Biological control

Plants that have become weeds in Australia are rarely invasive and troublesome in their natural range. This is often because natural populations are regulated by a variety of natural enemies such as insects and pathogens (disease-causing organisms like fungi and bacteria) that attack the seeds, leaves, stems and roots of a plant. If plants are introduced to a new region that does not have these natural enemies, their populations may grow unchecked to the point where they become so prevalent that they are regarded as weeds.

What is biological control?

The biological control approach makes use of the invasive plant’s naturally occurring enemies, to help reduce its impact. It aims to reunite weeds with their natural enemies and achieve sustainable weed control. These natural enemies of weeds are often referred to as biological control agents.

It is critical that the biological control agents introduced into Australia do not become pests themselves. Considerable testing is done prior to the release of biological control agents to ensure they will not pose a threat to non-target species such as native and agricultural plants.

Although in the long term, biological control can be cost effective and can reduce the need for less desirable management practices, not all weeds are suitable for biological control. Developing a biological control project requires a substantial investment, sometimes costing millions of dollars over many years.

An early success in biological control of weeds in Australia was the use in the 1920s of the Cactoblastis Moth (Cactoblastis cactorum) to control Prickly Pear (Opuntia stricta), which at the time was smothering large tracts of north-east Australia, and spreading rapidly each year. The larvae of the Cactoblastis Moth eat the leaves and seed pods of the Prickly Pear. The release and spread of Cactoblastis Moth in Australia virtually destroyed Prickly Pear populations.

There have been several other successful biological control programs in Australia. Insects that attack leaves, fruits or stems have been released, following stringent screening, to control weeds such as Skeleton Weed, Bridal Creeper, and Salvinia. There is also major research being undertaken on biological control for a number of other weed species.

Procedure to import and release a biological control agent

There are some well known examples of biological control programs that have been unsuccessful (such as the introduction of Cane Toads to control Cane Beetles).  To avoid such problems in future, the process for approving biological control agents is much more rigorous now than it has been in the past.

Before commencing the search for biological control agents, agreement needs to be sought with initial application through the Invasive Plants and Animals Committee to target the weed species for biological control.

The next step is to undertake host specificity testing. This is a requirement under the Australian Government Department of Agriculture, Water and The Environment to ensure that the agent will not damage native flora or agricultural stock or crops. Once host specificity testing is completed, biological control agents must be assessed to meet the requirements of the Quarantine Act 1908 and the Environment Protection and Biodiversity Conservation Act 1999 (the EPBC Act) before approval can be given for their introduction and release in Australia.

To meet the requirements of the Quarantine Act 1908, the Department of Agriculture and Water Resources assesses the risk regarding the proposed importation and release of biological control agents via a risk analysis, based on the results of host specificity testing.

The Australian Government Department of the Environment and Energy has a parallel approval process under the EPBC Act, whereby a report produced by the Australian Government Department of Agriculture and Water Resources – Biosecurity may be used by the responsible minister in making a determination to include the item on the list of specimens suitable for live import (the live import list).

Cultural control

Cultural control is usually associated with farming systems, although some elements are relevant to landscape and bushcare practices. It largely involves manipulating farming practices to suppress weed growth and production, while promoting the development of the desired plant.

The principles and techniques used to prevent weed spread are relevant to cultural control methods to limit the spread of weeds between different land areas.

Cultural control methods

Encourage the competitiveness of desired species that are more competitive and fast growing. This suppresses weed growth by reducing access to available sunlight, nutrients and moisture and can include:

  • Choose plant and crop species or cultivars that are naturally more competitive. This can include using plant species that suppress other plant species by the release of toxins.
  • Use high quality (large and plump) seeds, as they are more likely to produce vigorous and competitive plants.
  • Use increased seeding rates and narrow row spacing.
  • Use shallow seeding techniques, where possible, to allow the desired species to grow above the soil surface more quickly.
  • Ensure the desired plant is placed in the optimum growing environment.
  • Use fertilisers in the optimal growth period to encourage rapid growth of the desired species.
  • If possible use plant species that are native to the local environment.

Make it hard for weeds to adapt to weed management techniques. Using the same land management routines year after year may result in weeds adapting to these practices. Some practices that make it hard for weeds to adapt and therefore reduce their spread and vigour include:

  • Rotate crops: if a weed has adapted to grain crops continuously being sown, then alternating with a broadleaf crop will remove the environmental condition to which the weed has adapted.
  • Rotate species with different seasonal and growing cycles.
  • Rotate herbicides with different modes of action to help delay the development of herbicide resistance.

Physical control

Physical control is the removal of weeds by physical or mechanical means, such as mowing, grazing, mulching, tilling, burning or by hand. The method used often depends on the area of weeds to be managed, what the land is used for, physical characteristics and the value of the land.

It is important that, when using physical control, any item that can move from a weed-infested site to an un-infested site, such as machinery, vehicles, tools and even footwear, is cleaned free of weed seed before moving, to stop the spread of weeds to new areas.

As with most control methods long-term suppression of weeds requires follow up weed prevention.

Hay making, mowing and grazing

Hay making, mowing and grazing before weeds produce seeds restrict the amount of weed seed in an area and reduce the spread of weeds.

Mulching

Mulching, by covering the ground with a layer of organic material, suppresses or kills weeds by providing a barrier between the weeds and sunlight. Mulching has an added advantage in that it improves the condition and moisture level in the soil. Planting competitive and desirable plants that provide a dense cover over the weeds suppresses weed growth in a similar way to mulching.

Tilling

Tilling, the ploughing or cultivation method that turns over the soil, buries the weed beneath the soil. This provides a barrier to the sun, therefore killing the weeds. Tilling is a form of physical control that can be easily undertaken over a wide area, using agricultural machinery. This method is useful for making soil ready for planting new crops, but it can lead to damage in soil structure and exposes the soil to erosion and further invasion by weeds.

Burning

Burning removes the above-soil body of the weeds killing most of the plants. If carried out before seed is set it can prevent the further spread of weeds. Burning can be undertaken over a wide area with minimal human input. As with tilling, burning exposes the soil surface to erosion. If burning is used as a control method, caution should be exercised to minimise the risk of harm to the environment and to those undertaking the activity.

Hand removal

Removal by hand, including hoeing, is a good method for selective removal of weeds without disturbing the surrounding desirable vegetation. It is very labour-intensive and is often only used in small areas, such as gardens or in larger areas during bush regeneration.

Chemical control

Although the use of chemicals is not always essential, herbicides can be an important and effective component of any weed control program.

In some situations herbicides offer the only practical, cost-effective and selective method of managing certain weeds. Because herbicides reduce the need for cultivation, they can prevent soil erosion and water loss, and are widely used in conservation farming.

In some cases, a weed is only susceptible to one specific herbicide and it is important to use the correct product and application rate for control of that particular weed. Common mistakes include incorrect identification of the weed or using inappropriate products.

In most cases, weeds must be actively growing to be vulnerable to herbicide treatments.

Herbicide resistance can also be an issue with some species.

Conditions such as wind speed and direction, the possibility of rain and proximity to waterways should also be taken into account when preparing to use herbicides.

Label information

It is extremely important to read and follow the information contained on the herbicide label. This includes:

  • the signal heading (indicating the product’s hazard level)
  • the trade name
  • the claims for use
  • the active constituent
  • the net contents
  • directions for use
  • limitations for use
  • withholding period
  • important notes
  • storage instructions
  • safety directions and first aid
  • dangerous goods notification
  • expiry date
  • mode of action (type of herbicide)

By law, herbicides can only be used in accordance with the label.

The Australian Pesticides and Veterinary Medicines Authority (APVMA) is the Australian Government authority responsible for the independent assessment and registration of pesticides and veterinary medicines. The APVMA keeps a record of all registered pesticides in Australia, and their approved uses and also reviews older chemicals to make sure that they continue to meet contemporary high standards.

How herbicides work

A herbicide is a chemical that affects plants. Although there are a large variety of herbicides available they function in a limited number of ways, known as modes of action. These modes of action determine how the herbicide controls weeds:

  • by speeding up, stopping or changing the plant’s normal growth patterns
  • by desiccating (drying out) the leaves or stems
  • by defoliating the plant (making it drop its leaves).

The well-known herbicide glyphosate, for example, prevents the target plant making key amino acids.

As well as using different modes of action, herbicides can be classified according to how they are taken up by the plant. The main types are:

  • Contact – these kill plant tissue at or near the point of contact with the herbicide (they do not spread around the plant). Therefore, they require even coverage in their application.
  • Systemic – these move through the plant tissues via the plant’s circulation system, and can be injected into the plant.
  • Residual – these can be applied to the soil in order to kill weeds by root/shoot uptake. They remain active in the ground for a certain length of time, and can control germinating seedlings.

Herbicides also have differing selectivities, and can be categorised as either broad spectrum (working on a wide variety of plants) or selective (working on a specific range of plants).

For example some herbicides are effective on grasses, whereas others are more effective on woody weeds and will leave grasses intact to provide competition against re-establishment of the weeds.How to apply herbicides

There are several techniques that can be used to apply herbicides. Some of the most common are outlined below.

Foliar spraying

Here, the herbicide is diluted with water or another diluent as specified on the product label, and sprayed over the foliage to point of runoff (until every leaf is wetted, but not dripping).

The method is most suited to shrubs, grasses and dense vines less than 6 m tall so that complete coverage is achieved. Advantages include speed and economy. Disadvantages include the potential for spray drift and off-target damage.

Foliar spraying can be done in a number of ways, depending on the size of the weed plant or the infestation. Blanket spraying, using a boom spray from a tractor or aircraft, can be used to treat areas completely infested with weeds, especially with selective herbicides.

For large infestations that need targeted applications of herbicide, a hose and handgun can be used to spray solution from a herbicide tank and pump carried by a tractor or vehicle. Smaller infestations can be sprayed using a backpack/knapsack spray unit. Spot spraying is used to treat individual weed plants or areas that only have small clumps of weed infestations.

Basal bark spraying

This method involves mixing an oil soluble herbicide with a diluent recommended by the herbicide manufacturer and spraying the full circumference of the trunk or stem of the plant. Basal bark spraying is suitable for thin-barked woody weeds and undesirable trees.

Basal bark spraying is also an effective way to treat saplings, regrowth and multi-stemmed shrubs and trees. This method works by allowing the herbicide to enter underground storage organs and slowly kill the targeted weed.

The whole circumference of the stem or trunk should be sprayed or painted with herbicide solution from ground level to a height of 30 cm. It is important to saturate the full circumference of the trunk, and to treat every stem or trunk arising from the ground.

Basal bark spraying is a very effective control method and is a good way to tackle inaccessible areas such as steep banks. This method will usually kill difficult-to-kill weeds at any time of the year, as long as the bark is not wet or too thick for the solution to penetrate. The work is often best performed by contractors.

Stem injection

Stem injection involves drilling or cutting through the bark into the sapwood tissue in the trunks of woody weeds and trees. Herbicide is immediately placed into the hole or cut. The aim is to reach the sapwood layer just under the bark (the cambium growth layer), which will transport the chemical throughout the plant.

It is essential to apply the herbicide immediately (within 15 seconds of drilling the hole or cutting the trunk), as stem injection relies on the active uptake and growth of the plant to move the chemical through its tissue.

Stem injection methods kill the tree or shrub where it stands, and only trees and shrubs that can be safely left to die and rot should be treated this way. If the tree or shrub is to be felled, allow it to die completely before felling. The use of chainsaws, particularly in the felling of trees, is a dangerous activity that should only be undertaken by an appropriately trained person.

One method is the ‘drill and fill method’ also referred to as tree injection, and is used for trees and woody weeds with stems or trunks greater than 5 cm in circumference. A battery-powered drill is used to drill downward-angled holes into the sapwood about 5 cm apart. The placement of herbicide into the hole is usually made using a backpack reservoir and syringe that can deliver measured doses of herbicide solution.

Another method is the ‘axe cut method’ which involves cutting through the bark into the sapwood tissue in the trunk, and immediately placing herbicide into the cut. This method can be used for trees and woody weeds with stems or trunks greater than 5 cm in circumference. Using an axe or tomahawk, cuts are made into the sapwood around the circumference of the trunk at waist height. While still in the cut, the axe or tomahawk is leaned out to make a downward angled pocket which will allow herbicide to pool. The herbicide is then immediately injected into the pocket. Cuts should be made no further than 3 cm apart. This method of using an axe to make the cut is often referred to as frilling or chipping. A hammer and chisel can be used to make the pocket cuts, or a spear to make cuts in the trunk closer to ground. It is important not to entirely ringbark the trunk, as this will decrease the uptake of the herbicide into the plant.

Cut stump application

Here, the plant is cut off completely at its base (no higher than 15 cm from the ground) using a chainsaw, axe, brush cutter or machete (depending on the thickness of the stem/trunk). A herbicide solution is then sprayed or painted onto the exposed surface of the cut stump emerging from the ground, with the objective of killing the stump and the root system.

It is imperative that the herbicide solutions are applied as soon as the trunk or stem is cut. Refer to the product label instructions for information on timing, as delayed application will give poor results.

Two operators working as a team can use this method effectively. The herbicide can be applied from a knapsack, or with a paint brush, drench gun or a hand-spray bottle. It is a good idea to use a brightly coloured dye in the solution to mark the stumps that have been treated.

For trees with large circumferences, it is only necessary to place the solution around the edge of the stump (as the objective is again to target the cambium layer inside the bark). The stump circumference should be bruised with the back of an axe and each successive blow treated with herbicide.

This method has the appeal of removing the weed immediately, and is used mainly for trees and woody weeds. This method is also referred to as cut and spray or cut and paint.

Cut and swab

This method is similar to the cut stump method, but is suited to vines and multi-stemmed shrubs. Here, the plant stems are cut through completely, close to the ground. Herbicide is then applied immediately to the cut surface emerging from the ground, via spray or brush application.

In the case of Madeira Vine and some other vines with aerial tubers, both ends of the cut stems must be treated with herbicide. An effective way of doing this is to hold both ‘bunches’ of cut stems in a container of herbicide for 15 seconds after cutting, so that maximum translocation occurs to both ground and aerial tubers. Extra care should be taken when doing this to ensure spillages do not occur.

Stem scrape

Stem scraping is used for plants and vines with aerial tubers. A sharp knife is used to scrape a very thin layer of bark from a 10 cm section of stem. Herbicide is then immediately applied to the exposed soft underlying green tissue.

This method is also called bark stripping or stem painting. Some woody weeds can have their bark surface peeled away and the exposed wood painted or sprayed with herbicide.

Wick application

This method of applying a herbicide consists of a wick or rope soaked in herbicide from a reservoir attached to a handle or assisted with 12 volt pump equipment. The wetted wick is used to wipe or brush herbicide over the weed.

Ongoing maintenance

An important aspect of the success of any weed control program is the ongoing maintenance, that is the follow up actions that occur after the initial weed control has occurred. The lack of ongoing maintenance can often be the major failure of weed control programs.

Ongoing maintenance requires a long-term commitment but with careful management weed problems will reduce over time. An example of a long term commitment is planting well designed farm forestry and revegetation belts to effectively catch weed seed-heads and prevent them from spreading.

Leaving a disturbed space after the removal of a weed often results in the same or another weed filling the space. For example, it is important to assist natural regeneration or revegetate treated area in natural bushland areas or ensure that appropriate crop or pasture species are planted using correct methods.

Monitor the site for reinfestation and treat the weeds as they appear. Photo points, records and maps of work done will help to determine the success or failure of weed control work.

Select suitable plants for revegetation. The success of bush regeneration programs is increased by using species that are native to the local area. Greening Australia’s FloraBank website, provides information on species selection for bush regeneration projects.

If you find a weed, report it to the local weed control agency or those managing the area so that infestations can be treated where feasible.