Worms destroy grass
An army of worms is on the march across paddocks in the top half of the North Island and DairyNZ is advising farmers to check their new pastures for armyworm.
DairyNZ consulting officers have been receiving reports of damage to new pastures by armyworm in Northland, Waikato and the Bay of Plenty.
These pests can devastate pastures in a very short time.
‘‘ Pastures most at risk are those planted after a crop, especially maize, or in recently flooded areas,’’ said DairyNZ Farm Systems Specialist Phillipa Hedley.
‘‘All new pastures have some risk as there is always a resident population of armyworm. Given the right conditions there can be a population explosion. These conditions usually occur in seasons with the cyclone-like events we have had recently. Pastures that have been flooded are high risk as the eggs get washed to the flooded areas, concentrated and caterpillars hatch just in time to attack the re-sown new grass.
‘‘During the extended dry period preChristmas it was often too dry for the pre-emergence herbicides to be totally effective when the maize was established. Weed control was therefore poor in maize crops. This combined with an extra generation of armyworm due to warm temperatures has resulted in the current population explosion.’’
If they have an infestation DairyNZ is advising farmers to contact their merchant or spray contractor for advice on what to spray.
‘‘Check your new pasture now and if you have an infestation hit it fast and hit it hard,’’ said Ms Hedley.
Paul Addison from Nufarm said that sprays with the active ingredient chlorpyrifos or diazinon were registered for use in pasture.
‘‘There are also other sprays that are effective that are not registered for use in pastures,’’ he said.
The name armyworm comes from the way the caterpillars ‘‘march’’ across a field in formation, eating all suitable plant material in their path. Colours vary. Caterpillars grow up to 50mm long. Populations decline as winter starts.
In summer the eggs hatch within a week and the caterpillars take about 3 to 4 weeks to fully develop. The pupal stage lasts about two weeks and the female moth begins laying eggs about four days after it emerges from the pupa.
Several generations occur during spring to autumn depending on the average temperature; the exact number is not known.
Over-wintering is known to take place at the larval stage, although pupae may also over winter.