PestFacts WA

Seeing chewing damage in sprayed canola crops but can’t find the culprit?

• Bluff Point
• Tarin Rock
• Amelup

Canola with feeding damage. Photo courtesy of: Kim Gooding (Kukerin Rural Services).

The PestFacts WA team are receiving reports of germinating canola crops across the grainbelt having chewing damage but growers are having difficulty finding the culprit pests to make an identification.

Growers need to bear in mind that applying an insecticide with the knockdown, and even applying a follow up insecticide post sowing pre-emergence (PSPE) bare earth, does not ensure total pest control.   

The main reasons for this are usually:

1. Pests such as redlegged earth mites (RLEM) and lucerne flea hatch from eggs in autumn and these eggs are not killed by insecticide sprays. If this occurs late, relative to PSPE spray application, then these pests hatch when plants have little to no protection. A post emergent spray may be required, but identification of the pest species is important as effective insecticides and rates differ.
2. Where PSPE sprays effectively protect seedlings from ground dwelling pests such as mites and lucerne flea, the germinated plants have no insecticide coverage and are susceptible to flying pests such as moths. Moths can fly in at night out of sight, lay eggs, and emerging small caterpillars then begin to damage seedlings.
3. Some pests such as weevils, European earwigs and snails can be very difficult to kill with insecticides and require a specific approach.
4. Insecticide resistance in pests such as RLEM and Green peach aphids is well known. If you experience a spray failure and suspect resistance, please contact the PestFacts WA team via email or on +61 (0)404 819 534.

Checking emerging crops for insects

So as crops emerge it is important to remember that differences in pest biology, as well as seasonal differences, means that not one tactic or approach will effectively control pest issues in all paddock situations.

The most effective strategy is to inspect crops during the first three to five weeks after sowing. This is key to early detection of pest issues and early intervention, if necessary, to ensure crops get away to a healthy start.

The following tips can be used to identify damaging insect pests in emerging crops:

• Look up and down the seeding rows for missing plants that may have been chewed off at the base or plants with visible chewing damage.
• Look for insects around and under chewed or missing plants. Don't forget to check under the stubble in the inter-rows.
• What type of damage has the plant sustained?

Understanding the preferred feeding methods and mouth parts of insects can help narrow the lists of culprit insects. For example chewing damage above or below ground is the feeding method favoured by caterpillars (such as webworm or cutworm), European earwigs, weevils and other beetles. Though European earwigs usually leave jagged cuts in the leaf which can be confused with slug/snail damage; whereas caterpillars if immature, leave windows in the leaf which can be confused with lucerne flea damage.

Sucking-type damage appears as wilting and yellowing of plants – it’s usually due to aphids and Rutherglen bugs who have piercing and sucking mouthparts. These pests are easily found.

Still can't find a pest?

Mites and lucerne flea can be smaller than you think, especially when just recently hatched so consider using a hand lens when checking crops.

Place a pitfall trap near plants with damage, this is just a plastic cup dug into the ground so the top is flush with the soil surface. Put about 10mL of water into it and come back after 24 hours.

This has been especially useful in detecting the presence of weevils, slaters and European earwigs. European earwigs are one of the few pests that if numbers are high, can damage well established crops especially if they chew through the stem.

However, a single pitfall trap may not be sufficient, look to place at least three to five pitfall traps. A single nights trapping might not be enough-so consider keeping the trap out for longer.

If damage is still occurring, keep in mind it might not be invertebrates, there are many reports of mouse activity in the regions. To view recent mouse activity or make a report refer to Feral Scan’s Mouse Alert.  

Have you found an insect in the paddock that you have not seen before? It could be an incidental or it could be a new pest species. Consider taking the time to get it identified. It is hard to identify an insect from a verbal description. But there are a few things you can do. Have a phone or tablet that takes photos? Take a photo and email it to pestfax@dpird.wa.gov.au . We also encourage taking a photo and attaching it to your report when using the PestFax Reporter app.

For more information on monitoring insects in emerging crops refer to;

• DPIRD’s Monitoring insects and other crop pests page
• GRDC’s Monitoring for insects in canola. Part 1 video
• GRDC’s updated I SPY manual.

For more information contact Research scientist Svetlana Micic, Albany on +61 (0)8 9892 8591.

Article authors: Svetlana Micic (DPIRD Albany) and Dustin Severtson (DPIRD Northam).