Weeds can invite insects

Weeds can promote insect problems

Opinion
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Vegetable growers who have received early summer rainfall should be on the lookout for weed growth that can harbour insect pests.

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BE ALERT: Budworm larvae can be real pests.

BE ALERT: Budworm larvae can be real pests.

Vegetable growers who have received early summer rainfall should be on the lookout for weed growth that can harbour insect pests. 

Weeds are an important resource for a wide range of insects, whether they are beneficial or pests. 

Depending on which insects they are, the presence of these weeds can have either a positive or negative impact on crop productivity. 

Whether or not to apply a herbicide to these weeds can be a difficult call to make depending on the pest threshold and the presence of beneficial insects.

BE MINDFUL: Keep an eye out for Helicoverpa moths.

BE MINDFUL: Keep an eye out for Helicoverpa moths.

Despite the multitude of interactions between weeds and insects there is to consider, some aspects of the relationship are quite predictable in that most crop-pests are specialists. They tend to be adapted to feed only on some plants, often within a single plant family.  Therefore, weeds that are closely related to crops do tend to harbour insects that attack those crops.

For example, if you are growing cabbage, cauliflower, broccoli, Brussels sprouts or any of the brassica cole crops, the emergence of brassica weeds like wild radish, turnip weed, Indian mustard would be reason for concern. Some major pests to keep an early lookout for in summer are heliothis and diamondback moth.

Diamondback moth (DBM) Plutella xylostella is a particularly destructive pest of brassica crops and resistance to a number of registered insecticides is now a challenging problem.

Look for

Grey-brown and white markings on each moth wing forming a diamond shaped pattern when the wings are held together at rest.

Moths grow up to 10 millimetres long and lay greenish coloured eggs in groups of up to six on the lower surface of the leaf.

Full-grown larvae are smaller (about 10-12mm) than most other common caterpillar pests found on brassica crops. With the body being tapered at either end and the pro-legs on the last rear segment form a distinctive V-shape.

When disturbed the larvae wriggle and jump violently.

DBM management 

It is recommended to inspect randomly selected crop plants and brassica weeds surrounding any crops at least weekly.

Use spray thresholds. It is a good idea to have a planned approach with your neighbours (within several kilometres) to have a total break from crucifer production for two to three weeks in early to mid-summer to starve out DBM.

Heliothis (Budworms) Helicoverpa armigera, Helicoverpa punctigera are particularly important for sweet corn, lettuce and tomato growers, but also damage many other crops.

Look for

Larvae that can vary in colour from green, yellow, red, brown to almost black. They grow to about 35mm long and have stripes down their length. The adult moths are buff coloured to red-brown and have a wing span up to 40mm.

Heliothis management 

It is recommended to apply recommended insecticides with the aim of not disrupting beneficial species. Ensure thorough coverage of insecticide sprays.

Make sure insecticides are timed correctly, i.e. before larvae can enter the plant and when the larvae are small. The moth pupates in the soil and may be destroyed by cultivation.

Use effective crop hygiene practices. Early cultivation and burial of weed or crop residues will reduce insect numbers.

Resistance to synthetic pyrethroid insecticides has been identified in DBM populations from vegetable growing areas in all states as well as resistance to organophosphate insecticides.

Given this, it is essential to implement an anti-resistance strategy. This is achieved by rotating insecticide products with a different mode of action so to prevent overuse of any one chemical class and so they can all remain effective for as long as possible.

CropLife Australia, in consultation with researchers, has devised an resistance management strategy for DBM. Details are on the CropLife website.

  • Scott Mathew is senior technical services lead at Syngenta Australia. Email him at scott.mathew@syngenta.com
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