Whiteflies on Spring Crops

As spring passes by, several growers will likely observe the presence of the greenhouse whitefly (Trialeurodes vaporariorum) and/or the silverleaf whitefly (Bemisia argentifolii) on their crops. Whiteflies have a wide host range of plants they can be found feeding on.  

In most instances, there is no detectable damage to the plants from their piercing, sucking mouth parts. However, under rare circumstances and when their populations are high more significant injury is likely and the plants may turn chlorotic and appear mottled from whitefly feeding. Although, they do not usually cause visible injury symptoms from feeding, there is virtually zero tolerance for the presence of any adults on crops in today’s marketplace.
 

Like their common name suggests, whiteflies are small white flies; the adults are typically 1.5 to 2.0 millimeters in length. The silverleaf whitefly is slightly smaller than the greenhouse whitefly, has a slight yellow coloration and holds its wings roof-like over its abdomen at approximately a 45 degree angle with the leaf surface. The greenhouse whitefly appears nearly all white and holds its wings fairly flat over its abdomen, appearing nearly parallel to the leaf surface. Apart from these most obvious characteristics, it can be difficult to distinguish between the various species and biotypes of whiteflies.  

All life stages are most commonly observed on the undersides of plant leaves. Each female adult typically lays 200 to 300 eggs in her lifetime. Their lifecycle progresses through four nymphal stages, a pupal stage, and finally the adult stage, where they may live for one or two months.  

The complete life cycle (from egg to adult) takes an average of 32 days for greenhouse whiteflies and 39 days for silverleaf whiteflies. Females begin laying eggs as early as 1 to 4 days after emerging as adults. The eggs are most commonly observed in a crescent shaped pattern on the undersides of the younger, upper leaves of the plant. The spindle-shaped eggs are white at first and turn gray with time. Depending on temperature, the eggs of greenhouse whiteflies hatch in about 8 to 10 days and silverleaf whitefly eggs take slightly longer (10 to 12 days) to hatch. Whitefly nymphs are oval and have a pale green to yellow coloration.

The pupal case of the greenhouse whitefly has parallel sides that are perpendicular to the leaf surface, giving the pupa a disk- or cake-shaped appearance; there is also a fringe of wax filaments around the edge of the pupal case. The pupal case of the silverleaf whitefly is yellowish and appears more rounded or dome shaped; the sides are not parallel and lack filaments along the outer edge. The pupal cases of both whiteflies have several pairs of filaments arising from the top of the pupa. 

Growers should scout or monitor their crops at least once per week to detect the presence of whiteflies (and other pests) and to evaluate the effectiveness of their management strategies. When monitoring, it is useful to use a combination of methods including sticky cards and random plant inspections. While examining plants, check the undersides of older, more mature leaves for the presence of whiteflies- all life stages can be observed there. 

To reduce the occurrence of whiteflies in production areas, it is best to begin with a clean greenhouse prior to starting a new crop of poinsettias. The production facility should be free of any plant materials including weeds and ‘pet’ plants, which may harbor whiteflies. If possible, remove all living plant materials from the greenhouse for at least 7 to 10 days between crops- any surviving adults will soon die from starvation.
 
Next growers should take precautionary steps to prevent whiteflies from entering the production area. Inspect all incoming plant materials (such as cuttings, rooted liners, or prefinished materials) for the presence of whitefly immatures and adults before introducing them into the production facility.  

If neither of the above strategies is possible, scout any plant materials present and consider implementing one or more of the control strategies discussed below. Due to the low threshold levels our industry tolerates many growers implement preventative programs during production.  

Whiteflies can be effectively managed by using biological control agents including the use of microbial insecticides containing Beauveria bassiana, Isaria fumosorosea, Metarhizium aniospliae or Paecilomyces fumosoroseus. Parasitic wasps (Encarsia formosa) can be released for controlling greenhouse whitefly, Eretmocerus californicus and Eretmocerus mundus for controlling the silverleaf whitefly, or the parasitic wasp Eretimocerus eremiccus or predatory mite Amblyseius swirskii can be used for controlling either type of whitefly. Products containing azadirachtin, petroleum oil (horticultural oil), neem oil, pyrethrins can also be used to biologically control whiteflies.

Systemic insecticides containing acetamiprid, clothianidin, dinotefuran, imidacloprid, and thiamethoxam are very effective at controlling whitefly populations, provided they are applied properly. All of these systemic products are in the same chemical class and have a similar mode of action; do not rotate between these products during production. Contact insecticides containing abamectin, acetamiprid, bifenthrin, buprofezin, clothianidin, cyfluthrin, fenpropathrin, flonicamid, pymetrozine, pyriproxyfen, and pyridaben are all effective and can be used to control any hot spots that arise.  
 

Perennial Solutions Consulting
paul@perennialsolutions.com

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