Whiteflies

Several species of whiteflies infest cucurbits. Proper identification of the whitefly species is important because the silverleaf whitefly, and occasionally the greenhouse whitefly, represent the greatest damage potential to cucurbits. Silverleaf whitefly is relatively new to California and has all but displaced the sweetpotato whitefly, which was a problem in cucurbits because of its ability to vector viruses. Silverleaf whitefly is a major problem in California's southern desert and an increasing problem in the southern San Joaquin Valley.

Distinguishing whitefly species is difficult; use a hand lens to examine both immatures and adults. Whiteflies are small insects that are about 0.06 inch (1.5 mm) long. The body and wings of adults are covered with a fine, whitish powdery wax that is opaque in appearance. Silverleaf whitefly adults hold their wings somewhat vertically tilted, or rooflike, over the body and generally the wings do not meet over the back but have a small space separating them. Greenhouse whitefly (Trialeurodes vaporariorum) adults, the species most similar in appearance to silverleaf whitefly, hold their wings flatter over the back and there is no space where the two wings meet in the center of the back.

Whiteflies colonize the underside of leaves; adults and eggs are commonly found on the lower surface of younger leaves and the scalelike nymphal stages on somewhat older leaves. The tiny, oval eggs hatch into a first larval stage that has legs and antennae and is mobile. The legs and antennae are lost after the first molt and subsequent stages remain fixed to the leaf surface. The last nymphal stage, often called the pupa or the red-eyed nymph, is the stage that is easiest to identify. Silverleaf whitefly pupae are oval, whitish, and soft. The edge of the pupa tapers down to the leaf surface and has few to no long waxy filaments around the edge. In contrast, greenhouse whitefly pupae have many long waxy filaments around the edge, and the edge is somewhat vertical where it contacts the leaf surface. Most other whiteflies found on cucurbits produce a lot of white wax in their colonies; silverleaf whitefly has almost none.

Desiccation of plants occurs with moderate-to-heavy populations and the production of honeydew gives rise to sooty mold. The plant becomes unthrifty and nonproductive, and the fruit is rendered unmarketable. Sweetpotato whitefly has historically been a serious problem in cucurbits by transmitting lettuce infectious yellows virus and squash leaf curl virus. Recently, sweetpotato whitefly has all but disappeared in California, displaced by the silverleaf whitefly. The silverleaf whitefly has become especially damaging in southern California growing areas and also threatens cucurbits in northern California. In light-to-moderate infestations of silverleaf whitefly, leaves show no distinctive symptoms as a result of their feeding; however, copious quantities of honeydew are deposited on leaves, resulting in a sticky, shiny appearance. Silverleaf whitefly has become a serious pest because of its high reproductive capability, wide host range, high rate of feeding, and exudation of sticky honeydew. Its feeding on squash frequently causes crop leaves to turn whitish or silver, hence the name silverleaf whitefly.

Whiteflies, with the exception of the silverleaf whitefly, rarely require chemical control. Natural or introduced biological controls provide the best long-term solution to keeping most of the whitefly species at low levels along with crop host absence in the areas of heavy infestations. Key cultural controls to prevent the buildup of this pest include row covers in the low deserts, silver reflective mulches, noninfested transplants, and good field sanitation.

Several wasps, including species in the Encarsia and Eretmocerus genera, parasitize whiteflies. Whitefly nymphs are also preyed upon by bigeyed bugs, lacewing larvae, and lady beetles. Silverleaf whitefly is an introduced pest that has escaped its natural enemies. Some indigenous native parasites and predators do attack it, but do not keep it below damaging numbers. The lady beetle, Delphastus pusillus, is being introduced into southern California to assist in biological control.

Populations peak in late summer and begin to decrease by November. Delaying planting or using host-free periods may decrease severity of attack.

Do not plant melons during fall in the low deserts of southern California unless row covers are applied to beds at planting. Row covers are not recommended in the San Joaquin Valley.

1. Vent the row covers and remove them gradually at first bloom or before, if needed. Do not remove the entire cover at one time because a drastic reduction in humidity will shock plants and can lead to collapse. Covers made of spun-bonded materials do not need venting; hot air is able to escape.
2. Remove row covers if the air temperature underneath reaches 104°F before bloom.
3. Also remove plastic row covers before plants grow high enough to touch hot plastic.

Silver reflective plastic mulches applied at planting have been shown to be effective in reducing the number of silverleaf whiteflies landing on melon leaves. This, in turn, delays the buildup of whitefly populations on melons. They help plants off to a healthy start and are effective until expanding foliage covers the reflective surface. In desert areas, remove reflective mulches when summer temperatures are excessive for optimal growth of plants. In the Central Valley and cooler areas, mulches have not caused plant damage; in fact, they improve soil moisture and nutrient retention, which may increase plant productivity.

Avoid whitefly infested transplants; this is how the silverleaf whitefly has been transported to other areas of the state. When possible, plant cucurbits at least one-half mile upwind from other key whitefly hosts such as cole crops and cotton. Maintain good sanitation in winter/spring host plants and weeds. Remove field bindweed and other weeds in and adjacent to the crop field as well as crop residues. Attempt to produce the crop in the shortest season possible; proper management of irrigation and nitrogen will assist in this.

Biological and cultural controls, as well as soap and certain oil sprays are acceptable to use in an organically certified crop.

The key to management is preventing whiteflies from colonizing plants in and around the crop. This is often done by targeting the adult population to prevent them from ovipositing on the undersurface of leaves. Once oviposition takes place it is difficult and often expensive to prevent damage caused by nymph feeding. Failure to minimize the establishment of whitefly colonies can result in significant yield loss due to delayed maturity and reduced quality and fruit size.

If allowed, a soil application of imidacloprid (Admire) or thiamethoxam (Platinum 75 SG) at planting effectively controls whiteflies. Foliar treatments with bifenthrin (Brigade WSB) or spiromesifen (Oberon) during the growing season effectively controls whiteflies when thresholds are reached.

Presence-absence Sampling

A presence-absence sampling method developed in Arizona provides information on whitefly distribution and economic control, particularly for spring melons. According to the research, visual observation of adults on leaves offers a more practical and accurate method to estimate whitefly populations than sticky traps. While sticky trap monitoring is used to detect initial migration into the field, evidence shows that trap catches near recently treated fields had artificially inflated whitefly numbers.

Because whitefly populations have shown a relatively even distribution in melon fields, the presence-absence sampling relates the total number of leaves with one or more adults to the average number of whiteflies per leaf throughout the field.

Set up yellow sticky traps right before planting to monitor whiteflies and aphids. Start monitoring the traps after transplanting or when seedlings emerge. Once traps begin to catch whiteflies, start presence-absence sampling.

• Monitor during early morning hours, within 2 hours of sunrise (before adults are active).
• Sample 50 leaves per field (25 leaves from 2 separate areas) by carefully turning the terminal leaf (adults tend to aggregate at terminals) of each plant and looking for adult whiteflies on each leaf.
• Count leaves with one or more whitefly adults.
• Note presence of red-eyed nymphs on the crown portion of the plant. Nymph presence can indicate a rapid increase of adult numbers in the near future.

If 50% or more of the leaves have one or more adults, the estimated infestation level in the field is at least 2 to 3 adult whiteflies per leaf.

Apply an insecticide before whitefly numbers reach more than 2 adults per leaf and honeydew contaminates fruit. Fall melon plants are usually small when adults migrate, therefore insecticide applications may be needed more frequently. Whitefly adults move between crops, so be sure to manage them in all nearby crops.

Continue presence-absence sampling after treatment to determine its success and to follow populations migrating into the field later in the season.

Nymph Monitoring

Although more complicated, nymph monitoring can be used as a supplement to adult monitoring or to manage whiteflies with an insect growth regulator (IGR).

• Collect 50 leaves per field (25 leaves from 2 separate areas).
• Place a quarter between the two major leaf veins to delineate a 1-cm² disc.
• Using a hand lens, look at the disc area and count visible nymphs, including 3^rd and 4^th instars, which look like flattened, egg-shaped scales.
• Count and record the number of nymphs for two 1-cm² discs per leaf.

Apply IGRs like buprofezin (Courier) when there is an average of 0.5 large nymphs or more are per disc.