HortFACT
The smooth, shiny eggs, under a millimetre long and three times longer than wide, cylindrical, and rounded at both ends, are laid in small clusters, end to end or diagonally beside one another, tucked into the leaf sheath tissues. When first laid they are yellow, but darken within a day to a smoky black. These dark eggs are quite visible and can be felt as small bumps if the left sheath is run between the fingers.
The newly hatched larva is about 1 mm long. Larvae mine in a tiller, and will move from one stem to another in search of food. They grow to about 5 mm long, moulting three times before leaving the plant to pupate in the soil. The larva is legless with a creamy white body and small but distinct straw-coloured head. It is crescent-shaped, wider in the middle than at either end.
The pupa is white and less than 3 mm long. It has the "beak" characteristic of all weevils. Its small size distinguishes it from the pupae of other weevil species occurring in pasture.
Host plants Grasses, especially Italian and endophyte-free perennial ryegrasses, and cereals, including seedling maize, are the main hosts. Adults may also feed on germinating cruciferous crops.
Damage Adult feeding is characterised by "windowing" of the grass leaf near its tip. Windows are typically rectangular and appear as clear areas, the pane being the lower leaf cuticle which is left intact. Veins encountered while the adults feed are cut at the base and displaced. They appear as pigtail-like threads attached at the upper edge of the window. Adult feeding damage is not normally significant except in summer-sown pasture. Larvae mining the stem do the greatest damage to vegetative tillers, which wilt and yellow from the centre outwards. Damaged tillers contain larval frass and may show small, circular exit holes near the base. Flowering tillers when attacked whiten and may snap off owing to mechanical weakening. Where infestation is heavy, severe damage is done to the sward, particularly in dry conditions when pasture growth is retarded.
The presence of an endophytic fungus in perennial ryegrass makes it less susceptible to Argentine stem weevil attack.
Distribution Argentine stem weevil is present throughout New Zealand.
Life cycle
| Egg |  |
| Larvae | |
| Pupae | |
| Adult | |
| Month | |
| Note: Coloured bars indicate periods of peak activity in each of the life cycle stages | |
There are typically two generations per year, and larval damage may be apparent from mid October to mid April. Annual ryegrass may be completely eaten out by the larvae of the first generation, but in perennial ryegrasses the second generation is usually larger and more damaging.
Adults of the second generation go into a reproductive diapause [resting stage] in autumn which is triggered by photoperiod [day length]; egg laying ceases in the first week in March. These adults over-winter in plant litter at the soil surface and become active, mate, and lay eggs in the early spring of the following season.
RJ.B. POWER