NZPPS pesticide resistance management strategy

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Whitefly insecticide resistance management strategy

Buprofezin resistance has recently been
demonstrated in NZ Trialeurodes vaporariorum.
(Photo: Whitney Cranshaw, Colorado State
University, www.insectimages.org)

N.A. Martin
Crop and Food Research, Private Bag 92 169, Auckland, New Zealand

(Revised T.J.B. Herman, January 2025)

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 strategies (Martin 1996 and 2005).

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 sometime 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.

In the past 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, E. 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 found in Auckland in October 2000 and is now throughout the North Island and top of the South Island (Martin 2018). 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

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.

Oils may be used as insecticides, or to improve the spread and penetration of labelled insecticides.  UK research shows that the addition of oils improves the spreading and therefore the chance of contact with sessile scale stages.

Additives such as oils and spreaders and stickers should only be used if the product label recommends the use of a spreader/sticker.

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.

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 (puparium). See Martin (1999) for more information and pictures of the life cycle stages.

The egg and puparium stages are less susceptible to insecticides compared to the adults and nymphs. It is important to direct pesticide applications to where and when the adult and nymphal stages are on the plant.

Note: Control failure does not always imply resistance.

Guidelines to reduce the risk of whitefly resistance include the following points.

• Use integrated pest management to control pests in/on your crop.
• 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 three sprays of one insecticide to kill all stages of a generation. Then use an insecticide from a different MoA group the next time whitefly control is required.
• Ensure thorough spray coverage, including the underside of leaves. Where possible time sprays to follow plant training, i.e. removal of excess leaves and shoots, to improve 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 in existing crops and throughout the property.
• Control weeds prior to planting to minimise whitefly infesting the new crop.
• When pulling out the old crop, prevent as many whitefly as possible from escaping. 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.
• Remove the plants from the property immediately or cover them with black plastic (seal the edges). Leave plants covered for 4 (summer) to 8 (winter) weeks.
• 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 (also thrips and other insects)
• keep batches of plants separate to prevent cross infection if whitefly get in and breed.
• After planting out cover the door and vents with fine mesh netting to prevent entering whitefly. Ensure there is still adequate air flow to cool the greenhouse.
• If whitefly is seen, spray the plants (before E. formosa is introduced).
• Subdivide ornamental production areas to prevent whitefly movement between groups of plants, where practical. Keep mother plants for cuttings separate from production areas.
• Maximise biological control by either introducing E. formosa into greenhouse crops and/or encouraging parasite activity by not using insecticides 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 weeds prior to planting to minimise whitefly infesting the new crop.
• Get rid of the old crop immediately.
• When pulling out the old crop prevent as many whitefly as possible from escaping. 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.
• Remove the plants from the property immediately or cover them with black plastic (seal the edges). Leave covered for 4 (summer) to 8 (winter) weeks.
• If ploughing in the old crop, make sure all plant material is completely buried.
• 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. 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.
• Kill the whitefly on the prunings:
• remove prunings 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.

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

Citrus - citrus whitefly

Whitefly numbers need to be kept low if sooty mould build up is to be avoided. Monitoring should take place through the spring for whitefly (as well as other insect pests). Some citrus types appear to be less attractive to whitefly e.g  Afourer and Encore, whereas Navel, Valencia and Satsuma tend to be more attractive.

Good pruning is an important cultural practice that can reduce whitefly populations. A well pruned block has better air and wind flow, which disturbs whitefly and they are less likely to settle. They prefer more shelter, and so higher infestations are often found in thicker trees. Good pruning also allows for better spray penetration so insecticide applications are likely to be more effective.

Whitefly have some natural enemies in citrus, the most common being the steelblue ladybird (Halmus chalybeus) which is a predator of many other citrus pests as well. Serangium maculigerum is another, smaller ladybird which has appeared in New Zealand's citrus growing regions. It is now commonly found in the Bay of Plenty and Northland, but has been slow to spread in Gisborne.

Targeted insecticide applications are usually required where whitefly numbers are high. These applications can usually be timed to coincide with those required for other pests.

Citrus NZ IPM manual. Available via the Citrus NZ website (www.citrus.co.nz). Access to the electronic manual is via a member login. Please contact Citrus NZ (info@citrus.co.nz) for access.

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.

• Minimise whitefly infestation on young plants.
• Monitor whitefly adult populations.  Spot spray localised problems and treatments while populations are small.
• 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 MoA 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.
• After completing a cluster of pesticide applications, a different MoA group should be selected when the next cluster is required for whitefly control.

Greenhouse flower crops

Biological control could be suitable for other flowering crops produced for the local market. See also above for greenhouse fruit and vegetables without biological control.

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 movement 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.

Outdoor annual flower, fruit and vegetable crops

For transplants grown from seed or cuttings, take all the precautions listed above.

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. Check with your exporter prior to using any insecticides.

Reference

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