How to Manage Pests

UC Pest Management Guidelines

Carrot

Whiteflies

Scientific Names:
Greenhouse whitefly: Trialeurodes vaporariorum
Iris whitefly: Aleyrodes spiraeoides
Sweet potato whitefly: Bemisia tabaci
Silverleaf whitefly: Bemisia argentifolii (=B. tabaci, Biotype B)

(Reviewed 1/09, updated 4/10)

In this Guideline:


DESCRIPTION

Whiteflies are small insects that are about 0.04 inch (1 mm) long. The body and wings of adults are covered with a fine whitish powdery wax that is opaque in appearance. 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. Distinguishing whitefly species is difficult; use a hand lens to examine both immatures and adults. Adult silverleaf whiteflies hold their wings somewhat vertically tilted like the peaked roof of a house, instead of flat over their bodies like the greenhouse whitefly. During the last part of the fourth larval stage, often called the pupa, the whitefly develops red eyes and stops feeding. This is the stage that is easiest to identify silverleaf whitefly; pupae have no waxy filaments around their edges as do most other species of whiteflies.

DAMAGE

Extremely dense populations of silverleaf whitefly may immigrate into late August or September plantings of carrots in the low deserts of southern California and damage seedlings. In light to moderate infestations, leaves show no distinctive symptoms as a result of whitefly feeding; however, copious quantities of honeydew are deposited on leaves, resulting in a shiny, sticky appearance.

MANAGEMENT

Whiteflies can be very difficult to control with insecticides. Natural or introduced biological methods provide the best long-term solution to keeping whitefly numbers at a minimum along with reducing host plants in areas of heavy infestation. If you treat for silverleaf whitefly, make applications before pests build up and contaminate carrot tops with honeydew.

Biological Control

Several parasitic wasps, including species in the Encarsia and Eretmocerus genera, control whiteflies. Whitefly nymphs are also preyed upon by bigeyed bugs, lacewing larvae, and lady beetles. The lady beetles, Delphastus pusillus and Serangium parcesetosum, were introduced into the lower desert region to assist in biological control, but their impact has yet to be determined. Encourage buildup of beneficial insects by avoiding the use of nonselective pesticides and by protecting their habitat.

Cultural Control

Remove field bindweed and other weeds in and adjacent to the crop field as well as crop residues. Host-free periods (PDF) are valuable for controlling several of the whitefly species.

Organically Acceptable Methods

Biological and cultural controls, as well as soap sprays, are acceptable for use on organically grown produce.

Monitoring and Treatment Decisions

Carrots are not a preferred host of whiteflies and in some areas treatment with insecticides is rarely justified. Where silverleaf whitefly is a chronic problem, a treatment with imidacloprid (Admire) at planting will effectively control whiteflies during the critical period of stand establishment. If treatment is not applied at planting, occasionally dense colonies of silverleaf whitefly nymphs can necessitate treatment of bunching carrots to prevent honeydew and sooty molds from severely contaminating the carrot tops.

For foliar treatment, combinations of a pyrethroid insecticide (esfenvalerate) with either methomyl or endosulfan provide superior control over applications of a single insecticide. Rotate pyrethroid treatments with a treatment of insecticidal soap to help slow development of insecticide resistance. There is some evidence of endosulfan resistance in populations of silverleaf whitefly in the Imperial Valley. Insecticidal soaps control all immature stages of whiteflies, including eggs, whereas the other materials control first instar nymphs and adults only; thus, treatment timing is critical.

Common name Amount per acre** R.E.I.‡ P.H.I.‡
(example trade name)   (hours) (days)

  Calculate impact of pesticide on air quality
 
The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least harmful to natural enemies, honey bees, and the environment are at the top of the table. When choosing a pesticide, consider information relating to air and water quality, resistance management, and the pesticide's properties and application timing. Not all registered pesticides are listed. Always read the label of the product being used.
 
A. IMIDACLOPRID
  (Admire Pro) 4.4–10.5 fl oz 12 21
  MODE-OF-ACTION GROUP NUMBER1: 4A
  COMMENTS: Apply at planting to control migrating whitefly adults during stand establishment. Has minimal effect on beneficial insects. Do not exceed 10.5 fl oz/acre per season.
 
B. INSECTICIDAL SOAP# 1% solution or less 12 0
  MODE OF ACTION: A contact insecticide with smothering and barrier effects.
  COMMENTS: This material has no residual activity and requires frequent applications and thorough coverage. Provides poor control of adults.
 
C. METHOMYL*
  (Lannate 90SP) 0.5–1 lb 48 1
  MODE OFACTION GROUP NUMBER1: 1A
  . . . PLUS . . .
  ESFENVALERATE*
  (Asana XL) 5.8–9.6 fl oz 12 7
  MODE-OF-ACTION GROUP NUMBER1: 3
  COMMENTS: Apply as needed for control, but do not exceed 0.5 lb a.i. (3 qt esfenvalerate/acre per season). Use ground application only.
 
** See label for dilution rates.
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.
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/.
# Acceptable for use on organically grown produce.
* Permit required from county agricultural commissioner for purchase or use.

[Precautions]

PUBLICATION

[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Carrot
UC ANR Publication 3438

Insects

  • E. T. Natwick, UC Cooperative Extension, Imperial County
  • D. R. Haviland, UC IPM Program, Kern County
  • C. G. Summers, Entomology, Kearney Agricultural Center, Parlier
Acknowledgment for contributions to Insects:
  • W. E. Chaney, UC Cooperative Extension, Monterey County

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