Nursery sites for biological agents, established in heavy thistle infested pastures, remain undisturbed for 3 to 4 years

Nursery sites are established in heavy thistle infested pastures

Benefits from biological control of Onopordum thistles

Biological control of Onopordum thistles is leading to greater productivity from formerly heavy thistle-infested pastures in South-Eastern Australia.

  • 27 May 2010 | Updated 14 October 2011

Background

CSIRO Entomology has been searching for effective biological agents to control the spread of Onopordum thistles for the past 20 years. Onopordum thistles have a major impact on agricultural productivity in south-east Australia, where heavy weed infestation reduces pasture growth and livestock production.

The biological control program is aimed at reducing the density of thistle infestations in pastures to a point that makes their economic impact insignificant on farmers. The process involves long lead times, with agent populations taking many years post-release to reach damaging levels.

The biological control program is aimed at reducing the density of thistle infestations in pastures to a point that makes their economic impact insignificant on farmers.

Bio-control typically requires more than one agent to control the target. For Onopordum thistles, a suite of agents that attack different parts or growth stages of the plant have been released.

Four agents released into the field during the 1990s, seed-head weevil (Larinus latus), stem-boring weevil (Lixus cardui), crown weevil (Trichosirocalus briesei), and petiole moth, (Eublemma amoena), have successfully established widespread populations.

In 2000, the crown-feeding fly (Botanophila spinosa) and the thistle flower-head gall-forming fly (Urophora terebrans), were released and their rate of establishment and distribution is being monitored.

A further three agents have been released through the program, two fly species and a rosette fly species, but these have failed to establish in the field.

Biological agents

CSIRO has used a number of biological controls.

Seed-head weevil (Larinus latus)

First released in 1992, adult weevils are black in colour and grow up to 20 mm long. The larvae are the destructive stage, hatching from eggs during the summer months and boring into the flower head, feeding on the plant tissue and developing seeds.

A single larva has the capacity to destroy all the seeds in a 3 cm diameter flowerhead. Mature larvae pupate late summer and the newly emerged adults seek suitable refuges, often the dead stem of the thistle, to over-winter.

The weevil has been become widespread through active redistribution programs initiated by farmers and land owners. Infested thistle heads are readily collected prior to adult emergence and moved to a 'nursery' site that remains undisturbed for an extended period (for example, three to four years).

Stem-boring weevil (Lixus cardui)

First released in 1993, adult weevils grow up to 15 mm long and are brownish to black in colour. Adults emerge in early spring and feed on leaves of Onopordum rosettes. Females lay their eggs into holes bored into the stem, which are plugged after egg-laying.

Newly hatched larvae bore directly into the stem and feed on pith and cambium tissue. Many larvae may develop in a single plant stem. Adults continue to feed on leaf tissue and oviposit until plants senesce and the weevils die. Pupation occurs in the dead stems and new adults hibernate there until the following spring.

Large numbers of adults can cause significant defoliation, weakening the growing plant.

Crown weevil (Trichosirocalus briesei)

First released in 1997, this weevil has one adult generation per year. Adults are 3 to 5 mm long and are mottled brown in colour. Adults emerge from their summer dormancy following autumn rains, and feed on rosette leaves.

Once feeding begins, females lay eggs at the base of the main leaf vein of the Onopordum rosettes. Following egg hatch, larvae eat the heart of the rosette, destroying the growing bud, which may kill the plant or lead to greatly reduced vigour. Mature larvae leave the plant to pupate in the soil. The new adults emerge in spring and estivate in the soil in summer.

Petiole moth (Eublemma amoena)

First released in 1998, the adult moth is mottled light brown and white and 15 mm long. There are three adult generations per year, spring, mid to late summer, and early autumn. 

The larvae attack both bolting plants and perenniating rosettes. The emerging larvae feed within the leaf petiole, eat the collar and bore into the crown of the plant and the root, then leave the plant to pupate on dry leaves or on the soil.

The summer generation has the greatest impact on the Onopordum population as development of the larvae is rapid and the perenniating rosettes cannot compensate with a comparable growth rate.

Crown-feeding fly (Botanophila spinosa)

First released in 2000, adults lay eggs in the heart of Onopordum rosette from March to until they die in mid-spring. The developing larvae feed on the rosette bud and form pupae at the end of spring.

The adults emerge then estivate and hibernate over the summer months. Feeding by larvae reduces the growth of Onopordum rosettes during spring, which results in less vigorous growth of flowering stems and reduced seed production.

The impact of this fly complements the feeding behaviour of the crown weevil by extending the period of attack by biological control agents.

Thistle flower-head gall-forming fly (Urophora terebrans)

First released in 2000, adults lay eggs in the flowering heads in spring. Larvae develop in a gall formed on the receptacle of the head and mature pupae pupate in the gall.

The newly emerged adults mate and lay eggs in secondary buds of the thistles. This second generation over-winters in the galls as pupae and emerge as adults the following spring. Heavy gall set can lead to weakness and loss of vigour in the growing plant.

Learn more about Ecology and management of Australian weeds.