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Gabriella Caon (SARDI) and Paul De Barro (CSIRO)

Ash whitefly is a small white sap sucking insect which occurs in both temperate and Mediterranean climates. The first record of this pest for Australia was in Adelaide in autumn of 1998. A subsequent survey in June 1998 of greater metropolitan Adelaide and important fruit growing country regions found this new pest widely distributed in Adelaide from Hallet Cove in the south to the Barossa Valley in the north. There was only one record from the Adelaide Hills and none from the country regions surveyed. Interstate surveys in late June 1998 also found the whitefly in Wagga Wagga, Dubbo, Griffith, Gundagai in NSW and Canberra.

Identification

It is difficult to distinguish the common greenhouse whitefly from the ash whitefly. Both are small (1-2mm) white, winged insects found predominantly on the underside of host plant leaves. When heavily infested foliage is disturbed, a "cloud" of whiteflies is often observed with both whiteflies. Ash whitefly however, has the unusual behaviour of swarming around a host. In addition, this whitefly attacks hosts not normally attacked by other whitefly species (see table 1). Both of these behaviours are useful when trying to detect new outbreaks.

The whitefly has three growth stages: egg, nymph (immature phase consisting of four instars, in the later part of the fourth instar stage the nymph will cease to feed and form a puparium, also referred to as a ‘pupa’) and adult. The white (cream) eggs are laid on the underside of the leaf and soon turn grey. They are laid flat on the surface in a circular pattern. The first-instar nymph (crawler) is about 0.3mm long, flat and almost transparent. As the nymph develops it produces little waxy spines around it’s upper ridge, the fourth instar nymph also has a conspicuous ridge of white waxy material running longitudinally along the upper (dorsal) surface from top (anterior) to bottom (posterior). It is generally pale, but has a dark (melanic) stripe under this waxy ridge (dorsomedially).

Produced by the Entomology Unit SARDI
Information may be used with acknowledgment

Damage

Heavy infestations of ash whitefly can cause stress to trees and lead to premature leaf drop, wilting and smaller fruit. The whitefly does not attack the fruit but trees and shrubs grown for fruit can experience high percentage crop loss when numbers are high. It is recorded as a pest on apples and pears in Europe, but only when natural enemies are lacking.

Heavy infestations of ash whitefly can also produce large amounts of honeydew which in turn promotes the growth of sooty mould. Honeydew/sooty mould contamination of vehicles parked under ornamental and street trees has proved to be a problem in other parts of the world.

Ash whitefly is highly polyphagous and can be found on many small trees and shrubs. It is common for polyphagous whiteflies to broaden greatly host range during outbreaks. The species marked with an * in Table 1 are those that reflect the primary host range.

Life cycle

Ash whitefly reproduces quickly in summer and may pass through approximately three generations. Adults lay eggs on the underside of most leaves, so the whole canopy can be infested. The first instar is the only mobile juvenile stage, but even this stage rarely moves more than a centimetre. After settling, the insect remains in the same position until it becomes an adult .The nymphs and adults feed on sap. The ‘pupal’ stage is the resting stage from which the adult emerges. There are many overlapping generations in a year, and at any time all stages may be present on leaves.

The preferred hosts in spring, summer and autumn are the deciduous ash, pomegranate, pear and apple. In autumn, when these trees loose their leaves swarms of adult whiteflies can be seen moving to their winter hosts. Citrus and other non-deciduous trees (olive and photinia) make up the winter hosts. Development in winter is very slow with little occurring below 10^OC but it is this overwintering population that leads to large infestations early in the season. Ash whitefly will only develop on leaves. There is no evidence that it can develop on fruit.

Control

Chemical Control

There are many chemicals registered for whitefly but as a general rule pesticides rarely provide more than moderate reductions in population size. It is important to get good spray coverage including the undersides of leaves. On ornamental species in domestic settings reinvasion from untreated areas occurs rapidly and populations quickly return to prespray levels. Chemical pesticide treatment should be followed by a thorough visual inspection a few days after treatment, and further sprays may be required when attempting to disinfest nursery stock that is to be moved to a whitefly free area.

Some of the active constituents registered for whitefly control include acephate, bifenthrin, carbaryl, chilli spay, chlorpyrifos, cyfluthrin, dimethoate, endosulfan, fenitrothion, garlic spray, maldison, omethoate, permethrin, pyrethrins and soap sprays. It is important to observe label directions and seek advice from experienced advisers if unsure of the best choice(s).

Biological Control

There are a number of natural enemies of ash whitefly that are important in its ecology. These include parasitoids, predators and diseases. The most successful of these is a small parasitic wasp, Encarcia inaron. In the USA and NZ, very good biological control has been achieved by this wasp. Initial surveys of the Adelaide area have not found E. inaron but as it is the best candidate, it is the highest priority task to introduce this parasitoid into Australia, after AQIS approval, if it has not already accompanied its host into the country

Table 1: Host range of Ash whitefly

Ash Whitefly. Siphoninus phillyeae (Haliday)

Disclaimer

Use of the information/advice in the Fact Sheets is at your own risk. Primary Industries and Resources South Australia, the South Australian Research and Development Institute, CSIRO and their employees do not warrant or make any representation regarding the use or results of the use of the information contained herein as regards to its correctness, accuracy, reliability, currency or otherwise. The entire risk as to the results from the implementation of the information/advice which has been given to you is assumed by you. All liability or responsibility to any person using the information/advice is expressly disclaimed by PIRSA, SARDI, CSIRO and their employees.