ornamental potted plants in greenhouses in Canada. In July 1999 the registration was extended to greenhouse cucumbers and tomatoes. The insecticide gives excellent, long lasting (up to 16 weeks) control of whiteflies, aphids and sciarid flies. Below is a list of considerations when using imidacloprid:
Every year more poinsettia growers are switching to IPM (integrated pest management). This is very encouraging since the amount of chemicals used to control pests is greatly reduced which has a very positive impact on the environment. IPM uses all types of methods including physical, mechanical, manual, and chemical to keep pests at the level which will not cause economic losses. The change from chemical to IPM control takes at least one growing season and could be disappointing at the beginning. Some chemicals are used when changing the crop or to treat hot spots but most of the pest control is done by biologicals and by other non-chemical methods. Here are some strategies that can be used in the control of whiteflies.
- it should be applied when the crop is in a rapid growth stage and roots have reached the edge and the bottom of pots i.e. in poinsettias 10 days after the pinch. If applied too late, the effectiveness is lowered because it cannot be translocated to older bottom leaves. Please, remember Impower 60 WP works for almost 16 weeks, so applying to the crop after October 1-15 will be a waste of efforts and money
- it should not be applied as a foliar spray. This is a systemic insecticide and if applied as a drench it provides long lasting systemic control of insects. The drip irrigation is ideal to apply this chemical.
- it will take about a week for the most of the product to be taken by roots. Insect populations will start to decrease two weeks after application.
- Impower 60 WP does not control spider mites!!! Predatory mites are not affected by the chemical and parasitic insects are moderately affected.
- 100 g package will treat 5,000 pots size 15 cm.
- read and follow the label. Impower 60 WP is highly effective and works in very low concentrations.
Remember that whiteflies are particularly prone to develop resistance to insecticides. Whitefly resistance to organophosphates, carbamates and pyrethroids has already been documented. Insecticides should be rotated to prevent insect resistance.
A very effective method of trapping adults is the yellow sticky traps. Place traps close to affected plants (1 card (3"x5") for every 400-500 ft² of the greenhouse). However, the traps should be taken down when populations are low and bio-controls are used because the bio-control agents tend to be attracted to the traps when the whitefly population drops.
In small greenhouse operations, hand held vacuum cleaners proved to be effective at removing adults off leaves. This should be done when the temperature in the greenhouse is cool (i.e. early morning) because the whiteflies are sluggish and unable to move out quickly. Note: place the used vacuum bag in the plastic bag in the freezer overnight; it will kill the insects inside.
Remove and destroy leaves which are heavily infested with larvae and pupae (a good method for small greenhouse operations).
Incorporation of natural enemies into an integrated pest management program for whiteflies on poinsettias gives an opportunity to diversify whitefly control options, reduce reliance on insecticides and provide more sustainable whitefly control system for the crop. Encarsia formosa, a parasitoid specific to whiteflies, has been successfully used by greenhouse vegetable growers for years. Whitefly populations should be reduced to no more than one adult per leaf prior to releasing Encarsia (the lower the better, i.e. one adult per plant). Greenhouses should be screened and the cracks/holes filled to keep Encarsia inside. Optimum conditions in the greenhouse are: 26/27°C, relative humidity 70%, light intensity greater than 650 footcandles (conditions also ideal for optimum growth of poinsettias).
For those who are interested in biologicals, Koppert Canada in co-operation with Pacific Agriculture and Agri-Food Canada Research Station, Agassiz, BC are working on the introduction of new predators for whitefly control. Macrolophus caliginosus and Dicyphus hesperus, both from the family Miridae, have been used successfully against whiteflies in European greenhouses for several years. Both are generalist, omnivorous bugs that feed on both plants and small insects. Both will cause small damage to plants but will provide excellent control of whiteflies. Macrolophus caliginosus cannot be introduced to North America because it is not native to the environment and could cause problems in competition with native Miridae predators. Dicyphus hesperus is native to the Okanagan valley in British Columbia and could be introduced as a whitefly predator. For more information, contact Dr. Dave Gillespie from Agriculture Canada, Agassiz: GillespieD@em.agr.ca.
K. Pruski and M. Mirza, CDCN
Greenhouse Coverings - October 1999