Maize for Silage
Insight 316


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Argentine stem weevil (Listronotus bonariensis) ASW occurs throughout New Zealand and is a major insect pest of grasses and cereals including maize. It is responsible for a substantial decrease in plant populations in some maize crops. This bulletin gives details on the life cycle of the ASW, describes the symptoms of ASW damage in maize and gives a range of control options.


Adult weevils are 3 mm long, hard bodied and light to dark grey brown in colour. They have a distinct snout and three whitish stripes on the thorax (Fig 1).

Figure 1: Argentine stem weevil adult

Eggs are small, 0.5 - 0.75 mm long, cylindrical, and greenish black in colour. They are laid in the leaf sheath tissue close to the base of the plant.

Figure 2: Argentine stem weevil larva

Pupae are white with non-functional legs. They are found in the top 1 – 2 cm of the soil surface.


ASW completes two generations per year in most parts of New Zealand. The first generation develops during the spring-early summer and the second in summer-autumn. The weevil over-winters as an adult. First generation egg laying begins any time from late July to September depending on climate. First generation larval numbers peak in October to November, coinciding with the main planting period for maize. New adults begin emerging and laying eggs from early December onwards. Egg laying ceases in early March when the females enter a winter resting stage. It is this summer generation which can severely damage pastures.


Maize can become infested with ASW larvae if it is sown too soon (less than four weeks) after cultivation from pasture, annual ryegrass or cereal crops, or in previously cropped ground where grass weeds, especially Poa annua, are present. Partly developed larvae transfer from the decaying grass tillers and tunnel into the young maize plants. Plants can be killed before they emerge while emerged plants have symptoms similar to that of moisture stress. The centre leaves turn a blue-grey colour and if they are gripped, they pull out of the plant very easily. Removal of the plant from the soil will reveal a small round hole (about the size of a pin head) at the base of the plant where the larva has entered or exited the plant. Almost all ASW damage occurs during the first four weeks of maize plant growth.


A fallow period of 6-8 weeks prior to planting will reduce ASW numbers.  During the fallow period the ASW larvae move to the soil surface where they die from starvation and desiccation. It is important to note that the fallow period starts only when all vegetation, especially the grass, is completely buried. Cultivate to achieve a fine, even seedbed ensuring that all clods are broken up to hasten vegetation breakdown.

The best method of ASW control in maize crops is to plant insecticide treated seed. Poncho® is the preferred choice because it controls greasy cutworm and black beetle as well as ASW.

Poncho® should always be used in crops that are planted into paddocks that were in pasture. Many maize grain growers also choose to use insecticide treated seed when they are planting maize after a previous maize crop, especially if grass weeds are present.

Pioneer Premium Seed Treatment® offers a number of industry-leading insecticide options to protect your crop from insect damage. To carry the Pioneer Premium Seed Treatment stamp of approval. Every bag of Pioneer® brand maize seed must meet stringent quality control standards which ensure the precise dose of seed treatment is applied to each and every seed. For your protection every bag of Pioneer® brand seed is mechanically stitched closed with green and white bi-colour tamper proof string. This locks- in the Pioneer warranty and Seed Replant Risk Cover and guarantees there are a minimum of 80,000 kernels in each and every bag.


Argentine Stem Weevil Biology, Damage, Control (1985) Aglink FPP 48, MAF
New Zealand Pests And Beneficial Insects (1984) Scott, R.R (Ed) Lincoln University
The advice of Paul Addison (AgResearch Ruakura) in the preparation of this bulletin is gratefully acknowledged.

Poncho® is a registered trademark of Bayer

Pioneer® brand products are provided subject to the terms and conditions of purchasing, which are part of the labeling and purchase documents.

®, SM, Trademarks and service marks of Pioneer Hi-Bred International, Inc.

The information in this publication is general in nature only. Although the information in this publication is believed to be accurate, no liability (whether as a result of negligence or otherwise) is accepted for any loss of any kind that may arise from actions based on the contents of this publication.

© 2021, Genetic Technologies Limited. No part of this publication can be reproduced without prior written consent from Genetic Technologies Limited.

Revised: June 2015
Expires: June 2018