Pioneer Technical Insight

 316

Argentine Stem Weevil (Hyperodes 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).

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.

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.

The best method of controlling ASW numbers is the implementation of a fallow period of 4-6 weeks prior to planting. 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.

In all crops that have been planted out of a grass use insecticide treated seed (e.g. Poncho,^® ).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.

Even if you have used insecticide treated seed, you should monitor your crop on a daily basis. Currently there are no insecticides registered in New Zealand to control Argentine Stem Weevil in a germinated maize crop. If you notice ASW damage in your maize crop call the Pioneer Technical Advice Line on 0800 PIONEER (0800 746 633) or contact your local seed merchant or chemical company representative for assistance.

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.

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