Pests - Argentine Stem Weevil
Peak Activity: October-December & February-April
Argentine Stem Weevil (ASW) is a major pest of ryegrass and is also a threat to
forage brassica crops. ASW can be found throughout New Zealand and is a
potential pest every year with climatic conditions having little effect on
numbers. Generally there are two generations per year, one during spring and the
other during late summer – early autumn. Usually the summer/autumn generation is
the most damaging to ryegrass plants because it coincides with new pasture
establishment in many regions. All ryegrasses both spring and autumn sown are at
risk of ASW attack during plant establishment. Ryegrass seed established ‘grass
to grass’ by no-tillage following a glyphosphate spray out or seed ‘undersown’
into existing pasture is particularly vulnerable to ASW attack, because the
existing plant material may be hosting the pest.
Both the adult and larval stages of ASW attack establishing ryegrass plants.
Larvae appear from October through until March, feeding on seedlings by
burrowing into the ryegrass stem at emergence, causing the developing plant to
turn yellow and then brown and ultimately die. Adult ASW feed on foliage
throughout the year leaving narrow rectangular holes in the leaves (usually near
the tips). Of the two stages the larval stage is the most damaging. The damage
caused by ASW is often overlooked because the pest is difficult to see and the
impact on pasture can be more visual from other pests such as Grass Grub and
Black Beetle. ASW damage and the poor resulting growth of pasture is often
mistakenly attributed to other factors such as drought.
Prevention and Control
- Superstrike Grass or Ultrastrike Brassica seed treatments
- Removal of host ryegrass plants
- Recognise flight periods and control infestations early
- Sowing after peak ASW flights
- Contact insecticide
- Both seed treatment and a contact insecticide is recommended in no-tillage situations
- Use of tolerant species e.g. cocksfoot, tall fescue
- Use of grasses with standard, AR1, Endo5, AR37 or MaxP endophytes for long term protection
