How to Manage Pests

UC Pest Management Guidelines

Strawberry

Whiteflies

Scientific Names:
Greenhouse whitefly: Trialeurodes vaporariorum
Iris whitefly: Aleyrodes spiroeoides
Strawberry whitefly: Trialeurodes packardi

(Reviewed 6/08, updated 5/10)

In this Guideline:


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 0.04 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. 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

Greenhouse whitefly can transmit viruses and is known to vector pallidosis-related decline of strawberry in California. 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.

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 in field situations.

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 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 whitefly populations 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 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.

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.

Common name Amount/Acre R.E.I.+ P.H.I.+
(trade name)   (hours) (days)

  Calculate impact of pesticide on air quality
The following materials are listed in order of usefulness in an IPM program, taking into account efficacy and impact on natural enemies and honey bees. When choosing a pesticide, also consider information relating to environmental impact. Not all registered pesticides are listed. Always read label of product being used.
 
A. IMIDACLOPRID
  (Admire Pro) 10.5–14 fl oz 12 14
  MODE OF ACTION GROUP NUMBER1: 4A
  COMMENTS: A neonicotinoid insecticide. Best used as a preventive treatment. Can be applied through drip lines; see label for restrictions. When applying through drip, preirrigate so soil is moist. This material must move into the root zone and be taken up by the plants to be effective, so sufficient water must be applied to promote its movement through the soil. Only one application/year but has a long residual activity; more moves into plant with each irrigation.
 
B. PYRIPROXYFEN
  (Esteem) 0.86EC 10 fl oz 12 2
  MODE OF ACTION GROUP NUMBER1: 7C
  COMMENTS: Use allowed under a Supplemental Label. Apply after an application of imidacloprid (Admire) and when whitefly populations just begin to increase. Control of adult whitefly populations will take about 2-3 weeks following application so apply before populations build.
 
C. FENPROPATHRIN*
  (Danitol) 2.4EC 10.66 fl oz 24 2
  MODE OF ACTION GROUP NUMBER1: 3
  ...PLUS...
  DIAZINON* 50WP 2 lb 3 days 5
  MODE OF ACTION GROUP NUMBER1: 1B
  . . . or . . .
  MALATHION 5EC 1.5–3 pt 12 3
  MODE OF ACTION GROUP NUMBER1: 1B
  COMMENTS: Fenpropathrin is a pyrethroid and diazinon and malathion are organophosphates. Use of fenpropathrin is limited to 2 applications/year and multiple applications of pyrethroids can lead to resistance in target populations, so use the material only when population levels warrant its use. Applications made early in the season can lead to severe mite outbreaks later on. Diazinon has been found in surfaces waters at levels that violate federal and state water quality standards; avoid runoff into surface waters or choose alternative materials. For tank mixes, observe all directions for use on all labels, and employ the most restrictive limits and precautions. Never exceed the maximum a.i. on any label when tank mixing products that contain the same a.i.
 
D. THIAMETHOXAM
  (Actara) 3–4 oz 12 3
  MODE OF ACTION GROUP NUMBER1: 4A
 
E. AZADIRACHTIN#
  (Neemix) 4.5 4–16 fl oz 12 0
  MODE OF ACTION GROUP NUMBER1: un
  COMMENTS: An insect growth regulator.
 
F. NARROW RANGE OIL#
  (Omni Oil) 6E 1–2% 12 0
  MODE OF ACTION: Contact including smothering and barrier effects.
  COMMENTS: The potential for phytotoxicity has not been fully evaluated. Growers are encouraged to test product or product mixes for phytotoxicity before field applications to determine safety margins. Apply in 60 gal water/acre with air-assist, low-volume ground equipment or 200 gal water/acre. Moderately effective against low to moderate populations when coverage is excellent. Make applications only during winter months when plants are semi-dormant to reduce the risk of phytotoxicity. Do not use oil from peak bloom through fruiting period or when air temperatures are expected to exceed 75°F within several days following application. Do not apply from Jan 16 to May 30 in Orange and San Diego counties or the Oxnard Plains; do not apply from Feb 1 to June 15 in the Santa Maria Valley, and do not apply from Mar 1 to Jun 30 in Monterey and Santa Cruz counties.
 
G. INSECTICIDAL SOAP#
  (M-Pede) 2.5 oz/gal water 12 0
  MODE OF ACTION: A contact insecticide with smothering and barrier effects.
  COMMENTS: The potential for phytotoxicity has not been fully evaluated. Growers are encouraged to test product or product mixes for phytotoxicity before field applications to determine safety margins. Do not exceed 1 application/month or 2/season. Can burn plants so apply only when temperatures are cool. Moderately effective against nymphs only. Requires excellent coverage.
 
+ Restricted entry interval (R.E.I.) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (P.H.I.) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest.
# Acceptable for use on organically grown produce.
* Permit required from county agricultural commissioner for purchase or use.
1 Rotate chemicals with a different mode-of-action Group number, and do not use products with the same mode-of-action Group number more than twice per season to help prevent the development of resistance. For example, the organophosphates have a Group number of 1B; chemicals with a 1B Group number should be alternated with chemicals that have a Group number other than 1B. Mode of action Group numbers are assigned by IRAC (Insecticide Resistance Action Committee). For additional information, see their Web site at http://www.irac-online.org/.

[Precautions]

PUBLICATION

[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Strawberry
UC ANR Publication 3468

Insects and Mites

  • F. G. Zalom, Entomology, UC Davis
  • M. P. Bolda, UC Cooperative Extension, Santa Cruz County
  • S. K. Dara, UC Cooperative Extension, Santa Barbara County
  • S. Joseph, UC Cooperative Extension, Monterey County
Acknowledgment for contributions to the Insects and Mites:
  • P. A. Phillips, UC IPM Program and UC Cooperative Extension, Ventura County
  • N. C. Toscano, Entomology, UC Riverside

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