Managing crop pests and diseases

Managing crop pests

One of the most effective ways to minimising the risks and impacts of crop pests is to remove green bridges from the cropping area.

A ‘green bridge’ refers to weeds and crop volunteers that serve disease and insect pests by providing a habitat to survive on from one cropping season to the next. This vegetation can grow in the paddock, alongside fence lines, headlands, roadsides and non-crop land.

Green bridges can include:

• germinated seed from the previous crop
• local weeds germinating from summer rainfall
• germinating weeds blown in from nearby areas. 

Grasses and broadleaf weeds are both significant, however particular pest species are known to have preferred hosts to use as green bridges over the summer. 

It takes a community effort to control green bridges. Rust spores, aphids and mites all travel on the wind, easily crossing farm boundaries. 

Ideal timing of crop pest control should interrupt the life cycles of a number of pests and should be at least 2-3 weeks before sowing begins. 

Failure to control green bridges increases pressure on insecticides, fungicides and genetic resistance in crops. As it allows potentially resistant populations to continue to breed over the summer and pass on their resistance to following generations.

What to look for

This video explains how to identify the key crop pests, the benefits of parasitic insects, and the importance of integrated pest management.

videocam 5:28

Common crop pests and diseases

Armyworm – are responsible for the majority of larval damage to crops. Monitor crops at least fortnightly during vegetative stages and then increase to weekly if larvae are found as the crops begin to deteriorate. Look for signs of caterpillar droppings (frass) on the ground, damage to leaves and plant heads.

Locusts – can migrate over large distances. Plague locust swarms can devastate crops and cause major agricultural damage. It is important to know how to identify, monitor and control locust populations before they form swarms.

Crown rot – is a major disease affecting cereal crops in NSW and has been known to cause significant yield losses, particularly in seasons where favourable conditions early in the crop season are followed by a dry finish. The fungus survives on cereal stubble and grass weeds and is passed onto susceptible hosts. Managing crown rot involves a combination of variety selection, break crops and good crop management.

Lupin anthracnose – is a serious disease of lupins. It caused by the fungus Colletotrichum lupini, previously known as Colletotrichum gloeosporioides. The disease has potential to cause complete crop losses in susceptible varieties.

Controlling weeds in crops

The aim of all weed control operations is to reduce the existing seed bank of weed seeds over the long term. 

Weed management strategies are often different to those for insects where control measures are avoided until populations reach an economic threshold at which the cost of insect damage exceeds cost of control.

Low insect populations are tolerated, however even very low populations of weeds still have implications for the farming system as they replenish the seed bank and may encourage development of herbicide resistance.

Pre-harvest herbicides

Spraying and chemical applications are the main form of weed control in crops pre-harvest.

videocam 4:53

Just like any investment, it is important that every step is taken to maximise the chance for each herbicide application to succeed. This involves getting a couple of things right including:

• spraying when weeds are small or more susceptible
• using high water rates
• incorporating appropriate nozzles for the situation
• spraying in appropriate conditions
• rotating chemistry
• using the correct herbicides for your weed spectrum.

Cold temperatures and frosts can impact in-crop sprays. Environmental conditions slow down the initial absorption of herbicides into the plant, meaning less chemical is taken in.

Frosts can shut down plants for extended periods of time and applying herbicides while this occurs can lead to increased crop damage. This is because crops rely on quickly metabolising herbicides to avoid incurring damage.

Due to reduced crop growth, frost events impact the production of P450 enzymes. These enzymes are responsible for the breakdown of toxic compounds which means that when they are impacted, herbicides are deactivated more slowly within crop plants and damage occurs.

On the other hand, herbicides applied close to frost events do not work as well because of poor absorption rates by targeted weeds. Cold temperatures are particularly impactful on glyphosate, group A and group G herbicides as they require leaf uptake.

Generally, as temperature increases, the rate of uptake of herbicides increases as long as the conditions do not limit respiration or photosynthesis of plants.

At harvest

Harvest is an important time for weed control. Weeds which have “escaped” chemical applications throughout the season are producing seeds. Reducing the spread of these seeds will potentially reduce the weed burden for following seasons. 

Weed seeds are often picked up by headers during harvest, depending on the weed seed size and the crop being harvested. A lot of these weed seeds end up being blown out the back of the header and redistributed across the paddock with the chaff.

Harvest weed seed control (HWSC) includes several strategies which can include:

• windrow burning
• chaff carts
• seed destruction
• chaff lining and chaff decking.

Chaff lining and chaff decks

Weed seeds can be captured in the chaff as they pass through the header by separating it from the straw. Chaff is then compiled either on the wheel tracks, in the case of chaff decks, or in a singular row behind the header, in the case of chaff lining.

Chaff is placed in the same spot each harvest forcing weeds to migrate over multiple seasons. These weeds are effectively placed in “jail” and strips of weeds begin to appear across the paddock where producers can use heavy-handed tactics to obliterate weed numbers, such as:

• shielded sprayers
• higher chemical rates
• stock.

When placed on controlled traffic tracks, weeds also encounter a hostile environment, where they compete less with the crop. Escapes from these tracks will need to be monitored.

The process to modify a header for chaff lining or decking is relatively straightforward but can be slightly different depending on the colour and model of the header.

A baffle is placed to catch chaff as the fan blows it out the back. This baffle is angled to force chaff to fall straight down while the straw can pass over the top.

Chaff decks or chaff liners come in a variety of shapes and sizes. As a result, cost can vary depending if the setup is homemade or bought as part of a kit.