Silverleaf whitefl y alert for soybean growers March 2003 Natalie Moore1, Robin Gunning2, Hugh Brier3, Which whitefly is it? and oval in shape with red ‘eyes’. Most SLW Bernie Franzman4, Greg Mills3, Don McCaffery5. nymph stages are immobile and pupate in situ on the leaf. 1 NSW Agriculture, Grafton, NSW, Australia; Silverleaf whitefl y (SLW) is the B-biotype of the whitefl y Bemisia tabaci. The B-biotype is a 2 NSW Agriculture, Tamworth, NSW, Australia; pesticide-resistant strain of B. tabaci that has Why is the B-biotype of SLW so gained entry into Australia, most likely from 3 Department of Primary Industries, Kingaroy, the United States of America. SLW is a major difficult to manage? Qld, Australia; global pest of horticultural and agricultural SLW has resistance to many common 4 Department of Primary Industries, crops and is highly resistant to most current pesticides. SLW can occur in uncontrollable pesticides, including organophosphates, Toowoomba, Qld, Australia; plagues with billions of individuals per carbamates and synthetic pyrethroids. 5 NSW Agriculture, Orange, NSW, Australia hectare of crop. Australian experience and research shows SLW can develop resistance to pesticides extremely rapidly, due in part to its short What do SLW look like? generation time (egg to adult in as little as 12 days in summer) and to the huge populations Silverleaf whitefl ies are small, fragile sap- that can develop (billions/ha). It generates sucking insects in the same bug sub-order resistance to chemicals much faster than as aphids and scale insects. Adults have Helicoverpa armigera. powdery white wings and are only 1.5 mm long (see Fig. 1). SLW is similar to, but smaller than, greenhouse whitefl y (Trialeurodes vaporariorum) and has a distinct gap between its forewings. SLW can only be distinguished from other strains of B. tabaci by biochemical testing (see Monitoring and identifi cation below). Eggs are laid on the underside of leaves, are very small and sit on a pedicle or stalk. When laid en masse, the eggs can look like brown velvet. Whitefl y nymphs settle Fig. 1. Adults (1.5mm long) and nymphs of silverleaf whitefly on the under leaves and become sedentary like scale underside of a soybean leaf. insects. Nymphs are pale yellow-green, fl at Photo N. Moore, NSW Agriculture. What damage can it cause? establish in Australia are still unknown. Where to look for SLW Predictions have been made about potential areas for spread of SLW but more SLW sucks sap from the plant and can SLW survives on a wide range of crops, information needs to be gathered from rapidly generate large populations, which weeds and ornamental plants including, but soybean cropping areas to determine which reduce crop yields. Like aphids, SLW also not limited to, the following: ones are likely to be most at risk. Although secretes ‘honey dew’, a food source for SLW may spread throughout soybean sooty mould that covers leaves with black ■ Highly preferred hosts such as soybean, growing areas, it is not known whether it growth and reduces photosynthesis (Fig. 2). melon, sunfl ower (young), tomato, will necessarily cause bladder ketmia (Hibiscus trionum - see economic damage in all Figs. 3 and 4), native rosella (Abelmoschus regions.regions. fi culneus), Convolvulaceae vines, for example, cow vine (Ipomoea lonchophylla) In general, areasareas with and bell vine (Ipomoea plebeia), sow hot summers, mild or milk thistle (Sonchus oleraceus), winters and a year-roundyear-round Cucurbitaceae and Euphorbia weeds, abundance of susceptible and many ornamental plants. host plants areare consideredconsidered to be at highest risk. ■ Less preferred hosts that may still However,However, some areasareas at support large populations include cotton, lesser risk may experience sunfl ower (mature), pumpkin, peanuts, outbreaksoutbreaks when and pasture legumes (including lucerne). conditions areare favourable. Numerous other plants may support Soybean productionproduction SLW, especially when populations fl are. areasareas consideredconsidered to be at greatestgreatest risk include Fig. 2. Soybean crop severely affected by sooty mould due coastal Queensland to silverleaf whitefly infestation. Photo N. Moore, NSW (Qld) (including the Lockyer and Fassifern Agriculture. valleys), central Qld, and the Northern Rivers district of New South Wales (NSW). Once SLW populations explode little can Production areas at lesser risk include the be done to control them and crops can be inland Burnett and Darling Downs in Qld decimated in a short time if conditions are and the North West slopes in NSW. favourable (for example, hot, dry weather). Many cotton and soybean crops in Central Queensland were devastated by SLW in the We need to identify your 2001-2002 season. whiteflies! If you suspect an outbreak of Please contact your local District SLW, please DON’T SPRAY! Agronomist or Entomologist to coordinate the identifi cation of specimens. Biochemical Fig.Fig. 3.3. AdultAdult silverleafsilverleaf whiteflywhitefly onon thethe weedweed hhostost bbladderladder ketmiaketmia The fi rst line of action for a SLW outbreak analysis is required to distinguish the (Hibiscus trionum) Photo courtesy N. Forrester, Deltapine. should be identifi cation to confi rm the B- B-biotype - this is done by Dr Robin biotype (see Monitoring and identifi cation Gunning’s team at the Tamworth Centre below). This will determine available for Crop Improvement, NSW Agriculture, control options. If it is the B-biotype, Calala Lane, Tamworth, NSW 2340. don’t be tempted to spray with pesticides. SLW is known to be moderately to highly Your District Agronomist or Entomologist resistant to all commonly used broadacre will also assist you to collect the specimens crop pesticides currently used in Australia. correctly. Spraying with pesticides will likely promote the development of further pesticide ■ Adult fl ies are preferred for analysis. resistance in SLW, kill benefi cial insects and ■ Late-stage nymphs are also acceptable. probably cause the SLW population to fl are, ■ along with other pests such as mites. Send specimens as quickly as possible to arrive in good condition for analysis. Monitoring and identification ■ Keep SLW specimens cool and send alive. Fig.Fig. 4.4. SilverleafSilverleaf whiteflywhitefly nymphsnymphs (left)(left) andand adultsadults (right)(right) uundernder The areas to which this pest will spread and ■ Do not kill or put in alcohol. bladder ketmia leaves. Photo courtesy N. Forrester, Deltapine. Implications for soybean growers in northern NSW Presently there are no chemical treatments registered for use against SLW in soybean in Australia. Until effective control measures are found, SLW has the potential to prevent highly susceptible crops, such as soybean, being grown in areas where large populations of the pest become established. After devastating losses to soybean crops in Central Fig. 6. Generalist predators such as Lady beetles (Family Coccinellidae, above), lacewings (order Neuoptera) and big-eyed bugs (Geocoris sp.) Queensland (Emerald) and Bundaberg in feed on SLW and can reduce SLW populations provided they are not 2002, soybeans should not be grown in overwhelmed by SLW ‘flares’. Photo N. Moore, NSW Agriculture. these high-risk areas until control measures are developed. This will bring production pressures (and opportunities) to production Pesticides? areas where SLW does not establish in large numbers. The cotton industry in Australia is pursuing Since it was fi rst detected in Darwin and the registration of several new pesticides Tamworth in 1994, monitoring of SLW for SLW as components of an Insecticide has shown that it is still spreading to new Resistance Management Strategy (IRMS). cropping areas in the Northern Territory, The strategy involves pesticides from Queensland, and New South Wales, (and Fig. 5. An average of 25% yield loss resulted from this infestation at least four different chemical groups in glasshouses in Adelaide and Perth) (R. of SLW (B. tabaci B-biotype) in soybeans north of Ballina in northern (including insect growth regulators) for use Gunning, pers. comm.). The southern and NSW. The pest appeared in February 2002 in an otherwise healthy, at different stages of the season. Because of vigorous crop. Feeding by the pest and extensive sooty mould, which western limits for its future establishment SLW’s great capacity to develop resistance developed on honey dew produced by the pest, caused the pods not to are unknown. Cold winters are thought to complete filling. Photo N. Moore, NSW Agriculture. to pesticides, the success of such a strategy reduce SLW populations, so the boundaries needs all growers in a region to comply for damaging populations may shift with strict usage patterns so that only one depending on seasonal conditions, but this spray with one type of chemical is allowed cannot be predicted with certainty at this in any one season and no mixes are allowed. stage. Management strategies Coordination is essential so that everyone does ‘the same thing at the same time’ for Whilst SLW has previously been recorded in northern NSW on a variety of horticultural In the absence of pesticides registered for greatest impact on the local SLW populations. crops, only one severe outbreak has been SLW in soybeans, the key strategies at present The success of this strategy will be known in confi rmed in a soybean crop north of Ballina are: the coming seasons. in the 2002 season (see Fig. 5). 1. Weed control year round to remove host Registration of chemicals for use against The impact of control measures used on plants, especially winter hosts SLW in soybean in the future will require the other pests (for example, early season generation of residue (MRL) data. For some organophosphate or synthetic pyrethroid 2. Awareness of the pest, frequent inspection products this may mean a minimum of two sprays) must be considered in areas where of crops, volunteer plants and weeds for years’ trial work. The high cost of some new SLW is known to occur, as these can fl are early detection and identifi cation of SLW pesticides may prove to be prohibitive.