Management Of Whiteflies In Greenhouse Crops

Agdex#:	290/621
Publication Date:	September 2003
Order#:	03-067
Last Reviewed:	September 2003
History:
Written by:	Gillian Ferguson - Greenhouse Vegetable IPM Specialist/OMAFRA; Graeme Murphy - Greenhouse Floriculture IPM Specialist/OMAFRA; Les Shipp - Greenhouse Entomologist/Agriculture and Agri-Food Canada

Table of Contents

1. Introduction
2. Management Strategies
3. Biological Control
4. Physical Control
5. Chemical Control

Introduction

Whiteflies are a major pest of greenhouse crops including tomatoes, cucumbers, and many ornamental species, particularly poinsettia, gerbera, and a number of spring crops. For a description of the whitefly see OMAFRA Factsheet 03-065, Biology of Whiteflies in Greenhouse Crops. Many weed species are also hosts of whiteflies and often serve as sources of infestations. Whiteflies damage crops by reducing their vigour, facilitating growth of sooty mould on leaves and fruits, transmitting viral diseases, and by reducing crop quality by their visible presence.

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Management Strategies

General Recommendations

• Implement a routine monitoring program using yellow, sticky cards and plant inspections. Use host plants that are more attractive to whiteflies than the main crop as an early detection tool or as a trap plant. Examples include eggplant and tobacco. Mark infested plants so they can be used as indicators of the success of control strategies.
• Identify the whitefly species present on the crop.
• React promptly when whitefly adults are observed either on cards or on terminal leaves.
• Maintain good weed control.
• Quarantine new seedlings, transplants or cuttings until satisfied that they are virus-free.

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Biological Control

There are currently 4 commercially-available biological control agents for whiteflies, 2 parasitic wasps, Encarsia formosa and Eretmocerus eremicus, a small black ladybeetle, Delphastus pusillus, and a predatory bug, Dicyphus hesperus. Encarsia Formosais more effective against the greenhouse whitefly (GWF) but will still provide some measure of control against the silverleaf whitefly (SLWF). Eretmocerus eremicus is effective against both species and a better choice in warmer conditions. Good sanitation and weed control are essential for successful biological control of whiteflies. Introduce biological control agents when whitefly populations are low.

Encarsia Formosa

Adult Encarsia are about 0.6 mm long, and have a black thorax and yellow abdomen (Figure 1A). They kill whiteflies mainly by laying eggs into the immature stages or scales (mostly the 3rd and 4th instars). Adult Encarsia can live for a few days to a month depending on temperature, and females lay 50-350 eggs during their lifetime. A greenhouse whitefly scale turns black 10-14 days after being parasitized (Figure 1B). On average, another 2 weeks are needed before the adult wasp emerges. When the adult is fully developed, it cuts an opening in the top portion of the black scale before emergence. Parasitism by Encarsia of SLWF is not as effective as in GWF. Parasitized SLWF turn a tan to brown colour and are not as easily noticeable (Figure 1C). For biological control of SLWF with Encarsia, consider higher introduction rates.

Figure 1A. Adult Encarsia Formosa

Figure 1b. Greenhouse whitefly pupa parasitized by Encarsia Formosa

Figure 1C. Silverleaf whitefly pupa parasitized by Encarsia Formosa

Successful use of Encarsia can be improved by:

• avoiding use of pesticides with long residual effects for at least 3 months before initial release (for specific information on the effects of pesticides on biocontrol agents refer to OMAFRA Publication 370, Greenhouse Floriculture Production Recommendations or OMAFRA Publication 371, Greenhouse Vegetable Production Recommendations or contact your biocontrol supplier).
• monitoring all stages of crop production for whitefly presence and to follow trends in population levels. This allows for more accurate placement and timing of parasitoid releases.
• introducing Encarsia at first sign of whitefly, or preventively before whiteflies are detected. If whiteflies are present before the release of Encarsia, it may be necessary to reduce the population with a low-residual pesticide.
• avoiding using Encarsia between December and February unless light and temperature conditions are adjusted to suit the parasitoid.
• distributing Encarsia in shaded parts of the plant where the parasitoids are protected from direct sunlight.
• Distributing cards relatively evenly, but placing more in areas where whiteflies are observed. Purchase cards with fewer parasitized scales per card to achieve more distribution points.
• Avoiding removal of leaves with immature parasitized scales during deleafing or pruning operations. Such removal slows the build-up of Encarsia numbers and parasitism rate.
• continuing introductions of Encarsia until at least 80% of the scales on older leaves are black. During the warmer months when there is immigration of whiteflies into the greenhouses, introductions may have to be maintained as well as increased.

Eretmocerus eremicus

Adult Eretmocerus resemble Encarsia in shape and size but are entirely yellow (Figure 2A). Although this wasp species will host feed on all 4 whitefly instars, it prefers to parasitize 2nd and 3rd instars. Adults live for 1-2 weeks and lay approximately 100 eggs during their lifetime. The majority of these eggs are laid within the first 6 days after emergence from their pupal cases. Because this wasp thrives at higher temperatures than Encarsia, they are best released later in the year around March. Parasitized whiteflies are a slightly different colour but not noticeably so (Figure 2B and C). Other tips for increasing success in using this parasitoid are similar to those listed for Encarsia. Eretmocerus is very attracted to yellow sticky cards and tape. It may be necessary to reduce the use of these tools particularly when trying to establish an Eretmocerus population.

Figure 2A. Adult Eretmocerus eremicus.

Figure 2B. Greenhouse whitefly pupa parasitized by Eretmocerus eremicus.

Figure 2C. Silverleaf whitefly pupa parasitized by Eretmocerus eremicus.

Delphastus pusillus

Both adult (Figure 3A) and larval (Figure 3B) Delphastus feed on whiteflies, particularly on the eggs and nymphs, but it is reported to avoid parasitized scales. This behaviour makes it compatible with the use of parasitic wasps. Use these beetles to supplement the activity of Encarsia for reducing whitefly populations. Adult Delphastus live for 6-9 weeks and are reported to require a minimum diet of 10 whitefly eggs per day in order to reproduce.

Figure 3A. Adult Delphastus pusillus feeding on whitefly larva.

Figure 3B. Immature stage or larva of Delphastus pusillus.

Dicyphus hesperus

Dicyphus (Figures 4A and 4B) is a predatory bug with piercing/sucking mouth-parts. These bugs are general feeders but they appear to prefer whiteflies, feeding particularly on the eggs and larval stages. To a lesser extent, they feed on aphids, spider mites, thrips, leaf-miner larvae and moth eggs. In the absence of prey, a high population of Dicyphus may cause damage to tomato fruits. For management of whiteflies, it is best to use Dicyphus in conjunction with the parasitic wasps, Encarsia Formosaand Eretmocerus eremicus. The use of banker plants such as mullein (Verbascum thapsus) (Figure 4C) may help establish this predator when whitefly population densities are low.

Figure 4A. Adult Dicyphus Hesperus.

Figure 4B. Light green recently-emerged adult Dicyphus Hesperus.

Figure 4C. Mullein banker plant in tomato crop.

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Physical Control

• Sticky Traps: Yellow sticky traps in various forms can be used to trap large numbers of adult whiteflies. Large, yellow, sticky boards or 30 cm yellow sticky tapes are used in 'hot spots' at a rate of about 1/plant. Alternatively, reams of yellow, sticky tape can be draped between posts along plant rows (Figure 5). Such tapes will also trap thrips, fungus gnats, shore flies, and parasitic wasps (especially under low whitefly populations).

Figure 5. Use of yellow tapes for mass trapping.

• Vacuuming: Hand-vacuuming of adults in 'hot-spots' is very effective in rapidly removing adult whiteflies (Figure 6).

Figure 6A. Hand-vacuuming of adult whiteflies in tomato crop.

Figure 6B. Hand-vacuuming of adult whiteflies in Gerbera.

• Insect barriers: Installing fine screens over vents and doorways significantly reduces the movement of outdoor populations into greenhouses. Where whiteflies move in from adjacent field crops (e.g. field tomatoes) in large numbers at certain times of the year (e.g. during harvest), screening should be the first line of defence. See OMAFRA Factsheet, Order No. 00-021, Screening of Greenhouses for Insect Exclusion.

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Chemical Control

Whiteflies have shown an ability to develop resistance to many pesticides. Judicious use will extend the useful life of pesticides and at the same time, delay build up of resistance. Rational and judicious use of pesticides must be used in conjunction with a regular monitoring program, using action thresholds, rotating chemical classes, and at the same time, making use of all available control strategies.

For specific chemical recommendations, refer to OMAFRA Publication 370, Production Recommendations for Greenhouse Floriculture or OMAFRA Publication 371, Growing Greenhouse Vegetables.

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