How to Manage Pests

UC Pest Management Guidelines

DESCRIPTION OF THE PESTS

Populations of iris whiteflies and, to a lesser extent, strawberry whiteflies have always been present in low numbers in strawberry fields in California. These species are usually kept below damaging levels by naturally occurring beneficial insects. In recent years, however, a third species, the greenhouse whitefly, has become a major pest in certain areas on the Central Coast and in southern California. The greenhouse whitefly has a large host range including alfalfa, avocados, beans, blackberries and other berries, cucumbers, eggplants, grapes, lettuce, melons, peas, peppers, potatoes, tomatoes, and many ornamentals, and these alternate hosts serve as sources for whiteflies that enter strawberry fields.

Whiteflies go through six stages in their development: eggs; first, second, third, and fourth instar immatures; and the adult. Eggs are microscopic and laid on the underside of leaves. Whiteflies do not have a true pupal stage, but the last part of the fourth instar, when the red eyes of the adult whitefly begin to appear, is often referred to as the "pupa." Only adults and the newly hatched nymphs (i.e., crawlers) are mobile. Greenhouse whiteflies tend to build up in fall, reaching peak densities in late fall through winter in central coast plantings. In warm weather, whiteflies can complete a generation in as little as 18 days.

Whiteflies are easy to distinguish from other insect pests: adults of all species are about 0.01 inch in size with four membranous wings that are coated with white powdery wax. Whitefly species are most reliably identified from each other by examining the late fourth instar or red-eyed "pupal" stage. The greenhouse whitefly has long, waxy filaments around the margins in this stage. When seen from the side, the greenhouse whitefly "pupae" are circular with flat tops, with the filaments emerging from the tops. Adult greenhouse whiteflies are solid white and hold their wings parallel (flat) to the top of the body. Both adults and nymphs look similar to the strawberry whitefly, but the strawberry whitefly nymphs never have the long filaments often found on the greenhouse whitefly "pupae." Iris whitefly "pupae" also lack long filaments but have short waxy ones around their bodies. Iris whitefly adults hold their wings flat over their backs and have a dot on each wing.

DAMAGE

Whiteflies may reduce crop yields directly through their feeding on leaf tissue, which removes plant sap, stunts plant growth, and decreases sugars in fruit. They also produce sticky honeydew that they excrete during feeding. The honeydew may cover plants and support the growth of black sooty mold fungus. Greenhouse whitefly can also transmit diseases and is known to vector strawberry pallidosis-related decline of strawberry in California.

MANAGEMENT

Successful management of greenhouse whiteflies requires an integrated program that focuses on prevention and relies on cultural and biological control methods when possible. Treatments are often necessary if strawberries are grown so that continuous plantings are present in areas where greenhouse whiteflies have become established (summer plantings or second-year plantings adjacent to new plantings), if whitefly biological controls are disrupted by the use of a nonselective pesticide, or if adult whiteflies invade the strawberry plantations from adjacent crop hosts or from backyards. No precise treatment threshold has yet been developed for greenhouse whiteflies on strawberries, but even feeding at relatively low densities after transplanting can result in yield loss. Treatment may be necessary when honeydew or moderate to heavy whitefly populations are apparent during periods of warmer weather for summer- and fall-planted berries. Select treatments carefully and use them only when monitoring indicates a need.

Biological Control

In most crops, greenhouse whiteflies and iris whiteflies are generally kept under control by naturally occurring parasitic wasps and general predators. Their natural enemies include parasitic wasps of the genera Encarsia, Eretmocerus, and Prospaltella, bigeyed bugs, pirate bugs, and lacewing larvae. In summer, in certain areas on the Central Coast and in Ventura County, 30 to 40% of greenhouse whiteflies are parasitized by native parasites.

Encarsia formosa is used worldwide for greenhouse whitefly suppression in greenhouses, but more research is necessary to determine if the release of this or other parasites can be helpful in preventing whiteflies from increasing in numbers.

Cultural Control

For summer-planted strawberries, the practice of topping in spring helps to reduce overwintering immature populations. Monitor whiteflies on adjacent hosts and initiate control there, if possible, before these crops are harvested to prevent the whiteflies from moving to strawberries. Minimizing dust by keeping field roads watered or oiled allows biological control to work effectively.

The source of infestation for new plantings on the central coast appears to be adjacent strawberry fields that are being maintained for a second year of berry production. It is important that berries being held for a second year are monitored beyond the last day of harvest. If whitefly populations are observed, the plants need to be treated to protect new plantings in adjacent areas. Early pruning may be beneficial to reduce source populations. When berries are pruned it is important that the discarded material is removed from the field. It may not be economically feasible to maintain second-year plantings when severe infestations have been experienced in new plantings in previous years.

Organically Acceptable Methods

Preserving naturally occurring biological control agents, cultural controls, sprays of narrow range oil, azadirachtin (Neemix), and insecticidal soaps, and releases of Encarsia formosa into hot spots against low-to-moderate populations of greenhouse whitefly are acceptable for use on organically certified strawberries.

Monitoring and Treatment Decisions

There are two monitoring methods for whiteflies: yellow sticky traps and leaf counts. Sticky cards are useful for detection of whitefly infestation and determining relative infestation levels, but the number of whiteflies may not correlate closely with the number of immature whiteflies on leaves. Back up sticky trap counts by inspecting strawberry foliage throughout the field on a weekly basis. Place one yellow sticky trap every ten acres and next to field edges to catch adult whiteflies as they move into the strawberry fields. Put the sticky cards vertically on stakes, just above the crop canopy. Count the number of adults trapped on each card weekly and record counts to track population numbers. Replace sticky cards as necessary.

Monitor plants by counting the number of adults on 20 midtier leaflets in each quarter of a field and determine the average number. Also examine nymphs to determine what proportion of the nymphs are black, indicating they are parasitized.

When monitoring indicates that adult populations are increasing rapidly and nymphs that are detected on leaves have no indication of parasitism (i.e., are not black in color), begin treatments with products that control adult whiteflies. Insecticides (except imidacloprid), oils, and soaps are most effective against adults and early instar whiteflies but not against eggs. Very few materials are effective against the fourth instar "pupal stage." Try to time treatments when monitoring indicates that most of the population is in the adult and first-, second-, or third-instar stage.

If there is high risk of new plantings from nearby summer plantings or second-year fields that already have whiteflies, consider a preventive application of imidacloprid (Admire) at planting by injection into the planting hole or through the drip system. If application is through drip irrigation, it is best to preirrigate to make sure that the soil profile is well watered, then apply enough water to move the material into the root zone. Imidacloprid (Admire) must be taken up by the plant to be effective.

Good coverage of the underside of leaves is essential for effective use of insecticides against whiteflies, but dosage applied is also important. When treating whiteflies, use lower volumes of water than would normally be used against pests like spider mites and drive the sprayer more slowly over the field if possible. An air-assist sprayer might help. More than one application may be required for heavy populations or if monitoring indicates that populations are continuing to increase. Rotating between chemical classes when making multiple applications is recommended to reduce the development of resistance.

PUBLICATION

UC IPM Pest Management Guidelines: Strawberry
UC ANR Publication 3468
Insects and Mites
F. G. Zalom, Entomology, UC Davis
P. A. Phillips, UC IPM Program, UC Cooperative Extension, Ventura Co.
N. C. Toscano, Entomology, UC Riverside
M. Bolda, UC Cooperative Extension, Santa Cruz Co.

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