Description of the Pest
The most common species of whitefly infesting potato is the sweetpotato whitefly. The adults are tiny (0.06 inch, 1.5 mm long), yellowish insects with white wings. Sweetpotato whiteflies hold their wings somewhat vertically tilted, or rooflike, over the body and the wings do not meet over the back but have a small space separating them. Other species of whitefly have been observed in potatoes but their populations tend to be localized within the field and do not cause damage. Populations of sweetpotato whitefly can be found relatively uniformly throughout the field in fall plantings.
Whiteflies are found mostly on the undersides of leaves. They fly readily when plants are disturbed. The tiny, oval eggs hatch into a first larval stage that has legs and antennae and is mobile. Both 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-eye nymph, is the stage that is easiest to identify. Mature nymphs of sweetpotato whitefly are oval, whitish, soft, and have few to no long waxy filaments. In contrast, greenhouse whitefly, Trialeurodes vaporariorum, has many long waxy filaments and the edge of the body is somewhat vertical where it contacts the leaf surface.
Sweetpotato whitefly damages leaves by feeding, which causes leaves to yellow and curl, and by the production of honeydew, which causes leaves to appear shiny or blackened (from sooty mold growing on the honeydew). Damage is similar to that caused by aphid feeding: they debilitate the plants. Whiteflies cause the most damage to winter-harvested potatoes in the southern San Joaquin Valley. Fields near defoliated cotton can be severely infested.
Whitefly populations are often held in check by beneficial insects. If populations do reach high levels, it may be necessary to apply insecticide in fall.
Several wasps, including species in the Encarsia and Eretmocerus spp., parasitize whiteflies. Whitefly nymphs are also preyed upon by bigeyed bugs, lacewing larvae, and lady beetles. Sweetpotato 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.
When possible, plant potatoes at least one-half mile upwind from key sweetpotato whitefly hosts such as melons, cole crops, and cotton. Maintain good sanitation in areas of winter/spring host crops and weeds by destroying and removing all crop residues as soon as possible. Control weeds in noncrop areas including hedge rows and fallow fields and harvest alfalfa on as short a schedule as possible. In addition, allow the maximum time between whitefly host crops and produce vegetables and melons in the shortest season possible.
Organically Acceptable Methods
Use biological and cultural controls on an organically certified potato.
Monitoring and Treatment Decisions
Routinely check field margins for whiteflies; these areas are usually infested first. Record your results (example form—PDF). Be especially alert for rapid increase of whitefly numbers when nearby host crops are in decline. During these critical periods, check fields twice weekly. If beneficials are present, allow them an opportunity to control light whitefly infestations. If higher populations are present at the field margins than the field centers, then apply insecticide only the field margins. This approach will reduce treatment costs and help preserve beneficials in the field. Treatment thresholds have not been determined for sweetpotato whitefly in potato, but potatoes can take large populations of whitefly before treatment is necessary. If populations reach high levels in fall, a treatment may be warranted.
|Common name||Amount per acre**||REI‡||PHI‡|
|(Example trade name)||(hours)||(days)|
|Not all registered pesticides are listed. 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. Always read the label of the product being used.|
|(Venom, soil application)||6.5–7.5 oz||12||NA|
|(Venom, foliar application)||1–1.5 oz||12||7|
|MODE OF ACTION GROUP NUMBER1: 4A|
|COMMENTS: Do not apply more than 0.754 lb/acre per season. If an application of imidacloprid (Admire Pro) was made at planting, choose another treatment material with a different mode of action group number to help prevent the development of neonicotinoid resistance.|
|MODE OF ACTION GROUP NUMBER1: 23|
|(Oberon 2SC)||8–16 fl oz||12||7|
|MODE OF ACTION GROUP NUMBER1: 23|
|COMMENTS: Apply early at the first sign of psyllid presence. Do not exceed two applications/crop or make applications at less than 7-day intervals.|
|**||See label for dilution rates.|
|‡||Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) 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.|
|*||Permit required from county agricultural commissioner for purchase or use.|
|1||Rotate insecticides 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; insecticides with a 1B group number should be alternated with insecticides that have a group number other than 1B. Mode-of-action group numbers for insecticides and miticides (un=unknown or uncertain mode of action) are assigned by IRAC (Insecticide Resistance Action Committee)|