Silverleaf whitefly management in vegetable crops

Subra Siva-Subramaniam, Jerry Lovatt, Brendan Nolan and Peter Deuter, Department of Primary Industries and Fisheries, Queensland

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Introduction

Silverleaf whitefly (SLW), Bemisia tabaci Biotype B, (also named Bemisia argentifolii in USA) is a serious pest of many vegetable crops, including tomato, eggplant, cucurbits, sweetpotato, brassicas and beans.

SLW life cycle

Adult whiteflies are more active during the morning. They often concentrate on the undersides of younger leaves. Figure 1 shows the SLW life cycle. 

Figure 1. SLW life cycle

The eggs have a pointed end that is attached to the underside of the leaf surface, usually the youngest leaves where the adults congregate. Each female lays between 50 and 300 eggs (average 160) on the leaf surface. New eggs are whitish yellow. They turn brown and hatch within seven to 10 days.

There are four nymphal stages. Crawlers or first instar nymphs are greenish-yellow and flattened. They crawl a short distance until they tap into a sap source in the plant tissue. Second and third instar nymphs are attached to the leaf surface and suck sap from the plant. They are light yellow and their legs are not visible. Fourth instar nymphs are referred to as red-eyed nymphs or pupae and their body is dark yellow. Late in the fourth instar they stop feeding and develop into an adult before emerging from the pupal case. The empty white cases (exuviae) the adults emerged from can be seen on the leaf surface.

It takes 18 to 28 days from egg to adult in warm weather and 30 to 48 days in winter. The number of eggs produced peaks in warm weather and can reach 300 eggs per adult female. SLW populations build up rapidly during spring and summer. In Queensland the pest can complete eight to 12 generations in a year.

SLW damage

SLW can cause severe damage to vegetable crops in four different ways:

1. Direct feeding: SLW adults and nymphs (Figure 2) suck the sap from the plant causing reduced plant vigour, stunting, poor growth, defoliation and reduced yields. High populations may result in plant desiccation and death.

Figure 2. SLW nymphs feeding on a broccoli leaf

2. Injecting toxic saliva: While feeding, SLW inject toxic saliva into the plant causing physiological changes to plant tissue. On the outer skin of tomato fruit the external symptoms are green, yellow or orange streaks or blotches (Figure 3a). Internally, the affected fruit have white or yellow tissues (Figure 3b). In some tomato varieties the external symptoms may not be obvious, but internal damage is often very apparent once the fruit is cut open. 

Silvering of leaves is a common symptom on pumpkin, zucchini and squash (Figure 4) and fruit discolouration occurs in cucurbits and beans, pale stalks on broccoli and leaf yellowing and blanched stalks of lettuce.

3. Honeydew contamination: Both adults and nymphs excrete honeydew. This sugary substance can promote the growth of sooty moulds which affects the marketability of product. Sooty mould also reduces plant photosynthesis and the effectiveness of insecticides (Figures 5 and 6).

Figure 5. Sooty mould on a sweetpotato leaf

Figure 6. Sooty mould on tomato fruit

4. Transmission of viruses: SLW adults are efficient vectors in spreading Gemini viruses from infected plants into healthy crops. Gemini viruses include tomato leaf curl virus (TLCV) and tomato yellow leaf curl virus (TYLCV) that infect beans, capsicums, tomatoes and a wide range of ornamentals and weeds. More information on these diseases can be found in the DPI&F Notes listed below under Further information.

Monitoring and sampling

Regular counts of adults and nymphs on leaves should form the basis of deciding how and when to apply control measures. Adults congregate and lay eggs predominantly on the undersides of younger leaves. The majority of the eggs and young nymphs are found on the young leaves while older nymphs are usually found on older leaves.

To determine the numbers of adult SLW gently turn over young leaves and count the adults on the underside. Adults should be sampled during early morning (7 to 9 a.m.). Rapid adult migration usually occurs when infested crops are in decline or about to be destroyed.

To assess nymph populations, sampling should focus primarily on the older leaves. A hand lens (10 x) is necessary when inspecting leaves for the presence of eggs or small nymphs. Large nymphs can be counted with the unaided eye.

Yellow sticky traps

Yellow sticky traps (Figure 7), are useful for monitoring whitefly adult movement or dispersal, especially the movement of SLW from mature or older neighbouring crops and host weeds. 

Yellow plastic boards (Norwood^®) size 15 x 10.5 cm, coated with sticky glue (e.g. Tangle-trap Brush On^®) can be used to detect the adults. Around three to five traps should be placed in a crop of 2 to 3 ha. Place them level with the tops of the plants, as whiteflies are most attracted to young foliage (Figure 8).

Adult numbers on the traps will give an early warning of population increases within crops and an indication of the need for regular monitoring for nymphs.

Managing silverleaf whitefly

Controlling whitefly populations before they reach large numbers in crops is very important for successful management. If the adults occur in large numbers it becomes more difficult to control the nymphal stages. Adults move between successive crops, so management approaches must be employed in all crops within the area.

To reduce early season populations, best management practices require consideration of several management approaches including the use of pest-free seedlings, weed management, chemical control and cleaning up crop residue.

1. Pest free (clean) seedlings

Seedlings are potentially a major means of spreading whiteflies and leaf curl viruses into new plantings. Young plants are more susceptible to damage from SLW, so early infestations need to be avoided. Clean seedlings can be the first line of protection against the development of damaging populations.

Growers should check their suppliers to determine how the seedlings are grown and what measures are being used to protect against whitefly infestation. Inspect transplants carefully upon arrival for whitefly eggs, nymphs and adults.

2. Weed management

The availability of a continuous source of hosts, whether they are crops, weeds or abandoned crops, is the major contributing factor to a severe whitefly problem. Even a small area of a favoured host can maintain a significant whitefly population.

Minimising whitefly hosts is important in reducing the base population at the start of the cropping season. A smaller base population then will delay the time it takes for SLW numbers to reach significant levels, reducing the number of sprays needed to control whitefly.

Common weed species that carry high numbers of SLW include sow thistle (Figure 9), bladder ketmia, bell vine, burr gherkin, native rosella (Figure 10) and star burr. Milk or sow thistle is a regular weed host for whitefly and is common in Queensland vegetable production areas. Control these weed species in farming areas and seedling nurseries to minimise a build-up in SLW populations.

3. Chemical control

Selecting the correct insecticides and applying them at the appropriate time is very important, both for achieving good SLW management and minimising the development of resistance to the insecticides. A spray program should be based on the results of monitoring.

Insecticides vary in their efficacy on adult and immature SLW. Select insecticides according to the growth stage of whitefly, the infestation level, the age of the crop and the type of crop. Information on how to select insecticides for controlling SLW is provided in Table 1. A link to the table in a printable PDF format is provided under Further information.

Good spray coverage, particularly of the underside of leaves, is very important when using foliar insecticide applications as SLW adults, eggs and nymphs are found predominantly on the underside of leaves. Spray equipment should be correctly calibrated so that the correct amounts of insecticide are applied efficiently.

Imidacloprid is a systemic insecticide that can be applied to some crops as a foliar spray, or as a soil treatment through sub-surface drip irrigation tubing, as a furrow spray or as a plant hole drench. Soil applications are more efficient than foliar sprays and are made shortly before, at or shortly after planting. READ THE LABEL before use for crops, application directions, rates and timing.

Table 1.  A guide to choosing insecticides for silverleaf whitefly control in vegetables - 2007

Product (active)	Action	Activity against life stages			Comments
		Eggs	Nymphs	Adults
Admiral (pyriproxyfen)	translaminar	***	***	N M	Slow acting and interrupts SLW life cycle. Apply to young crops. Safe on parasitoids and bees.
Applaud (buprofezin)	contact vapour	*	**	N M	Use 600 mL/ha rate for SLW control. Slow acting. Safe on parasitoids and bees.
Confidor Guard (imidacloprid)	systemic	N M	***	**	Only for soil application. Apply at planting.
Chess (pymetrozine)	translaminar	N M	N M	***	Stops adult feeding. Use at flowering stage in cucurbits. Less toxic to bees and parasitoids.
DC Tron (petroleum oil)	contact	*	**	**	Good coverage essential. Take care when mixing with soap and fungicides.
Talstar + Synergy (bifenthrin + piperonyl butoxide)	contact + synergist	*	*	***	Controls SLW adults. Use only in later part of crop if SLW in high numbers. Toxic to beneficial insects.

NM = no direct mortality; *** = best; ** = good; * = fair

Disclaimer: The information in Table 1 above is based on HAL funded experiments conducted in Bowen over the last 5 years. It is provided solely on the basis that the readers will be responsible for making their own assessment of the content and seek professional advice as needed. Chemical registrations and APVMA permits for SLW control do not apply to all vegetable crops. Users must check the label for registration or permit status of the insecticides before use on a specific crop.

Short residual contact insecticides (such as Talstar^®), mainly control adults and are less effective against immature stages. Systemic insecticides (Confidor^®) can control both adults and nymphs. Organophosphate insecticides used alone provide no control for silverleaf whitefly.

Under the APVMA emergency permit system several chemicals have been approved against SLW for use in a range of vegetable crops. A few chemicals are registered for SLW control. Before use read the APVMA permit (click link) and product label for directions. Table 2 lists the chemicals registered or with APVMA permits at the time of publication. A link to the table in a printable PDF format is provided under Further information.

Table 2.  Insecticides for silverleaf whitefly control in vegetables in Queensland - 2007

Active ingredients	Product(s)	Chemical group	Crops	WHP (days)	Rate of product
REGISTERED INSECTICIDES There are two formulations (200 SC for foliar spray and 350 SC for soil application) of imidacloprid registered for use against silverleaf whitefly on some vegetables. Bifenthrin (Talstar) is registered oncucurbits and tomatoes and thiamethoxam (Actara) is registered as a plant hole drench at planting. There are also a number of other chemicals registered on a range of vegetables for use against any of the many types of whiteflies. These chemicals may or may not be effective against silverleaf whitefly Bemisia tabaci Biotype B.
PERMITS						Permit No. & expiry date
bifenthrin (100 g/L)	Talstar 100 EC and others containing 100 g/L bifenthrin	pyrethroid (3A)	broccoli, brussels sprouts, cabbage (head)*, cauliflower, lettuce (head)*	7	40-60 mL/100 L of water or 600 mL/ha plus wetter	Per 9243 (31/03/08)
			cucumber, gherkins, melons, pumpkin, squash, zucchini	3
			beans	2
buprofezin (440 g/L)	Applaud (IGR)	chitin inhibitor (17A)	cucumbers (greenhouse)	3	30 mL/100 L of water	Per 8963 (01/07/10)
buprofezin (440 g/L)	Applaud (IGR)	chitin inhibitor (17A)	cucumbers, eggplant, tomatoes, zucchini	3	600 mL/ha	Per 9178 (31/03/10)
imidacloprid (200 g/L)	Confidor 200 (foliar spray for use against whitefly)	chloronicotinyl (4A)	bok choy, pak choy, choi sum, gai lum, kai choi, wong bok, mizuna, amsoi, taishona, komatsuma, tatsoi, mustard, turnip and rutabaga greens, and mibuna	3	25 mL/100 L of water, or 300 mL/ha	Per 5735 (28/02/08)
imidacloprid (200 g/L)	Confidor 200 (seedling foliar drench to cell trays)	chloronicotinyl (4A)	seedlings of: tomatoes & peppers (excluding seedlings for hydroponic production)	NA	40 mL per 1000 seedlings	Per 9269 (31/01/10)
imidacloprid (200 g/L)	Confidor 200 (seedling foliar drench to cell trays)	chloronicotinyl (4A)	seedlings of: broccoli, cauliflower, head cabbage	NA	40 mL per 1000 seedlings	Per 9184 (30/09/08)
	Confidor 200 (soil applied)		broccoli, cauliflower, common bean (Phaseolus vulgaris), head cabbage, lettuce, okra*	NA	25 mL/100 m of row
imidacloprid (350 g/L)	Confidor Guard and others (soil applied)		broccoli, cauliflower, common bean (Phaseolus vulgaris), head cabbage, lettuce, okra*	NA	14 mL/100 m of row
imidacloprid (350 g/L)	Confidor Guard(soil applied)	chloronicotinyl (4A)	potatoes*	NA	14 mL/100 m of row	Per 9160 (31/12/10)
petroleum oil (839 g/L)	DC Tron Plus Spray Oil	insecticide/ spreader	capsicum, cucurbits, eggplant, okra, tomatoes	1	500 mL/100 L of water	Per 8249 (31/03/10)
piperonyl butoxide (800 g/L) plus bifenthrin (100 g/L)	Synergy  plus Talstar	synergist pyrethroid (3A)	broccoli, cabbage (head)*, lettuce (head)* cucurbits (cucumbers, melons, pumpkins, squash, zucchinis) green beans tomatoes	7 3 2 1	Low Volume: Synergy 300 to 400 mL per ha plus Talstar 600 mL/ha High Volume: Synergy 30 to 40 mL per 100 L of water plus Talstar 40-60 mL/100 L of water	Per 9569 (31/12/07)
pymetrozine (500 g/kg)	Chess	feeding inhibitor (9A)	head lettuce	7	200 g/ha	Per 9244 (31/03/08)
			broccoli	5
			cucurbits, eggplant, tomatoes	3
pyriproxyfen (100 g/L)	Admiral (IGR)	juvenile hormone mimic (7C)	cucurbits, eggplant, tomatoes	1	500 mL/ha	Per 10205 (30/06/08)

NA = not applicable; * = Read the permit for more information on permitted crops and application methods; IGR = Insect Growth Regulator
Note: The information above is a guide only. All users should read, or have read to them, the details and conditions of the permit and/or product label before using the product. The permits are available on the Australian Pesticides and Veterinary Medicines Authority (APVMA) website.

4. Clean-up crop residues

Movement of SLW adults from older crops and crop residues is the main source of infestation for younger crops. Post-harvest destruction of heavily infested crops often causes mass migration of SLW adults into adjacent crops. Therefore it is important to control adult whiteflies before they move into young crops.

Clean-up strategies for old crops/crop residues:

. For moderate whitefly infestations, use an insecticide effective against adults or 2% D-C-Tron Plus^® oil.

. Use high spray volumes, normally around 600 to 1000 L/ha for better coverage.

. Plough in the crop within three days to kill all remaining nymphs on the crop foliage.

. Remember that withholding periods still apply and that produce should not be taken from the fields for consumption. Don't feed crop residues to livestock.

Further information

Further information on SLW management can be obtained from:

. The silverleaf whitefly in Queensland home page.

Information on tomato leaf curl disease is available in the following DPI&F Notes:

. Tomato leaf curl virus (TLCV)   

. Tomato yellow leaf curl virus (TYLCV)

Follow the links below for PDF copies of the insecticide Tables 1 and 2.

Table 1. A guide to choosing insecticides for silverleaf whitefly control in vegetables - 2007 (  77 KB)

Table 2. Insecticide permits for silverleaf whitefly control in Queensland - 2007 (  45 KB)

For more specific information contact:

. Subra Siva-Subramaniam, Entomologist, DPI&F, Bowen Research Station - (07) 4761 4036

. Brendan Nolan, Senior Experimentalist, DPI&F, Gatton Research Station - (07) 5466 2222

. Iain Kay, Senior Entomologist, DPI&F, Bundaberg Research Station - (07) 4155 6244

DPI&F information and services

• To access DPI&F's information and services, Queensland residents can contact the DPI&F Business Information Centre on 13 25 23 for the cost of a local call, from 8 am to 6 pm Monday to Friday (excluding public holidays). E-mail callweb@dpi.qld.gov.au. Non-Queensland residents phone (07) 3404 6999.

• Current national information on agricultural chemicals registered for use on all crops is available on the Infopest CD-ROM. Write to DPI&F, GPO Box 46, Brisbane, Qld 4001, E-mail infopest@dpi.qld.gov.au, visit the Infopest web page, or phone (07) 3239 3967 for further information.

Industry links

• Industry links is a page of links to various sites of interest to horticultural growers.

Acknowledgements

Much of the information included in this document has been developed through projects facilitated by HAL in partnership with AUSVEG and funded by the vegetable levy. The Australian Government provides matched funding for all HAL's R&D activities.

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Last updated 07 December 2007