Listronotus bonariensis

Listronotus bonariensis is a species of weevil that is native to South America and is commonly known as the Argentine stem weevil. It is a pest of grasses and cereals, with the larvae being more destructive than the adult insects. It has spread to Australia and New Zealand, where it is regarded as a pest species.

The Argentine stem weevil is a small grey insect up to 3 mm (0.12 in) long, with a waxy covering. Soil particles adhere to this wax, and while the insect remains stationary, which it often does, this makes it well-camouflaged and difficult to spot. The larvae are cream-coloured grubs with brownish heads.^[1]

The Argentine stem weevil is native to South America where it is present in Brazil, Uruguay, Argentina, Bolivia and Chile. In these countries it is not considered to be economically important.^[2] It has spread to Australia and New Zealand, in both of which countries it is regarded as a pest species. In New Zealand, it is considered to be the most important insect pest in the country, causing a total loss estimated in 1991 to be NZ$78–251 million per year.^[2]

This weevil feeds on a number of crop plants including perennial ryegrass, on which it is a major pest, Italian ryegrass, other grasses, wheat, barley, oats and maize.^[2] The adult beetle cuts small window-like holes in the foliage but the larvae do more damage. Batches of up to five eggs are laid, usually in a single row. The eggs are less than 1 mm (0.04 in) in length and are yellow at first, but darken with age. They hatch after one to two weeks.^[1]

In wheat, young plants are often killed by the larvae and plants at later stages of growth are stunted, have smaller root systems and fewer tillers. Attacks are more prevalent in reduced tillage systems because this encourages the growth of wild grasses and weeds which can also act as hosts. Seedlings of maize are also badly affected.^[1]

In perennial ryegrass, the eggs are laid on the leaf sheaths and the newly hatched larvae bore into the stem below. As each larva passes through its four instar stages it burrows through the plant and may destroy up to five tillers in the process.^[3] It is usual for only one larva to be present in each tiller.^[1]

En Nueva Zelanda se han realizado intentos para controlar biológicamente el gorgojo del tallo argentino. La avispa bracónida Microctonus hyperodae es un parasitoide de esta plaga en Sudamérica y se ha introducido para controlarla. La avispa se ha establecido en la Isla Norte y parece estar reduciendo las poblaciones del gorgojo. Otra avispa, Microctonus aethiopoides, se había introducido previamente para controlar el gorgojo de la raíz del trébol ( Sitona lepidus ), y también se ha descubierto que parasita a L. bonariensis . Varias especies de hongos endófitos , especialmente Acremonium spp., también se han estudiado con miras al control biológico en Nueva Zelanda. Su presencia dentro de la planta de cultivo disuade la puesta de huevos del gorgojo y reduce el daño causado por las larvas; sin embargo, Acremonium lolii hace que el raigrás perenne sea tóxico para el ganado en algunas etapas de crecimiento, lo que los pone en riesgo de desarrollar la enfermedad conocida como temblores del raigrás perenne . ^{[ 2 ]}