This report is intended to provide guidelines for growers and pest control advisors concerning the management of the sweet potato whitefly, Bemisia tabaci (Gennadius), on spring melons. These recommendations are based primarily on experiments conducted in commercial fields and at University of Arizona experiment stations. Some statements are the result of personal observations.

Sampling Methods

To make appropriate decisions concerning pesticide applications, a reliable sampling scheme must be available. Currently, some control decisions are based on information generated from sticky traps. Sticky traps, although useful in certain situations, have not been found to accurately reflect population levels within melon fields. We have also found that trap catches near sprayed fields are artificially inflated following pesticide applications. Visual observations of adults on leaves provide a more accurate and practical method for estimating field populations.

Important Considerations

• Experimental evidence to date indicates that whitefly populations are relatively evenly distributed in melon fields
• Whitefly adults aggregate on the plant terminals (i.e., distally from the fourth node)
• You should sample adults during early morning hours (within 2 hours of sunrise) by carefully turning leaves before adults are active

Action Thresholds

To prevent problems at harvest, it is necessary to minimize the establishment of whitefly colonies. Failure to do so may result in significant yield loss due to delayed maturity, and reduced fruit size and quality. Whitefly colonies have relatively lower densities during the early part of the growing season, but if left unchecked, can reach unmanageable levels at harvest. Experimental fields where whitefly populations reached 25 adults per leaf experienced a yield reduction of 52 percent, a 2 percent loss in total sugars and 50 percent of the fruit was excessively sticky.

On the basis of preliminary field data, we recommend that whitefly populations not be allowed to exceed three adults per leaf. We believe that a control regimen based on this threshold would prevent large numbers of whiteflies from infesting plants later in the season.

Presence/Absense Sampling Scheme

We have developed a presence-absence sampling scheme similar to those that have become popular in integrated pest management (IPM) programs because they minimize the time required for estimating populations. Presence-absence sampling involves counting the number of leaves with one or more whiteflies (i.e., are they there [present] or not [absent]) and relating these to the mean number of whiteflies per leaf across the field. For example, we recommend that a grower sample 200 leaves in a field (50 from each quadrant). If he finds at least 61 percent of the leaves have at least one adult then he will know that the estimated level of infestation for the entire field is two to three whiteflies per leaf (Table 1). This estimation is based on a 200-leaf sample; reduction of the sample size will reduce the accuracy of the estimation. Three whiteflies per leaf is the action level for whiteflies on spring melons and control measures should be taken when populations reach that level.

The presence-absence sampling scheme for adults should not preclude monitoring the crown portion of the plant for red-eyed nymphs. Presence of these nymphs may indicate that adult numbers may increase rapidly in the near future.

Chemical Management

• It is recommended that growers use ground application to obtain the necessary coverage whenever possible. This should be accomplished with narrow nozzle spacing, high pressure and maximum volume. This is particularly important considering the prostrate characteristics of melons. New technology such as hydraulic air-assist, and electrostatic sprayers are currently being evaluated and show promise commercially. Experimental results will be published when available.
• Effective pesticide combinations include bifenthrin (a pyrethroid) with endosulfan (a chlorinated hydrocarbon relative), bifenthrin with methamidophos (an organophosphate), abamectin (a new chemistry) with endosulfan and esfenvalerate (a pyrethriod) with endosulfan. Assurances should be obtained that these materials are registered before application.
• The limited materials available for melons should be rotated according to chemical class. The continual use of a given combination will likely lead to enhanced pesticide resistance.
• It is imperative that pollinator loss due to pesticides be minimized. Failure to do so has been shown to dramatically reduce yields.
• Be aware of surrounding whitefly crop hosts (e.g., broccoli, cauliflower, alfalfa, and lettuce) and overwintering weed hosts that are present near your melon fields. These weeds should be controlled and crop residue should be removed as soon as possible.
• Post harvest management of crop residue will reduce the impact of whiteflies in subsequent crops. It is recommended that vines be destroyed within 21 days following the initiation of harvest.

Tonhasca, A., J. C. Palumbo and D. N. Byrne. 1994. Binomial sampling plans for Bemisia tabaci (Homoptera: Aleyrodidae) populations in cantaloupes. Research in Population Ecology in press.

Tonhasca, A., J. C. Palumbo and D. N. Byrne. 1994. Population dynamics of Bemisia tabaci (Homoptera: Aleyrodidae) in cantaloupe fields in Arizona. Environmental Entomology in press.

Tonhasca, A., J. C. Palumbo and D. N. Byrne. 1994. Aggregation patterns of Bemisia tabaci (Homoptera: Aleyrodidae) in response to insecticide applications. Entomologia Experimentalis et Applicata in press.

Tonhasca, A. and D. N. Byrne. 1994. The effects of crop diversification on herbivorous insects: a meta-analysis approach. Ecological Entomology in press.

Full Disclaimers

Issued in furtherance of Cooperative Extension work, acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, James A. Christenson, Director Cooperative Extension, College of Agriculture and Life Sciences, The University of Arizona.

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