Melon thrips in potatoes
John Maltby & Bronwyn Walsh, Department of Primary and Industries and Fisheries, Queensland.
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Background
Melon thrips (Thrips palmi) were first detected in the Northern Territory in 1989 and in Queensland in 19933. In Queensland melon thrips have been found in potatoes, capsicums, chillis, eggplant, beans, cucumbers, melons and pumpkins. Melon thrips mainly cause damage by feeding but are also a weak vector of tomato spotted wilt virus (TSWV).
Description
Melon thrips are small (1.3 mm long) cigar shaped insects (Figure 1) that can barely be seen with the naked eye. The pest is pale green to orange in colour and is most commonly found on the undersides of leaves but can also be found in flowers3.
- Figure 1. Melon thrips (enlarged)
- Figure 2. Bronzing along veins of a potato leaf (Photo: C. Freebairn, DPI&F Qld)
Damage
Melon thrips feed on the potato leaves, killing cells with their piercing and sucking mouthparts. At low levels there may be no visible sign of leaf damage however in high numbers they can produce silvering, yellowing and bronzing of affected areas (Figure 2). Leaves may crinkle and die and the overall effect is a loss of plant vigour and a reduction in marketable produce3. Up to 30% yield losses have been recorded in autumn Sebago potato crops from melon thrips in the Lockyer Valley.
Life cycle
Their life cycle can take from 20 days at 30°C to 80 days at 15°C2. The life cycle consists of egg, larva, pupa and adult (Figure 3). The adult, egg and larval stages are found on the host plant. Each female produces an average of 50 eggs, which are inserted into plant tissue and hatch in 4 to 16 days. Larvae emerge, feed on plant tissue for 4 to 14 days and then crawl to the soil where they enter a resting stage as pupae for 3 to 12 days. The winged adult emerges from the pupae, feeds on leaves, stems and flowers and can live for 7 to 30 days.
- Figure 3. Thrips lifecycle (Life cycle modified A. Medhurst DPI Vic)
Managing melon thrips
Australian and overseas experience has shown that a number of natural enemies have been shown to impact on the level of melon thrips in crops. The natural enemies vary according to the crop and the country, they include predatory bugs, predatory mites and predatory thrips as well as lady beetles and parasitic wasps2,4.
Managing melon thrips should not be done in isolation from managing other pests in potatoes. What you spray against one pest can result in an increase in numbers of another pest. For example the pesticide you spray against aphids may kill the natural enemies that are keeping thrips at a low level. You will remove your aphid problem but create a thrips problem.
The following steps should be considered for managing melon thrips (as well as other pests in your crops).
Step 1. Identifying
It is important to identify the type of thrips present in the crop. Due to their small size melon thrips can barely be seen with the naked eye and can only be positively identified by skilled personnel with the aid of a suitable microscope. If you do not know the type of thrips present in your crop you may select an ineffective control measure.
Step 2. Monitoring
Just as important as the type of thrips is the level of thrips present in the crop. Monitoring for thrips can done using sticky traps and crop inspections.
Blue and white sticky traps are attractive colors for thrips and have been used to trap melon thrips however yellow has also been suggested as an attractive colour2. Hang sticky traps just above the crop, check them weekly and have thrips identified by a skilled practitioner.
For crop inspections look at 20 leaflets per planting using a 10x magnifying lens. Choose plants at random from throughout the planting, take leaflets from the middle of the plant. On the underside of leaves look for the white-yellow larvae or yellow-brown adults. Adults tend to move toward young foliage, with larvae tending to be clustered on foliage inhabited by adults several days earlier.
Results from monitoring an autumn Sebago crop in the Lockyer valley suggest:
a) less than one thrips per leaflet can be tolerated without yield loss;
b) if levels reach four thrips per leaflet early in the crops growth (tuber initiation), pesticide control methods should be considered.
Step 3. Control
Field experience has shown that relying on pesticides alone can lead to an increase in thrips numbers. Other management practices, such as farm hygiene, breaks in production of susceptible crops, protecting natural enemies, plastic mulch and windbreaks, have been used successfully in the Northern Territory4.
Spraying
Only the larvae and adults can be killed by insecticides as the eggs and pupae do not come into contact with insecticide (see Figure 3). For this reason a series of sprays is normally required to cover the time taken for eggs to hatch into larvae and pupae to develop into adults.
The period between sprays will vary with temperature. During summer 5 to 7 days is recommended between sprays. During winter this interval can be increased to 10 to 14 days.
Continue to monitor thrips to determine if you need to spray again.
If after the first series of sprays thrips build up again, you should change to another chemical group for the next series of sprays, to reduce the risk of thrips becoming resistant to the chemical.
Further information
The following references have been cited above.1. Horne, P. de Boer, R. and Crawford D (2002), A field guide to insects and diseases of Australian potato crops. Melbourne Press. www.mup.com.au.
2. Capinera, J.L. (2000), Melon thrips, Thrips palmi Karny (Thysanoptera: Thripidae). EENY-135 Information sheet Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. http://edis.ifas.ufl.edu.
3. Planck, J (2000), Emerging plant pests - Melon thrips. DPI&F Note.
4. Young, G. and Zhang, L. (1998), Control of melon thrips, Thrips palmi. Agnote 753. No. I45. Agdex 230/612. Department of Primary Industries and Fisheries, Northern Territory.
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.
Information contained in this publication is provided as general advice only. For application to specific circumstances, professional advice should be sought. The Department of Primary Industries and Fisheries, Queensland has taken all reasonable steps to ensure the information in this publication is accurate at the time of publication. Readers should ensure that they make appropriate inquiries to determine whether new information is available on the particular subject matter.
Last reviewed 19 September 2007
