Thrips insecticide resistance management and prevention strategy

Western flower thrips damage on English daisy
(Photo: Whitney Cranshaw, Colorado State University,
www.insectimages.org)

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

(Revised February 2005)

Reasons for strategy and update

Several species of thrips (Thysanoptera) including two economically important species, onion thrips and western flower thrips, that are present in New Zealand, have become resistant to pesticides. Pest management strategies aimed at reducing or preventing resistance will help conserve existing products for ongoing effective use. This is an update of the earlier resistance management strategy (Martin 1996).

Background

A wide variety of thrips, mainly exotic, but also including indigenous species, can cause unacceptable damage to plants. In addition, the presence of thrips, especially indigenous species, on export produce can cause consignments to be rejected or treated for reasons of quarantine security imposed by importing countries.

The eggs and pupae of thrips are usually protected from insecticide sprays (see below). This, together with their rapid development at higher temperatures, makes them particularly difficult to control with insecticides and often results in frequent applications or use of persistent insecticides.

Thrips species and strains

Thrips (Thysanoptera) are grouped into two suborders, Terebrantia and Tubulifera, with most pests in the Terebrantia.

Terebrantia

A large number of minor pests and a few important pests in this group belong to the family Thripidae. Plants from most families may be attacked by these thrips, especially when grown in a greenhouse. The most important species in New Zealand are described below.

Intosa flower thrips (Frankliniella intonsa (Trybom)) has been found in the Auckland region on flowers, capsicums and strawberries (Gill 2002). In Europe this pest can have a negative economic impact on strawberry crops, lucerne and nectarines (Gill 2002). It is also a pest of a wide variety of crops in Asia (e.g. Murai 1988; Fang & Fang 1996).
Gladiolus thrips (Thrips simplex (Morrison)), a serious pest of gladioli and closely related genera, can also be found on other flowers. It is primarily a pest of commercial and homegrown gladioli. Most saleable gladioli flowers would require pesticide application.
Greenhouse thrips (Heliothrips haemorrhoidalis (Bouche)) is a pest of trees and shrubs, including citrus, many ornamentals and young pine trees. It inhabits the older leaves of trees and shrubs, and can feed on young and almost-mature fruit, especially where there is direct fruit to fruit or fruit to leaf contact. The brown, corky damage to citrus and avocado fruit reduces quality. These thrips causes severe silvering to leaves of several shrubs including camellias and rhododendrons, and to young pines. A parasitoid, Thripobius semiluteus (Boucek) (Hymenoptera: Eulophidae), has been released to control this thrips (Brook 2001; Froud & Stevens 2004). Pesticides are used in orchards, plant nurseries and gardens.
Kelly's citrus thrips (Pezothrips kellyanus (Bagnall)) is found in citrus flowers. Feeding can scar young fruitlets although damage occurs primarily while the plants are flowering and while fruit are small. Cultivars that flower over a long period are vulnerable throughout that time.
New Zealand flower thrips (Thrips obscuratus (Crawford)) is an indigenous species and is primarily associated with damage to flowers. Even if it is not breeding in flowers its presence causes a quarantine problem on export produce. This species has a wide host range and at certain times of the year it can readily infest crops. The thrips is attracted to the flowers of several summer/stone fruit types and subsequent feeding can mark nectarine fruitlets. Adult females are also attracted to ripening peaches and nectarine fruit on which they feed, producing a silvering of the skin. Their presence causes a quarantine problem for export fruit. No resistance to pesticides has been recorded.
Onion thrips (Thrips tabaci Lindeman) can cause severe damage to onion crops if not controlled. It is also a pest on young growth of a variety of plants including capsicums and cucumbers and it can damage flowers of flower crops. The thrips is a vector of tomato spotted wilt virus, a disease with a wide host range and a regular problem on autumn-planted greenhouse tomatoes and spring-planted field tomatoes. Pesticides are used to control the thrips in all the affected crops and to keep it off flowers for export. Biological control with a predatory mite, Amblyseius cucumeris, is becoming available for some greenhouse crops.
Western flower thrips (Frankliniella occidentalis (Pergande)) is a pest on a wide variety of vegetables, ornamental crops and weeds overseas. A greenhouse strain, first found in New Zealand during 1992, is resistant to many pesticides. It has been found damaging greenhouse crops of capsicums, cucumbers, eggplant and flower crops including chrysanthemums, carnations, gerbera and roses. It is also a vector of tomato spotted wilt virus. Another strain (the lupin strain) of western flower thrips has been in New Zealand since at least 1934, and is common on yellow lupins. It causes no economic damage to crops. Biological control with a predatory mite, Amblyseius cucumeris, is available for some greenhouse crops.

Tubulifera

Bottlebrush thrips (Teuchothrips disjunctus (Hood)) causes unsightly twisted leaves on some species of red bottlebrush and requires pesticide application in nurseries.
Lily thrips (Liothrips vaneeckei Preisner) feeds on the subterranean scales of lilies such as Asiatic lilies. It is rarely a problem at present.
Red clover thrips (Haplothrips niger (Osborn)) has damaged clover seed crops, but is rarely a current problem on clover seed crops. No pesticides are specifically used to control this thrips.

Products with label claims for thrips control in New Zealand

Some insecticides have label claims specifically for "thrips" control, but only two products claim control for particular species of thrips. However, some label claims are restricted to crops that have a distinctive complex of thrips species. Claims for thrips control on one crop with a particular pest complex may not be relevant to another crop with a different thrips pest complex. One chemical, methamidophos, which gives effective control of the pesticide-resistant greenhouse strain of western flower thrips, has no label claim for thrips control. Pesticides with label claims for thrips control are listed in Table 1. A biological control agent is also available for greenhouse crops.

In addition 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 stages. The products with insecticide enhancement label claims for fruit and/or vegetables in New Zealand are Agpro Crop Oil, C-Dax Oil, Codacide Oil, D-C-Trate, Hasten, Rapid Plus and Spray-Sure Kwickin.

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

**Table 1:** Products with label claims for control of thrips in New Zealand (September 2002). Not all products listed for each pesticide may have a label claim for all crops indicated.
	Type of label claim for each crop group¹
Pesticide category and IRAC chemical group Pesticide common and (product) names
Biological control
Amblyseius cucumeris (Mite-A, Thripex)						T in GH	T in GH
Hypoaspis aculifera (Entomite, Hypo-Mite)						SF in GH	SF in GH
Mineral oils
mineral oil (Caltex, D-C Tron)		T
Carbamates 1A
carbaryl (Carbaryl, Sevin)	T		T	T
Organo-phosphates 1B
acephate (Lancer, Orthene)		XX
azinphos-methyl (No longer registered)		T
chlorpyriphos (Chlorpyriphos, Lorsban, Pychlorex)		T	T	T in GR
diazinon (Basudin, Dew, Diazinon, Diazinyl, Gesapon)				T	T	T	T
dichlorvos (Nuvos)					T	T	T
dimethoate (Dimezyl, Rogor)		T	T	T
maldison (Malathion, Yates Maldison)		T	T	T			T
methamidophos (Metafort)					T
Cyclodiene 2A
endosulfan (no longer registered)					OT	T
Fiprols 2B
fipronil (Ascend)		KCT
Pyrethroids 3
alpha-cypermethrin (Bestseller, Dominex)					T
cyfluthrin (Baythroid)					T
deltamethrin (Decis, Deltaphar)					T
lambda-cyhalothrin (Karate Zeon)				T	OT
tau-fluvalinate (Mavrik)			T	T	T		T
tau-fluvalinate + fungicide (Guardall, Supershield)							T
Pyrethrins 3
pyrethrum (Garlic & Pyrethrum)		T	T	T			T
Chloronicotinyls 4A
imidacloprid (Confidor)						T

¹T = thrips, OT = onion thrips, KCT = Kelly's citrus thrips, SF = sciarid fly, GH = greenhouse crops, GR = grapes, XX = label claim for control of aphids, leaf rollers and mealy bugs in citrus.

Current status of thrips resistance in New Zealand

Some thrips species present in New Zealand are major pests overseas, but of little consequence locally. The pesticide resistance status of these species has not been investigated.

Western flower thrips. Overseas populations of this thrips are resistant to most groups of pesticides, but it is not possible to be sure that overseas data apply to the New Zealand greenhouse strain due to the variability in this species. In New Zealand, the susceptibility to 13 pesticides has been tested (Martin & Workman 1994). The greenhouse strain was less susceptible than the lupin strain to all 13 pesticides but maldison and methamidophos still gave acceptable control, and dichlorvos gave partial control. Pesticides that gave inadequate control in the 24 hour bioassay were: acephate, abamectin, chlorpyrifos, endosulfan, methomyl, methiocarb, omethoate, pyrazophos, and tau-fluvalinate.
Onion thrips. In New Zealand, resistance to deltamethrin is present in the North and South Islands, but resistance to diazinon and dichlorvos has only been found near Auckland (Martin et al. 2003). Onion thrips is reported to be resistant to many pesticides in the USA, but still susceptible to synthetic pyrethroids (Grossman 1994).
New Zealand flower thrips. No resistance to pesticides has been recorded.
Gladiolus thrips. No resistance to pesticides has been recorded in New Zealand.
Greenhouse thrips. No resistance to pesticides has been recorded in New Zealand.
Kelly's citrus thrips. Resistance to chlorpyrifos has been reported from South Australia (Purvis 2002).

Resistance management and prevention strategy

Thrips have life stages that are resistant to pesticides. For this reason a single application of a pesticide is rarely very effective. A brief understanding of thrips life cycle will help improve control of the pest.

Most pest thrips in New Zealand belong to the family Thripidae, whose life cycle will be described first. The winged adults (male and females) are mainly found on flowers and the youngest leaves. The females insert eggs into plant tissue, usually flowers or leaves. There are two larval stages which tend to occur where they can hide, e.g. between leaves or flower petals. There are two "pupal" stages, which are usually off the plant in the soil or litter. The adults hatch from the last pupal stage and fly back onto a plant. The egg and pupal stages are protected from pesticides and single applications of most pesticides will not kill these stages. Individuals may emerge from these stages when pesticide residues will no longer kill them.

The greenhouse thrips has a similar life cycle, except that the "pupal" stages remain on the leaves and all stages tend to be on old leaves. Similarly all stages of the bottlebrush thrips stay on the plant, but are protected in curled up leaves. The life cycles of the clover and lily thrips have significant differences, but the principles described for Thripidae apply.

Note: Control failure does not always imply resistance.

In New Zealand, the risk of pesticide resistance appears to be confined to onion thrips and western flower thrips, though possibly also Kelly's citrus thrips.

Guidelines

Guidelines to reduce the risk of the development of resistance to pesticides by gladiolus thrips, Kelly's citrus thrips, onion thrips, western flower thrips and to manage resistance to pesticides include:

Adopt or continue to use integrated pest management which incorporates pesticide resistance prevention/management strategies for specific crops, e.g. processing tomatoes and greenhouse vegetables.
Use pesticides from several chemical groups (see Table 1 for pesticides to control gladiolus thrips, Kelly's Citrus thrips and onion thrips). See above and Crop & Food Research Broadsheet No. 36 (Martin 1993) and its updates for pesticides that will control western flower thrips. For greenhouse crops on which onion thrips and western flower thrips breed, use a cluster of three or four sprays of an insecticide applied 3-5 days apart to kill susceptible larval and adult stages as all individuals of a generation pass through a generation. Each time a cluster of insecticides is required for thrips control, use one insecticide for all sprays of a cluster, but for the next cluster use an insecticide from a different chemical group.
Ensure thorough spray coverage, including the underside and between leaves (e.g. onions), and flowers and fruitlets (e.g. citrus). Use sufficient insecticide and, if necessary and if recommended, use a wetter. Where possible, time sprays to follow plant training, i.e. removal of excess leaves and shoots, to facilitate good spray coverage.
Monitor crops by plant inspection or with yellow or blue sticky traps. Note that blue traps do not catch many other insects so that thrips are easier to see. Examine traps at least weekly.
Maximise biological control by either introducing Amblyseius cucumeris into greenhouse crops and/or by encouraging predator activity by avoiding sprays toxic to parasites and predator.
Crop and plant management can assist pesticide applications.
Arrange plants in the greenhouse (or outside) so that the person spraying can move easily and concentrate on spraying, e.g. leave wide paths for people and machinery.
Arrange plants so that the spray can be directed at the appropriate parts of the plants e.g. provide 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.
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-pesticide controls for greenhouse crops or crops propagated in greenhouses. These controls include:
- covering all vents and doorways with insect proof netting
- practising good hygiene between crops by disposing of the crop remains carefully so that thrips are not left on the property and cannot infest other properties
- maintaining good hygiene around the greenhouse; removing all weeds
- propagating in a separate house and ensuring all plants in the propagation house remain thrips free
- disposing of all plant prunings so that thrips cannot escape from infected material onto plants outside the greenhouse or into a greenhouse.
- moving from a clean greenhouse to an infested greenhouse, and not the other way round
- picking all flower blooms and not leaving unwanted flowers for the thrips to breed in

Guidelines to reduce the risk of resistance development by greenhouse thrips include:

Use pesticides from several chemical groups alternatively (see Table 1)
For short lived pesticides, apply each pesticide twice about 7 days apart to kill larvae hatching from eggs.
Monitor the crops by plant inspection. Search the older, internal leaves or where fruit and/or leaves touch.
Apply pesticide at first signs of the pest.
Ensure thorough spray coverage on the under side of all leaves and between all fruit.
Manage the crop so that fruit do not touch and leaves do not touch fruit. Keep the plant structure open to allow good air movement to lower the humidity.

Crop specific recommendations

Crop specific IPM manuals which include pesticide resistance management and prevention strategies for thrips and other insects, mites and diseases, are available for greenhouse beans, capsicums, cucumbers, cymbidium orchids, roses and tomatoes.

Information on the life cycle of thrips and non-pesticide and pesticide controls for thrips grown in greenhouse or under protection is found in Crop & Food Research Broadsheets Nos 35 & 36.

Greenhouse crops

The strategies for greenhouse crops depend on whether the crop is one where pesticide residues are of concern (e.g. fruit and vegetables) or not.

The strategies in general are the same for all thrips infesting fruit and vegetable crops, but the selection of pesticides is severely limited where western flower thrips is present.

Crops that are susceptible to tomato spotted wilt virus have a lower tolerance of the two thrips vectors, onion thrips and western flower thrips.

Greenhouse vegetable and fruit crops - western flower thrips and other thrips

Biological control is practical for some greenhouse vegetable crops in New Zealand. It has been tested on capsicums and cucumbers, but may be suitable for other crops. Contact the suppliers of Amblyseius cucumeris for details on how to use and integrate the predator with control of other pests and diseases and for information on biological control of other pests. Incorporate non-pesticide methods of pest control suitable for the crop. IPM Manuals are available from Crop & Food Research for beans, capsicums, cucumbers and tomatoes. These programmes can also 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 should be adopted to minimise thrips infestation on young plants (see above and Broadsheet No. 36)
adult populations of thrips should be monitored during routine plant management and with blue sticky traps. This will enable pesticide treatments to be applied while populations are small and so reduce selection for resistance
plants should be managed to allow easy and thorough pesticide application (see above)
during each crop or 12 month period, pesticides from more than one chemical group should be used
each insecticide should be applied as a cluster of three sprays 3-5 days apart in order to kill the majority of a population as it passes through the susceptible life stages
monitor the crop for thrips and do not apply pesticides for thrips control until thrips are seen or reach an action threshold. Pesticides that control western flower thrips should only be used when western flower thrips control is required
after completing a cluster of insecticide applications, an insecticide from a different group should be selected when the next cluster is required for thrips control. Note that with regard to western flower thrips, maldison and methamidophos belong to different chemical groups

See Table 1 for a list of insecticides registered for thrips control in each crop.

Greenhouse flower crops - western flower thrips and other thrips

The only IPM manuals for a flower crop with thrips as a pest are greenhouse roses and cymbidium orchids.

Biological control could also be suitable for crops such as chrysanthemums produced for the local market.

The strategy is as above for greenhouse fruit and vegetables, but without biological control.

See Table 1 for a list of pesticides registered for thrips control in each crop.

Outdoor flower crops

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

Onion crops - onion thrips

Follow the onion thrips management strategy for onion crops published by the New Zealand Onion Exporters Association

Inspect 50 or 100 plants per crop in groups of 5 plants from October onwards.
Apply a cluster of 3 or 4 sprays of an insecticide when thrips numbers exceed 5 thrips per 50 plants.
Ensure thorough coverage of the plants and that the pesticide runs down between all the leaves. If the pesticide label specifies the use of such a product, use a spreader or oil to enhance penetration of the pesticide between plant leaves and enhance pesticide contact with the thrips.
If further applications of pesticide are required select a spray from a different chemical group. Use synthetic pyrethroids as late as possible.

See Table 1 for a list of pesticides registered for thrips control in onion crops.

Summer fruit and stone fruit - New Zealand flower thrips

New Zealand flower thrips can infest stone fruit flowers and ripening fruit. Nectarine flowers and fruitlets require protection, as do fruit of all export summer fruit at harvest time, and peaches and nectarines for local market. New Zealand flower thrips usually invade the crop from sources outside the orchard and the risk of pesticide resistance developing is low.

See Table 1 for a list of pesticides registered for thrips control in each crop.

Ornamentals/pot plants/pine nurseries - western flower thrips, greenhouse thrips and other thrips

The strategies in general are the same for all thrips infesting these crops but the selection of pesticides is severely limited where western flower thrips is present.

Crops that are susceptible to tomato spotted wilt virus have a lower tolerance of the vectors, onion thrips and western flower thrips.

For plants grown from seed or cuttings, follow the precautions listed 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 thrips from one area to another. It is also important to keep mother plants from which cuttings are taken isolated and free from both thrips and virus by non-pesticide means.

See above for greenhouse fruit and vegetable crops.

See Table 1 for a list of pesticides registered for thrips control in each crop.

Note: see above with regards to western flower thrips.

Citrus and other fruit crops - Kelly's citrus thrips and greenhouse thrips

Kelly's citrus thrips damages young fruit and pupates in the ground, whereas greenhouse thrips damages mature leaves and older fruit, especially where fruit touch or fruit touch leaves, and pupates on the tree. Descriptions of life cycle and control options for citrus are in Mooney (2001).

For Kelly's citrus thrips see guidelines in "General Strategies" above. Start monitoring crops during flowering and continue until the fruit are safe from damage. Apply the first of a cluster of sprays when larvae are seen. If more than one cluster of insecticide sprays is required, use a product with a different active chemical.

For greenhouse thrips, see guidelines in "General Strategies" above.

See Table 1 for a list of pesticides registered for thrips control in each crop.

Berryfruit and other fruit crops not listed above.

Damage can occur from thrips feeding at flowering or on young fruit. Thrips presence on fruit for export can cause consignments to be declined or treated.

See Table 1 for a list of pesticides registered for thrips control in each crop. Note: see above with regards to western flower thrips and intonsa flower thrips.

Acknowledgements

Thanks for help from P. Stevens and G. McLaren, HortResarch.

Reference

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