Whitefly Morphology and Control

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Whitefly Morphology and Control

Whitefly is the most serious pest on poinsettias. Prior to 1986 the Greenhouse whitefly (GHWF) was the predominate species but since the early 1990’s, the silverleaf whitefly (SLWF), also known at the B strain of the sweet potato whitefly has also become a serious concern for poinsettia growers. While the SLWF is considered more difficult to control, the biology and control of both species is similar. Neither species is known to overwinter outside in northern areas and development does not occur at temperatures below 50F (10C). Several other species of whitefly including additional strains of the sweet potato whitefly have been identified on poinsettias.

Greenhouse whitefly (Trialeurodes vaproariorum)

There are some subtle differences in the morphology of the two species. Greenhouse whitefly (GHWF) adults are 2mm long and covered with a white, waxy powder. They cluster on the undersides of leaves. The immature stages are oval, light in color, and have a fringe of white wax threads or filaments. Pupae have a translucent marginal fringe that is elevated in profile with sides perpendicular to the leaf. Eggs are often laid in a circular pattern and are attached to the leaves on short stalks. Growth and development of both whitefly species depend on temperatures. Eggs take five to ten days to hatch at which time a crawler emerges, travels a short distance, then settles down to feed. After feeding for about two weeks the pre-pupa and pupa stage form and last about a week. Once the adult female emerges she will lay an average of 50 to 150 eggs. The adult lifespan varies depending on temperatures but the average lifespan is six days. Adult GHWF feed with sucking mouthparts that extract sap from the phloem. Heavy infestations can leave the foliage mottled and chlorotic. In addition the excrement known as honey dew left behind by the whiteflies is an excellent growing medium for sooty mold. While unsightly, heavy growth can also lead to chlorosis, deterioration, and eventually plant death.

Silverleaf whitefly (Bemisia argentifolii, also known as strain B of the sweet potato whitefly, B. tabaci)

Silverleaf whitefly (SLWF) adults are smaller and generally more active than the GHWF. The adults have less white wax and a more yellow appearance. The easiest and most consistent way to identify the two species is to examine the later-stage larvae or pupae. SLWF have a very distinct yellow color while GHWF have a whitish translucent appearance. SLWF immatures are ellipsoidal, appear to have a “waist” and no fringe is present. The pupal stage is convex and flattened in shape while the GHWF pupas have a strong fringe and are more elevated in profile. SLWF adults’ wings lay at a 45 degree angle to their body while GHWF adults’ wings are flat. At normal greenhouse temperatures, SLWF adults live an average of 22 days during which time they can lay up to seven times more eggs than the GHWF. High levels of SLWF feeding can cause white stems on a poinsettia which is thought to be from a toxin released by the adults. Bract color may also be lighted by a heavy infestation.

 

Figure 1 Whitefly life cycle

Unfortunately, even after whiteflies have been controlled, the remains, including sooty mold, dead whiteflies, and empty pupal cases, may reduce the marketability of a poinsettia crop. For this reason it is imperative the growers make every effort to control populations and prevent infestations. Integrated pest management programs should focus on whitefly control.

Poinsettia IPM Program for Whiteflies

Sanitation

  • Remove weeds in and around the greenhouse as they can be host to whiteflies/ Remove plants that are attractive to whiteflies such as hibiscus, lantana, vegetables, etc.
  • Allow for a host free period – Ideally the greenhouse should be emptied and disinfected for a period of 5 to 7 days prior to introducing a poinsettia crop. If this isn’t possible, every effort to control existing whitefly population should begin 1 to 2 months prior to planting the poinsettia crop.
  • Continually remove debris from pinching or leaf drop throughout the crop as whitefly can continue to develop on detached leaves.

Exclusion

  • Inspect cuttings upon arrival for whiteflies and other pests or diseases. Any leaves with immature or whitefly eggs should be discarded. Do not take infested plants into the greenhouse. Ideally 100% should be inspected but if this isn’t possible check randomly throughout the cuttings.
  • Microscreening can be an effective form of exclusion and may reduce the number of pesticide applications needed. Screened greenhouses must begin with a pest free house and all employees must be trained to keep screens in place and doors closed.

Cultural Practices

  • Cultural practices will effect whitefly populations. Space plants appropriately; good spay coverage is difficult at tighter spacing. Just after pinch the canopy of the plant is fairly open and immature leaves are not favored for egg laying. Take advantage of this time to control existing populations. Continually remove heavily infested leaves and remove them from the greenhouse.
  • Avoid the use of anything yellow as whiteflies are attracted to the color. Don’t allow employees to wear yellow and don’t use yellow trash cans or signs etc. Train employees to enter whitefly infested houses last to avoid moving whiteflies to non-infested areas.

Monitoring

  • Place yellow sticky cards just above the plant canopy with 1 card per 1,000 sq.ft. Additional cards should be placed near doors or outside walls.
  • Inspect plant samples for all stages weekly from all areas of the greenhouse. Leaves from the top, middle, and bottom of the plant should be inspected. Train employees to recognize all stages.
  • If infested plants are found they can be tagged and used as indicator plants. Whitefly development rate and effectiveness of chemical and biological controls can be monitored.
  • Keep accurate records of whitefly counts/ monitoring costs/ control costs.
  • Involve all employees in informal scouting. Employees that frequently handle plant material should be familiar with the life stages.

Biological Control

  • Biological control can be an effective preventative measure when used properly. Used in conjunction with all the other components mentioned biological controls can reduce the need for chemical controls.

Chemical Control

  • Chemical control is still a major component of an IPM program. Proper choice on insecticides is very important.
  • Whiteflies are known for their ability to develop resistance to chemicals. Proper rotation of chemicals is necessary in reducing resistance. Chemicals should be rotated by mode of activity (MOA) and not by chemical class as some chemical classes have a similar mode of activity.
  • Always use an insecticide for the duration of one generation of whiteflies before switching to a chemical with a different MOA.
  • Resistance management should also minimize the use of excessive tank mixes. Combining pesticides with different MOA will expose the insect to multiple MOA which may lead to resistance of more than one insecticide. If tank mixes are used, consider combining a nonspecific MOA with a specific MOA, or two chemicals with known synergistic activity.
  • The use of non-specific modes of activity is also encouraged. Most conventional insecticides have site-specific modes that target or inhibit one vital function so only one of the insects genes has to be altered for resistance to develop. Non-specific modes target multiple functions. Because several of the insects genes have to be altered to develop resistance non-specific modes are less susceptible to resistance activity. Non-specific modes of activity include horticultural oils, insect growth regulators, insecticidal soaps, and beneficial fungi or bacteria. Conventional insecticides should be rotated with non-specific modes of activity in order to preserve each chemicals longevity.
  • Always use minimum label rates. Beginning with the lower rates gives growers the ability to use higher rates if necessary but constant use of maximum rates may increase the amount of resistance BY honey, strain, they, hibiscus, whitefly, whiteflies, indicator, someBY honey, strain, they, hibiscus, whitefly, whiteflies, indicator, someBY honey, strain, they, hibiscus, whitefly, whiteflies, indicator, someBY honey, strain, they, hibiscus, whitefly, whiteflies, indicator, someBY honey, strain, they, hibiscus, whitefly, whiteflies, indicator, someBY honey, strain, they, hibiscus, whitefly, whiteflies, indicator, someBY honey, strain, they, hibiscus, whitefly, whiteflies, indicator, someBY honey, strain, they, hibiscus, whitefly, whiteflies, indicator, someby whiteflies. It also limits the amount of options when higher rates fail to control populations.
  • Know which life stage is most prevalent and choose a chemical and mode of activity accordingly. Whitefly nymphs and adults are more susceptible to pesticide applications than the eggs and pupae. Chemical applications should not be made on a frequent basis or by the calendar regardless of whitefly population dynamics.
  • Proper application techniques will also help reduce resistance. Good coverage is essential because if not obtained, pests not reached within the canopy may later come in contact with reduced dosage of the chemical residue and begin developing resistance.
  • Always consult the label of the chemical you are working with for recommended rates and method of application. Many labels today also include useful information about resistance management.
  • Monitor control after pesticide applications and contact your local extension agent if whitefly resistance appears to be a problem or if you need help with a pesticide rotation program.

The Paul Ecke Ranch encourages you to contact your local experts, cooperative extension agents, local Departments of Agriculture, Universities, and others, if you detect any problems controlling insects, or if in your estimates the degree of difficulty in controlling the insect goes beyond the reasonably expected degree of difficulty you have experienced in the past. They are qualified to recommend better management strategies that currently exist even when resistance has been detected or reported.
You may also go to http://www.mrec.ifas.ufl.edu/LSO/ for more information about Bemisia Q and B types.

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