Emex (Emex australis)

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Alternative names: doublegee, three-cornered jack or spiny Emex

Background

Emex australis, commonly called doublegee in Western Australia (but also referred to as three-cornered jack or spiny Emex in the eastern States of Australia), is originally from South Africa. It was intentionally imported into Western Australia (WA) in 1830 as a vegetable (Cape spinach). Once in Australia, it rapidly spread so that it now occurs throughout Australia's southern-temperate regions. It is an annual weed that competes with crops and pastures and is estimated to cost $40 million a year in crop losses/production costs in WA alone. A single plant is capable of producing over 1000 burrs which can contaminate agricultural produce such as wool, grain and dried fruit.

Lesser jack (Emex spinosa) is also a problem in restricted areas within southern Australia and looks similar to E. australis, but has more erect stems and smaller burrs. Seeds from both Emex species can remain dormant in the soil for more than seven years and this dormancy, together with rotational cropping-grazing farm practices, can make control of this weed a problem. Typically, seedlings from both Emex species are selectively and effectively killed by broadleaf herbicides within the crop phase of a cropping-grazing farming system but seed banks build-up during the pasture phase, when suitable herbicides are often not applied. Many herbicides that control the Emex species will also damage other beneficial broadleaf-pasture species and/or the growers are not willing to expend resources on weed control during the non-cropping phase due to inherent lower economic returns due this phase.

The difficulty experienced in controlling E. australis is one reason why a biological control program was started in 1974.

The Project

Biological Control of Emex
Biological control is a long term control option for weed management and should be seen as part of an integrated weed management strategy. Biological control never eradicates a weed problem but reduces infestations to levels where they are no longer of economic importance. Such reductions in intensity of infestations are a gradual process and can take at least 10 years before weed populations begin to decline. This is because most weeds in pasture/cropping systems have been in Australia for many decades and have built up vast numbers of long lived seed in the soil. This seed bank must be depleted before any impact on overall weed intensity can be achieved. The old adage of one year's seeding, seven years weeding is certainly true in the case of Emex.

The biological control program for Emex started with the release of the weevil Perapion antiquum. Although this species was highly successful in controlling Emex in Hawaii, it failed to establish in Australia due mainly to our harsh summers. A second weevil, Lixus cribricollis, was collected from E. spinosa in Morocco and released in Western Australia in 1981. This species also appears to have failed to establish.

More recent studies have concentrated on E. spinosa populations in Israel where summer extremes are similar to those in Australia. One such potential biological control agent, red apion (Apion miniatum), was subsequently imported into an Australia quarantine facility and screened to ensure it was safe to release in Australia. The Australian regulator bodies approved the insect's release within Australia and the inaugural release onto E. australis field populations occurred in 1998.

The Red Apion, Apion miniatum
The red apion appears ideally adapted for our Australian conditions as the adults become dormant over summer. In this state they do not need to eat and it is believed they hide, inactive, in remnant vegetation and wait until the season breaks in the following autumn. This adaptation has enabled them to withstand the harsh dry summers in habitats such as the Negev Desert, Israel. Red apion is highly specific, attacking only E. australis, E. spinosa and some introduced weedy docks (Rumex spp.).

When newly emerged (late spring/early summer), red apion adults are light tan in colour and are about 3mm long. For the first few weeks they feed on Emex foliage causing a 'shot hole' effect. They gradually change from tan to red. At this time, the host plants are senescing and young red apion adults fly away to seek over summering refuge sites (tree trunks). In the mediterranean-type environment, no Emex (or Rumex) foliage is available to red apion until the autumn rains commence. The rains, together with a suitable after-ripening period, cause the Emex seeds to germinate. Red apion adults fly back onto the Emex plants, feed, mate and start to lay eggs. Each female is capable of producing several hundred eggs which are laid into the leaves, stem and petioles of the plant. The development from egg to adult takes up to four months and there is one generation per year.

The eggs hatch a few days after being laid and the larvae tunnel throughout the plant damaging the roots, crown, stems and petioles. Under heavy infestation the stems collapse, the leaves wilt and the flowers die. Although red apion adults damage the leaves whilst feeding, it is the larvae that are considered the most destructive stage, reducing plant vigour and thus indirectly decreasing seed production.

Laboratory studies with another biological control agent (the dock aphid) found that the E. australis produced less dormant seed when attacked by the insect. Assuming seed dormancy reduction also occurs as a result of red apion attack, control of Emex becomes feasible if an integrated weed management system can be developed; biological control agents controlling the weeds in the pasture phase and the farmer spraying out the remnant plants in the crop phase.

Establishment of red apion within Australia
CSIRO developed methods of mass rearing red apion and then conducted release trials in an effort establish the insect within Australia. The majority of trials were within the WA wheat belt but releases also occurred in other areas of WA and in NSW and SA. Between 1998 and 2003, over 55,000 red apion adults were distributed over more than 60 sites with E. australis populations. In most cases, the newly emerged, red apion adults were stored within CSIRO laboratories over the summer period and then between 50 and 8,000 individuals released as young egg-bearing adults at the E. australis sites in the autumn. These red apion adults produced up to 113,000 offspring per site during the year of their release but the resulting offspring appeared to have been unable to survive the following summer as, to date, there have been no confirmed reports of established populations of red apion anywhere within Australia. For many other biological control programs, agents were also thought to have not established but they had actually just remained below detectable levels (due to the vast expanse of host plants) for several years. It is hoped that this is also the case for red apion but as time passes the likelihood of this diminishes.

The summer conditions of the sites where red apion were collected from in Israel are drier and hotter than the sites where red apion have been released within Australia. In both countries, red apion must survive the entire summer in the absence of its winter growing host plant, Emex. We know they can survive these conditions in Israel so why are they apparently not surviving in Australia?

Recent studies on the behaviour of red apion over summer discovered that although the adults are in a reproductive diapause (ie. don't mate) over summer, they will feed on nectar if it is available. Populations of red apion provided with access to Tamarix plants (summer flowering trees) over summer had 92% survival compared to only 1% survival in populations kept under identical conditions but without access to nectar-bearing plants. In Israel, Tamarix plants are common, often growing in areas near E. spinosa. In Australia, all releases prior to and including 2003 have been in areas without Tamarix plants. It is not known whether other species of plants also produce nectar suitable for red apion to survive upon over-summer, but in general summer flowering plants are not common in Australia.

All Australian releases have also only been on populations of E. australis whereas the natural host of red apion in Israel is E. spinosa. Under laboratory conditions, both E. spinosa and E. australis are suitable hosts for red apion however it is possible that subtle differences exist between the species and that this results in offspring that are not as adapt to survive the summer period (eg. inadequate fat reserves etc.).

Mass rearing of red apion ceased in 2004 but, in a final attempt to establish red apion in Australia, 7000 red apion have been released at a South Australian location that contain co-existing populations of E. spinosa, E. australis and Tamarix aphylla trees. The results of this release have not been assessed yet.

Future
Although mass rearing of red apion has ceased, if it becomes established anywhere within Australia, then it is anticipated that individuals can be recollected from these sites and redistributed to other similar locations. Information gained during the red apion release program will be invaluable for improving the screening process used for selecting and/or establishing any future potential biological control agents for Emex.

CSIRO has not given up hope for finding an biological control agent for Emex that can survive Australian conditions. We are currently seeking funds to import and test a Moroccan stem feeding weevil, Perapion neofallax, that seeks refuge from the summer heat beneath the ground, and a fungus, Cercospora tripolitana, that has been observed devastating young E. spinosa plants in Tunisia.

Key People

More Information

Emex australis Biology, Management and Research -Weeds CRC (PDF 640KB)

Background | The Project | Key People | More Information | Publications