With the fall brings preparation for the holiday season. For growers, this may mean poinsettia production. Proper irrigation, fertilization, climate control, disease control, and insect control are some of the many factors that go into growing a beautiful poinsettia crop. When it comes to insect problems on poinsettia crops, a grower’s main concern is whitefly infestation. Whiteflies affect both the vigor and sale ability of this staple holiday plant. How does one prevent or control a whitefly population? The first step is identification.
Whiteflies are small, white, moth-like flying insects. Greenhouse whiteflies (trialeurodes vaporariorum) and Silverleaf whiteflies (bemisia argentifolii) are the two species of whiteflies that attack greenhouse crops. The differences in the two species can be found mostly in their immature stages, seen on the underside of the leaves. The immature Greenhouse whiteflies are oval, pale, and have an almost hairy like white, waxy thread surrounding their body. The immature Silverleaf whiteflies are a more vivid yellow color than the Greenhouse whitefly. Immature Silverleaf whiteflies also have less pronounced hairs around their body and appear to be smooth. There are also differences in the adults of these two whitefly species. The Greenhouse whiteflies are larger and also have more white wax covering them. The Silverleaf whitefly has more yellow coloring and is also more visibly active than its relative. When the population level is high enough or goes unchecked, then damage on the crop might also be visible. Tell-tale signs of whitefly damage are a glossy sticky honeydew secretion which leads to the growth of sooty mold, stem blanching, chlorotic spots, leaf chlorosis and leaf abortion.
Taking preventative measures is the best way to control a whitefly population. When the poinsettia cuttings arrive, make sure that they are looked over for pests before introducing them into the growing area. Prevention also means implementing a good IPM program, with a strict scouting schedule. Before scouting, a grower should know what the pest level tolerance is for the crop they are going to sell. For example a grower selling poinsettias to a retail florist shop would have very little to no tolerance for pest present, whereas a grower growing and selling their poinsettias for the end user could have a slightly higher tolerance for pest present in their crops. A good method when scouting crops with whiteflies in mind is to first set up sticky cards. Since whiteflies are attracted to the color yellow, it is recommended to use yellow sticky cards. Keep in mind whitefly are so attracted to yellow that some growers will actually ban wearing the color yellow in their greenhouses and enter infested greenhouses last to prevent spreading the pest.
Growers cannot depend on using sticky cards alone, it is also recommended to pick up a random sampling of the crop and check the under sides of the leaves for the immature whiteflies. It may be beneficial to use a magnifying glass to easily identify the pest. If there are any adults present, the moving of the plant will stimulate them to fly out which should be noted when scouting. Scouting isn’t solely observing the crop, growers should also take note where in the greenhouse the infestation started, the crop or variety the pest was first seen on, and also note if there are pests on weeds present on the floor of the greenhouse.
Since the majority of the damage inducing lifecycle of whiteflies is located on the underside of the leaf, it is often hard to control the insects by spraying. Usually an application of Marathon or Mantra (imidacloprid) is used as a systemic insecticide to prevent whitefly infestations. Imidacloprid is usually applied after the poinsettia crop is potted and when the roots can be seen when the plant is taken out of the pot. This is about 2-3 weeks after the crop is potted up. Only one application of imidacloprid is recommended but if there are whiteflies present after the application then using an insect growth regulator (Enstar) and/or a knockdown (Mavrik) is suggested. A good combination of two insecticides would be to use Enstar AQ and Mavrik Aquaflow as a combination spray. When spraying, ensure that all parts of the plant are covered with the chemical, making sure the undersides of the leaves are covered. One thing to take note when applying chemicals to poinsettia crops is that once flowering, the bracts are very susceptible to bract burn when chemicals are applied. It is best to control whiteflies earlier on to prevent having to apply chemicals when the poinsettia crop is in flower.
Controlling whiteflies doesn’t solely have to be reliant on chemicals. There are also great biological controls, as well as physical. Using insect screen at the vents or openings of the greenhouse will prevent the pests from being able to enter the greenhouse during ventilation. There are also several whitefly predators and parasitoids such as lacewing larvae or encarsia which are used mainly for prevention or low levels of infestation. An insect killing fungus called beauveria bassiana can be applied to the plants as a foliar spray. Common label name is mycotrol or bontanigard and can be used as a resistance management application.
The key to controlling whiteflies, especially on poinsettia crops, is early detection and prevention. A good scouting program and knowing what the adults look like, as well as the juvenile stages, is the first step in controlling the population. There are many controls including physical, biological, and chemical that can be used. No greenhouse or growing condition is the same. The grower should implement the practices that best fit with their needs to create a beautiful and profitable poinsettia crop for the holidays.