Whiteflies

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 their natural enemies. 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 avocados, beans, blackberries and other berries, cucumbers, eggplants, lettuce, melons, peas, peppers, 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 numbers tend to increase 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 0.04 inch (1 mm) in size with four membranous wings that are coated with white powdery wax. Whitefly species are most reliably distinguished from each other by examining the late fourth instar or red-eyed "pupal" stage.

Greenhouse Whitefly

The greenhouse whitefly has long, waxy filaments around the margins in the "pupal" 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.

Strawberry Whitefly

Both adults and nymphs of the strawberry whitefly look similar to the greenhouse whitefly, but the strawberry whitefly nymphs never have the long filaments often found on the greenhouse whitefly "pupae."

Iris Whitefly

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.

Greenhouse whitefly can transmit viruses and is known to vector virus decline of strawberry in California. Whiteflies may reduce crop yields directly through their feeding on leaf 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.

Successful management of greenhouse whiteflies requires an integrated program that focuses on prevention and relies on cultural and biological control methods when possible. Insecticides 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 insecticide, 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. Apply insecticides when honeydew, moderate to high numbers of whiteflies, or both are apparent during periods of warmer weather for summer- and fall-planted berries. Select insecticides carefully and use them only when monitoring indicates a need.

In most crops, greenhouse whiteflies and iris whiteflies are generally kept under control by naturally occurring parasitic wasps and general predators. Natural enemies of whiteflies 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 in field situations.

For summer-planted strawberries, the practice of topping (cutting of old leaves, stems, runners, and old fruit spurs) 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 and summer plantings that have become infested from the previous season's fall plantings. It is important that berries held for a second year be monitored beyond the last day of harvest. If whiteflies are observed in the previous year's plantings once new fields are transplanted, the older 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 plant material is removed from the field. It may not be economically feasible to maintain multiple-year plantings when severe infestations have been experienced the area.

Preserve naturally occurring biological control agents, and use cultural controls, sprays of narrow range oil, azadirachtin (Neemix), Isaria fumosorosea (PFR-97), Beauveria bassiana (Botanigard), and insecticidal soaps on organically certified strawberries. Releases of Encarsia formosa into hot spots against low-to-moderate numbers of greenhouse whitefly are also acceptable for use on organic strawberries.

There are two monitoring methods for whiteflies: yellow sticky traps and leaf counts.

Sticky traps 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. Therefore, in addition to sticky trap counts, inspect 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 whitefly numbers. Replace sticky traps as necessary.

Monitor plants by counting the number of adults on 20 mid-tier 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 whitefly numbers are increasing rapidly and nymphs that are detected on leaves have no indication of parasitism (i.e., are not black), begin insecticide applications 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 insecticides are effective against the fourth instar "pupal stage." Try to time insecticide applications to 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 insecticide 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 high numbers of whiteflies or if monitoring indicates that whitefly numbers are continuing to increase. Rotating between chemical classes when making multiple applications is recommended to reduce the development of resistance.