Description of the Pest
Greenhouse whitefly. The greenhouse whitefly adult is 0.9 (male) to 1.1 mm (female) long, with four wings, sucking mouthparts, a powdery waxy coating over the body, and wings that give the otherwise yellow body a white color. The wings are held nearly parallel to the leaf and cover the abdomen when the adult is at rest. There are seven life stages: egg, four nymphal instars, pupal stage, and the adult. Females occasionally lay eggs in circles on the undersides of leaves of plants with smooth leaves. Egg laying on plants with pubescent leaves results in random placement of the eggs. Eggs are partially inserted into the leaf, initially they are yellowish, but close to hatching they turn a purplish brown. The first nymphal instar is called a crawler and has functional legs, while the remaining instars are attached to the underside of the leaf and do not move. The end of the fourth instar is called a pupa. The pupal stage is the most important for determining whitefly species identification. Greenhouse whitefly pupae are oval and have vertical sides, giving the pupa a cakelike appearance from the side. Along the perimeter of the upper surface is a fringe of filaments and relatively large wax filaments project above the upper surface of the body. The greenhouse whitefly can complete one generation in 21 to 26 days at 81°F.
Silverleaf whitefly. The silverleaf whitefly adult is 0.8 (male) to 1.0 mm (female) long, with four wings, and sucking mouthparts. The white, waxy coating covering its body and wings is not as thick as it is on the greenhouse whitefly and its yellow body has a whitish hue from the wax rather than a white color. The wings are held at the sides of the body, partially exposing the back of the abdomen when the adult is at rest. There are seven life stages: egg, four nymphal instars, pupa, and the adult. Eggs are not laid in a circular pattern, are partially inserted into the leaf, and remain yellowish until they hatch. The first nymphal instar is called a crawler and has functional legs, while the remaining instars are attached to the leaf and do not move. The end of the fourth instar is called a pupa. Silverleaf whitefly pupae are ovoid, but with a slightly pointed hind end and red eye spots easily visible from above. The pupa is fairly flat and does not have a marginal fringe of filaments. The length of wax filaments projecting above the upper surface of the body varies on different hosts. Silverleaf whiteflies can complete development in 16 (86°F) to 31 (68°F) days.
Whitefly adults and immatures feed on sap. As they feed, they excrete honeydew, a sticky substance that causes unsightly glistening and supports the growth of black sooty mold. Very large populations of whiteflies cause stunting of plant growth, and leaves may senesce and die. Physiological abnormalities, such as white stem on poinsettia, may also occur. Usually populations are not high enough to stunt ornamentals, and damage is mostly caused by honeydew, sooty mold, and nuisance populations of flying adults.
Encarsia formosa, a tiny, stingless parasitic wasp, is as an effective biological control for greenhouse whiteflies. Wasps are released once a week at a rate of two to five parasites per plant for 8 to 10 weeks of the growing season. This sort of release program can be effective if long residual insecticides have not been applied in advance of the parasite release, and where the initial population of whiteflies is quite low (only a few whiteflies per plant). Greenhouse whitefly pupae turn black when parasitized by Encarsia, which emerge as adults through circular exit holes. Delphastus pusillus, a whitefly predator, has been used against silverleaf whiteflies. Eretmocerus eremicus is a commercially available whitefly parasite for silverleaf whitefly. It will also attack greenhouse whitefly, whereas Encarsia is ineffective against silverleaf whitefly. For more information, see BIOLOGICAL CONTROL.
Because whiteflies feed on a large variety of plant species, keep production areas free of weeds, which can serve as hosts for whitefly populations. Exclusion of winged adults can be accomplished by covering openings to the greenhouse with screens that have a pore width of 405 microns or smaller.
Before starting a new crop, carefully inspect plants to ensure that they are free of whiteflies and other pests. Treat or discard any infested plants.
In open field production, mulching with silver reflective plastic has been shown to repel whiteflies, thus greatly reducing their presence in and around the plant canopy. Apply the plastic mulch at the time of planting or transplanting for best results. In addition to repelling insects and reducing the incidence of insect-transmitted diseases, reflective mulch may increase plant growth, cut-flower yield, and reduce the irrigation water and fertilizer requirement. This control method is acceptable for organic production.
Monitoring and Treatment Decisions
Yellow sticky cards placed in greenhouses will capture adult whiteflies. However, traps need to be used at a greater density, one per 1,000 sq. ft., than for other pests. Trap monitoring should be supplemented with inspection of leaves for nymphs and pupae. When monitoring plant samples, it is imperative to look on the undersides of leaves for adults and nymphs. For more information, see MONITORING WITH STICKY TRAPS.
Treatment thresholds vary with the crop. For example, cut flowers such as gerberas can tolerate more whiteflies than poinsettias because only the flowers are harvested in the cut flower crop whereas the entire poinsettia plant is marketed. For more information, see ESTABLISHING ACTION THRESHOLDS.
Selected Materials Registered for Use on Greenhouse or Nursery Ornamentals
Read and follow the instructions on the label before using any pesticide. Before using a pesticide for the first time or on a new crop or cultivar, treat a few plants and check for phytotoxicity. Also consider pesticide resistance management and environmental impact.
|Manufacturer||REI1||Mode of action2||Comments|
|Laverlam||4||—||Treat every 7 days while insects are active. Do not tank mix with most fungicides and wait 48 hours after application to apply a fungicide.|
(PT Pyrethrum TR)
|Whitmire MicroGen||12||3/—||An aerosol.|
|insect growth regulator||A.||azadirachtin
||Must contact insect. Repeat applications as necessary. Label permits low-volume application.|
|SePRO||12||un||Do not exceed 22.5 oz/acre/application.|
|Chemtura||12||15||Whitefly suppression. May damage poinsettias if used over labeled rate. Also effective on fungus gnat larvae and lepidopteran larvae.|
|Chemtura||12||15||Use no more than twice per year and don't exceed 52 oz/acre/year. Don't use on poinsettia.|
|Valent||12||7C||Do not apply more than 2 times per cropping cycle or per 6 months.|
|Wellmark||4||7A||Apply prebloom. Also labeled for low volume use.|
|Syngenta||12||6||Label permits low-volume application.|
|OHP||12||4A||Not to be used more than once every 16 weeks. Do not apply to soils that are water logged or saturated. Do not apply to bedding plants intended to be used as food crops.|
|(Marathon 60 WP)||12||4A||As above. Apply only as a drench.|
|oil4||A.||clarified hydrophobic extract of neem oil#
|OHP||4||un||Do not spray plants under stress. Target pest must be completely covered with spray. Repeat application as necessary. Check label for list of plants that can be treated. May cause injury to flowers.|
(JMS Stylet Oil)
|Use as above for neem oil. Also, do not use with sulfur fungicides; check label for tank mix restrictions.|
(Orthene T, T&O Spray)
|Valent||24||1B||A number of chrysanthemum varieties have exhibited phytotoxic reactions. In the greenhouse, only labeled for use on anthurium, cacti, carnation, rose, orchids, some foliage plants, young poinsettia and some varieties of chrysanthemum. Can stunt new growth in roses.|
(PT 1300 Orthene TR)
|24||1B||An aerosol for greenhouse use only.|
|various||12||1B||Not for greenhouse use.|
|12||3||Check label. A fogger for greenhouse use only.|
|FMC||12||3||Label permits low-volume application.|
|OHP||12||3||Label permits low-volume application.|
|Valent||24||3||Label permits low-volume application.|
|Wellmark||12||3||Label permits low-volume application. Also labeled as a cutting dip at 5 fl oz/100 gal.|
|FMC||12||3||Direct application to blooms may cause browning of petals. Marginal leaf burn may occur on salvia, diffenbachia and pteris fern. Label permits low-volume application. Do not apply more than 2 lb a.i./acre/year.|
|Syngenta||12||9B||Apply as foliar spray at 7-14 day intervals. For outdoor use, do not apply more than 48 oz/acre/year; for indoor use, do not use more than 100 oz.|
|12||—||Must contact insect, so thorough coverage is important. Repeat weekly as needed up to 3 times. Test for phytotoxicity. Do not spray new transplants or newly rooted cuttings. Do not add adjuvants.|
|1||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.|
|2||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).|
|3||PBO = piperonyl butoxide|
|4||Note that single doses of soaps or oils can be used at anytime in a pesticide rotation scheme without negatively impacting resistance management programs.|
|5||Check with certifier to determine which products are organically acceptable.|
|*||Restricted use pesticide. Permit required for purchase or use.|
|#||Acceptable for use on organically grown ornamentals.|