Controlling whiteflies

Eretmocerus eremicus; Encarsia formosa; Whitefly damage on poinsettia leaf

Whiteflies have historically been problematic pests of ornamental greenhouse and nursery crops. For many years, greenhouse whitefly (Trialeurodes vaporariorum) was the primary pest. However, since the late 1980s, another species of whitefly has become a major pest of ornamentals — the sweetpotato or silverleaf whitefly (Bemisia tabaci). Both species can develop large populations and cause significant damage to plants. Recently, drought conditions and high temperatures across the U.S. have caused an increase in whitefly populations for many growers.

Integrated crop management (ICM) is vital to whitefly control and overall plant quality. ICM programs incorporate several factors of plant production, such as plant protection products, biological control agents (BCAs) and environmental factors. Plant protection products include chemical controls, and BCAs include beneficial insects, mites, fungi and cultural practices.

Plant damage
Whiteflies belong to the sub-order Homoptera, which also includes aphids and scale pests. Adult whiteflies and immature stages have piercing-sucking mouthparts, which are inserted in the phloem and extract plant sap. Whiteflies feed on a large number of plant species, but damage on ornamentals is most noteworthy on poinsettias, gerbera daisies, hibiscus, mandevilla, lantana and verbena.

Injury is generally observed as chlorotic plant tissue, and the presence of black sooty mold growing on honeydew excretions of the whiteflies. Some whitefly species cause more distinctive plant damage symptoms, including stunting and shedding of leaves in addition to chlorotic plant tissue. Sweetpotato whitefly is particularly notorious for transmitting numerous viruses. Large whitefly populations can cause stunting of plant growth and senescence of leaf blades.

Prevention is crucial
Pests thrive in greenhouse environments because there is an abundance of plant material available for feeding. Greenhouses also foster an ideal environment for pests to live and breed, which often results in the rapid reproduction of pests. It is important to monitor plants regularly in order to locate the adult and immature whiteflies before the population becomes too large to easily manage. Growers should take the following precautionary steps to prevent whitefly infestations:

• Maintain weed-free production areas to eliminate potential hosts.
• Inspect new plant shipments, and remove or treat plants infested with whiteflies.
• Eliminate or monitor points of entry to prevent invasion of adults from outside the greenhouse.
• Monitor egg and nymph populations. Check the underside of leaves with a hand lens.
• Monitor for adults with yellow sticky cards.
• Use appropriate water volume to cover the underside of plant leaves when applying spray applications. The addition of an adjuvant is beneficial with some products. Read and follow all product label recommendations.

Whitefly detection
Yellow sticky cards are commonly used to detect and monitor movement of adult whitefly populations on greenhouse crops. However, the best method for following population development and movement is strategically sampling a crop by turning leaves to look for immature whiteflies. A population can advance to high levels on the undersides of leaves and still remain undetected. Once a population has increased to damaging levels, it will be very difficult to control.

Manage whiteflies with an ICM approach
In the past, growers have used insecticide spray programs to control whiteflies. However, intensive spray programs can lead to the development of resistant populations. Rotating insecticides with different modes of action (MOAs) and incorporating BCAs will help manage pest resistance and provide control necessary for producing a successful crop.

The use of BCAs, such as whitefly parasitoids Encarsia formosa and Eretmocerus eremicus, must be carefully planned. A thorough understanding of each greenhouse, including pest and disease history, prior insect control methods and cropping systems, is necessary to implement a successful ICM program. When considering whitefly control, the use of biological controls can be incorporated with physical and chemical controls.

Paying careful attention to details will help ensure ICM programs start successfully and result in a quality crop. When integrating BCAs into greenhouse operations, consider the following factors:

• Pests and diseases in crop
• Production history
• Crop conditions
• Products used during the past three to four months; some products have long residual activity, which can affect the establishment of beneficial insects

Environmental conditions
ICM is based on the pests and beneficial insects that are present in the crop and the pests that are generally problematic. Knowing the life cycles for both the pests and beneficial insects will aid in the program’s success. The life cycle of a whitefly from egg to adult can be as short as 21 days.

Whiteflies on underside of a leaf

It is also important to consider climatic and environmental conditions during production, including temperature, humidity, light intensity and day length. By collecting this information, growers will be able to make informed decisions.

Beyond pest control
Integrated crop management is a pragmatic approach to crop production. Unlike integrated pest management (IPM), which focuses on crop protection, ICM encompasses multiple aspects, including IPM, growing media and social and environmental management. An ICM approach should combine all aspects of crop inputs and management to achieve the needs of both the producer and consumer.

Marla Faver and Paul Winski work for Syngenta Technical Services.