Agnote 752 No. I44 May 1998 Agdex No: 230/612 ISSN No: 0157-8243 Redbanded Thrips on Fruit Trees (Selenothrips rubrocinctus) G. Young and D. Chin, Entomology, Darwin Redbanded thrips (RBT) are found in many areas of the tropical and sub-tropical parts of the world. The pest is thought to have originated from northern South America. A wide variety of ornamental shrubs, fruit trees and native trees and shrubs are attacked by redbanded thrips. Avocado, cashew, guava, mango, mangosteen and rambutan are the main fruit trees attacked in the Top End. Mango trees can suffer severe damage to recently hardened flush between April and August. DESCRIPTION Adult redbanded thrips are dark brown to black in colour and about 1.3 mm long. They possess two pairs of narrow feathery wings which are folded along the back when the insect is at rest. The immature stages or nymphs are creamy yellow with two characteristic bright red bands around the abdomen. One band is directly behind the hind legs and the other at the tip of the abdomen. DAMAGE Nymphs and adults are mostly found on the undersides of leaves where they feed by rasping leaf tissue and sucking the contents of the leaf cells. Developing fruit is also attacked in a similar fashion. Adults and nymphs frequently feed together. Nymphs usually carry a drop of liquid at the tips of their upturned abdomens. The excrement periodically falls giving rise to dark brown spots on the leaf. In the early stages, feeding activity produces a silvery sheen on the leaves and skin of fruit. The margins of leaves tend to curl downward as a result of feeding on the underside of the leaf. 2 In the case of severe infestations, leaves take on a brown sun-scorched appearance while the skin of fruit becomes scarred, making it unmarketable. RBT prefer recently hardened flush to new flush and older leaves. There is some evidence that trees that are not growing well are more susceptible to attack by RBT than healthy trees. LIFE HISTORY Redbanded thrips, like most species of thrips, avoid direct light and, when not feeding, the adults shelter on the undersides of leaves, in curled up leaves and the buds of developing flush. The female inserts eggs into the lower surface of the leaf and covers the point of insertion with a drop of fluid which dries to a black disc like cover. Females lay up to 50 eggs and live for up to one month. The eggs take about 12 days to hatch. After hatching there are two nymphal stages lasting 8 to 16 days. During these two stages the nymphs feed widely on several plants. Full grown second stage nymphs are about 1 mm long. The two nymphal stages are followed by two resting stages (pre-pupal and pupal stages), which congregate towards the end of the underside of the leaf. The resting stages last 5 to 10 days before adults emerge. The average time from egg laying to adult is about 30 days. MONITORING Redbanded thrips are present from the end of February to November. Pest numbers are highest in the dry season. Monitoring should commence at the end of February and be carried out every 7-10 days. RBT may be transferred by prevailing winds and are often first noticed in fruit trees on the edges of the orchard. The best time to monitor for RBT is during the cooler part of the day, either in the early morning or late afternoon. Examine the underside of flush leaves with a x10 magnification hand lens. RBTs are usually on the outer foliage of the tree. Look for either RBT adults or nymphs. Adults and nymphs may be difficult to find on the older leaves, although there may be signs of damage or excretal remains (small black spots) from previous feeding. Fully matured leaves are less susceptible to damage by RBT. When RBT are noticed on the trees, they should be carefully monitored for any change in populations. Spot spraying should be carried out while the numbers are still limited to a few trees. Control becomes difficult if RBT are allowed to spread throughout the orchard. An infestation is best controlled before the start of mango flowering. Try to avoid using any insecticide sprays at the time of flowering as this will result in a reduced fruit set. Spraying may physically damage the flowers and the insecticide may repel or kill pollinators. CONTROL Chemical The registered chemicals for control of RBT are trichlorfon (eg.Dipterex®) dimethoate (eg.Rogor®), and endosulfan (eg.Thiodan®). Endosulfan is a S7 chemical and requires a permit issued by the NT Health Department before it can be used. Petroleum spray oils (P.S.Os.) (eg DC Tron plus®) are also used in the control of RBT. PSOs are used at 2% oil in water. PSOs work by suffocating rather than poisoning the insect. In order for PSOs to be effective, the insect has to be coated with a thin film of the oil- water solution. As a result, 2 to 2½ times more spray mixture is applied when using PSOs, than would be used with a chemical poison. 3 The Entomology Branch has had some promising results in the laboratory using potassium soap (eg. Natrasoap®) against RBT. Potassium soap works in a similar manner to PSOs. It is recommended for use against RBT on mango seedlings and young trees. The surfaces of the leaves need to be thoroughly wetted with the soap solution. Biological Redbanded thrips are attacked by natural predators such as spiders, lace wings, predatory thrips and predatory bugs. Commercially raised predatory insects are available to assist with the control of RBT. To control RBT with commercially raised predators, the predators should be introduced in March when RBT first start to appear. The Entomology Branch is currently carrying out trials on the control of RBT using commercial predators. For further information on redbanded thrips contact the Entomology Branch, Berrimah Agricultural Research Centre, Department of Business, Industry and Resource Development, phone 8999 2260. For further information on spraying see Agnote D36; Preparation and Setting of Air Blast Sprayers. Please visit us on our website at www.primaryindustry.nt.gov.au Published: Wednesday 27 May 1998. While all care has been taken to ensure that information contained in this Agnote is true and correct at the time of publication, the Northern Territory of Australia gives no warranty or assurance, and makes no representation as to the accuracy of any information or advice contained in this publication, or that it is suitable for your intended use. No serious, business or investment decisions should be made in reliance on this information without obtaining independent/or professional advice in relation to your particular situation.