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Plantwise Technical Factsheet

Argentine stem weevil (Listronotus bonariensis)

Host plants / species affected
Agrostis capillaris (common bent)
Anthoxanthum puelii (Annual vernalgrass)
Avena sativa (oats)
Bromus (bromegrasses)
Cynodon (quickgrass)
Dactylis (orchardgrass)
Digitaria (crabgrass)
Echinochloa spp.
Eleusine coracana (finger millet)
Festuca pratensis (meadow fescue)
Festuca rubra (red fescue)
Hordeum vulgare (barley)
Lolium multiflorum (Italian ryegrass)
Lolium perenne (perennial ryegrass)
Paspalum
Phleum pratense (timothy grass)
Poa annua (annual meadowgrass)
Poaceae (grasses)
Triticum aestivum (wheat)
Zea mays (maize)
List of symptoms/signs
Leaves  -  abnormal colours
Leaves  -  external feeding
Leaves  -  yellowed or dead
Roots  -  reduced root system
Seeds  -  discolorations
Stems  -  external feeding
Stems  -  internal feeding
Symptoms
Adult feeding on leaves produces narrow, rectangular holes near the leaf tips, giving a windowed appearance. Spots or stripes may also result. Instances of 'silvering' of foliage (resembling slug damage) have also been recorded. Adults produce fibrous frass deposits on leaves. Larvae feeding in the lower parts of stems cause yellowing of young leaves of Festuca rubra tillers. Larval feeding in the tillers can be confused with damage by grass grubs (scarabaeids), although L. bonariensis leaves the roots intact and damages pasture uniformly, rather than in distinct circular areas (EPPO, 1992).
Prevention and control

Cultural Control

Maize seedlings in New Zealand are highly susceptible to L. bonariensis if planted too quickly after cultivation of infested pasture or crop residues, up to 90% of the plants being attacked. It has been shown from tests in the field during 1975-77 that control can be obtained <10% of plants attacked) from a cultivation period of at least four weeks before planting (Watson et al., 1978).

The damage caused by L. bonariensis to maize seedlings sown following different methods of tillage was investigated in New Zealand during 1976-78. In the first year, the tillage techniques most prone to insect attack were one pass with plough and disc; minimum tillage and direct drilling were also followed by considerable damage, but conventional tillage and rotary hoeing were not. Even with insecticidal protection, the direct-drilled and one-pass cultivation treatments suffered more insect damage than the untreated rotary hoeing and conventional cultivation methods (Carpenter et al., 1978).

Barker et al. (1982) found that damage to maize seedlings in New Zealand by larvae was affected by cultivation and sowing dates in relation to the life-history of the weevil. There was a close relationship between egg and larval numbers in pasture before cultivation and subsequent damage in maize that was sown there, with crops sown in October and early November at the greatest risk. Control <10% damage) was obtained from a period of cultivation before sowing at 15 days or more for sowing in the period from mid-November to mid-December.

In field trials of perennial ryegrass (Lolium perenne) pastures in New Zealand during 1984-1986, nitrogen fertilizer (50 kg N/ha) applied in autumn had no effect on damage levels, but increased the ryegrass content of the sward. Hunt et al. (1988) therefore recommended that nitrogen fertilizers should be applied to low-endophyte ryegrass swards in autumn to enhance recovery from damage by L. bonariensis.

In field trials of mixed swards of pasture grasses in New Zealand during 1985-1991, L. perenne cultivar Yatsyn-1 was not attacked by L. bonariensis and annual DM production was significantly higher than that of Festuca arundinacea and Phalaris aquatica (Johnson et al., 1994).

Biological Control

Barker et al. (1991) tested the pathogenicity of the fungal pathogen Beauveria bassiana collected from L. bonariensis, Vespula spp., Otiorhynchus sulcatus, Sitona discoideus, Heteronychus arator and Syrphus novaezelandiae [Melangyna novaezelandiae] to adults of L. bonariensis in the laboratory at 100% RH. Mortality was higher at 25°C than at 15°C. Isolates of B. bassiana from O. sulcatus, S. discoideus and M. novaezelandiae were as pathogenic as those collected from L. bonariensis.

Goldson et al. (1995) studied the release of 100,000 L. bonariensis adults parasitized by the braconid, Microctonus hyperodae, in New Zealand during 1988-1990. In most places, especially in the north, the parasitoid became established and rapidly built up high levels of parasitism (80%), although its rate of spread was modest at approximately 3 km per year. Preliminary results from both Canterbury and the northern North Island showed that the weevil populations were affected in the release areas, indicating that M. hyperodae may play a major role in the future management of the pest.

South American populations of M. hyperodae were introduced into New Zealand during 1991 (Phillips et al., 1997). Populations of M. hyperodae originating from east of the Andes were more prevalent during later surveys. During 1972-1973, 19.2-38.7% of L. bonariensis adults collected in October to December at Bariloche, Rio Negro, Argentina, were parasitized by M. hyperodae; in other months parasitism rates ranged from 4.9 to 8.1% (Loan and Lloyd, 1974).

M. aethiopoides was originally released in New Zealand for the control of Sitona discoideus (Barratt et al., 1998), but also parasitizes L. bonariensis. In New Zealand pasture, high levels of parasitism (22-33%) by M. aethiopoides were found in trapped weevils on three occasions during the winter. Peak parasitism in weevils obtained by sweeping was much lower (under 14%) and it is suggested that the electric blanket method of collecting may have selectively attracted parasitized weevils (Goldson and Proffitt, 1991).

The role of endophytic fungi, particularly Acremonium spp., in protecting host plants from attack by L. bonariensis, has been studied by several authors. Although this is often described in terms of host plant resistance, it is a case of classical biological control (Prestidge et al., 1994).

A. starii infection in Bromus anomalus reduced feeding and oviposition of L. bonariensis adults in the laboratory, compared to healthy plants (Bell et al., 1991). A. lolii, in association with Lolium perenne, produces peramine, a feeding deterrent to L. bonariensis, and lolitrem B, a neurotoxin causing ryegrass staggers in sheep. In New Zealand field evaluations, Endosafe, a zero lolitrem B, high peramine strain of A. lolii, protected its host against L. bonariensis attack and did not cause ryegrass staggers in lambs grazing on three to four L. perenne cultivars (Fletcher et al., 1991). Popay and Wyatt (1995) found that fungal endophytes associated with L. perenne that produced the alkaloids peramine, ergovaline and lolitrem B reduced adult feeding and oviposition, compared with endophyte-free plants or plants infected with an endophyte which produced none of these alkaloids.

Host-Plant Resistance

Field trials in New Zealand by several authors have investigated resistance of forage and pasture grasses to L. bonariensis damage.

Kain et al. (1982) discovered a line of ryegrass (designated Takapau) collected from Central Hawkes Bay, that suffered minimal damage from adult weevils and harboured low numbers of eggs and larvae, compared with commonly-sown cultivars. Swards of Takapau persisted as well-balanced grass-clover mixtures, rather than swards of Grasslands Nui and Grasslands Ruanui which were severely affected by weevil damage and became dominated by white clover (Trifolium repens).

Goldson (1982) found that short-rotation cultivars of Lolium multiflorum showed higher susceptibility to larval and adult attack than ryegrass cultivars, which was related to cellulose levels and tissue toughness. Of the other grasses studied, Festuca arundinacea was highly resistant to oviposition, whereas Phleum pratense was tolerant to larval mining. Resistance to adult feeding in young seedlings of a low-cellulose selection of ryegrass cultivar Grasslands Ariki was not expressed as ovipositional or larval resistance in mature plants. It was suggested that seedlings of this ryegrass initially have high levels of the alkaloid perloline, which confers resistance, but this compound rapidly dissipates as the plants develop.

Barker (1989) found that feeding intensity by L. bonariensis adults on 19 grasses was negatively correlated with fibre (cellulose, hemicellulose and lignin) content of foliage, possibly indicating an effect of leaf toughness on feeding. Oviposition preferences were correlated with feeding intensity; 29-86% of the variation in egg numbers per plant was accounted for in the numbers of feeding scars on these plants. The number of eggs deposited was negatively correlated with the density of intercostal silica deposits (inclusive of trichomes) in the abaxial surface of the grass sheaths. A causal relationship between silification and oviposition preference was confirmed in a pot experiment where increased silica uptake and deposition reduced egg laying on two ryegrass cultivars (Barker, 1989).

Chemical Control

Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources:


Impact
L. bonariensis is a serious pest of pastures in New Zealand (Ferro, 1976; Goldson and Emberson, 1980; Goldson et al., 1982). Adults and larvae are oligophagous, feeding on most representatives of the Poaceae. They can severely damage pastures, particularly perennial ryegrass (Lolium perenne). Although adult damage per se is usually assessed as insignificant, an infestation of 200 adults/m² leads to a large subsequent larval infestation. The high migratory potential of the adults means that pastures of any age can harbour large infestations. Newly-sown pastures are most seriously affected, especially in dry seasons; the adults eating through seedlings at their bases. Systemic insecticides have to be used to prevent this, or sowing is delayed to ensure emergence after the weevils enter diapause (Goldson and Penman, 1979).

On grass hosts, larvae damage both vegetative and reproductive tillers by feeding inside the stems and around the nodes causing tiller death, 'whiteheads' (non-fertile seeds), breaking of stems and lodging. Yield loss attributable to whiteheads is usually not serious, however. Larvae can also induce premature ripening or pinching of seed. Examples of complete destruction of first-and second-year stands of perennial ryegrass (L. perenne) have been recorded. Short-lived ryegrass cultivars, such as Manawa, Patoe and Tama, are often unable to tiller and keep ahead of larval attack; losses of up to 98% have been recorded. During the summer, when first-generation larval attack on pastures is most severe (exacerbated by the retarding effects of drought on plant growth), the long-lived ryegrasses (for example, cultivars Ariki and Ruanui), Dactylis glomerata and Phleum pratense are the least affected. Second-generation larval attack on pastures is often masked by growth.

Larvae cause severe damage to establishing maize and wheat crops and are an important pest of maize throughout the Waikata, South Auckland and Bay of Plenty areas of the North Island of New Zealand.

L. bonariensis is not known to be of any economic importance in South America, where it originated (Chadwick, 1963), but it does damage cereal crops in Brazil and probably short-rotation ryegrasses in Argentina and Chile (SL Goldson, AgResearch, personal communication, 1998). It was reported attacking wheat shoots in Argentina in 1995 (Anon., 1996), which was the first report of the pest in this country since 1972. The damage was caused mainly by the larvae, which fed on shoot and root buds. Damage resulted in death of the shoots, a smaller number of shoots per plant, a smaller number of ears and a reduction in root volume.

L. bonariensis was one of the major pests of barley, wheat and oats in the Wairarapa region, New Zealand, during 1974-80 (Cromey et al., 1980).

Damage caused by L. bonariensis was assessed on tall fescue (Festuca arundinacea) cultivar Grasslands Roa in young pastures of New Zealand during 1985-86. The pest occurred in all the pastures studied, adult populations ranging from 19 to 279/m² in autumn 1985 and 1986, respectively. The percentage of tillers killed by larvae ranged from 4 to 39%. Studies showed that without insecticide protection, tiller densities decreased throughout the summer and autumn (Prestidge et al., 1989).

The economic cost of L. bonariensis in the 7 million hectares of improved pasture in New Zealand was assessed by Prestidge et al. (1991). Damage to L. perenne (perennial ryegrass) resulted in reduced animal carrying-capacity and the need for pasture renovation was estimated as costing NZ$ 46-202 million annually. Other costs attributable to L. bonariensis include those associated with reduced animal health due to changes in pasture quality, such as ryegrass staggers, facial eczema and bloat. These economic losses totalling NZ$ 78-251 million per annum make L. bonariensis the most important insect pest in New Zealand.

A greenhouse pot trial and two field trials were performed in New Zealand during 1991-92 to determine the susceptibility of early autumn sowings of ryegrass to L. bonariensis. Adult population densities of 35/m² reduced seedling survival and DM yield of L. multiflorum by 33.1 and 31%, respectively, after 28 days, compared with 22.6 and 26.3%, respectively for L. perenne. Most seedling mortality occurred within 7 days of germination (Prestidge et al., 1994).

The potential of L. bonariensis as a vector of cocksfoot mottle sebemovirus and ryegrass mosaic rymovirus (RgMV) was investigated in the laboratory at 20°C by Smales et al. (1995). One individual of this weevil transmitted RgMV to perennial ryegrass (L. perenne) and an unknown virus to cocksfoot (D. glomerata).
Related treatment support
 
External factsheets
AgPest Factsheets, AgResearch, English language
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