Field Scouting Guide: Whitefly

Field Scouting Guide: Whitefly

Whitefly-adults-on-a-melon-leaf.-Photo-by-John-Palumbo

Whitefly adults on a melon leaf. Photo by John Palumbo

This month’s field scouting guide concentrates on Bemisia tabaci biotype B: silverleaf whitefly or sweet potato whitefly (SWF). Each month, we bring you a different crop protection issue, ranging from weeds and diseases to insects.

Two entomologists discuss how to spot and treat this insect — John C. Palumbo, University of Arizona; and Hugh A. Smith, University of Florida, Institute of Food and Agricultural Sciences.

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Basics

  • Scientific name: Bemisia tabaci biotype B
  • Common name: Silverleaf whitefly or sweet potato whitefly (SWF)
  • Geographical Range: The pest is globally present in warm regions, such as the tropics and sub-tropics. In temperate regions, SWF is an issue in greenhouses and in the field. It’s also a problem on vegetable crops grown throughout the Southern U.S.
  • Crops affected: Both experts agree that SWF has hundreds of hosts, including many important horticultural, agronomic, ornamental crops. It even impacts weeds.
    “SWF is a very polyphagous pest, feeding on more than 500 species of plants in 74 families,” explains John C. Palumbo, University of Arizona entomologist. “Of the important vegetables crops grown in Arizona, SWF is a primary pest of most leafy vegetables (lettuce, spinach, and celery), all cole crops, and melons (cantaloupes, honeydews, and watermelons).”

Pest Impact

Melon-field-infected-with-cucurbit-yellow-stunting-disorder-virus.-Photo-by-John-Palumbo

Although whiteflies are a major pest themselves, their propensity for transmitting viruses also has a big economic impact. Here is a field infected with cucurbit yellow stunting disorder virus. Photo by John Palumbo

Palumbo: SWF is an annual pest that requires control to prevent economic yield losses in Arizona and Southern California. In the past 25 years, SWF has shifted from an occasional virus vector to now being one of the primary pests in vegetables and melons in the desert Southwest.

All nymph stages feed and remove plant assimilates (carbohydrates). Adults also feed, but they are more important for transmitting plant viruses.

In leafy vegetables, damage by large whitefly populations can result in reduced head size, delayed harvest, and leaf chlorosis (yellowing). SWF also causes economic damage through contamination associated with the insects themselves, and honeydew and sooty mold accumulation on harvested leaves.

In cole crops, adult and nymph feeding causes localized spotting and can lead to wilting, yellowing, leaf drop, stunting, or plant deformity. Heavy infestations on broccoli can cause a chlorosis, or blanching effect, on the main stem (“white stalk”) and petioles that can render the crop unmarketable.

In melons, adults and nymph feeding often results in reduced fruit production and size, as well as affecting fruit quality by lowering soluble sugars in the fruit and by the contamination of fruit with honeydew and sooty mold. SWF adults can also transmit important viruses on melons.

Smith: SWF is a problem throughout the Southeastern U.S., mostly Florida and Georgia, as well as the desert Southwest.

The primary impact of this pest in vegetables is to vector viruses. SWF transmits tomato yellow leaf curl virus (TYLCV) in tomatoes and three important viruses in cucurbits: squash vein yellowing virus, which causes watermelon vine decline; cucurbit leaf crumple virus; and cucurbit yellow stunting disorder virus. In addition, SWF is the causal agent of silverleafing of squash and irregular ripening in tomatoes, both of which stem from SWF nymphs feeding.

Feeding by adults and nymphs can debilitate plants. Both produce honeydew, a sugar-rich excreta that serves as a substrate for sooty mold. Nymphs and adults can acquire the virus from feeding on infected plants; however, only adults are mobile, so only adults are responsible for transmission of virus to uninfected plants.

Identification

Adult-and-nymph-whiteflies

Whiteflies are generally not confused with other pests. But there are several
species of whiteflies. The species affecting vegetables most is silverleaf whitefly (SWF), which can be identified by its yellow abdomen and how it holds its wings in a roof-like way over its body. In the final nymphal stage, the SWF is relatively flush with the surface of the leaf. Photos by John Palumbo

Smith: Whiteflies are not generally confused with other pests, although it can be challenging to distinguish among species of whitefly in some instances. SWF looks like Trialeurodes vaporariorum, the greenhouse whitefly, which tends to predominate at higher elevations in the tropics and in greenhouses in temperate regions.

The primary whitefly that you will find on field-grown vegetables in the U.S. is the SWF. You may find the greenhouse whitefly in temperate or Mediterranean-growing regions of the U.S., such as the Salinas Valley in California, but it’s rarely a problem.

Here are a few ways to differentiate between the two:

  • The SWF holds its wings “roof-like” over its body; the greenhouse whitefly holds its wings flat or “fan-like” over its body.
  • Under magnification, SWF’s yellow abdomen is visible.
  • The final nymphal stage of the greenhouse whitefly has a marginal fringe of waxy rods that look like hairs. The SWF’s final nymphal stage is relatively flush with the surface of the leaf.

There are several whiteflies affecting landscape and ornamental plants that might be confused with these two.

Recommended Treatment

Whiteflies-recommended-treatments-chart-LOW-RES

Palumbo: The best treatments are selective insecticides that are easy on natural enemies.

Smith: Plants should be treated at planting (or as soon after planting as is feasible) with a systemic insecticide. Systemic insecticides available for whitefly management include neonicotinoids, butenolides, and diamides.

Plants should be treated at least once during the first 5-week treatment window with systemic insecticides for protection against virus transmission.

As whiteflies become established, it’s important to include materials to control nymphs in the insecticide rotation.

Organic Controls

Smith: Insecticidal soaps such as M-Pede and Beauveria bassiana products, including BotaniGard, can be included in rotations and applied repeatedly throughout the season. Most biopesticides are acceptable for use on certified organic farms. Repellent mulches and virus-resistant varieties are other management tactics that certified organic growers can use.

IPM Recommendations

Smith: Clean culture is key for reducing viral inoculum. Growers should be sure to start with virus-free tomato transplants, and to end the season by promptly destroying harvested fields that can serve as a source of SWF and virus for crops that are still in the field.
When possible, tomato growers should plant at least a portion of their acreage with TYLCV-tolerant tomato varieties. There are presently no varieties of cucurbits with tolerance to SWF-transmitted viruses.

Metalized plastic mulches contribute to early season virus suppression by repelling SWF. SWF nymphs are immobile and vulnerable to attack from predators. SWF nymphs are also parasitized by many species of parasitic wasp. Generally, SWF nymph numbers must reach high levels before they are impacted by natural enemies such as predators and parasitoids.

Since SWF can transmit viruses in a matter of minutes, natural enemies are generally considered to have a limited role in field-grown crops. Greenhouse-vegetable production offers greater opportunities for using natural enemies of SWF, although most commercially available biocontrol agents are deterred by the sticky exudates and leaf hairs on tomato.

Palumbo: Sampling for adult SWF should be concentrated on young leaves, whereas sampling for nymphs should be on older foliage. Adults should be sampled during early morning hours by carefully turning leaves before adults are active. Sampling for immatures should include visual observations of the underside of leaves using a 10X-20X hand lens.

Certain cultural practices can help to avoid or minimize problems with SWF before they have the chance to occur:

Crop Management. It is important to use optimal growing practices to avoid stressing newly emerged seedlings. This includes proper irrigation management, plant nutrition, and salinity. Experience has shown a vigorously growing plant is better able to withstand external stresses like SWF.

Crop Scheduling. Careful crop sequencing, crop placement, and planting dates can have significant impacts on adult SWF migration. When practical, growers should avoid planting fall melons and vegetables near significant host crops such as alfalfa and cotton.

Source Reduction. The most important area-wide cultural practices are sanitation and clean culture. Rapidly destroying host crops postharvest can reduce the magnitude and duration of whitefly movement in an area. Also, eliminate weed hosts in and around fields, particularly in the winter, since they can serve as overwintering reservoirs for SWF and viruses.