Oriental Scale in Papayas
David Astridge and Rod Elder
|
|
Description
Adult
The adult female is a yellow shield-shaped insect covered by a pinkish-greyish-brown, semi-translucent scale 2 mm across. The insect is soft-bodied, without appendages and remains in the one position feeding by means of long tubular mouthparts. It produces living young referred to as crawlers. These escape from under the female covering. After about 2 weeks the male scale becomes mussel-shaped and can be distinguished from the circular female scale. The adult male emerges as fragile, winged insect.
Immature stages
Crawlers cannot be seen with the naked eye, are yellow-cream, have legs and disperse over the tree to a position where they settle permanently and begin to feed. Once the crawlers settle they quickly form immature scales which are similar to adult females except smaller.
Distribution
Northern Australia as far south as Gladstone (24oS) on the east coast.
Life history
Both males and females are ready for mating about 3 weeks after the crawlers settle. The male dies soon after mating. The fertilised female begins to produce crawlers 2 weeks later then dies after a further 3 to 4 weeks. These times are greatly extended in winter so that about 5 generations are produced in a year.
Host range
Plants from the following families: Anacardiaceae, Apocynaceae, Caricaceae, Combretaceae, Euphorbiaceae, Musaceae, Palmae, Rosaceae, Salicaceae, and Vitidaceae. Hosts of horticultural interest include banana, coconut, castor oil, frangipani, grape, mango, oleander, papaya, rose and willow. Although other hosts have been recorded in Queensland, heavy infestations have only been recorded on castor oil and papaya
Management
Importance
Minor and infrequent since release of a wasp (Comperiella lemniscata Compere and Annecke) for biological control. Population increases are common in late summer and autumn. Oriental scale is not a pest south of Yarwun.
Damage
Scales suck the sap. Heavy scale infestations on papaya trunks cause tissue damage allowing fungal infection. Infested trees become very unthrifty and may snap off in wind or die. Both the skin and fruit of infested fruit fail to ripen in the area surrounding the individual scales so that the ripe fruit has green spots and is inedible. Fruit infested while still expanding develop a sunken area around the scale.
Action level
Examine the trunks and 5 fruit on 20 widely separated trees throughout each 0.5 ha of crop. Order and release Chilocorus circumdatus (see below) if 1 or more trees have a large flush of (medium sized) scale. A release is unnecessary if high levels of parasitism -(70% Encarsia citrina (Craw) in medium (2nd instar) sized scales, and 70% Comperiella lemniscata in fully-grown female (3rd instar) scales)- are already occurring.
Assessment of parasitism levels requires training and equipment such as a microscope or high magnification hand lens. Twenty large and 20 medium scales should be examined for parasitoids on each of 10 fruit. Pick the fruit with the most scale or collect more than 10 fruit if scale populations are too low to give 20 of each sized scale with only 10 fruit.
Control methods
Chemical
Apply petroleum oil (narrow range) if C. circumdatus predators are not available or if the scale problem rapidly increases. To avoid plant/fruit damage apply in cooler weather and ensure continual agitation in the tank. One or two oil applications should enable the parasitoids to regain control of the scale without greatly reducing the parasitoid numbers. If the problem is restricted to a few trees only spray these.
Recommended chemicals
Petroleum oil (narrow range).
Note: This information was correct at the time of publication. However, it is the user's responsibility to ensure that registered agricultural chemicals are used in accordance with legal requirements. (See Further information). Always read the label.
Cultural
Cut out the occasional seriously infested trees. Usually the scale does not spread to many nearby trees and it may be preferable to leave the infested trees to allow parasitoids to breed up on the scale.
Biological
The parasitoids, Encarsia citrina, and C. lemniscata are very important in the control of this pest. The 1 caterpillar and 4, ladybeetle predators, Batrachedra arenosella and Telsimia sp., Lindores lophanthae Chilocorus circumdatus and C. baileyi respectively, are usually only present and effective when scale populations are very dense. Because of the importance of these parasitoids and predators, every effort should be made to avoid the use of pesticides over the whole plantation.
As soon as a large increase in medium sized scales is noticed, order C. circumdatus predators. Apply at the rate of 750 per hectare, (300 per acre). If part of the papaya crop is infested, only make releases in that area. Another occasional parasitoid, Aphytis melinus was used as a biological pesticide but it often did not persist for longer than 12 weeks and it did not survive the winter. The 2 important parasitoids above are established throughout Queensland although it may occasionally be necessary to introduce them in newer more remote papaya growing areas.
Their populations decline with scale populations during winter. Most parasitoids prefer healthy well foliaged trees to provide them with shelter from extremes of heat and low humidity. Prolonged periods of temperature extremes, and dust are harmful to many beneficial insects. Growing healthy plants, using cover crops or mulches between rows and avoiding the use of dusty tracks will enhance parasitoid activity
Further information
-
Contact the DPI&F Business Information Centre on 13 25 23 -
local call 8 am to 6 pm Monday to Friday (non-Queensland residents phone
07 3404 6999). E-mail: callweb@dpi.qld.gov.au
- Visit the DPI&F website www.dpi.qld.gov.au for other DPI&F notes
- Contact the DPI&F bookshop on 07 3239 3772 or DPI&F Shop-Online or at some DPI&F offices throughout Queensland for the DPI&F publications
- Infopest CD-ROM contains current national information on registered agricultural
chemicals and is available from Infopest, DPI&F, GPO Box 46, Brisbane Qld 4001
or by email from infopest@dpi.qld.gov.au
Recommended Reading:
Elder, R.J., Smith, D. and Mayer, D.G. 1988, Trials of insecticides against Aonidiella orientalis (Newst.) on papayas, Internal Department Primary Industries Final Report.
Elder, R.J., Smith, D. and Bell K.L. (1998). Successful parasitoid control of Aonidiella orientalis (Newstead) (Hemiptera: Diaspididae) on Carica papaya L. Australian Journal of Entomology. 37:74-79.
Elder, R.J. and Bell K.L. (in press). Establishment of Chilocorus spp. (Coleoptera: Coccinellidae) in a Carica papaya L. orchard infested by Aonidiella orientalis (Newstead) (Hemiptera: Diaspididae) Australian Journal of Entomology.
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 updated 13 May 2005
