Whitefly insecticide resistance management strategy
Buprofezin resistance has recently been
demonstrated in NZ Trialeurodes vaporariorum.
(Photo: Whitney Cranshaw, Colorado State
University, www.insectimages.org)
(Revised February 2005)
Reasons for strategy and update
Several species of whitefly (Hemiptera: Aleyrodidae), including two species present in New Zealand, are capable of readily becoming resistant to pesticides. Pest management strategies aimed at reducing or preventing resistance will help conserve existing products for on-going effective use. This is an update of the earlier resistance management strategy (Martin 1996).
Background
The whitefly fauna is summarised in Martin (1999) and economic species are illustrated. Since 1999, an Australian species that attacks citrus has been found in Auckland (Gill 2001). Five species of whitefly are found on crops of economic significance. A sixth species is associated with ferns, but its systematic and economic status is uncertain.
Greenhouse whitefly, Trialeurodes vaporariorum, is the main pest species of greenhouse and outdoor crops. It has a wide host range and some time in the late 1970s a strain adapted to tamarillos and became a major pest of the crop. Normal greenhouse whitefly breed poorly on tamarillos. Both strains are also found on a wide variety of weeds and there are indications that local populations have adapted to some weed species. Reports indicate that in England a strain of whitefly has adapted to chrysanthemums.
Greenhouse whitefly can transmit a disease, beet pseudo yellows virus, to cucurbits, but this has not been confirmed in New Zealand. Control is principally by insecticides although an increasing proportion of greenhouse vegetable growers is using biological control. Unsatisfactory control with pesticides is often due to a poor understanding of the life cycle (Martin 1989), poor between-crop practices and a lack of basic hygiene during plant propagation.
Until recently, most growers relied solely on pesticides to control greenhouse whitefly. Some would spray once a week but most about every two weeks. Few growers applied pesticides in clusters to try and eliminate a generation of the pest. These conditions have been suitable for the development of resistance to pesticides, but there have been no reports of insecticide resistance. Pesticides with label claims for whitefly control in New Zealand all refer to greenhouse whitefly control only.
Two parasites and a fungal pathogen of greenhouse whitefly are present in New Zealand. One parasite, Encarsia formosa, is available for sale to growers. It will also parasitise cabbage whitefly and silver leaf whitefly. A second parasite, E. pergandiella, has a complex life cycle. It will parasitise greenhouse whitefly to produce females, but in order to produce males it parasitises whitefly previously parasitised by itself or E. formosa. High numbers of E. pergandiella can be found outdoors in late summer and autumn. The fungal pathogen, Verticillium lecanii, is present in New Zealand and is used overseas to control whitefly and aphids.
The consequences of resistance to pesticides by greenhouse whitefly are very significant to growers of particular crops and may make growing some crops impossible or economically very difficult. This would include crops where whitefly is, at this stage, a minor pest. This strategy is designed to reduce selection pressure for resistance as far as possible without jeopardising produce quality.
Sweet potato whitefly, Bemisia tabaci, adults look similar to greenhouse whitefly, but at rest hold their wings parallel to their body (Martin 1999). The larvae can be readily distinguished under a microscope. When B. tabaci was first found in New Zealand it was thought to be sweet potato whitefly, B. tabaci (poinsettia strain or strain B), however, this is not certain. B. tabaci strain B is now also called B. argentifolii. This species or strain has a wider host range (including tomatoes and vegetable brassicas), causes physiological disorders in plants and transmits viruses and has a high tendency to express resistance to pesticides. Most overseas literature does not clearly define the species or strain of Bemisia so that it is difficult to relate published information to the strain present in New Zealand.
In New Zealand, sweet potato whitefly has only been found on ornamental plants, principally poinsettia and begonia, and appears to be established only at two nurseries, one in South Auckland and one in Christchurch. No virus diseases associated with the whitefly have been found in New Zealand. Only a few pesticides are known to give effective control in New Zealand. Overseas, Encarsia formosa can control Bemisia in some ornamental greenhouse crops.
Cabbage whitefly, Aleyrodes proletella, is distinguished by the black mark on its wing (Martin 1999). It is mainly found on brassicas and Sonchus and rarely needs to be controlled.
Ash whitefly, Siphoninus phillyreae, was found in Auckland in May 1995. It is widespread in the Auckland region and is spreading to other parts of New Zealand. Hosts include pipfruit and many deciduous trees including ash. A host specific parasitoid, Encarsia inaron, which is self-introduced to New Zealand, usually controls the whitefly to very low numbers.
Citrus whitefly, Orchamoplatus citri, an Australian species, was discovered in Henderson, Auckland in October 2000 and was also present in Auckland, North Shore and Waitakere cities and Kumeu. It is now present in Nelson (R. Henderson, pers. comm.). It can infest all varieties of citrus (Gill 2001) and can breed on the native trees, titoki (Alectryon excelsus Gaertner (Sapindaceae)) and juvenile leaves of pohutukawa (Metrosideros excelsa Gaertner (Myrtaceae)) (R. Henderson & N.A. Martin, unpubl. data). It is a minor pest causing leaf distortion and is associated with honey dew and sooty mould.
Other whitefly found on commercial plants include Aleuroclava eucalypti on Eucalyptus and Pealius azaleae on Rhododendron species.
Products with label claims for whitefly control in New Zealand
Very few registered insecticides have specific label claims for whitefly control, including pesticides primarily used for whitefly control by greenhouse tomato growers (Martin 1989). For this reason all products have been listed that have label claims for insect control on crops that commonly require applications of pesticides for whitefly control. In addition, few pesticides have label claims specifically for use on greenhouse crops, where whitefly problems tend to be most severe, although some products are registered for use specifically on field crops. Those products that have specific label claims for whitefly control were all registered for greenhouse whitefly although most of the products would have overseas data showing efficacy against sweet potato whitefly.
The pesticides for whitefly control can be grouped into 6 categories as shown in Table 1. In addition, a biological control product is available to growers.
Several oils claim to enhance insecticide efficacy. UK research shows that the addition of oils improves the spreading and therefore the chance of contact with sessile scale stages. The products with insecticide enhancement label claims for fruit and/or vegetables are Agpro Crop Oil, C-Dax Oil, Codacide Oil, D-C-Trate, Hasten, Rapid Plus and Spray-Sure Kwickin.
Additives such as oils and spreaders and stickers should only be used if the pesticide product label recommends the use of a spreader/sticker.
| Pesticide category | IRAC chemical group | Pesticide common and (product) names |
|---|---|---|
| Parasites | Encarsia formosa (En-force, En-strip) | |
| Carbamates | 1A | methomyl (Lannate) |
| Organo-phosphate | 1B | diazinon (Basudin, Dew, Diazinon, Diazinyl, Gesapon) |
| dichlorvos (Nuvos) | ||
| pirimiphos methyl (smoke) (Actellic) | ||
| Organophosphates + pyrethroids | 1B/3 | pirimiphos-methyl + permethrin (Attack) |
| Cyclodiene | 2A | endosulfan (No longer registered) |
| Pyrethroid | 3 | deltamethrin (Decis Forte, Deltaphar) |
| taufluvalinate (Mavrik) | ||
| taufluvalinate + fungicide (Guardall, Supershield) | ||
| Pyrethrins | 3 | pyrethrum (Garlic & Pyrethrum) |
| Chloronicotinyl | 4A | imidacloprid (Confidor 5GR) |
| Thiadiazine | 16 | buprofezin (Applaud) |
Current status of whitefly resistance in New Zealand
There have been reports of insecticide resistance to greenhouse whitefly, but only the most recent, to buprofezin, has been confirmed (Workman & Martin 1995).
Overseas, greenhouse whitefly has developed resistance to organochlorine, organophosphate, carbamate and pyrethroid insecticides (e.g. Georghiou 1981; Anis & Brennan 1982; Elhag & Horn 1983; Wardlow 1985; Hommes 1986). Resistance has also been found in newer insecticides, buprofezin and teflubenzuron (Gorman et al. 2000).
Resistance management and prevention strategy
Whitefly have two life stages that are resistant to pesticides. For this reason a single application of pesticides is rarely very effective. A brief understanding of the whitefly life cycle will help improve control of the pest.
The winged adults (male and females) are mainly found on the youngest leaves. The females lay eggs on young leaves upon which they are feeding. All subsequent stages stay on the leaf upon which the egg was laid. As a result the older whitefly stages are found on the older plant leaves. There are four larval stages that appear like flat oval scale insects. The first larva has legs and crawls a short distance from the egg. The last larval stage swells up and when fully grown, stops feeding and forms a pupa within the nymphal skin. This is called a puparium. Some growers call this stage an "egg" because the adult hatches from it. See Martin (1999) for more information and pictures of the life cycle stages.
The egg and puparium stages are least susceptible to pesticides, while the adults and youngest nymphs are the most susceptible to most pesticides. It is usually important to direct pesticide applications to where these two stages are on the plant and to time applications to when these stages are on the plants.
Note: Control failure does not always imply resistance.
Guidelines to reduce the risk of whitefly resistance include the following points.
- Adopt or continue to use integrated pest management that incorporates pesticide resistance prevention/management strategies for specific crops.
- Avoid repeated applications of insecticides.
- Buprofezin should be used no more than twice in any one crop per 12 month period. Buprofezin should be used with another pesticide to control adult whitefly.
- For other insecticides, use a cluster of two to six sprays of one insecticide to kill all stages of a generation. Then use an insecticide from a different chemical group the next time whitefly control is required.
- Ensure thorough spray coverage, including the underside of leaves. Use sufficient insecticide. Where possible time sprays to follow plant training, i.e. removal of excess leaves and shoots, to make it easier to achieve good spray coverage.
- Maximise non-insecticide controls, especially in the transition between crops or cropping seasons for outdoor crops (the notes below are adapted from IPM manuals for greenhouse crops).
Greenhouse crops and nursery crops
To keep whitefly numbers very low or undetectable during propagation and after planting out:
- Control weeds outside the greenhouse or on the property. Control the weeds well ahead of planting out so that whitefly do not go straight from the dying weeds to the newly-planted crop. Do this 3-4 weeks before pulling out an old crop.
- When pulling out the old crop, prevent as many whitefly as possible from escaping. Adult whitefly can move to other greenhouse crops, outdoor crops or onto weeds. They will eventually get back into the newly-planted greenhouse. Note that adult whitefly will continue to emerge from puparia after the plant has been pulled out.
- Spray or fog with a pesticide to knock down the adult whitefly.
- If possible remove the plants from the property immediately. Do not leave the plants uncovered for the emerging adult whitefly to escape.
- If the plants are dumped on the property, cover them immediately with a sheet of black plastic and seal the edges. Leave plants covered for 4 weeks in summer and 8 weeks in winter.
- Be a responsible neighbour.
- To remove all whitefly from the empty greenhouse you can:
- kill all weeds
- spray the house with insecticide
- use yellow sticky traps.
- To prevent whitefly from getting onto the seedlings or cuttings:
- grow seedlings or cuttings in a separate greenhouse
- cover all openings with netting that is too fine for adult whitefly (and thrips and other insects)
- keep batches of plants separate in order to prevent cross infection if whitefly get in and breed.
- After planting out cover the door and vents with fine mesh netting if the greenhouse is of a suitable size and your plants are likely to be invaded by whitefly from outside. Make sure there is still adequate air flow to cool the greenhouse.
- If whitefly is seen, spray the plants. Young plants do not need much time or product for a thorough application of pesticide. Control at this stage is easy and cheap compared with later. This activity is recommended before Encarsia formosa is introduced.
- Subdivide ornamental production areas. Prevent whitefly movement between groups of plants by subdividing greenhouses where practical. Keep mother plants for cuttings separate from production areas.
- Maximise biological control by either introducing Encarsia formosa into greenhouse crops and/or encouraging parasite activity by avoiding sprays toxic to parasites and not removing crop leaves before parasites have emerged.
Outdoor annual crops
Follow these guidelines to keep whitefly numbers very low or undetectable during propagation and after planting out.
- Control weeds in existing crops and throughout the property. Control the weeds well ahead of planting out so that whitefly do not go straight from the dying weeds to the newly-planted crop. Do this 3-4 weeks before pulling out old crops.
- Get rid of the old crop immediately it is not needed. Do not leave a redundant crop to breed whitefly to infest new crops, weeds or neighbours.
- When pulling out the old crop prevent as many whitefly as possible from escaping. Adult whitefly can move to other crops or onto weeds. They will eventually get back into newly-planted crops or into propagation areas. Note that adult whitefly will continue to emerge from puparia after the plant has been pulled out.
- Spray with a pesticide to knock down the adult whitefly.
- If possible remove the plants from the property. immediately. Do not leave the plants uncovered for the emerging adult whitefly to escape.
- If the plants are dumped on the property, cover them immediately with a sheet of black plastic and seal the edges. Leave plants covered for 4 weeks in summer and 8 weeks in winter.
- If ploughing in the old crop make sure all plant material is completely buried.
- Be a responsible neighbour.
- To prevent whitefly from getting onto your seedlings or cuttings:
- grow seedlings or cuttings in a separate greenhouse
- cover all openings with netting that is too fine for adult whitefly (and thrips and other insects.
- After planting out, if whitefly is seen, spray the plants. Young plants do not need much time or product for a thorough application of pesticide. Control at this stage is easy and cheap compared with later.
Perennial outdoor crops - Tamarillo
The only perennial outdoor crop with persistent whitefly problems is tamarillo. At this stage it is not known how serious ash whitefly will be. See above for protection of new plants grown from seed or cuttings. The following guidelines will help to control whitefly between growing seasons.
- Control weeds in the orchard and throughout the property. Control the weeds well ahead of pruning so that whitefly do not go from the prunings to weeds and then on to the new spring growth. Complete weed control 3-4 weeks before pruning and keep them controlled until pruning has been completed. Ideally keep weeds controlled throughout the year.
- Kill the whitefly on the prunings. Adult whitefly can continue to hatch from puparia (white scale) on dead or dying leaves for weeks after removal from plants and can therefore infest new tamarillo growth or new growth on weeds. Ways to reduce the risk from prunings include:
- remove them from the property the same day (cover them up if left overnight)
- leave the pile of prunings covered up with black plastic for 6 to 8 weeks; seal the edges of the covers tightly
- mulch the prunings at the end of each day, making sure the leaves are thoroughly smashed up
- be a responsible neighbour; don't send your whitefly to others.
- After pruning, spray the pruned trees to kill remaining whitefly. Pruned plants with few leaves do not need much time or product for a thorough application of pesticide. Control at this stage is easy and cheap compared with later.
Plant management to assist pesticide application
- Arrange the plants in the greenhouse or outside so that the person spraying can move easily and concentrate on spraying, e.g. wide paths for people and machinery.
- Arrange plants so that the spray can be directed at the appropriate parts of the plants, e.g. adequate space between plants.
- De-leaf or leaf prune so that the sprays can be applied to the difficult to reach parts of the plants, e.g. if foliage is too dense, thin the leaves.
- Where possible, time sprays to follow plant training, i.e. removal of excess leaves and shoots, to make it easier to achieve good spray coverage.
- Remove leaves close to the ground. It is very difficult to spray the underside of leaves when they are close to the ground. Remove them as soon as possible.
- Train plants to keep leaves away from the ground. For example, make tomato plants grow up straight and do not allow them to trail on the ground.
- Select cultivars that allow easy spray application. The older leaves of the cabbage cultivar lie horizontal to the ground and are difficult to spray underneath. Select a cultivar with leaves that remain more vertical but still open to allow sprays to penetrate.
Plant management to assist biological control of whitefly
- Careful leaf management, especially de-leafing, is essential in order to fully benefit from the parasite. This can also reduce the number of E. formosa parasites required and save money.
- Ideally leaves should be left on the plants until the E. formosa have hatched from the black scale.
- To maximise E. formosa breeding, it is helpful to know when the parasite has emerged from the black scale and where this is happening on the plants. Ask an advisor to demonstrate how to recognise the signs of the parasite emergence hole in the black scale.
- Monitor adult whitefly using yellow sticky traps or by crop monitoring. Crop monitoring is particularly important for greenhouse crops and during the four months following pruning of tamarillos.
Crop specific recommendations
Crop-specific IPM manuals that include pesticide resistance management/prevention strategies for whitefly and other insects, mites and diseases, are available for greenhouse beans, capsicums, cucumbers, roses and tomatoes.
Citrus - citrus whitefly
No insecticide products have label claims for whitefly control on citrus, but insecticides with label claims for whitefly in other crops and a claim for control of other insects in citrus are shown in Table 2.
Greenhouse crops - greenhouse whitefly
The strategies for greenhouse crops are divided into those crops where chemical residues on the crop are of concern (fruit and vegetables) and other crops. Biological control is practical for most greenhouse fruit and vegetable crops. IPM manuals published by Crop & Food Research (available from Publications Coordinator, Crop & Food Research, Private Bag 4704, Christchurch) that include information on biological control and incorporate non-pesticide methods of pest control suitable for the crop, are available for beans, capsicums, cucumbers and tomatoes. These programmes can be adapted for zucchinis, eggplant, melons and pepinos. If biological control is not practical or appropriate, the following procedures should be incorporated into the crop protection programme.
- All steps to minimise whitefly infestation on young plants should be adopted.
- Whitefly adult populations should be monitored during routine plant management (see IPM Manual No. 1 Revised January 1995 for example of monitoring chart). This will enable spot spraying of localised problems and treatments while populations are small (this will reduce selection for resistance).
- Plants should be managed to allow easy and thorough pesticide application.
- Buprofezin should not be applied more than twice in any one crop within 12 months and should be used with another insecticide to kill adult whitefly.
- During each crop or 12 month period, pesticides from more than one chemical group should be used.
- Each pesticide should be applied as a cluster of sprays or in a mixture in order to kill a majority of a population as it passes through susceptible life stages. For example, where there are moderate populations of whitefly prior to the hot season, use two applications 2-3 weeks apart of a persistent pesticide such as buprofezin. Whereas if a short lived pesticide such as methomyl is used, it should be applied weekly or even twice weekly for 3-4 weeks.
- After completing a cluster of pesticide applications, a different pesticide group should be selected when the next cluster is required for whitefly control.
See Table 2 for list of pesticides registered for each crop.
Greenhouse flower crops
The only IPM manual for a flower crop for which greenhouse whitefly is a pest is greenhouse roses.
Biological control could be suitable for crops such as Bouvardia produced for the local market. Overseas researchers are testing E. formosa for whitefly control on gerberas.
See also above for greenhouse fruit and vegetables without biological control.
See Table 2 for list of pesticides registered for each crop.
Ornamentals/pot plants - greenhouse whitefly, sweet potato and ash whitefly
For plants grown from seed or cuttings follow the precautions above. However, if continuous batches of cuttings or seeds are planted it is most useful to organise the propagation and production units to minimise transmission of whitefly from one area to another. It is also important to keep mother plants from which cuttings are taken isolated and whitefly free by non-pesticide means. Also see above for greenhouse fruit and vegetable crops. See Table 2 for list of pesticides registered for each crop.
Outdoor annual flower, fruit and vegetable crops
For transplants grown from seed or cuttings, take all the precautions listed above. See Table 2 for a list of pesticides registered for each crop.
Tamarillos - greenhouse whitefly tamarillo strain
It is important to ensure whitefly numbers are very low during the first half of the growing season when it is still possible to achieve good spray coverage. Monitor plants carefully and spot-spray plants if localised whitefly infestations are found.
Most pesticides can be used close to harvest on fruit destined for the local market but some pesticides must not be used several months before harvest on fruit for export. See Table 2 for a list of pesticides registered for the crop.
| Type of label claim for each crop group1 | ||||||||
|---|---|---|---|---|---|---|---|---|
| Pesticide category and IRAC chemical group Pesticide common and (product) names |
 |
 |
 |
 |
 |
 |
 |
 |
| Parasites | ||||||||
| Encarsia formosa | GHWF in GH | GHWF in GH | GHWF in GH | GHWF in GH | ||||
| Carbamates 1A | ||||||||
| methomyl | WF in GH | WF in GH | WF in GH | |||||
| Organo-phosphates 1B | ||||||||
| diazinon | XX | GHWF | ||||||
| dichlorvos | WF in GH | WF in GH | WF in GH | WF in GH | GHWF | WF in GH | WF in GH | |
| pirimiphos methyl | WF in GH | WF in GH | WF in GH | |||||
| Organo-phosphates + Pyrethroids 1B/3 | ||||||||
| pirimiphos methyl+ permethrin | XX | WF in GH | WF in GH | WF in GH | ||||
| Cyclodiene 2A | ||||||||
| endosulfan (No longer registered) | WF | |||||||
| Pyrethroids 3 | ||||||||
| deltamethrin | GHWF | |||||||
| taufluvalinate | WF | GHWF | ||||||
| taufluvalinate + fungicide | WF | |||||||
| Pyrethrins 3 | ||||||||
| pyrethrum | GHWF | GHWF | ||||||
| Chloronicotinyl 4A | ||||||||
| imidacloprid | WF in TSP | WF in TSP | ||||||
| Thiadiazine 16 | ||||||||
| buprofezin | XX | GHWF in GH | GHWF in GH | GHWF in GH | GHWF | GHWF in GH | GHWF in GH | |
1GHWF = greenhouse whitefly where this species is specified or is the only species on the crop, WF = whitefly, GH = greenhouses, TSP = transplants, XX = no label claim for whitefly, but claim for control of other insects in citrus.
Reference
Anis AIM, Brennan P 1982. Susceptibility of different populations of glasshouse whitefly Trialeurodes vaporariorum, Westwood to a range of chemical insecticides. Faculty of General Agriculture University College of Dublin, Research report 1980-1981: 55.
Elhag EA, Horn DJ 1983. Resistance of greenhouse whitefly (Homoptera: Aleyrodidae) to insecticides in selected Ohio greenhouses. Journal of Economic Entomology 76: 945-948.
Georghiou GP 1981. The occurrence of resistance to pesticides in arthropods, an index of cases reported through 1980. FAO of UN, Rome. 172 pp.
Gill G 2001. Citrus whitefly blows in. Biosecurity Issue 26 (15 March 2001): 19.
Gorman K, Devine GJ, Denholm I 2000. Status of pesticide resistance in UK populations of glasshouse whitefly, Trialeurodes vaporariorum, and the two-spotted spider mite, Tetranychus urticae. The BCPC Conference: Pests and diseases: 1: 459-464.
Hommes M 1986. Insecticide resistance in greenhouse whitefly (Trialeurodes vaporariorum, Westw.) to synthetic pyrethroids. Mitteilungen aus der Biologischen Bundesanstalt fur Land-und Forstwirtschaft 232: 376.
Martin NA 1989. Greenhouse Tomatoes - A survey of pest and disease control. DSIR Plant Protection Bulletin No. 1. 42 pp.
Martin NA 1996. Whitefly resistance management strategy. In: Bourdot GW, Suckling DM ed. Pesticide Resistance: Prevention and Management. New Zealand Plant Protection Society, Lincoln, New Zealand. Pp. 194-203.
Martin NA 1999. Whitefly: biology, identification and life cycle. Crop and Food Research, Broadsheet No. 91. 8 p.
Wardlow LR 1985. Pyrethroid resistance in glasshouse whitefly (Trialeurodes vaporariorum, Westw.). Mededelingen van de Faculteit Landbouwwetenschappen, Rijksuniversiteit, Gent 50 (2b): 164-165.
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